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ISSN 2354-7065Vol.16, February.2015

Journal of Ocean, Mechanical and Aerospace -Science and Engineering-

Vol.16: February 2015

ISOMAse

International Society of Ocean, Mechanical and Aerospace -Scientists and Engineers-

Contents

About JOMAse

Scope of JOMAse

Editors

Title and Authors PagesSea Level Rise and the Impact for the Port

Muhammad Zikra, Kriyo Sambodho Defian Naturezza

1 - 5

A New Implementation of Vortex Lattice Method Applied to the Hydrodynamic Performance of the Propeller-Rudder

Hassan Ghassemi, Farzam Allafchi

6 - 11

Numerical Study on the Fluid Forces of a Rigid Cylinder Covered by Helical Rods with Gap Due to the Variations of Incoming Flow Direction and Pitch at Reynolds Number 1000

Arief Syarifuddin, Rudi Walujo Prastianto, Silvianita

12 - 17

Air Flow Characteristics and Behaviour of Main Rotor Blade of Remote Controlled Model Scale Helicopter

Mohd. Shariff bin Ammoo, Ziad Bin Abdul Awal, Jaswar Koto

18 - 22



ISOMAse


About JOMAse

The Journal of Ocean, Mechanical and Aerospace -science and engineering- (JOMAse, ISSN: 2354-7065) is an online professional journal which is published by the International Society of Ocean, Mechanical and Aerospace -scientists and engineers- (ISOMAse), Insya Allah, twelve volumes in a year. The mission of the JOMAse is to foster free and extremely rapid scientific communication across the world wide community. The JOMAse is an original and peer review article that advance the understanding of both science and engineering and its application to the solution of challenges and complex problems in naval architecture, offshore and subsea, machines and control system, aeronautics, satellite and aerospace. The JOMAse is particularly concerned with the demonstration of applied science and innovative engineering solutions to solve specific industrial problems. Original contributions providing insight into the use of computational fluid dynamic, heat transfer, thermodynamics, experimental and analytical, application of finite element, structural and impact mechanics, stress and strain localization and globalization, metal forming, behaviour and application of advanced materials in ocean and aerospace engineering, robotics and control, tribology, materials processing and corrosion generally from the core of the journal contents are encouraged. Articles preferably should focus on the following aspects: new methods or theory or philosophy innovative practices, critical survey or analysis of a subject or topic, new or latest research findings and critical review or evaluation of new discoveries. The authors are required to confirm that their paper has not been submitted to any other journal in English or any other language.



ISOMAse


Scope of JOMAse

The JOMAse welcomes manuscript submissions from academicians, scholars, and practitioners for possible publication from all over the world that meets the general criteria of significance and educational excellence. The scope of the journal is as follows:

• Environment and Safety • Renewable Energy • Naval Architecture and Offshore Engineering • Computational and Experimental Mechanics • Hydrodynamic and Aerodynamics • Noise and Vibration • Aeronautics and Satellite • Engineering Materials and Corrosion • Fluids Mechanics Engineering • Stress and Structural Modeling • Manufacturing and Industrial Engineering • Robotics and Control • Heat Transfer and Thermal • Power Plant Engineering • Risk and Reliability • Case studies and Critical reviews

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ISOMAse


Editors

Chief-in-Editor

Jaswar Koto (Ocean and Aerospace Research Institute, Indonesia Universiti Teknologi Malaysia, Malaysia)

Associate Editors

Abyn Hassan (Persian Gulf University, Iran) Adhy Prayitno (Universitas Riau, Indonesia) Adi Maimun (Universiti Teknologi Malaysia, Malaysia) Agoes Priyanto (Universiti Teknologi Malaysia, Malaysia) Ahmad Fitriadhy (Universiti Malaysia Terengganu, Malaysia) Ahmad Zubaydi (Institut Teknologi Sepuluh Nopember, Indonesia) Ali Selamat (Universiti Teknologi Malaysia, Malaysia) Buana Ma’ruf (Badan Pengkajian dan Penerapan Teknologi, Indonesia) Carlos Guedes Soares (Centre for Marine Technology and Engineering (CENTEC), University of Lisbon, Portugal) Dani Harmanto (University of Derby, UK) Iis Sopyan (International Islamic University Malaysia, Malaysia) Jamasri (Universitas Gadjah Mada, Indonesia) Mazlan Abdul Wahid (Universiti Teknologi Malaysia, Malaysia) Mohamed Kotb (Alexandria University, Egypt) Priyono Sutikno (Institut Teknologi Bandung, Indonesia) Sergey Antonenko (Far Eastern Federal University, Russia) Sunaryo (Universitas Indonesia, Indonesia) Tay Cho Jui (National University of Singapore, Singapore)

Published in Indonesia.

ISOMAse, Jalan Sisingamangaraja No.89 Pekanbaru-Riau Indonesia http://www.isomase.org/

Printed in Indonesia.

Teknik Mesin Fakultas Teknik Universitas Riau, Indonesia http://ft.unri.ac.id/

Journal of Ocean, Mechanical and Aerospace -Science and Engineering-, Vol.16

February 20, 2015

1 Published by International Society of Ocean, Mechanical and Aerospace Scientists and Engineers

Sea Level Rise and the Impact for the Port

Muhammad Zikra,a,*, Kriyo Sambodho,a and Defian Naturezza,b

a)Department of Ocean Engineering, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia b)Operational Research Laboratory, Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia *Corresponding author: [email protected] Paper History Received: 1-February-2015 Received in revised form: 12-Februay-2015 Accepted: 17-Februay-2015 ABSTRACT The problems in port infrastructures related with climate change, especially sea level rise, have continued to receive a high level of attention. Knowledge of climate change should become a major interest for engineer, stakeholders, and decision makers or the port authorities in port industry for developing mitigation and adaptation strategies in the future. The objective of this paper is to measure sea level change in the Indonesian sea from satellite altimetry. In this study, satellite altimetry mission of Jason-2 are used to obtain altimetry data from NOAA server databse. These data are processed by using software Basic Radar Altimetry Toolbox (BRAT). Analysis of sea level change is done for 4 years period from 2009 to 2012 in 4 locations, which are Medan, Pemangkat, Ambon, and Manokwari. The results showed that the highest sea level rise is in Manokwari 14.10 mm/year, and the lowest is in Ambon with trend of sea level rise 1.17 mm/year. KEY WORDS: Sea Level Rise; Satellite Altimetry; JASON-2; Port; BRAT 1.0 INTRODUCTION Indonesia is known as one of the largest archipelagic country in the world. The Indonesian seas are four times greater than its land area. Indonesia also located in strategic position locates in the cross road of East-West and North-South shipping route. Sea transportation as one element in the transportation system plays

key roles in servicing and promoting the economic growth in Indonesia.

Ports as main elements of sea transportation are important infrastructure for supporting economic development in Indonesia. Until 2011, Indonesia has more 1031 public ports and special ports and wharfs, where 111 public ports to be managed commercially by four Indonesian Port Corporations, PT Pelabuhan Indonesia (PELINDO) I, II, III and IV in order to improve effectiveness and efficiency of public port management.

As the largest archipelago nation in the world, Indonesia is one of the countries that are most vulnerable to the negative impacts of climate change. Generally, the global climate change will affect public and private transportation in coastal areas through sea level rise, intensity of rainfall that will increase the risk of floods, increased frequency of extreme wave climate or storm events [1]. One of the largest threats of climate change to the Port is known the increase of the sea surface level.

In general, the increase in sea level provides a huge potential threat to Indonesia areas which is an archipelagic country with many islands and ports as main element of sea transportation as seen in Figure 1. In 2050, sea level rise due to global warming is projected to reach 35-40 cm relative to year 2000. Based on these projections, the maximum sea level rise in Indonesia can reach up to 80-175 cm in 2100 [2]. At this magnitude of rise, many ports will face inundation in the future.

