Frunzulica ARA 35

8/3/2019 Frunzulica ARA 35

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ACTIVE AND PASSIVE CONTROL METHODS

ON THE AERODYNAMIC SURFACES

Florin FRUNZULICA*,**

, Alexandru DUMITRACHE**

,

Octavian PREOTU***

, Horia DUMITRESCU**

* POLITEHNICA University of Bucharest, Faculty of Aerospace Engineering,Polizu 1-7, 011611 Bucharest, ROMANIA, e-mail: [email protected]

** Gheorghe Mihoc-Caius Iacob Institute of Statistics and Applied Mathematics, Calea 13

Septembrie no. 13, 050711 Bucharest, ROMANIA, e-mail: [email protected]***

University of Craiova, st. A. I. Cuza no.13, 200585, Craiova, Romania

Abstract.Flow control refers the ability to alter flows with the aim to achieve a desired effect: examples include delay of

boundary layer separation and drag reduction, noise attenuation, improved mixing or increased combustion efficiencyamong many other industrial applications. The main objective of this paper is to investigate ways of keeping the flow

attached to a larger length of a Coanda surface, with application in aerospace and wind energy. We investigated two

possibilities: one passive, which uses a slot, and an active one, based on the principle of synthetic jet. Reynolds averaged

Navier-Stokes simulations (RANS) with shear stress transport k- (SST model) of Menter have been used to compute the

two-dimensional turbulent flow. The numerical results are presented for the two methods considered.

Keywords: active control, passive control, turbulent flow, RANS

1. IntroductionIn the field of aerospace engineering, the

aerodynamic design of future civilian and military

aerospace vehicles will be greatly influenced by flow

control technologies available for high-lift devices, flight athigh angle of attack, jet engine inlet and exhaust systems,

thrust vectoring, jet noise reduction, etc. The flow control

devices will be used in a variety of flow situations: to

energize the boundary layer and to control of boundary

layer transition in low and high speed regimes, to modify

the shear layers, to produce jet deflections and to control

oscillations of different structural parts of the aircraft.

In the field of wind energy, aerodynamic control is

achieved by variable pitch blades and in present by stall

control; however, other control concepts for efficiency

improvement become a substantial source of study.

The approaches for separation control can be broken

in: (1)passive control(vortex generators, flaps/slats, slots,

absorbant surfaces and riblets) and (2) active control

(mobile surface, planform control, jets, advanced controls -

magnetodynamics).

The circulation control as active control technique is

know as beneficial in increasing the bound circulation and

hence the lift coefficient of airfoil. This technology has

been investigated both experimentally and numerically in

the last decade [1]. Circulation control is implemented,

usually, by tangential blowing a small high-velocity jet

over a highly curved surface, such as a rounded trailing

edge. This causes the boundary layer and the jet to remain

attached along the curved surface due to the Coanda effect

(the tendency of a moving fluid to attach itself to a surface

and flow along it) and causing the jet to turn without

separation.Forced jets have a few disadvantages:complexity of

internal piping from a source of pressure or vacuum, and

the parasitic cost to produce this pressure. A currently

challenge is to reduce the power consumption to produce

the jet and using efficiently the jet to control flow

separation.

The active control without additional net mass flow

can be achieved by synthetic jets or small vibrating flap. A

synthetic jet is a concept that it consists of an orifice or

neck driven by an acoustic source in a cavity [2]. At

sufficiently high levels of excitation by the acoustic source,

a mean stream of flow has been observed to emanate from

the neck. The excitation cycle increase the ability of theboundary layer to resist separation.

Another technique of increasing the lift of airfoils is

the use of passive devices, one of these being known as

Gurney flap [3]. The Gurney flap is a small tab attached

perpendicular to the lower surface of the airfoil in the

vicinity of the trailing edge, with a height that can vary

from 1% to 5%. The results showed a significant increment

in lift compared to the baseline airfoil.

Another passive device uses a slot between lower-

pressure and high-pressure points (near the separation


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point) on the upper surface of the thick airfoil (at positive

angle of attack). Mounting on the slot a controlled

hydraulic resistance we can control the separation point of

the boundary layer [4].

In this paper we investigate three issues related to

flow control with applicability in aerospace and wind

energy: appropriate turbulence model for the study of jets

on convex surfaces, passive control using a slot and active

control using synthetic jet at medium frequencies on

Coanda surfaces.

