Computational Evaluation of Controlling Flap-Edge Vortex and Tip Vortex Using Circulation Control...

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American Institute of Aeronautics and Astronautics

Computational Evaluation of Controlling Flap Edge Vortex and Tip Vortex Effects with Circulation Control Technique

Yi Liu1 National Institute of Aerospace, Hampton, VA 23666

Lakshmi N. Sankar2 Georgia Institute of Technology, Atlanta, GA 30332

[Abstract] This paper extends our previous work on the two-dimensional Circulation Control (CC) airfoil studies to three-dimensional applications so that the potential benefits of CC techniques on controlling vortex and reducing noise can be evaluated. Two numerical simulations have been done to identify the effects of CC techniques on controlling the flap edge vortex and the tip vortex. The first is a stream-wise tangential blowing on a wing-flap configuration. It is demonstrated that a gradually varied CC blowing can totally eliminate flap-edge vortex, thus reducing flap-edge noise. The second case involves a span-wise tangential blowing over a rectangular wing with a rounded wing tip. It is found that CC blowing cannot totally eliminate the tip vortex. However, it can control and modify the location of the tip vortex to increase the vertical clearances between the wing and the tip vortex, thus reducing the blade vortex interaction (BVI) and the BVI noise.

Nomenclature Ajet = area of jet slot Æ = aspect-ratio of the wing α = angle of attack CL = lift coefficient CD = drag coefficient Cµ = jet momentum coefficient e = efficiency coefficient of lift distribution m& = jet mass flow rate Pjet = pressure of jet S = wing area Tjet , T0,jet = temperature and total temperature of jet V∞ = free-stream velocity Ujet = jet velocity ρjet , ρ ∞ = densities

I. Introduction

urrent generation large commercial aircraft are dependent on components that can generate high lift at low speed during take-off or landing in order to use existing runways. Conventional high-lift systems include flaps

and slats, with the associated flap-edges and gaps. In addition to their contribution to noise, these high-lift systems also add to the weight of the aircraft, and are costly to build and maintain.

An alternative to conventional high-lift systems is the CC Wing technology [1]. This technology and its aerodynamic benefits have been extensively investigated over many years by Englar et al at Georgia Tech through experimental studies [2, 3]. A limited number of numerical analyses [4, 5] have also been done. Work has also been done on the acoustic characteristics of CC wings [6, 7]. These studies indicate that very high CL values (as high as

1 Research Scientist, National Institute of Aerospace. Member AIAA. 2 Regents Professor, School of Aerospace Engineering. Fellow AIAA.

45th AIAA Aerospace Sciences Meeting and Exhibit8 - 11 January 2007, Reno, Nevada

AIAA 2007-473

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8.5 at α=0°) may be achieved with CC wings. Because many mechanical components associated with high-lift systems are no longer needed, the new wings can be lighter and less expensive to build. Some of the major airframe noise sources, such as flap-edges, flap-gaps, and trailing/leading edge flow-separation can all be eliminated with the use of CCW systems.

Circulation Control technology has many potential applications for both fixed and rotary wing aircraft. All of these applications take advantage of the high lift benefits and the ability of directly controlling the flow field associated with the CC technology. For fixed wing vehicles, the high lift generated by CC wings makes them ideal candidates for short take-off and landing (STOL) and high lift aircraft. To find ways of improving the aircraft operation from carriers, the Navy sponsored a full-scale flight test program on an A-6/CCW STOL demonstrator in 1979 [8, 9]. The airfoil used was a rounded trailing edge CC airfoil. Using only available bleed air from the engines, it could achieve CL values that were 120% higher than a conventional Fowler flap, or a 140% increase in the usable lift coefficient at take-off/approach angles of attack. The researchers were also aware of the drag penalty, and improvements with use of smaller cylinder trailing edges and hinged flaps have been recommended [10]. For commercial aircraft, experimental evaluations were conducted on the use of blown high-lift devices and control surfaces on the High Speed Civil Transport (HSCT) aircraft [11]. These studies found that the advanced pneumatic high-lift devices produced large lift increases as well as significant drag reductions, and confirmed the effectiveness of combined pneumatic high-lift devices and control surfaces on these HSCT aircraft.