Based on that fact, this phenomenon requires further study in order to find out how much sea level rise is occurred in Indonesia waters. Because an accurate estimation of sea level data is difficult to obtain due to the lack of consistent data recording in Indonesia territory. Added, conventional observation methods such as using survey vessels and in situ observations with tide gauge are not an effective and efficient for a very large water area in Indonesia. Therefore, to overcome this problem the satellite altimetry is used to measure the sea level change from space. Analysis of sea level change is done for 4 years period from 2009 to 2012 in Indonesia seas. This kind of information is important for better understanding when and how to implement adaptation and mitigation strategies for port development in the future related to sea level change phenomena.


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Figure 1: Ports location around Indonesia seas. (Bokosurtanal, 2004) 2.0 SEA LEVEL TREND USING ALTIMETRY

SATELLITE Now days, there are several remote sensing technologies that can monitor the condition of the oceans continuously. Satellite altimeter technology is one of technique to monitor sea level change. During the past two decades, observations from satellite altimeters have demonstrated dramatic descriptions of sea level variability with higher spatial resolution than the traditional tide gauges [4]. High accuracy radar altimeter missions are uniquely capable to globally and continuously observe the ocean for better understanding long term changes of ocean circulation [5]. Sea Level Anomaly Sea Level Anomaly (SLA), also referred to as residual sea surface, is defined as the sea surface height (SSH) minus geophysical surface and geophysical effect, namely tidal and inverse barometer [5]. Geophysical surface in this case can be either the geoids or Mean Sea Surface (MSS). SLA is calculated using the formula SLA = SSH – MSS – Correction (1) where Sea Surface Height (SSH) is calculated by subtracting the corrected range from the Altitude, which is [5]:

SSH = Altitude – Corrected Range (2) Sea Surface Height (SSH) in the equation 2 still contains the effects of short-period variations such as tidal. Furthermore, these effects should be eliminated so that the phenomenon of sea level rise can be seen through the temporal analysis. Based on this, SLA was used to observe the phenomenon of sea level rise, because these effects have been eliminated in SLA data. The SLA contains information about real changes in ocean topography related to ocean currents, dynamic response to atmospheric pressure, differences between tides and the tide models, differences between the mean sea surface model and the true mean sea surface, unmodelled or mismodeled measurements effects and orbit errors.

SLA calculation is performed to obtain an average value of SLA per month at each observation point. In one month there were ± 3 cycles, monthly calculation is performed to get a definite value that will be used to analyze sea level rise phenomenon per year, for 4 years of observation (from 2009 to 2012) at each observation points.


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3.0 DATA In this research, we use GDR (Geophysical Data Record) data that obtained from Jason-2 satellite for period 2009-2012 which passed Indonesian waters. Cycle data which is used in this paper after selection is ranging from 018-165 cycles.

This data were obtained from NOAA server (http://data.nodc.noaa.gov/jason2/gdr/gdr site). These data will be used to calculate Sea Level Anomaly (SLA) for the analysis of sea level rise. There are 4 observation points deployed along the trajectory following the Jason-2 orbits that pass through the territory of Indonesia as seen in Figure 2 and 3.

Figure 2: Observation points in Indonesian waters.

Figure 3: Jason-2 orbits track

4.0 RESULT AND DISCUSSION Trend of Sea Level Rise In this study, Geophysical Data Record (GDR) data that obtained from altimetry mission of Jason-2 are processed and controlled by using software BRAT. In the BRAT, data that are not in accordance with proper reference will be automatically removed. Only data those are really appropriate with the reference which can be processed using this software. GDR data products are fully validated data and generated every 30 days [5]. GDR files contain sea surface height, ocean surface wind speed, significant wave height information and all required corrections. All files are available in NetCDF format.

For this study, GDR data will be used as an input in software BRAT to calculate Sea Level Anomaly (SLA). SLA images and graphics will be displayed for each observation point as well as a whole area. The output example of BRAT for SLA analysis is

given in Figure 4. Then SLA calculation is performed to obtain an average value of SLA per month at each observation point. Figure 5 shows the averaged SLA results for every observation points during 4 years of observation period. Then, linear trend analysis was conducted to determine the existence of the phenomenon of sea level rise on the data SLA Jason-2 satellite for the period 2009-2012 as shown in Figure 5.

Figure 4: SLA for November 2011


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Figure 5: Trend analysis of sea level rise

From the trends of analysis of 4 observation points, indicated

that there were increased trend of sea level in Indonesian seas as shown in Table 1. The highest trend of sea level rise is 14.10 mm/year at Manokwari (North West Papua) and the lowest is in Ambon with trend of sea level rise 1.17 mm/year.

Table 1: Trend of sea level rise from 2009 to 2012

Location Trend (mm/year)

1 Medan 12.90 2 Pemangkat 3.52 3 Ambon 1.17 4 Manokwari 14.10

Potential Impact of Sea Level Rise The 4th assessment report by the Intergovernmental Panel on Climate Change (IPCC) published in 2007 has warned that the warming of the earth’s climate system is unequivocal. Global warming as a result of the effects of greenhouse gases, have an impact on rise in sea levels. IPCC (2007) reported that the sea level has risen by an average of 2.5 millimeters annually.

The past few years, have witnessed an increase in damage due to the extreme weather events in Asia or Indonesia itself. Coastal erosion, inundation of land, sedimentation, coastal flooding, and increased salinity are common problems that affect man-made infrastructure and ecosystems in coastal areas. Based on this

study, there were increased trend of sea level in Indonesia, which make Indonesia particularly very vulnerable to sea level rise.

Even though, sea level rise would have no direct impact on navigation, but it would affect port infrastructure and port structure. Climate change could affect port environments in different of ways. Climate change will affect ports, depending on their geographic location and topographical area which the port is located. Ports that are located in low lying areas are sensitive to sea level rise, increased flooding, changes in the frequency and intensity of wind speed and increased storm surges.

Sea level rise may allow greater penetration of wave energy to the coastline and into port, causing increased coastal erosion in areas with a soft coastline. Increased sedimentation in harbors, inlets and channels will result in the need for more maintenance dredging to maintain exists access. Sea level rise also may reduce top clearance between ships and bridges due to a change in high and extreme sea levels.

Another potential impact on port structures include changes in overtopping and even stability of breakwaters due to increased force from wave action coupled with attack at a higher level on a structure due to sea level rise. The elevation at which the wave force attack a structure will increase the exposure of decks of wharfs and piers, which may increase the corrosion rate and the degradation of material designed for a particular range of sea level conditions.

The problems and potential impacts of sea level rise require serious attention from the scientific community, stakeholders, decision makers and the port authorities to take an active role to better understand when and how to implement proactive adaptation and mitigation strategies. With better data and information, especially the types of impacts that they can expect on their facilities, it can help the port assess their risk in the future. Because the data that ports typically use for planning purposes do not incorporate climate change forecasts. 5.0 CONCLUSION In this study, the sea level change in the Indonesian seas was measured from satellite altimetry. Satellite altimetry mission of Jason-2 are used to obtain altimetry data from NOAA server databse. These data are processed by using software Basic Radar Altimetry Toolbox (BRAT). Analysis of sea level change is done for 4 years period from 2009 to 2012 in 4 locations, which are Medan, Pemangkat, Ambon, and Manokwari. The results showed that the highest sea level rise is in Manokwari 14.10 mm / year, and the lowest sea level rise is in Ambon at 1,175 mm / year. This kind of information is important to study the impact of sea level rise to the port infrastructures and perhaps strategy for adaptation and mitigation in the future.

ACKNOWLEDGEMENTS The authors would like to thank NOAA for providing altimetry data for this study.


February 20, 2015


REFERENCES 1. Wirght, P. (2013). Impact of climate change on ports and

shipping, MCCIP Science Review 2. ICCSR. (2003). Basis Saintifik: Analisis dan Proyeksi

Kenaikan Muka Air Laut dan Cuaca Ektrim. Bappenas, Indonesia

3. Bokosurtanal (2004). Data Spasial Lokasi Pelabuhan. Jakarta

4. A. H. M., Din and K., Omar (2009) Derivation of sea level anomaly using satellite altimeter. In: Proceeding of 2009 East Asia Hydrographic Symposium & Exhibition (EAHSE), Kuala Lumpur.

