The Effect of Tip Clearance on Performance of a Counter ...

The Effect of Tip Clearance on Performance ofa Counter-Rotating Ducted Fan in a VTOL UAV*

Minhyoung RYU,1) Leesang CHO,2) and Jinsoo CHO3)†

1)Rotary-wing Flight Performance Team, Korea Aerospace Industries, Sacheon 52529, Korea2)Department of Mechanical & System Engineering, Hansung University, Seoul 02876, Korea

3)School of Mechanical Engineering, Hanyang University, Seoul 04763, Korea

This research analyzed the effect of tip clearance on counter-rotating ducted fans for vertical take-off and landing inunmanned aerial vehicles using commercial computational fluid dynamics tools. The computational results were verifiedusing the subsonic wind tunnel at Hanyang University. In this study, the tip leakage flow of the counter-rotating ducted fanwas analyzed in order to enhance the fan performance. The thrust coefficient decreases with increasingly larger front rotortip clearance due to the increase in tip leakage flow. The power coefficient is influenced by viscous and tip leakage lossesfrom the large tip clearances for the rear rotor. Smaller figures of merit are captured at larger tip clearances for the front andrear rotors because the direction of vortex core changes quickly. In conclusion, to improve the aerodynamic performanceof counter-rotating ducted fans, it is necessary to reduce the mass flow rate across the tip clearance and tip vortex loss.

Key Words: Counter-Rotating Ducted Fan, Tip Clearance, CFD, Wind Tunnel Test

Nomenclature

AP : cross-sectional area of the propellerAW : cross-sectional area of the wind tunnelCP : power coefficientCT : thrust coefficientD: inner diameter of the ductE: input voltage

FOM: figure of meritHn: normalized helicityI: currentJ: advance ratioL: input shaft powerN: rotational speed

Ptotal: total pressureP: static pressureR: rotor tip radiusSE: energy sourceSM : momentum sourcet: tip clearanceU: mean velocity vectorV 0: equivalent free airspeedV: wind tunnel datum velocity

V1: free stream velocityhtot: specific total enthalpy

x; y; z: coordinatexb; yb; zb: ducted fan body fixed coordinate

yþ: dimensionless wall distance

©: efficiency of ducted fan�m: efficiency of the motor: thermal viscosity®: molecular(dynamic) viscosity�i: absolute vorticityμ: air density¸: mean viscous stress tensorwi: relative vorticity

1. Introduction

Ducted fans are widely used in propulsion systems for ver-tical take-off and landing (VTOL) in unmanned aerial ve-hicles (UAVs). The flow fields around a ducted fan in aVTOL UAV are complicated due to the effects of the finitehub, annulus wall and leakage flows. Tip leakage flow isone of the most complex phenomena in turbo-powered ma-chinery. Because of the pressure difference between the pres-sure and suction sides of the blade, leakage flow is produced.Leakage flows in the tip clearance region of the rotor ad-versely affect the aerodynamic performance and noise. Thestrength of the tip vortex depends on the tip clearance leak-age. Therefore, reducing the tip leakage mass flow improvesthe efficiency of ducted fan systems.

In this research, a new concept diagram for a multi-ductedfan VTOL UAV was designed at Hanyang University asshown in Fig. 1. The vehicle has three ducted fans. Two ofthe ducted fans are part of the propulsion. The other sub-ductedfan can be tilted by a stepper motor, which is located on theaxis of the rotation (x-axis), giving the vehicle yaw motion.