The ability of controlling the lift directly without angle of attack change gives the CC airfoil potential of being used on rotary wing aircraft as well. This concept allows the use of higher harmonic control of helicopters, where cyclic lift variations are usually at frequencies higher than one per revolution. Suppression of these high frequency components can result in considerable reduction of rotor vibration, fatigue and noise. In 1979, a CC rotor flight demonstrator based on a Kaman H-2 helicopter was tested [12, 13]. Instead of using a conventional mechanical cyclic and collective blade pitch control system, a pneumatic aerodynamic and control system was applied. It was found that the CC rotor had the potential of eliminating the mechanical blade lift and control devices in hover and forward flight, and also had the ability of achieving higher harmonic control. It also suggested that the elimination of the angle of attack control could also result in reducing the hub complexity, number of mechanical parts, size, and drag. However, due to a control system phasing problem, the flight test envelope was limited. Another application of the CCW technology is the X-wing stopped rotor aircraft. In this design, a four-blade CC rotor would be used during vertical take-off and landing, and the rotor assembly would be locked into a stationary position during forward flight, and function as a fixed wing [14].

There are many other potential applications for the Circulation Control or Pneumatic Aerodynamic technology besides these mentioned above. The reader is strongly referred to the Reference [15], which gives a great overview of the development of CC technologies, and summarizes many of this effort.

Our previous work [16] numerically simulated the use of CC for enhancing the lift characteristics of 2-D airfoil configurations. It was demonstrated that both steady and unsteady (pulsed) jets are effective in achieving high values of lift without having to resort to the use of complex multi-element configurations.

For 3-D applications, CC has a number of other uses. It may be used to modify the span-wise lift distribution of wing sections, effectively altering the span loading of lift forces. Since the trailing vortex structures are directly affected by, and related to the bound circulation, one can modify the strength (or spatial distribution) of trailing vortex structures, including the strong vortex that forms at the wing tips.

This paper extends our previous work and addresses the uses and benefits of 3-D Circulation Control. Two configurations are simulated and studied. The first is a stream-wise tangential blowing on a wing-flap configuration. The second is a span-wise tangential blowing over a rectangular wing with a rounded wing tip. Some interesting results are obtained for both configurations, demonstrating that there are many potential practical applications for the Circulation Control technology, beyond high lift applications

II. Mathematic Formulation and Modeling Approach

In the present work, the Reynolds-averaged Navier-Stokes equations were solved using an unsteady 3-D viscous flow solver. A semi-implicit finite difference scheme based on the Alternating Direction Implicit (ADI) [17] method was used. The NS solver uses a multi-zone technique to model the wing and the flap separately for the tangential blowing case. For the span-wise blowing case, the rounded wing-tip is modeled separately with this technique. The flow around the airfoil is assumed to be fully turbulent, so no transition models are currently used. Two turbulence models are used: the Baldwin-Lomax [18] algebraic model and the Spalart and Allmaras[19] one equation model. To better represent separation and flap-edge vortex, the results shown in this paper are simulated with the Spalart-Allmaras model.

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In most Circulation Control Wing studies, the driving parameter is the momentum coefficient Cµ, defined as follows:

UmC jet

∞∞

Here, the jet mass flow rate is given by:

jetjetjet AUm ρ=& (2)

where Ajet is the area of the jet slot, and S is the area of the whole wing section. In 2-D simulations, Ajet is the height of the jet slot and S is the chord of the CC airfoil.

In the present study, the following boundary conditions are specified at the slot exit: 1) the total temperature of the jet T0, which is approximately equal to the total temperature of free-stream, 2) the momentum coefficient Cµ as a function of time, and 3) the flow angle at the exit. In this simulation, the jet velocity direction is normal to the jet slot exit and tangential to the surface. Since the jets are nearly always under-expanded, the jet slot exit location is assumed as the minimum area of the nozzle, i.e., the throat. The physics of the jet slot boundary conditions are shown in Figure 1. The detailed formulation about the governing equations, numerical differentiation procedure, and boundary conditions can be found in Ref. 20.