5. AVISO. (2011). OSTM/Jason-2 Products Handbook. CNES, EUMETSAT, JPL, NOAA/NESDIS

6. ESA, and CNES. (2011). Basic Radar Altimetry Toolbox v3.0 User Manual. ESA, CNES.

7. IPCC. (2007). Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press.


February 20, 2015


A New Implementation of Vortex Lattice Method Applied to the Hydrodynamic Performance of the Propeller-Rudder

Hassan Ghassemi,a,* and Farzam Allafchi, a

a )Department of Ocean Engineering, Amirkabir University of Technology (AUT), Iran

*Corresponding author: [email protected] Paper History Received: 2-February-2015 Received in revised form: 7-February-2015 Accepted: 17-February-2015 ABSTRACT This paper describes a new implementation of a vortex lattice method based on modified lifting line (VLML) for ship propeller-rudder. Method has been employed to estimate hydrodynamic performance and flow fields, for design and analysis. For this purpose, the results obtained using theoretical model are validated against experimental data, carried by Tamashima et al. [18], concerning to propulsor system. Comparison of these results indicates good agreement with those of the experimental data. Therefore the method can be used as a fast tool for preliminary design and analysis. KEY WORDS: Vortex Lattice Method; Hydrodynamic Performance; Propeller-Rudder. NOMENCLATURE

Vortex Strength Unit Vector Tangent to Vortex Distance Vector Differential Element of Length along the Vortex Line Axial Force Tangential Force Axial Induced Velocity Tangential Induced Velocity Rotational Velocity of Propellers

Radius of Blade’s Element Blade Pitch Angle

Hydrodynamic Pitch Angle Pitch Angle of The Helical In The Wake

Control Point Radius Vortex Point Radius Lift Coefficient

Thrust Torque Number of Blades Advanced Coefficient

Thrust Coefficient Torque Coefficient

Efficiency 1.0 INTRODUCTION Propellers work as the whole or the main part of the propulsion systems of marine vehicles, thus the prediction and the calculation of the propulsor performance is a momentous matter for hydrodynamic specialist and designers; because, these calculations and predictions play important roles in achieving the favorable design speed of vessels with the lowest consumption of energy.

For this purpose, considerably improvements have been fulfilled during the past decades, thus, several methods are available based on different levels of sophistication, each of them have their own benefits and disadvantages. The most precise methods are three-dimensional viscous flow models, which the three-dimensional Reynold-Average Navier-Stockes (RANS) equations have been solved iteratively. Following are the advanced lifting surface methods which solve RANS equations to account viscous effects near the solid walls [1]. The next methods are the boundary element method (BEM) which generates


February 20, 2015


elements just at the boundary of body which leads to the reduction of computing time and cost [2]. Other innovative procedures were presented which tend to reduce the cost of calculations. Momentum and blade element theory were combined to predict the performance of marine propellers compared results with those found using RANS equation, comparison indicates that simple theories are applicable as well as more complicated one [3].

The conventional vortex lattice method (VLM) is a subclass of the lifting surface method which replaces the continuous distribution of vortices by a set of concentrated line elements of constant strength [4]. This method was optimized by Olsen (2001) and was used by Kinnas and Coney (1988) and Coney (1989) for the modeling of ducted propellers [5,6,7]. Moreover, Performance analysis of podded propulsors has been made with a vortex lattice method [8].

Lifting line is another method which utilizes concentrated vortices to predict lifting body performances. Lifting line is known as a fast procedure with simple formulation, which is used for hydrofoils mainly [9,10]. This method is being used as a powerful method for studying hydrodynamic behavior near free surface [11,12].

A combination of VLM and lifting line was presented, which uses VLM concepts with the simplicity of the lifting line [13]. This method has been used for off design condition and ducted propellers separately, they showed good result from this method [14,15]. Driss et al. (2012) investigated the effect of multiple Rushton impellers configurations on hydrodynamics and mixing performance in a stirred tank by CFD code [16]. Recently, Rui and Koto (2014) carried out prediction of propeller performance using quasi-continuous method [17].

The main tasks involved in this method include the panel generation, calculation of the induced velocities per unit-strength vortices and the obtaining and solving the equations, to calculate total thrust, torque and efficiency. In this paper, the hydrodynamic analysis of some conventional propellers and a propeller-rudder are performed using VLML. Then the results are compared with experimental data of other works done. 2.0 MATHEMATICAL FORMULATION Considering a three-dimensional vortex of constant strength ( ), acting along the entire length of the line describing its path through space, the induced velocities are calculated using Biot-Savart’s law

| | (1) An element of blade is shown in Figure 1. which and are the axial and tangential velocities of input flow, is the total velocity and finally , and are the induced, frictional and total force acting on the element, respectively.

Using Kutta-Joukowski’s theorem induced force can be obtained and by suitable resolution of forces, the axial and tangential forces are fulfilled.

(2)

Figure 1: Velocity triangles and forces on a blade element.

2.1 Vortex Lattice Implementation The lattice is structured in two main zones, i.e. blade zone and wake region. Blade zone is modeled by bound vortices that are expanded between two vortex points with constant strength and radial direction. Wake is made up of some twin semi-infinite helical vortices (trailing vortex) with opposite directions, which are situated in bound vortices ends (vortex points). Fig. 2 shows the lattice explained. The force analysis is performed at control points which are determined with cosine spaced.

Figure 2: Vortex lattice modeling

2.2 Calculation of Induced Velocities The induced velocities are computed at control points, each horseshoe vortex induces a velocity at every control point, and thus the total induced velocity at control points is computed as follows:


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where , and , are the axial and tangential velocity induced at the ’th control point by a unit-strength horseshoe vortex surrounding panel. Whereas the control points are located at the bound vortex’s axis, they do not induce velocity at control points, therefore the induced velocity at control points are just due to trailing vortices as follows:

where , and , are velocity induced by a single unit-strength trailing vortex of the ’th panel at the ’th control point, which are computed using the approximation by Wrench for a constant-pitch helical vortex filament as follows: for : , 2

, , , 1,2, … ,

and for : ,

, 1 2 (5) where

(9)

where is the pitch angle of the helical in the wake for the

vortex points as computed below

where is the ratio of the pitch to propeller diameter, this equation is fulfilled through several trail and errors. 2.3 Governing Equations Based on Prandtl’s thin airfoil theory:

where indicates the quantities at the ’th panel and is the chord

(12) The following angles can be computed using geometrical equations. Replacing the values from the last equations, a nonlinear simultaneous equation is obtained with unknown ( , , … , , that should be solved iteratively. In the present study, Newton's method for systems has been employed to solve the mentioned equation. This procedure converges rapidly however, in some cases it may be difficult to choose an appropriate initial guess. Therefore, steepest descent method (SDM) was employed to improve it. 2.4 Propeller Formulation Solving the simultaneous equation results to the circulation distribution, which eventuate to the hydrodynamic forces. Fig. 3 shows circulation distribution computed by code for a propeller in different operating conditions. The total thrust and torque of the propeller is obtained from integrating the elements of axial force and torque over the radius.

, (13)

The hydrodynamic coefficients of the propeller are determined as

,

,

(3)

, , , 1

, , , 1 (4)

12

11

. 11

124

9 21 .

3 21 . 1

11

(6)

12

11

. 11

124

9 21 .

3 21 . 1

11

(7)

(8)

1.5

0.5 (10)

22 (11)


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follows:

(14)

2

where is the rotational speed.

Figure 3: Circulation distribution for B4,70, P/D = 1.0,

( ).

2.5 Rudder Formulation Rudder can be considered as a simplified propeller blade, which is placed in the wake of propeller therefore propeller induce different velocity at every rudder panel. For considering the effect of propeller blades and wake, the inflow velocity at each rudder panel is calculated using super-position. Different terms of rudder inflow are classified below:

• Axial inflow velocity of propeller, . • Tangential inflow velocity of propeller, . • Induced velocity due to helical vortices (propeller

wake). • Induced velocity due to bound vortices (propeller

blades). • Rudder’s trailing vortices induced velocity.