The sub-ducted fan is a counter-rotating fan, consisting oftwo counter-rotating rotors, namely, the front rotor and rearrotor. The rear rotor is rotated in the opposite direction to re-move the torque generated by the front rotor. The counter-ro-tating fan has higher performance characteristics, as well as a

Trans. Japan Soc. Aero. Space Sci.Vol. 60, No. 1, pp. 1–9, 2017

© 2017 The Japan Society for Aeronautical and Space Sciences+Presented as Paper 2014-0483 at the 29th Congress of the InternationalCouncil of the Aeronautical Sciences, St. Petersburg, Russia, September,2014Received 25 May 2015; final revision received 22 September 2016;accepted for publication 23 September 2016.†Corresponding author, [email protected]

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higher noise level compared to the other fan. The pitch of thevehicle can be controlled by adjusting the rotor rotationalspeed.

Tip leakage loss is a source of aerodynamic noise genera-tion. Understanding the effect of tip clearance is important inreducing tip leakage loss.

There is an abundance of research on tip leakage loss inturbo-powered machinery.1–5) Jang et al.6,7) analyzed the ver-tical flow field in a propeller fan using laser Doppler veloc-imetry (LDV) measurements and large eddy simulation(LES). They studied the structure and behavior of the verticalflow near the tip region in propeller fans, confirming thatthree vortex structures are formed near the tip: tip vortex,leading-edge separation vortex and tip leakage vortex.

Lee et al.8) investigated the structure of tip leakage flow inaxial flow fans using LDV measurements and computationalfluid dynamics (CFD). These results show the generationmechanism of tip leakage vortex-induced noise, as highmomentum flux was observed below the tip leakage vortex.

The ducted fan commonly consists of a rotor located in theduct. Therefore, the clearance between the blade and theshroud wall produces leakage flow. Reducing tip leakageloss is critical for improving the performance of a ductedfan.

Martin and Tung9) developed a wind tunnel test as adesign tool and validated the CFD analysis for ductedfans. They found that the hover stability a crosswind im-proved when the radius of the leading-edge in the duct wasdecreased. The smaller leading-edge radius of the duct shapeimproves the pitching moment under stall conditions. Fur-thermore, the tip clearance changed from 1% of radius to4.5% of radius as the suction pressure near the leading-edgewas suddenly disrupted due to the increase in tip clearance.This then caused a decrease in duct lift.

Cho et al.10) performed a computational analysis and windtunnel test on ducted fans in an attempt to improve perfor-mance. These researchers confirmed that the thrust perfor-mance was only improved in terms of the rotor because ofthe ducted effect. Furthermore, this research highlighted anaerodynamic design step for the rotor and stator blades ofthe ducted fans.

Akturk and Camci11–13) studied the large tip clearance ef-fect in ducted fans for VTOL UAV using a kiel total probe,six-component load cell, torque measurements, and 3D Rey-nolds-averaged Navier-Stokes (RANS) analysis. They sug-

gested new tip treatments for the ducted fan in order to im-prove the overall performance. Decreasing the tip clearanceincreased the thrust and hover efficiency. This research con-firmed the effectiveness of the new tip treatments, as they re-duce tip clearance leakage mass flow rates.

Various studies have focused on the aerodynamic charac-teristics of the ducted fan. The ducted fan is more efficientand safer than open rotor blades because the rotor is locatedin the duct. A counter-rotating ducted fan is used in theducted fan UAV.14,15)

Although tip clearance of the ducted fan has been studied,there is a lack of research concerning the effect of tip clear-ance on ducted fans. Understanding the tip leakage flowfields for ducted fans is required in order to improve the per-formance of a ducted-fan UAV. The effective radius of theducted fan can be increased by reducing the tip vortex. Theducted fan typically has good aerodynamic characteristics.In a previous study, the static performance of a counter-rotat-ing ducted fan was analyzed.16) However, the relation of tipclearance and performance was not clarified.

In the present study, the tip leakage flow of a counter-ro-tating ducted fan was analyzed in order to enhance the per-formance of the ducted fan. The CFD analysis included ap-plying various tip clearances of the counter-rotating ductedfan using commercial tools. The computational methodsolved the RANS equations using a finite volume method.The computational results were verified using the results ofwind tunnel tests.