Figure 1: The Jet Slot Boundary Conditions

1. Controlling Flap Edge Vortex with Tangential Blowing

The flap edge vortex is always a strong source of the airframe noise, especially when high lift devices are fully

deployed during take-off or landing. According to Prandtl’s classic lifting-line theory [21], a trailing vortex will be generated whenever there is a change in the bound circulation over the wing. For a wing-flap configuration, the lift and hence the bound circulation is much higher over the flap than on the main wing. Since the circulation is not continuous at the interface between the wing and the flap, a strong vortex is generated here. These vortices have been seen in many experiments and flight tests. For example, the experimental [22] and computational [23] studies indicated that a very strong vortex was generated at the flap-edge due to the sudden increase in the lift. This vortex, due to its interaction with the flap gap, will generate a strong noise, commonly labeled as “flap-edge noise”.

A number of approaches have been proposed to eliminate this noise source. Vortex fences and serrated flap edges [24] have been proposed and tested. These devices add to the weight and cost of manufacturing of the wing. Because these are passive devices, they can be best optimized for a single operating condition (e.g. a specified flap angle, flow angle of attack, and free-stream velocity), and cannot be expected to work for all conditions.

The purpose of the present research is to determine if the CC technology may be used to modify the lift distribution along the span, thereby weakening or eliminating the flap-edge vortex. Figure 2 below shows a sketch of

Flow angularity depends on slot geometries

P0 = Total pressure depends on upstream conditions

T0 = Total temperature also depends on upstreamconditions

These are specified.

In subsonic jets, P must be continuous.

In supersonic jets, P should be specified.

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this concept – a wing-flap configuration with tangential blowing over the main wing. Only the left half of this wing-flap configuration is simulated, and the flow is assumed to be symmetrical. In this region, the wing section within the first five chord-lengths from the central boundary has a 30-degree flap, and there is a weak jet blowing (Cµ ≅ 0.01) over the flap to suppress the separation and vortex shedding. The other part of the wing has no flap, but a scheduled CC blowing is used in this section of the wing to generate the high lift that is comparable with the lift generated by the 30-degree flap.

∞∞∞∞C

This region is modeled as shown in next figure

2-D BC

Symmetry BC

Small blowing to suppress vortex shedding

15 C 5 C 5 C

∞∞∞∞C

This region is modeled as shown in next figure

2-D BC

Symmetry BC

Small blowing to suppress vortex shedding

15 C 5 C 5 C

Figure 2: The Wing-flap Tangential Blowing Configuration

Figure 5 shows details of the grid around this configuration. There are two regions that are very important in these simulations. Region B is the interface between the blowing section of the main wing and the non-blowing section of the main wing, and region A is the interface between the blowing section of the main wing and the wing-flap section. 2. Controlling Tip Vortex with Span-wise Blowing

The vortex over the wing tip region is also a strong noise source. In rotary wing applications, this vortex can

interact with other blades, giving rise to BVI noise. Tip vortex is generated by the pressure differences between the upper and lower surfaces of the lift wing. Since in general, the pressure at the lower surface is much higher than that at the upper surface, the vorticity of the fluid particles within the boundary layer at the lower surface will flow around the wing tip, rollup, and form a tip vortex. The tip vortex formation may be drastically altered by generating a flow in a direction opposite that of the boundary layer. To investigate the feasibility of this concept, a wing-tip configuration is modeled to study the effects of tangential span-wise blowing on the flow field around the wing-tip region.

Figure 3 below shows a sketch of this concept for a rounded wing tip. The wing is a simple rectangular wing with NACA 0012 airfoil sections, but the wing tip is round. The angle of attack is 8 degrees, giving rise to sufficient lift and a strong tip vortex. The jet slot is located above the rounded wing tip edge, and the jet is blowing out in the span-wise direction. The multi-zone H and O grids are used to mesh the wing and the rounded wing-tip, respectively. Figures 6, 7, 8, 9 show the configuration and the body fitted grid in the vicinity of the rounded wing tip and the jet slot.