3.0 PROPELLER’S NUMERICAL MODELING In this study, to determine the open-water characteristics of the propeller, different types of propeller models has been performed by means of VLML. Undisturbed velocity ( ) was considered as a variable parameter in order to obtain different values of advanced coefficient ( ) while the rotational speed is fixed. To achieve the panel independency of the model, several models were created with different panel size. The panels’ size is reduced step wisely until the results converged to a same point after a specific step. Fig. 4 shows computed hydrodynamic parameters

versus number of panels. The propeller blade has been divided into thirty panels on both radial and chordwise directions. Therefore, all panels are for the propeller is about 2700 panels.

Figure 4: Hydrodynamic performances versus number of panels on both radial and chordwise directions

Comparison between VLML results and experimental data is given in Fig. 5. As a general indication, the accuracy in the prediction of propeller performance using VLML is acceptable, which depends on the value of and / . In particular, the best results are fulfilled when is rather close to propeller design point (maximum efficiency). In the mentioned region the maximum discrepancy from experimental results being in order of 5% in prediction of , and .

Results for / =0.5 are not as accurate as the other ones found for other pitch ratios. Increasing pitch ratio in minimum region causes an increase in discrepancy between calculated torque coefficient and experimental data. For / =0.5, the thrust is overestimated near minimum , this trend became inversed by increasing pitch ratio. This is due to the difference in operating condition of propellers. For a propeller with high pitch ratio operating at minimum in super heavy condition, the 3D effects are significant, but in this specific work were neglected.

The wake is modeled with elaborate helical vortex filaments, which are constant-pitch and constant-radius. In a real case, the wake angle and pitch are as the function of several parameters such as advanced coefficient ( ), which is neglected. These assumptions are for simplicity, but cause an error in the performance prediction of a propeller in off-design condition.


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Figure 5: Open water hydrodynamic characteristics of B:4-0.70.

The most prominent task in this procedure is prediction of the

wake angle, which used to be calculated iteratively. In this method, this angle is related to pitch ratio without iteration, which causes precipitation of the calculations, as it can be run in a conventional personal computer in a few seconds. 4.0 PROPELLER-RUDDER’SNUMERICAL

MODELING VLM was applied to predict hydrodynamic performances of rudder as a part of propulsion system. Rudders work in propeller wake, thus effects of propeller play important role in rudder performance. These effects are considered as induced velocity behind the propeller. Figure 6 shows induced velocity components in different positions, where X is the distance behind the propeller. The computed results are validated against experimental data carried out by Tamashima et al. (1993), which acceptable agreement was observed [18]. Discrepancies are not enough acceptable at low radius ratios; this is owing to neglecting the hub effects, whereas at higher radius predictions are admissible. The main dimension of the propeller-rudder (employed by Tamashima et al.) is presented in Tables 1 and 2.

The axial induced velocity component shows more accurate predictions rather than tangential velocity, particularly in maximum values. The error is due largely to simplifying assumption of helical trailing vortex such as constant-pitch and constant-radius. The axial component is the most important one for rudder analysis. Fig. 7 shows the contour of axial velocity at four distances behind the MP101 propeller. Tangential component of induced velocity causes forces on two region of rudder (up and down), which neutralize each other due to axis-symmetrical attributes of propeller’s wake. Thus the obtained result of force analysis of rudder in presence of propeller would be reliable, (Fig. 8). The results show good agreement in all operating conditions except the rudder angles higher than 30[deg.] and for high advance coefficient ( =0.55), which is attributed to cavitation, that is neglected in present method.

Table 1: Dimension of propeller models

Propeller type MP24 MP101

Diameter [mm] 210 220 Exp. Area Ratio 0.62 0.55 Pitch Ratio 0.89 0.80 Boss Ratio 0.1888 0.18 No. of Blades 5 4

Rake Angle [deg.] -12.03 10.0 Thickness Ratio 0.0405 0.050 Blade Section MAU MAU

Figure 6: Induced velocity components behind the propeller without rudder (MP24, J=0.3)

Figure 7: Axial velocity contour behind MP101 propeller at J=0.6; a) X/D=0.05 b) X/D=0.15 c) X/D=0.25 d) X/D=0.35

Figure 8: Rudder Lift at different operating conditions in presence of propeller (MP101-MR21)


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Table 2: Dimensions of rudder models Rudder Type MR21

Shape REC Span[mm] 240 Chord [mm] 160 Type of Section NACA 0015

The total thrust and torque of propeller-rudder system

calculated using present method. An increase in rudder angle causes increase in rudder drag, thus the whole propulsion thrust decreases, as seen in Fig. 9. In the present calculations, propeller-rudder is discretized by 2700 panels for the propeller and 600 panels for the rudder, totally is about 3300 panels.

Figure 9: Total thrust and torque of the whole propulsion system at different operating conditions (MP101-MR21). 6.0 CONCLUSIONS In conclusion, a new implementation of the VLML based on lifting line was presented, which estimates the wake angle as a function of pitch ratio. The prepared code analyzes propellers without iteration, which causes reduction in numerical cost and time.

In the case study, four types of different propellers, and one propeller along with rudder as a whole propulsion system were analyzed to predict hydrodynamic performance using the mentioned code. The results were validated against experimental data. The following results can be extracted from the comparisons: • The performance predictions are admissible especially at and

before design point (the appropriate region for designers) for propellers.

• Present method can be used as a fast method for preliminary propeller and propeller-rudder design and analysis, because it is inexpensive to run.

• In the performance prediction of propellers, the results for minimum and which is higher than design point are not acceptable.

• The source of the above errors within VLML is attributed to the poor prediction of the wake angle and neglecting 3D effects in this method, which is intrinsically two-dimensional.

REFERENCES 1. Black, S.D. (1997).Integrated lifting-surface/Navier–Stokes

design and analysis methods for marine propulsors, Ph.D. Thesis Massachusetts Institute of Technology.

2. Ghassemi, H., Ghadimi, P. (2011).Numerical Analysis of the high skew propeller of an underwater vehicle, J. Marine Science & Application 10(3), 289-299.

3. Benini, E. (2004).Significance of blade element theory in performance prediction of marine propellers, Ocean Engineering 31, 957-974.

4. Carlton, J. (2007).Marine propellers and propulsion, second edition, Butterworth-Heinemann Ltd, London.

5. Olsen, A. S. (2001).Optimization of propellers using the Vortex-Lattice Method, Ph. D. Thesis Technical University of Denmark.

6. Kinnas, S. A., Coney, W. B. (1988).On the optimum ducted propeller loading, In proc. propellers ’88 symposium, Jersey city 1.1-13, N. J: SNAME.

7. Coney, W. B. (1989).A method for the design of a class of optimum marine propulsors, Ph.D. Thesis Massachusetts Institute of Technology.

8. Bal, S., Guner, M. (2009).Performance analysis of podded propulsors, Ocean Engineering, 36, 556-563.

9. Breslin, J. P., Andersen, P. (1994).Hydrodynamics of marine propellers, University press, Cambridge UK.

10. Molland, A. F., Turnock, S. R. (2007).Marine rudder and control surfaces, Butterworth-Heinemann Ltd, London.

11. Liang, H. and Zong Z. (2011).A Lifting Line Theory for a Three-dimensional Hydrofoil, J. Marine Science & Applications, 10, 199-205.

12. Zong, Z., Liang, H., Zhou L. (2012).Lifting line theory for wing-in-ground effect in proximity to a free surface, J. Engineering Mathematic, 74, 143-158.

13. Kerwin, J.E., Coney, W.B. & Hsin, C.-Y. (1986).Optimum circulation distributions for single and muIti-component propulsors. In Proc. of 21st ATTC, ed. R.F. Messalle, 53-92. Washington, D.C.: National Academy Press.

14. Flood, K. M. (2009).Propeller performance analysis using lifting line theory, M.S. Thesis, Massachusetts Institute of Technology.