2. Analysis Model

2.1. Counter-rotating ducted-fan modelThe counter-rotating ducted-fan model consists of a front

rotor, rear rotor and duct. The rotor blades are designed ac-cording to the free vortex design method where the productof the blade radius and tangential component of absolute ve-locity is a constant. The axial velocity is constant along theradial direction of the rotor blades.10) The diameter of theducted fan is 150mm. The NACA 65 Series airfoil is usedin this study. The rotational speed is 10,000 rpm. The rota-tional direction is contrary for each rotor. Therefore, theanti-torques from each rotor are reduced. Detailed specifica-tions of the counter-rotating ducted fan are described inTable 1.

Figure 2 shows the counter-rotating ducted-fan model inthe wind tunnel. The Reynolds number of this model is9:49� 106, which was based on mid-blade chord length.

The tip clearance (t) is divided by the height (h) of therotor. The non-dimensional tip clearance ratio (t=h) of thebaseline model is 1.80% in the front and rear rotors.

3. Experimental Method

3.1. Experimental setupThe aerodynamic forces of the counter-rotating ducted fan

were measured using the subsonic wind tunnel at HanyangUniversity. Figure 3 shows the schematic diagram of the ex-

Fig. 1. Concept diagram of a multi ducted VTOL UAV.

Trans. Japan Soc. Aero. Space Sci., Vol. 60, No. 1, 2017

2©2017 JSASS

perimental apparatus for the counter-rotating ducted fan inthe wind tunnel. Table 2 shows the operating parameters ofthe wind tunnel. The blockage ratio is 4.27%, which is lessthan acceptable ratio 7.5%.17) The forces were measured us-ing an external balance system (CAS six-axis load cell). Themeasurements of force and moment are 5 kg and 0.5 kg � m,respectively. The accuracy of the load cell was 0.5% FS foreach of the components.

Figure 4 shows a schematic diagram of the counter-rotat-ing ducted-fan system. The front rotor and rear rotor are ro-tated in opposite directions by two motors. The rear motorshaft, which rotates the front rotor, is wrapped by the frontmotor shaft.

Figure 5 shows the counter-rotating ducted-fan modeltested in the wind tunnel. A 375W CR23L counter-rotatingBrushless DC electric motor (HobbyKing) was used to oper-ate the counter-rotating ducted-fan rotor. The BLDC motorwas controlled by a brushless speed controller (HobbyKing40A SBEC).

The input shaft power (L) of the BLDC motor, which turnsthe rotor of the ducted fan, is calculated from the motor inputvoltage (E), current (I) and motor efficiency (�m), as describ-ed in Eq. (1). The electric motor efficiency was acquiredfrom a calibration test performed by the motor manufacturingcompany.

L ¼ E� I � �m ð1ÞThe results of the wind tunnel tests for the ducted fan were

corrected using the Glauert method and are shown inEq. (2).18) The ducted fan was tested in a closed-throat windtunnel.

Table 1. Specifications for the counter-rotating ducted fan.

Parameter Front rotor Rear rotor

Blade design Free vortexDucted-fan inner diameter 150mmTip diameter 148mmTip clearance 1mmHub diameter 37mmAirfoil NACA 65 SeriesChord length Hub 39.3mm 42.7mm

Mid 35.8mm 35.1mmTip 34.0mm 34.0mm

Stagger angle Hub ¹41.7 deg 54.9 degMid ¹66.6 deg 68.6 degTip ¹73.6 deg 74.1 deg

Number of blades 3Rotational speed 10,000 rpm ¹10,000 rpm

Table 2. Operating parameters of the wind tunnel.

Classification Parameter

Wind tunnel Open circuitTest section size 0.8m � 0.8m � 1.6mVelocity 2 –70m/sTurbulence intensity 0.2%Non-uniformity 1%

Fig. 3. Schematic diagram of the counter-rotating ducted fan in the windtunnel.

Fig. 4. Schematic diagram of the counter-rotating ducted-fan system.