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Figure 3: The Wing Tip Configuration

0 5 10 15 20 25

Span, Y/C

Noblowing on Main Wing

Constant Blowing on Main Wing

Gradual Blowing on Main Wing

Figure 4: The Lift Coefficient Distribution along Span for the Wing-flap Configuration

III. Results and Discussion 1. Controlling Flap Edge Vortex with Tangential Blowing

For this configuration as shown in Figure 2, three cases are studied. In the first case, there is no blowing on the

main wing, so it is just a regular wing-flap configuration. In the second case, there is a constant blowing, which means the Cµ is constant along the span, over some sections of the main wing (from 15C to 20C). In the third case, a gradual blowing case is studied, wherein the Cµ is gradually increased along the span over some sections of the main wing (from 10C to 20C). Figure 4 shows the lift coefficient distribution along the span of this wing-flap configuration for these three cases. When there is no blowing, a steep increase in lift coefficient is found at the interface between the main wing and the flap. This is expected because the sectional lift generated in the vicinity of the 30-degree flap is much higher than of the main wing. In the second case, when a constant blowing is generated over a section of the main wing, the lift at these stations will be greatly increased due to the Coanda effect. Thus in Region A, the difference of lift between the blowing section of the main wing and the flap will be reduced, but a sharp increase in the lift is still found at the interface between the blowing section of the main wing and the non-blowing section (Region B). In the third case involving the gradual blowing, it is seen that the lift is smoothly

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increased along the span, from 0.25 to 1.4 over the flap without a sudden change. This is due to the gradual increase in the blowing momentum coefficient, Cµ.

According to the lift distribution and the Prandtl’s lift line theory, cases 1 and 2 should generate strong vortices in Region B and A, respectively, while in case 3, there is a weakening or a total elimination of the flap-edge vortex. This is observed in the vorticity contours shown in Figures 10, 11 and 12. In Figure 10, when there is no blowing on the main wing, there is a flap-edge vortex in region A, with no vortex in region B. If the constant blowing is produced on the main wing, as shown in Figure 11, the flap-edge vortex in Region A is eliminated, but another vortex is generated in region B. Only when a gradual blowing is conducted on the main wing, are both flap-edge vortex and in-board vortex eliminated, as shown in Figure 12.

2. Controlling Tip Vortex with Span-wise Blowing

For this configuration as shown in Figure 3, three other cases have been studied. In the first case, there is no

blowing, simulating a rectangular wing with a rounded wing tip. In the second case, there is a small amount of blowing with Cµ = 0.04. In the third case, there is a stronger blowing with Cµ = 0.18. Figures 13, 14 and 15 show the vorticity contours around the wing tip region at three different stream-wise locations, which are x/c = 0.81, 1.0, and 1.50, respectively. From those figures, it is proven that there is a strong tip vortex if there is no blowing. If there is a small amount of blowing over the wing tip in the opposite direction, the tip vortex is pushed away from the wing tip, but the vortex is not eliminated. Even when the amount of blowing is increased, the tip vortex is only pushed down and far away from the wing; another weaker vortex with an opposite rotation direction is generated.

Figures 16 and 17 show the lift and drag coefficients distribution along span for this wing tip configuration. It is seen that the tangential blowing over the wing tip can also increase the lift around whole wing, especially when there is a strong CC blowing. The calculated overall lift coefficient and drag coefficient for the whole wing are tabled as follows:

Table 1: The Total Lift Coefficient and Drag Coefficient for the Wing Tip Configuration

Total Lift

Coefficient

Total Drag Coefficient

Computed Drag from the

Inviscid Relation

CD,C = (CL)2/(πÆe)

Noblowing Case 0.4850 0.02997 0.02997

Less Blowing,

Cm = 0.04

0.5215 0.03078 0.03465

More Blowing,

Cm = 0.18

0.6064 0.04342 0.04685

Where Æ is the aspect-ratio of the wing, which is equal to 4 for this configuration, and the e is the efficiency of the lift distribution, which is set at 0.6246 from the non-blowing case calculation. It is seen that the total drag of the wing has been reduced by about 10% by CC blowing, compared to the invisicid calculations with the correction for the increase in CL.