15. Stubblefield, J.M. (2008).Numerically-Based ducted propeller design using vortex lattice lifting line theory, M.S. Thesis, Massachusetts Institute of Technology.

16. Driss, Z., Karray, S., Chtourou, W. , Kchaou, H., Salah Abid M. (2012).A Study of Mixing Structure in Stirred Tanks Equipped With Multiple Four-Blade Rushton Impellers, The Achieves of Mechanical Eng., Vol. LIX, No. 1.

17. Hao Rui, Jaswar Koto, (2014).Prediction of Propeller Performance Using Quasi-Continuous Method, Journal of Ocean, Mechanical and Aerospace-Science and Engineering, Vol.10 August 20.

18. Tamashima, M., Matsui, K., and Yamazaki, R. (1993).The method for predicting the performance of propeller-rudder system with rudder angle and its application to the rudder design. Transactions of the West-Japan Society of Naval Architects. 86, (in Japanese).


February 20, 2015


Numerical Study on the Fluid Forces of a Rigid Cylinder Covered by Helical Rods with Gap Due to the Variations of

Incoming Flow Direction and Pitch at Reynolds Number 1000

Arief Syarifuddin,a,* Rudi Walujo Prastianto,b and Silvianita,b

a)Graduate Program on Marine Technology, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia b)Department of Ocean Engineering, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia *Corresponding author: [email protected] Paper History Received: 28-January-2015 Received in revised form: 6-February-2015 Accepted: 18- February -2015 ABSTRACT Offshore structures such as a jacket platform, risers, conductors, mooring lines, Spars, and pipelines, are subject to severe vibration due to Vortex-induced vibration (VIV). This vibration can lead the structures to fatigue failure. One of a passive suppression device which effectively reduces the VIV is in the form of triple helical rods with gap covered to a cylinder. The present paper specially discusses the influence of incoming flow direction and pitch of helix on the induced fluid forces acting on the cylinder due to addition of helical rods at Reynolds number (Re) of 103. The configuration produced best reduction on drag and lift forces in CFD simulation are with 30D length of pitch for the incoming flow direction of 0° and 60°. Reduction on the drag and lift forces for incoming flow directions of 0° and 60° are respectively, 11.34% and 88.32%, and 10.99% and 97.94%. KEY WORDS: Vortex Shedding; Helical Rods with Gap, Incoming Flow Direction; Pitch; Fluid Forces. NOMENCLATURE Re Reynolds Number

Fluid Velocity Diameter of Structure Kinematic Viscosity of Fluid

Lift Force Drag Force Lift Coefficient Drag Coefficient

Fluid Density Frontal Area of Structure Eddy or Turbulent Viscosity

Turbulent Kinetic Energy Turbulent Frequency Velocity Vector Blending Function Production Rate of Turbulence , The Turbulent Prandtl Numbers for k and ω

Invariant Measure of The Strain Rate β*, α Constants of The SST Model 1.0 INTRODUCTION An offshore structure is a structure that has many cylindrical components such as risers, conductors, mooring lines, Spars, and pipelines. Each submerged cylindrical component subjected to fluid flow will undergo the phenomenon called Vortex Induced Vibration (VIV). The VIV is a phenomenon in a fluid flow caused by the shedding of vortices behind the structures due to the interactions of fluid and structure. Vortex is a fluid flow which its particles rotate around its central point. The release of this vortex is called vortex shedding, with its tangential and transversal velocity varies with respect to its radius [1]. The schematic process of vortex shedding can be illustrated in Figure 1.

The existance of VIV encouraged an amount of research to investigate how to reduce its impact. Jones and Lamb in 2003 explained that vortex suppressing devices can be divided into


February 20, 2015


three categories: topographic devices, shrouds, and wake stabilizers [2].

Figure 1: Schematic mechanism of vortex shedding on a cylinder in steady stream [1].

(a)

(b)

Figure 2: Model of cylinder covered by helical rods with gap (a) the variations of incoming flow direction; (b) the variations of pitch length.

This paper will discuss about the influence of installation of

helical rods with gap on a rigid cylinder to the induced drag and lift. Based on previous research that has been done by Sugiwanto, et al. (2013) which explained that the configuration of the model as follows: rods diameter 0.0625D, the length of pitch 15D, and gap 0.0625 m can reduce the drag force by 50% at Re 105 [3]. Furthermore, the research which also explained its passive control device has been done by Beu (2013). The best configuration that able to reduce the fluid forces in this research was helical rods with gap installed along 60% of the length of the model and gap 6 mm. It was able to reduce the fluid forces by ±45%. The length of the model used in this research was 4.28 m, and diameter of cylinder was 0.016 m [4]. Experimental study on its passive control device has been done by Arianti (2014) and Prastianto, et. al. (2014). Experiments were carried out in Laboratory of Aero-Gas Dynamics and Vibration (UPT-LAGG), The Agency for the

Assessment and Application of Technology (BPPT) in PUSPIPTEK area Serpong using LAGG Mini Wind Tunnel (LMWT). The research which has been done by Arianti (2014) shows that helical rods with gap can reduce the drag force on a cylinder by 47.1% at Re 2.8×104, while for the lift force by 43.8% at Re 2.5×104 [5]. Whereas, Prastianto, et. al. (2014) shows that the model successfully reduce drag and lift forces respectively by 50% at Re 2.36×104 and 25% at Re 2.19×104 [6].

This paper specially discusses the influence of variations of the incoming flow direction and the length of pitch on reducing the fluid forces (drag and lift forces) due to installation of helical rods with gap on rigid cylinder at Re = 103. Figure 2 shows the variations of the incoming flow direction and pitch. The modeling has been conducted using an Computational Fluids Dynamics (CFD) software known as ANSYS. 2.0 METHOD 2.1 Data Used in this Paper Some of data used for this paper were obtained from a journal written by Sugiwanto, et al. (2013) [3]. Detail of the data used is presented in Table 1.

Table 1: Cylinder and fluid data [3] Description Quantity Units

Length of cylinder 9.75 m Diameter of cylinder 0.325 m Diameter of rods 0.02 m Gap 0.05 m Temperature 25 °C Water density 997 kg/m3 Dynamic viscosity 8.899x10-4 kg/ms

Determination of the fluid domain for CFD analysis is based on

a thesis written by Beu (2013), as shown in Figure 3. The height of fluid domain is adapted from the height of cylinder [4]. The variations used for this paper is presented in Table 2.

Figure 3: Fluid domain used by the simulation model.

Table 2: Variations used in this paper

Description Quantity Length of pitch 30D, 15D, 10D Incoming flow direction

0°, 15°, 15°, 30°, 45°, 60°, 75°, 90°, and 105°

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February 20, 2015


distinguish laminar and turbulent flow. Re is defined as the ratio of inertial forces to viscosity force [8]. Re equation will be used to obtain the fluid velocity of the inlet and the outlet.

(1)

• Turbulent Model SST (Shear Stress Transport) Turbulent model used in this paper is SST. SST is a combination of two models of turbulence, namely k-ω and k-ε. SST formulation will replace k-ε behavior in the free stream. So it will avoid the common problem of k-ω models that sensitive for free stream inlet turbulence. SST models has good behavior in negative pressure gradient and flow separation. SST also not produce too much turbulence levels in areas with large normal strains, such as stagnation areas and areas with strong acceleration. The SST formulation according to Menter SST model [9] is:

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• Drag and lift forces As a result of changes in the period of vortex shedding, the pressure distribution on the cylinder due to the flow will also change periodically, then it will create periodic variation in the components of the force on the cylinder. The force components are divided into cross flow and in line direction. Component of force in the cross flow direction is called the lift force, while the in line direction called the drag force. Scheme for drag and lift force can be seen in Figure 8. The equation for the drag and lift forces as follow [10]:

(5)

(6)

Figure 8: Drag and lift forces on the cylinder [11].