(a) Side view (b) Front view

Fig. 5. Counter-rotating ducted fan model tested.16)

Fig. 2. Counter-rotating ducted fan unit.


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V 0

V¼ 1� �4�1

2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 2�4

p ð2Þ

where,

�4 ¼ �Fz=�APV2; �1 ¼ AP=AW:

4. Computational Method

4.1. Governing equations and turbulence modelThe steady, incompressible, three-dimensional and turbu-

lent flow in the counter-rotating ducted fan was calculatedusing the commercial solver ANSYS-CFX Academic Ver.14.5. This numerical simulation solves the RANS equationsusing a finite volume method. The continuity, momentumand energy equations were simultaneously calculated andare represented in Eqs. (3)–(5),19) respectively:

Continuity equation :@�

@tþ r � �U

� � ¼ 0 ð3Þ

Momentum equation:

@ �U� �

@tþr � �U � U

� � ¼ �rpþ r � � þ SM ð4Þ

Energy equation:

@ �htot� �

@t� @p

@tþr � �Uhtot

� �

¼ r � �rTð Þ þ r � U � �ð Þ þ U � SM þ SE ð5ÞThe k–! based shear stress transport model was used for

this computation. This turbulence model predicts the adversepressure gradient near the wall and flow separation well.20–22)

In order to evaluate the aerodynamic performance of acounter-rotating ducted fan, the following conventional per-formance parameters were used. The aerodynamic character-istics of the ducted fan are represented in Eqs. (6)–(10)23):

Advance ratio : J ¼ V1ND

ð6Þ

Thrust coefficient : CT ¼ Thrust�N2D4

ð7Þ

Power coefficient : CP ¼ Power�N3D5

ð8Þ

Efficiency : � ¼ JCT

CP

ð9Þ

Figure of merit : FOM ¼ C1:5Tffiffiffi2

pCP

ð10Þ

4.2. Computational domains and boundary conditionsThe tip clearance ratio (t=h) of the front and rear rotors was

1.80–7.62%. All of the models were calculated for hoveringconditions. Figure 6 shows the computational domain andboundary conditions for the CFD simulation of the analyzedmodel. The computational domain consists of the front rotor,rear rotor and an outer region. Multiple frames of reference

(MFR) were used in this study. The frozen-rotor interfacemodel (quasi-unsteady) was used to predict the interactionbetween the front and rear rotors. The frozen-rotor interfacemodel has been documented as showing large circumferen-tial flow variation.24,25) This technique is widely used in axialflow turbo-powered machinery. The rotational speed of thefront and rear rotors was fixed at 10,000 rpm.

The boundary conditions of the inlet for the outer domainwere set to the total pressure conditions for hovering and atthe velocity inlet for forward flight conditions. The outletboundary conditions were set at the pressure outlet(Pgage ¼ 0). The boundary conditions of the front rotor, rearrotor, hub and duct were determined to be no-slip boundaryconditions. The outer surface defines the opening boundaryconditions for the hovering and free-slip wall for forwardflight. In order to reduce the computing time, the outer do-main and front and rear rotor regions were divided by 120�

using periodic boundary conditions.

5. Results

5.1. Grid independent studyThe outer domain has a hexahedral grid, which was gen-

erated using ICEM CFD, as shown in Fig. 7. The grid onthe front and rear rotor domains were generated usingTurboGrid. All of the grids were composed of hexahedralmesh.