2. Coanda effect. Computational analysisThe main goal is to provide a systematic survey of

the performance of selected eddy-viscosity models in a

range of curved flows and to establish more clearly their

potential and limitations.

Reynolds averaged Navier-Stokes simulations

(RANS) with different turbulence models have been

employed to compute the two-dimensional turbulent wall

jet flowing around a circular cylinder: (1) Spalart and

Allmaras (SA - one equation turbulence model) [5], (2)Launder and Spalding k- model [6], (3) Wilcox k- model

[7] and (4) Menter k- SST model [8]. The predictions of

the simulations were compared to available experimental

measurements in the literature.

The particular configuration shown in figure 1 is

considered since cylindrical wall jet properties have been

reported by Neuendor and Wygnanski [9] and provide a

means for evaluation of simulation results (diameter

d=0.2032 m, nozzle height b=2.34 mm and jet-exit velocity

Vjet=48 m/s). For the turbulence models used in these

calculations the laminar sublayer needed to be resolved.

The y+

values of the wall-next grid points were between 0.2

and 1, and the x+ values were between 50 and 300. Thegrid resolution in the jet was between 40 and 180 times the

local Kolmogorov length scale. The computational grid

consists of 720 nodes (on cylindrical wall) x 150 nodes (on

radial direction).A fully developed channel velocity profile

was prescribed at the nozzle inflow. The ambient was

quiescent.

Figure 3 Configuration used in analysis

One weakness of the eddy-viscosity models is that

these models are insensitive to streamline curvature and

system rotation. Based on the work of Spalart and Shur a

modification of the production term has been derived,

which allows to sensitize the standard k-, SST model to

these effects.

a. b.

Figure 2 Streamlines: k- (a) and k- SST c.c. (b)

For the k- SST model (with curvature correction

c.c.) the separation location was slightly closer to the

experiment. When the k- and Spalart-Allmaras models

were used, the jet remained attached to the cylinder formore than 270 deg (figure 2).

For some of these turbulence models the jet-velocity

decay and jet-half-thickness are plotted in figure 3 against

streamwise angle. When the k- SST c.c. model was used

a close match of the jet-velocity decay with the measured

data was achieved.

Because the predicted half-thickness was small for

all models, the normalized velocity profiles do not match

the experimental velocity profiles, either in the mild

pressure region or in the adverse pressure region.

Figure 3 Jet velocity decay and jet-half-thickness


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3. Passive control using a slotThe first computational case uses a simple convex

surface and the second computational case uses the same

convex surface with a slot between over-pressure point on

the surface and under-pressure point on the surface (placed

in separation boundary layer region). The tendency of

equalization of pressures leads to blow in the first oriffice

of slot, while in the second oriffice we have the suction.

The jet is developed in a rectangular channel with 9

mm height and it has velocity Vjet=25 m/s. For

computation we use steady RANS with k- SST c.c.

turbulence model and computational grid has 219,300

nodes. The suction-blowing phenomenon has a beneficial

effect on keeping boundary layer attached on 82% of the

surface compared to the case without the slot when

boundary layer is attached to the 58% of the surface. The

figure 4 shows the two situations mentioned. The jet isdeflected by 20 degrees from the original direction. Using a

hydraulic resistance on the slot we can control the

separation point of the jet and the jet orientation (the

problem will be investigated in a future work).

a.

b.

Figure 4 Velocity vectors without (a) and with (b) slot

4. Active control using synthetic jet (SJ) concept4.1. Laminar boundary layer interacting with SJ on flat

plateThe concept is shown in figure 5, where the

synthetic jet is embedded in the wall of a boundary layer

for which separation control is desired. The cavity is

provided at the bottom with a mobile surface that oscillates

sinusoidally with 1 mm amplitude and 50 Hz frequency.

The flow (15 m/s) over plate is laminar with Blasius

velocity profile.

Figure 5 Boundary layer interacting with SJ

One parameter found useful in the normalization of

the jet velocity is the maximum inviscid jet velocity which

for the prescribed membrane motion (sinusoidal

oscillation) is given by max / (2 )inv

V AW d = =6.3 m/s.

The simulation includes the dynamic mesh model

using a spring network near the membrane for interacting

between membrane and adjacent fluid.