Drag force obtained from CFD simulations using SST

turbulence models for bare cylinder at Re 103 is 0.01315 N. The drag force converted to drag coefficient by using equation 6. The value of drag coefficient is 0.975. It can be validated by the drag

coefficient that obtained from experimental result. The value of drag coefficient that contained from Constantinides, et al. (2006) is 0.968 [7]. The difference between drag coefficient of simulation and experiment is 0.66%. This indicates that the configuration of bare cylinder models used in this simulation are valid. 3.0 RESULT AND DISCUSSION Having obtained the bare cylinder models were valid, then the next step is to perform a comparison between the fluid forces (drag and lift forces) generated by the bare cylinder and the cylinder covered by helical rods with gap. Several variations of the incoming flow direction and pitch performed on the cylinder covered by helical rods with gap to get the best configuration can reduce the fluid forces. 3.1 Effect Analysis in the Variations of Incoming Flow

Direction As shown in Figure 2 (a), each incoming flow direction will produce different rods configurations. It will be analyzed in the CFD software. The purpose of this analysis is to determine the configuration that generates the greatest reduction of fluid forces. The results obtained from the simulation due to variations of incoming flow direction presented in Table 6.

Table 6: Comparison of CD and CL between bare cylinder and cylinder covered by helical rods with gap due to variations of

incoming flow direction at Re 103 Incoming

Flow Direction

Length of Pitch

CD CL

Bare Helix Bare Helix

0° 30D

0.97486

0.864

0.00153

0.00018 15° 30D 0.865 0.00067 30° 30D 0.866 0.00568 45° 30D 0.865 0.00337 60° 30D 0.868 0.00003 75° 30D 0.866 0.00853 90° 30D 0.865 0.00298 105° 30D 0.863 0.00220

Table 6 shows the best configuration that generates the greatest

reduction of lift force are incoming flow direction of 0° and 60°. Whereas the results of drag force, obtained from simulation, showed little differences that generated in all variations of incoming flow direction. Figure 9 shows the percentage reduction of the fluid forces on the cylinder covered by helical rods with gap.

Figure 9 explains that there is no significant difference in the drag coefficient due to variations of incoming flow direction. The percentage reduction of drag coefficient is between 10.9-11.5%. The incoming flow direction which result the greatest reduction of drag coefficient is 105°, while that result the lowest reduction is 60°. However, it is not enough to conclude that this configuration with incoming flow direction of 105° is the best configuration, because lift coefficient have not been considered in that result. The results obtained for the percentage reduction in the lift coefficient in the variations of incoming flow direction as presented in Figure 10.


February 20, 2015


Figure 9: Reducing percentage diagram of the drag coefficient in all variations of incoming flow direction at Re 103.

Figure 10 explains that almost all of the rods configuration due

to the incoming flow direction on cylinder covered by helical rods with gap increased the lift coefficient (the reducing percentage of lift coefficient is minus) when compared with bare cylinder, except for the configuration of rods due to the incoming flow direction of 0°, 15°, and 60°. The configuration that produce the largest reduction of lift coefficient are the configuration rods due to the incoming flow direction of 0°, 15°, and 60° respectively by 88.32%, 56.32%, and 97.94%. Whereas the reduction of drag coefficient in the configuration rods due to the incoming flow direction of 0°, 15°, and 60° respectively by 11.34%, 11.24%, and 10.99%. It can be concluded that the configuration rods due to the incoming flow direction of 0° and 60° are the greatest reduction of drag and lift forces. The configuration rods due to the incoming flow direction of 0° is the best configuration to reduce drag force, while the configuration rods due to the incoming flow direction of 60° is the best configuration to reduce lift force.

Figure 10: Reducing percentage diagram of the lift coefficient in all variations of incoming flow direction at Re 103. 3.2 Effect Analysis in the Variations of Pitch Previous discussion explains that the configuration rods due to the incoming flow direction of 0° and 60° with length of pitch 30D, and gap 0.05 m (g/D=0.154) is the best configuration to reduce fluid forces. This section will strengthen the previous results that obtained by do variations in the length of pitch. The variations used in this paper are 10D, 15D, and 30D. The results obtained from CFD analysis for the effect of pitch in the fluid forces is presented in Table 7 and Table 8.

Table 7: Comparison of CD and CL between bare cylinder and cylinder covered by helical rods with gap due to variations of pitch for incoming flow direction of 0°

Length of Pitch

CD CL Bare Helix Bare Helix

10D 0.97486

0.86565 0.00153

0.00191 15D 0.87252 0.00058 30D 0.86431 0.00018

Table 8: Comparison of CD and CL between bare cylinder and cylinder covered by helical rods with gap due to variations of pitch for incoming flow direction of 60°

Length of Pitch

CD CL Bare Helix Bare Helix

10D 0.97486

0.86158 0.00153

0.00031 15D 0.87143 0.00214 30D 0.86776 0.00003

Table 7 shows that for all variations of pitch produce the

reduction in the drag force when compared with bare cylinder, the percentage reduction as presented in Figure 11 (a). Whereas for the lift coefficient, just length of pitch 30D which results the reduction of the lift coefficient in both incoming flow direction of 0° and 60°. Length of pitch 10D increase of lift coefficient (the reducing percentage of lift coefficient is minus) in the incoming flow direction of 0°, while the length of pitch 15D increase of lift coefficient in the incoming flow direction of 60°. Percentage reduction in the drag and lift in the variations of pitch for the incoming flow direction of 0° and 60° can be seen in Figure 11.

(a)

(b)

Figure 11: Variations of pitch for incoming flow direction of 0° and 60° at Re 103 (a) Reducing percentage diagram of the drag coefficient; (b) Reducing percentage diagram of the lift coefficient.

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February 20, 2015


From the above explanation, it can be seen that the length of pitch 30D produces the greatest reduction of the fluid forces when compared with the other pitch. The reduction in the length of pitch, it means that the area of cylinder covered by helical rods will be even greater, will lead to increase in the area of the cylinder that will be used in the calculation of force. In this condition, that helical rods would be considered as foreign object that will make area of the cylinder increase. Based on the equation 3, the addition of area that is subject to the flow will increase the fluid forces result. 4.0 CONCLUSION The configuration of installation helical rods with gap due to incoming flow direction of 0° and 60° with gap 0.05 m (g/D = 0.154) and length of pitch 30D is the best configuration in order to reduce fluid forces at Re 103. That configuration produce the greatest reduction of drag and lift forces in incoming flow direction of 0° respectively 11.34% and 88.32%. Beside that incoming flow direction, the greatest reduction of drag and lift forces also occurred in incoming flow direction of 60° respectively 10.99% and 97.94%. ACKNOWLEDGEMENTS The first author would like to convey a great appreciation to Directorate of Higher Education (DIKTI) – Ministry of Education and Culture, Republic of Indonesia for the financial support in his study in postgraduate. REFERENCE 1. Indiyono, P. (1994). Hidrodinamika Bangunan Lepas

Pantai. SIC, Surabaya, Indonesia. 2. Lubbad, R. K., Loset, S., Gudmestad, O. T., Torum, A. and

Moe, G. (2007). Vortex Induced Vibrations of Slender Marine Risers – Effects of Round-Sectioned Helical Strakes, Proceedings of the 16th International Offshore and Polar Engineering (ISOPE) Conference, San Francisco, California, USA, May 28 – June 2.

3. Sugiwanto, A., Prastianto, R. W., Murdjito and Djatmiko, E. B. (2013). A Numerical Study on Cylinders with Passive Control Device of Helical Rods with Gap for Reducing Vortex-induced Vibration, Proceeding of the Second International Conference on Sustainable Infrastructure and Built Environment (SIBE2013), Proc. Book Vol. III, Institut Teknologi Bandung, Bandung, Indonesia, November 19-20.

4. Beu, M. M. Z. (2013). Studi Numerik Pengaruh Bentuk Passive Control Device berupa Rods Bergap Berpola Helix terhadap Vortex Induced Vibration (VIV) pada Long Flexible Riser. Thesis: Program Studi Teknik Perancangan Bangunan Laut, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia.