In order to confirm that the computational results are inde-pendent for the number of meshes, grid convergence test wasperformed. Figure 8 shows the aerodynamic coefficients ofthe counter-rotating ducted-fan in the independent grid test.When the number of elements is 2:2� 106, the variation ofthe FOM is less than 1%. Consequently, 2:2� 106 elementsare chosen in this study using the independent grid test. Inorder to take account of the viscous sublayer in the typicalvelocity profiles for the turbulent boundary layer based onthe law of the wall,26) the dimensionless wall distance wasset less than 1. The expansion ratio of the gird is 1.2 and a30-prism layer is generated.5.2. Computational results validation

The comparison results of the computational and experi-mental ducted-fan behaviors are shown in Fig. 9, wherethe difference in thrust coefficient is less than 7%. The dis-crepancy was caused by experimental effects such as block-age and the model support strut of the ducted fan. The com-putational and experimental results showed good agreementwith respect to the advance ratio, except at a large advance

Fig. 6. Computational domain and the boundary conditions for the ductedfan.


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ratio (J > 0:4). When the free-stream velocity increased, thethrust coefficient of the ducted fan decreased, which is sim-ilar to previous research.27–29) Because the blade section an-gles of attack can be reduced by increasing the inflowvelocity, the power coefficient is nearly constant with alladvance ratio values. The dissidence of power coefficientin this research was less than 13%. However, a tendencytoward efficiency was also seen in the experimental results.The efficiency of the ducted fan increases as the advanceratio increases until it reaches 0.4. The difference in effi-ciency is increased at a high advance ratio because of the pre-diction of lower thrust in the CFD.5.3. Aerodynamic characteristics for tip clearance

Figure 10 shows the thrust coefficient of the counter-rotat-ing ducted fan for various tip clearance ratios of the front andrear rotors. The tip clearance of the rear rotor changed from1.80% to 7.62%, as did several tip clearance values of thefront rotor. The thrust coefficient decreases as the tip clear-ance of the front rotor increases. The thrust coefficientdecreases because the tip leakage flow increases. The tipleakage flow is determined by calculating the mass flow rate

during tip leakage. When the tip clearance of the front rotor isless than 3.67%, the thrust coefficient increases as the tipclearance of the rear rotor increases.

Figure 11 shows the power coefficient of the counter-rotating ducted fan for various tip clearances of the frontrotor. The increasing tip clearance of the rear rotor affectsthe power coefficient. Therefore, the large tip clearance ofthe rear rotor affects viscous loss and tip leakage loss. Con-versely, the power coefficient decreases with the tip clear-ance of the front rotor.

In order to evaluate the efficiency of the ducted fan forhovering conditions, we used FOM. Figure 12 shows theFOM of the counter-rotating ducted fan for various frontrotor tip clearances. As the tip clearance of the rear rotordecreases, the hover efficiency increases. Conversely, whenthe tip clearance of the front rotor increases, the efficiencydecreases. In the baseline model (ðt=hÞFront ¼ 1:80%to ðt=hÞFront ¼ 1:80%), FOM was reduced to 4.21%when the tip clearance changed from ðt=hÞRear ¼ 1:80% toðt=hÞRear ¼ 7:62%. In addition, FOM was reduced to 7.38%when the tip clearance changed from ðt=hÞFront ¼ 1:80% toðt=hÞFront ¼ 7:62%. This phenomenon indicates that the tipclearance effect of the front rotor is larger than that of the rear

Fig. 8. Characteristic curves for the number of element.

Fig. 9. Characteristic coefficient comparison of experimental and compu-tational analyses for baseline.

(t/h)Rear [%]

CT

1 2 3 4 5 6 7 80.36

0.38

0.40

0.42

0.44

1.80% (t/h)Front3.67% (t/h)Front5.61% (t/h)Front7.62% (t/h)Front

Fig. 10. Thrust coefficient of the counter-rotating ducted fan for varioustip clearances.

Fig. 7. Computational mesh for the ducted fan.


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rotor in the counter-rotating ducted fan.Figure 13 shows the variations in thrust coefficient for the

components of the ducted fan when the front rotor has a fixedtip clearance. The total thrust coefficient of the counter-rotat-ing ducted fan increases as the tip clearance of the rear rotorincreases. The part of the thrust coefficient for the rear rotorranges from ðt=hÞRear ¼ 1:80% to ðt=hÞRear ¼ 7:62%.