IV. Conclusion

In summary, for the 3-D tangential stream-wise blowing over the wing-flap configuration, several conclusions can be obtained: 1) the flap-edge vortex is generated by the sudden increase in the lift along the flap-edge interface; 2) CC blowing with a constant momentum coefficient cannot eliminate the flap-edge vortex, but can weaken and move the location of this vortex from the flap-edge towards the main wing; 3) a gradually varying CC blowing can totally eliminate the vortex.

The conclusions for the 3-D spanwise blowing over a rounded wing tip configuration are that the jet blowing around the rounded wing tip can modify and change the location of the tip vortex. It cannot totally cancel or eliminate the tip vortex, but can change or increase the vertical clearance between the wing and the vortex. Since the

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blade vortex interaction of rotors is strongly influenced by the clearance between the following blades and the tip vortex, this approach offers the potential of reducing BVI noise. It can also slightly reduce the drag of the whole wing tip configuration by pushing the tip vortex away from the wing and increasing the aspect-ratio.

It should be noted that these are just some simple numerical simulations, and that the models used here are not complicate and with full details. To fully understand the effect of the CC blowing on the flap-edge vortex and tip vortex, more detailed simulations and experiments are needed to be conducted.

References 1Englar, R. J., “Circulation Control Pneumatic Aerodynamics: Blown Force and Moment Augmentation and Modification;

Past, Present & Future”, AIAA Paper 2000-2541, June 2000. 2Englar, Robert J., Smith, Marilyn J., Kelley, Sean M. and Rover, Richard C. III., “Application of Circulation Control to

Advanced Subsonic Transport Aircraft, Part I: Airfoil Development,” Journal of Aircraft, Vol.31 No.5, pp. 1160-1168, Sep. 1994.

3Englar, Robert J., Smith, Marilyn J., Kelley, Sean M. and Rover, Richard C. III., “Application of Circulation Control to Advanced Subsonic Transport Aircraft, Part II: Transport Application,” Journal of Aircraft, Vol.31, No.5, pp. 1169-1177, Sep. 1994.

4Shrewsbury, G. D. and Sankar, L. N., “Dynamic Stall of an Oscillating Circulation Control Airfoil,” International Symposium on Nonsteady Fluid Dynamics, oronto, Canada, June 4-7, 1990, Proceedings. New York, American Society of Mechanical Engineers, pp. 15-22, 1990.

5Shrewsbury, G. D. and Sankar, L. N., “Dynamic Stall of Circulation Control Airfoils,” AIAA Paper 90-0573, January 1990. 6Salikuddin, M., Brown, W. H. and Ahuja, K. K., “Noise From a Circulation Control Wing with Upper Surface Blowing,”

Journal of Aircraft, Vol.24, pp55-64, Jan. 1987. 7Munro, S., Ahuja, K., and Englar, R., “Noise Reduction Through Circulation Control Technology,” AIAA Paper 2001-0666,

Jan. 2001. 8Englar, R. J., Hemmerly, R. A., Moore, H., Seredinsky, V., Valckenaere, W. G. and Jackson, J. A., “Design of the

Circulation Control Wing STOL Demonstrator Aircraft,” AIAA paper 79-1842, August, 1979. 9Pugliese, A. J. and Englar, R. J., “Flight Testing the Circulation Control Wing,” AIAA paper 79-1791, August, 1979. 10Englar, R. J. and Huson, G. G., “Development of Advanced Circulation Control Wing High Lift Airfoils,” AIAA paper 83-