5. Arianti, E. (2014). Studi Eksperimental dan Numerik Pengaruh Penambahan Helical Rod Ber-Gap terhadap Gaya Fluida pada Silinder Rigid Tertumpu Fleksibel. Thesis: Program Studi Teknik Perancangan Bangunan Laut,

Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia. 6. Prastianto, R. W., Musthofa, A. Z. A., Arianti, E.,

Handayanu, Murdjito, Suntoyo, and Fariduzzaman. (2014). Triple Helical Rods with Gap as A Passive Control Device for Reducing Fluid Forces on A Cylinder, The 9th International Conference on Marine Technology, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia, MT-27, October 24-25.

7. Constantinides, Y., and Oakley O. H. Jr. (2006). Numerical Prediction of Bare and Straked Cylinder VIV, Procedings of 25th International Conference on Offshore Mechanics and Arctic Engineering, Hamburg, Germany, June 4-9, pp. 1-9.

8. Munson, B. R., Young, D. F., and Okiishi, T. H. (2002). Fundamentals of Fluid Mechanics, ISBN 0-471-44250-X. John Wiley & Sons, Inc., New York, USA.

9. Menter, F. R. (1994). Two-Equation Eddy-Viscosity Turbulence Models for Engineering Applications, AIAA Journal, Vol. 32, No. 8, August 1994, pp. 1598-1605. Reston, USA.

10. Cengel, Y. A. and Cimbala, J. M. (2010). Fluid Mechanics: Fundamentals and Applications. The McGraw Hill, New York, USA.

11. Purwanti, L. (2008). Analisa Vortex Induced Vibration Pada Riser Tension Leg Platform. Bachelor Final Project: Jurusan Teknik Kelautan, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia.


February 20, 2015


Air Flow Characteristics and Behaviour of Main Rotor Blade of Remote Controlled Model Scale Helicopter

Mohd. Shariff bin Ammoo,a,* Ziad Bin Abdul Awal,a and Jaswar Koto,a,b

a)Department of Aeronautics, Automotive and Ocean Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia b)Ocean and Aerospace Research Institute, Indonesia *Corresponding author: [email protected] Paper History Received: 25-January-2015 Received in revised form: 7-February-2015 Accepted: 18-February-2015 ABSTRACT The airflow through the main rotor blade system of a helicopter is still not exceedingly well understood owing to its obscurity in aerodynamics. It is prognosticated that helicopter wakes can be significantly greater than those formed by a fixed wing aircraft of the same weight. Nuisance incidents such as brownout & noises are engendered from rotor wake. Study through flow visualization plays a key role in understanding the airflow distinctiveness and vortex interaction of a helicopter rotor blade. Inspecting and scrutinizing the effects of wake vortices during operation is a great challenge and imperative in designing effective rotor system. This study aimed at finding a suitable method to visualize the main rotor airflow pattern of a remote controlled subscale helicopter and seek for the vortex flow at the blade tip. The experimental qualitative data is correlated with quantitative data to perform meticulous study on the airflow behaviour & characteristics along with its distinctiveness generated by the main rotor in various flight conditions. Simulation is also performed in similar conditions to bequeath with comparability between the flow visualization results. Several dissimilar flow patterns were identified throughout the blade span. At the centre of the main rotor hub, the presence of turbulent flow was perceived. This is because of the low energy of air pooled in this region. Conversely, an apparent straight streamline pattern in the middle portion of the rotor blade was noticed as the air in this section encompassed high kinetic energy.

KEY WORDS: Main Rotor Blade; Hover; Rotor Wake; Induced Velocity; Dynamic Pressure. 1.0 INTRODUCTION Structural strength is a vital element in designing sustainable and burly aircrafts. With the significance of unambiguous materials, from wood to advanced aluminium, steel and composites, dexterous design plays a key role in performance as well. In the field of aeronautics the significance of an airfoil cannot be illustrated using vocabulary. Reducing drag but increasing lift and a smooth flight with better control and manoeuvrability can be achieved with a smart and efficient airfoil design according to the precise criteria. Consequently, a comprehensive knowledge of airfoil flow field plays a key role in achieving better performance. For helicopter performance and behaviour, an adept airfoil design is vital as well. Firdaus.et.al (2014) has studied on aerodynamic characteristics of helicopter tail rotor propeller using Quasi-Continuous Vortex Lattice Method. According to Bangalore and Sankar (1996), rotorcraft airfoil sections, designed by Dadone (1976), Noonan (1980) and others achieved high maximum lift coefficient (C1max) and low zero lift drag coefficient (Cdo).

A pure helicopter is an aircraft that uses rotating wing, which provides lift, propulsion and control forces that enables it to hover relative to the ground without forward flight. Aerodynamic forces are generated by the relative motion of a wing’s surface with respect to the air. The helicopter has the ability to take-off and land vertically. Moreover, the helicopter has the ability to climb, fly forward, cruise at a speed, descend and hover. In most cases, the rotor system of a helicopter is categorized as fully articulated, semi-rigid and rigid (Rotorcraft Flying Hand Book, 2000). Rotorcraft optimization is basically the design process of the main rotor system. It necessitates amalgamation of several disciplines


February 20, 2015


which includes aerodynamics, dynamics, structures, and acoustics. When handling qualities are considered into the design, broader integration may perhaps be required (Celi, 1999), (Guglieri, 2012).

Aerodynamics plays a vital role in understanding the helicopter behaviour with respect to the airflow. On the contrary, the aerodynamics of helicopter is greatly concealed. Decoding the airflow around a helicopter is a challenge because of the aerodynamic ambience is very much diverse unlike the flow environment around an airplane’s wing. Performing wind tunnel tests on small-scaled prototype is one of the most pertinent looms to obtain the aerodynamic characteristics of a helicopter. Though it requires quite a great investment of time, money and resources; still it is the most apt means to visualize the airflow. This study looks into the air flow behaviour of a controllable helicopter where various flow patterns acquired through visualization process are briefly described. 2.0 METHODOLOGY This study opted for the smoke flow visualization technique to envisage the air flow pattern over the main rotor blade system of Hirobo-Falcon 505, a remote-controlled subscale helicopter. Table 1 portrays some of the imperative stricture of the main rotor blade. The rotor radius for the main rotor blade of the Hirobo-Falcon 505 helicopter is 0.670m with a chord length of 0.05m which uses NACA0014 airfoil. Based on a conceptual design, Figure 1 lays bare the final assembly prior to experimentation. An electric motor is fixed on the test rig where the helicopter is then mounted on it. A drive shaft gets connected to the gear system of the main rotor head, establishing connection with the motor which is powered by the central battery of RotorWay Exec 162Fhelicopter. In order to visualize the air flow of the rotor blades, Aerotech smoke generator is used for providing white smoke. An indispensable point to be noticed here is that the background is painted in black color; the underlying principle behind doing this is to provide better flow visualization. The disparity between the colors of white smoke and black background would irrefutably offer better sight of the air flow-field. Design software named SolidWorks-2012 is used to carry out simulation in corresponding circumstances for result comparison. Table 1: Main rotor-head specifications of Hirobo-Falcon 505 controllable helicopter.

Rotor System Rigid with stabilizing bar Number of Blades 2 Rotor Radius 0.670m Airfoil Section NACA0014 Chord Length 0.05m

Figure 1: Final assembly prior to experimentation.

3.0 RESULTS AND DISCUSSION 3.1 Pressure distribution along the blade span As the helicopter operated at 450 rpm, the pressure distribution along the blade span was appraised using a measuring probe which moved parallel along the blade span. Pressure distribution data was taken at a distance of 0.15m below the rotor plane. At different blade pitch angles, the probe measured the dynamic pressure along the blade span. Using the obtained data, the graph of Cp versus Rotor Radius was plotted; which is demonstrated in Figure 2. From the graph of Cp versus Rotor Radius, it can be noticed that at about 50 percent of the blade span there is a sudden boost in the dynamic pressure; the main patron to this effect is the stabilizer bar of the rotor system. The stabilizer bar operates as the control surface of the helicopter which consists of an airfoil strip at each end. These airfoil strips unswervingly engenders downward airflow followed by the increment in dynamic pressure during hover. Consequent to this hasty rise of pressure, the lingering fraction of the blades demonstrate an invariable pressure gradient. In this situation, the slope ascended doggedly round about 85 percent of the blade span with impulsive staged alteration afterwards. The graph personifies the jagged pressure reduction to negative values approximately from 550mm to 680mm of the rotor radius, which however has repulsive correspondence with slower airflow. Subsequently, the average values of the dynamic pressure data at different pitch angles (α) throughout the blade span is then used to calculate pressure coefficient Cp.