The mass flow rate is calculated passing from the tip clear-ance region over the camber line as shown in Fig. 14. The

mass flow rate was calculated for various tip clearances ofthe rear rotor.

Figure 15 shows the mass flow rate across the tip clear-ance of the rear rotor for the counter-rotating ducted fan.The variation in mass flow rate increases significantly for alarge front rotor tip clearance. This variation changed from21.68% to 58.80% when the tip clearance of the rear rotor in-creased. When the mass flow rate of the tip leakage flow in-creases, the performance of the turbo-powered machinery de-creases. The variation in mass flow rate is greatly affected bythe rear rotor tip clearance.5.4. Flow field analysis

The total pressure coefficient measurement plane is de-scribed in Fig. 16. Figure 17 shows the total pressure coeffi-cient contour with the leakage flow patterns of various tipclearances for a counter-rotating ducted fan. The leakageflows from the left side (pressure side) to the right side(suction side) of each figure for the front rotor and in theopposite direction for the rear rotor.

The total pressure coefficient is defined as

Cp0 ¼Ptotal � Ptotal; inlet

1=2� �� V2inlet

ð11Þ

A small amount of tip leakage vortex was observed on thefront and rear rotors at ðt=hÞFront ¼ 1:80% and ðt=hÞRear ¼1:80%, as shown in Fig. 17(a). The front rotor pressure dif-ference was smaller than that of the baseline model, as shown

(t/h)Front [%]

CP

1 2 3 4 5 6 7 80.28

0.30

0.32

0.34

0.36

1.80% (t/h)Rear3.67% (t/h)Rear5.61% (t/h)Rear7.62% (t/h)Rear

Fig. 11. Power coefficient of the counter-rotating ducted fan for varioustip clearances.

(t/h)Rear [%]

FOM

0 1 2 3 4 5 6 7 80.45

0.50

0.55

0.60

0.65


Fig. 12. FOM of the counter-rotating ducted fan for various tip clearances.

(t/h)Rear [%]

CT

CT,

Tota

l

1 2 3 4 5 6 7 80.05

0.10

0.15

0.20

0.25

0.30

0.25

0.30

0.35

0.40

0.45

0.50CT, TotalCT, Front RotorCT, Rear RotorCT, Duct

(t/h)Front=1.80%

Fig. 13. Thrust coefficient of the ducted-fan elements for 1.80% tip clear-ance of the front rotor.

(a) Rear rotor (b) Front rotor

Fig. 14. The mass flow rate across the tip clearance for the rear rotor.

(t/h)Rear [%]

Mas

sflo

wra

te(k

g/s)

1 2 3 4 5 6 7 80.000

0.001

0.002

0.003

0.004

0.005


Fig. 15. The mass flow rate across the tip clearance for the front rotor.


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in Fig. 17(b). A wide region of tip leakage flow occurred onthe suction side of the front rotor with small tip clearance,indicating wide leakage flow, as shown in Fig. 17(c). Fur-thermore, the pressure difference was greater than that forthe other cases. A strong tip vortex was generated from alarge tip clearance for each rotor, as shown in Fig. 17(d).The smallest thrust coefficient was confirmed, as shown inFig. 9.

In order to analyze the vortex cores, the normalized helic-ityHn was used in the present study. The normalized helicityis necessary in order to identify the vortex structure1,6) and isdefined by:

Hn ¼�i � wi

�ij j wij j ð12Þ

where �i and wi are the absolute vorticity and relative veloc-ity vectors, respectively. The normalized helicity refers to thecosine absolute vorticity and relative velocity vectors andcan be used to predict the magnitude of normalized helicity.

Figure 18 shows the measurement plane for the normal-ized helicity on the counter-rotating ducted fan.