In figure 6.a the instantaneous profiles downstream

of the slot (40 mm) at the peak of the in-stroke and peak ofthe out-stroke are shown versus the baseline. The in-stroke

profile illustrates the wall removal of the low-momentum

fluid at the same time with a freestream velocity decrease,

and the out-stroke profile illustrates the high-momentum

injection at the same time with a freestream velocity

increase. The time-averaged controlled boundary versus the

baseline profile is given in figure 6.b which shows the

energization effect of the synthetic jet with a net diffusion

of the freestream.

a.

b.

Figure 6 The velocity profile: Instantaneous velocity

profiles on flat plate with and without control (a) and time-

averaged velocity profiles on flat plate with and without

control (b).


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4.2. Turbulent boundary layer interacting with SJ over

Coanda surfaceThe configuration is the same as in section 3 but it

has an actuator with lateral slot placed at the point of the

detached boundary layer. The diaphragm oscillates

sinusoidal with 100 Hz and 1 mm amplitude. For

simulation we used unsteady RANS, k- SST turbulence

model with curvature correction. The computational grid

has 160,000 nodes and the y+

values of the wall-next grid

points were between 0.05 and 1, and the x+values were

between 10 and 100. In this investigation did not

completely suppress the separation, boundary layer not

enough energized by the vortical structures generated. We

observed a small unsteady deviation on the jet, about 3

degrees (figure 7).

a.

b.

Figure 7 Velocity vectors at maximum expulsion (a) t =

0.02 s, and maximum ingestion (b) - t=0.03s

5. Conclusions

The Coanda wall jet developing on a circularcylinder was investigated numerically. This configuration

was used to evaluate turbulence models for steady RANS

of flows over curved surfaces. The main conclusion was

that none of the models tested correctly predicted all

relevant aspects of the flow. Relatively speaking, the k-

SST model with curvature correction performed best.

Two methods for flow control were presented:

- One passive, which uses a slot that connects the low

pressure and high pressure points on the Coanda surface.

Adding a controlled resistive device on the slot we can

change the jet orientation and the system become an active

control device.

- An active one, based on the principle of synthetic jet,

created through an orifice located near the point of

detachment of the jet.

The synthetic jet concept and a numerically

investigation of interacting jet with boundary layer were

investigated. Numerically investigation of medium

frequencies of synthetic jets hasnt led to expected results

on the Coanda surface.

Future work: we will introduce the active control on

the slot and we will investigate the using of the piezo-

actuators on the Coanda surface.

References1. Slomski, J.F., Chang, P.A.: Large Eddy Simulation of

a Circulation Control Airfoil. AIAA Paper 2006-3011,

3rd AIAA Flow Control Conference, 5 - 8 June 2006,

San Francisco, California

2. Glezer, A., Amitay, M.: Synthetic Jets. Annu. Rev.Fluid Mech., 34, 2002, p. 503-529

3. Van Dam, C.P., Yen, D.T., Vijgen, P: Gurney flapexperiments on airfoil and wings. Journal of Aircraft

(0021-8669), 36 (2), 1999, p.484486

4. Frunzulica, F., Dumitrache, A., Preotu, O.: Control oftwo-dimensional turbulent wall jet on a Coanda

surface. 82nd Annual Scientific Conference of the

International Association of Applied Mathematics and

Mechanics (GAMM), Graz, Austria, April 18 - 21,

2011 (to be published in PAMM J.)

5. Spalart, P.R., Allmaras, S.R.: A one-equationturbulence model for aerodynamic flows. AIAA Paper

92-0439, 1992

6. Launder, B.E., Spalding, D.B.: The numericalcomputation of turbulent flows. International Journalfor Numerical Methods in Fluids, 15, 127, 1974

7. Wilcox, D.: Simulation of transition with a two-equation turbulence model. AIAA Journal, 32, 1192,

1994

8. Menter, F.R.: Eddy viscosity transport equations andtheir relation to the k- model. ASME Journal of

Fluids Engineering, 119, 876, 1997

9. Neuendorf, R., Wygnanski, I.: On a turbulent wall jet flowing over a circular cylinder. Journal of Fluid

Mechanics, 381, 1, 1999

10. Shur, M. L., Strelets, M. K., Travin, A. K., Spalart, P.R.: Turbulence Modeling in Rotating and CurvedChannels: Assessing the Spalart-Shur Correction.

AIAA Journal, 38, No. 5, 2000, p. 784-792

Acknowledgement: This paper was supported by National

Research Program 543 IDEI/2008.

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