1847, presented at AIAA Applied Aerodynamics Conference, July, 1983. 11Englar, R. J., Niebur, J. S. and Gregory, S. D., “Pneumatic lift and control surface technology for high speed civil

transports,” Journal of Aircraft, vol. 36, no.2, pp.332-339, Mar.-Apr., 1999. 12Wilkerson, J. B., Reader, K. R. and Linck, D. W., “The Application of Circulation Control Aerodynamics to a Helicopter

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Symposium on Rotor Technology, August, 1976. 15Englar, R. J., “Overview of Circulation Control Pneumatic Aerodynamics: Blown Force and Moment Augmentation and

Modification as Applied Primarily to Fixed Wing Aircraft,” Application of Circulation Control Technology, Edited by Ronald D. Joslin and Gregory S. Jones, Progress in Astronautics and Aeronautics, Vol. 214, Chap 2, pp23-68, 2006

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17Briley, W. and McDonald, H., “Solution of Multi-Component Navier-Stokes Equations by Generalized Implicit Method,” Journal of Computational Physics, Vol.24, p. 372, 1977.

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21Johnson, W., Helicopter Theory, Dover Publications, INC, 1980. 22Radeztsky, R. H., Singer, B. A. and Khorrami, M. R., “Detailed Measurements of a Flap Side-edge Flow Field,” AIAA

paper 98-0700, Jan. 1998. 23Khorrami, M. R., Singer, B. A. and Radeztsky, R. H., “Reynolds Averaged Navier-Stokes Computations of a Flap Side-

edge Flow Field,” AIAA paper 98-0768, Jan. 1998. 24Herkes. W. H., and Stoker R. W., “Wind tunnel measurements of the airframe noise of a high-speed civil transport,” AIAA-

1998-472, 36th Aerospace Sciences Meeting and Exhibit, Jan. 12-15, 1998.

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Figure 5: The Grid of the 3-D Wing-flap Configuration with a 300 Partial Flap

Region A Region B

25 10 0 1520

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Figure 6: The H-Grid for the Wing Tip Configuration (Side View at Spanwise Station)

Figure 7: The O-Grid around the Rounded Wing Tip (Front View)

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Figure 8: The Surface Grid for the Rounded Wing Tip

Figure 9: The Detailed Grid Close to the Jet Slot

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Tangential Blowing Case 1: Non-blowing

Figure 10: The Vorticity Magnitude Contours for Tangential Non-Blowing Case

Main Wing

Unblown Section

Blowing Section

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Tangential Blowing Case 2: Constant-blowing

Figure 11: The Vorticity Magnitude Contours for Tangential Constant-Blowing Case

Flap Main Wing

Unblown Section

Blowing Section

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Tangential Blowing Case 3: Gradual-blowing

Figure 12: The Vorticity Magnitude Contours for Tangential Gradual-Blowing Case

Flap Main Wing

Unblown Section

Blowing Section

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Figure 13: The Vorticity Contours around the Wing Tip (x/C = 0.81) For Span-wise Blowing Case

No-Blowing Case

Less Blowing Case(Cµµµµ= 0.04)

More Blowing Case (Cµµµµ= 0.18)

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No-Blowing Case

Less Blowing Case(Cµµµµ= 0.04)

More BlowingCase (Cµµµµ= 0.18)

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No-Blowing Case

Less Blowing Case (Cµµµµ= 0.04)

More Blowing Case (Cµµµµ= 0.18)

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0 0.2 0.4 0.6 0.8 1 1.2

Span, Y/Ytip

Noblowing

Blowing, Cmu = 0.04

Blowing, Cmu = 0.18

Figure 16: The Lift Coefficient Distribution along Span for Wing-tip Configuration

0 0.2 0.4 0.6 0.8 1 1.2

Span, Y/Ytip

Noblowing

Blowing, Cmu = 0.04

Blowing, Cmu = 0.18

Figure 17: The Drag Coefficient Distribution along Span for Wing-tip Configuration

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Computational Evaluation of Controlling Flap-Edge Vortex and Tip Vortex Using Circulation Control...

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