Figure 19 shows the normalized helicity contour with thevortex cores of the counter-rotating ducted fan. The visual-ization planes were located at 0, 0.2, 0.4, 0.6, 0.8 and 1.0blade chords from the leading edges of the front and rearrotors at the tip region. As the tip clearance of the rear rotorincreased the thrust increased. The Hn � 1 region decreasedat ðt=hÞFront ¼ 7:62% and ðt=hÞRear ¼ 1:80% as shown inFig. 19(b). This indicates that the tip leakage flow of thecounter-rotating ducted fan is significantly affected by thetip clearance. An increase in tip clearance of the rear rotorcaused an increase in the region of Hn � 1, which affectedthe rear rotor, as shown in Fig. 19(c). In this case, the thrustcoefficient was larger than that of the baseline model. Atðt=hÞFront ¼ 7:62% and ðt=hÞRear ¼ 7:62 in Fig. 19(d), thenormalized helicity changed rapidly, causing FOM to de-crease. In the case ðt=hÞFront ¼ 1:80% and ðt=hÞRear ¼ 7:62%,the vortex core direction did not change. If the vortex coredirection is changed, the momentum loss increases as shownin Fig. 19(a), (b), (d).

6. Conclusions

We analyzed the aerodynamic performance of a counter-rotating ducted fan for VTOL UAV using computationalfluid dynamics. The computational analyses were validatedusing wind tunnel tests. The tip clearances of the front andrear rotors were changed. The following conclusions werederived.

(1) The thrust coefficient decreases as the tip clearance ofthe front rotor increases due to the increasing tip leakageflow.

(2) The power coefficient is influenced by the increase intip clearance of the rear rotor. This clearly shows that the

Fig. 16. Total pressure coefficients calculated location of the ducted-fan inthe mid-blade chord.

(a) (t/h)Front = 1.80% and (t/h)Rear = 1.80%

(b) (t/h)Front = 7.62% and (t/h)Rear = 1.80%

(c) (t/h)Front = 1.80% and (t/h)Rear = 7.62%

(d) (t/h)Front = 7.62% and (t/h)Rear = 7.62%

Fig. 17. Total pressure coefficients and leakage flow patterns for a coun-ter-rotating ducted fan in the mid-blade chord.

Fig. 18. Normalized helicity calculated location for a counter-rotatingducted fan.


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large tip clearance of the rear rotor incurs viscosity and tipleakage losses.

(3) A small tip clearance affects the amount of pressuredifference between the pressure and suction sides of the ro-tor. The largest thrust coefficient was observed at the largesttip clearance, which was located behind the smallest tipclearance.

(4) The smallest FOM occurred with large clearance of thefront and rear rotors. In this case, the normalized helicity

quickly changed.A small tip clearance for the front rotor and a large tip

clearance for the rear rotor were selected in this study. Thisenables the thrust coefficient to be increased in correlationwith the large tip clearance.

For the tip clearance of a rotor, the thrust increases at thecounter-rotating ducted fan. The tip clearance of the rear ro-tor has a significant effect on the total thrust of the counter-rotating ducted fan.

To reduce the amount of tip leakage vortex for each rotorin the counter-rotating ducted fan, further studies on tip treat-ment are needed.

Acknowledgments

This research was supported by the Basic Science Research Pro-gram of the National Research Foundation of Korea (NRF) fundedby the Ministry of Education (No. 2009-009072) for Jinsoo Cho.Also, this research financially supported by Hansung Universityfor Leesang Cho.

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(a) (t/h)Front = 1.80% and (t/h)Rear = 1.80%

(b) (t/h)Front = 7.62% and (t/h)Rear = 1.80%

(c) (t/h)Front = 1.80% and (t/h)Rear = 7.62%

(d) (t/h)Front = 7.62% and (t/h)Rear = 7.62%

Fig. 19. Normalized helicity contours with vortex cores on the blade-tipregion for a counter-rotating ducted fan.


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S. SaitoAssociate Editor


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