Figure 2: Graph of Cp vs. Rotor Radius.

Figure 3 exhibits the graph of Cp versus α. This graph assists in

determining the maximum pressure coefficient (Cp), the pitch angle which generates the maximum lift and the stall angle. From


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the graph, it is prominent that the maximum Cp occurs at around 11 degrees of pitch angle where the stall angle is apparently round about 12 degrees for the main rotor pitch.

Figure 3: Graph of Pressure Coefficient, Cp vs. Pitch angle, α (o). 3.2 Flow visualization at different blade sections of during flight conditions Using the smoke flow visualization technique, Figure 4 (a) exhibits the embodied image of airflow pattern for the main rotor blade in hovering provision. As the smoke was released from outer part just above the blade tip, it gradually moved inward in the form of a curved line. The airflow passed through the rotor plane at ¾ of the rotor radius as the smoke followed the streamline. Slip stream or otherwise known as ‘far wake’ is produced as the airflow rambled down the rotor. The smoke stream appeared to be moving away from the blade as the blade angle was pitched at about 10 degrees. As the smoke moved inward where it was released just about on the middle segment of the blade span; from Figure 4(b) it is understood that the airflow emerged to be more straighten whilst passing through the main rotor blades. The middle part of the blade plays an imperative role in enabling a helicopter to execute its functions as it generates effective lift and thrust. In this section, the airflow is uninterrupted, as it’s has adequate detachment from the blade tip. The blade tip is the vortex subjugated area. As fundamental rule, it is known that for a wing if there is high velocity present at one side and low on the other, the low velocity side will have higher pressure compared to another.

Around the rotor hub, the airflow was figuratively in serene state at the root of the blade. From Figure 4(c), it can be perceived that the smoke is released directly on top of the rotor hub. As the smoke moved through the rotor disc, the flow arched outward. The flow held in reserve its outward curvature pattern furthermore as it passed the helicopter before becoming straight. At about ¼ of the blade radius, low velocity air inhabits in the region just below the rotor hub. Here, as this air does not possess high kinetic energy, the smoke bungled to have explicit directional flow and just pursued the shape of the helicopter. It is also projected that the flow is very emaciated in terms of energy as this region didn’t reveal unequivocal streamline of flow. As a corollary, it can be asserted that the hub of the rotor system is ineffectual in lift generation.

(a) Smoke released on the superficial portion of the main rotor

blade during hover.

(b) Smoke released on the middle portion of the blade.

(c) Smoke released just above the rotor center hub.

Figure 4: Air flow visualization throughout the blade span. 3.3 Rotor wake formation during hover Figure 5, demonstrates rotor wake formation during hovering condition. From this representation, it can be noticed that a concentrated vortex has trailed from the blade tip. Generally, the inboard turbulent wake is referred as a vortex sheet. The blade tip vortex is set apart by exceedingly distinct dark seed void. This in fact is a consequence of the centrifugal forces generated by high swirl velocity surrounding the vortex core. The local velocity is required to be raised enough to cause centrifugal forces on the seed particles to move spirally outward. The particles attain equilibrium as soon as the centrifugal force and pressure force are in balance.


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Figure 5: Rotor wake configuration during hover.

Figure 6 represents the main rotor simulation using SolidWorks-2012 during hovering condition. This simulation is conducted using similar rotor properties and conditions. The airfoil used here is NACA0014 where the chord length is 0.05m and rotor diameter is 1.34 m. The simulation was conducted without the presence of the fly-bar. Rotating at 450 rpm, the simulation is carried out with both laminar and turbulent types of flow for Temperature 293.2k and Pressure 101325 Pa. Figure 6 portrays the flow trajectory for velocity. The crucial point to be distinguished here is that the velocity is at its supreme on the tip of the rotor blades’ upper surface compared to its bottom surface. The velocity amplifies exponentially from the inner to outermost section of the rotor blade. It is quite cogent that the chief ground for rotor wake is this immense contradiction in velocity at tip of the blade for the main rotor compared to other section of the blade.

(a) Front View

(b) Isometric View

Figure 6: Air flow simulation of the main rotor blade during hover (Flow Trajectory - Velocity). 3.4 Effective Flow on Blades The quest for finding the basic principles of the airflow pattern for the helicopter main rotor is carried out in this study. The smoke flow visualization process is exploited in order to get a better understanding of the flow behaviour and characteristics from a subscale helicopter’s rotor blade. For dissimilar flight conditions the flow pattern depicted peerless behaviour and individuality. Quite a few disparate flow patterns have been recognized throughout the blade span. The centre of the main rotor hub has turbulent flow. This is due to low energy of air amassed in this region. The air encompasses high kinetic energy with a clear straight streamline pattern in the middle portion of the rotor blade. Figure 7, summarizes and elucidates the effective region of the main rotor. It epitomizes that throughout the blade span; 25 percent is stall region, 45 percent is the driving region and 30 percent is driven region moving from the centre to the outer tip of the blade. Furthermore, this study scrutinized the rotor wake system where the vortices formed below the blade at the rotor tip. The tip vortex noticeably exhibited the vortex core and vortex sheet. The flow properties of the main rotor system of a sub-scale helicopter can be correlated to a full scale helicopter. This can aid in gaining enhanced perceptive and designing helicopters with improved performance.

Figure 7: Effective Blade Properties.


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5.0 CONCLUSION In conclusion, this paper studied on main rotor blade air flow characteristics and behaviour of a remote controlled sub-scale helicopter. The air flow pattern over the main rotor blade system of Hirobo-Falcon 505, a remote-controlled model scale helicopter was envisaged using the smoke flow visualization technique. ACKNOWLEDGEMENTS The authors would like to convey a great appreciation to Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia for supporting this research. REFERENCE 1. Anderson Jr. J. D., (2011). Fundamentals of Aerodynamics,

McGraw-Hill, New York. 2. Bangalore A., L. Sankar N., (1996). Numerical analysis of

aerodynamic performance of rotors with leading edge slats, Computational Mechanics (Springer-Verlag), 17, pp.335-342

3. Celi R., (1999). Recent applications of design optimization to rotorcraft-a survey, American Helicopter Society 55th Annual Forum, Montreal.

4. Dadone L., (1976). Helicopter Design DATCOM, Vol I-Airfoils. NASA CR- 153247.

5. Firdaus, Jaswar Koto, M.S Ammoo, I.S.Ishak, and Nofrizal, (2014). Review on Aerodynamic Characteristics of Helicopter Tail Rotor Propeller Using Quasi-Continuous Vortex Lattice Method, Journal of Ocean, Mechanical and Aerospace-Science and Engineering-, Vol.7, pp.8-17.

6. Firdaus Mahamad, Jaswar Koto, M.S Ammoo and I.S.Ishak, 2014, Application of Quasi-Continuous Vortex Lattice Method to Determine Aerodynamic Characteristics of Helicopter Tail Rotor Propeller, Proceeding of Ocean, Mechanical and Aerospace -Science and Engineering-, Vol.1, Sec.2, pp.44.52.

7. Guglieri G., 2012, Using of particle swarm for performance optimization of helicopter rotor blades, Scientific Research-Applied Mathematics, 3, pp. 1403-1408.

8. Noonan K. W., Bingham B. J., 1980, Aerodynamic characteristics of three helicopter rotor airfoil sections from model scale to full scale at mach numbers from 0.35 to 0.9, NASA Technical Paper 1701,AVRADCOM TR-80-B-5.

9. Rotorcraft Flying Hand Book, 2000, U.S. Department of Transportation, Federal Aviation Administration, Flight Standards Service.

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