Numerical Simulations of the Aerodynamic Characteristics of

Circulation Control Wing Sections

Ph.D Thesis Defense

By

Yi Liu

Advisor: Prof. Lakshmi N.Sankar

Supported by NASA Langley Research Center

Outline of Presentation• Motivation and Objectives• Circulation Control Wing Technology• Previous Research Work• Mathematical and Numerical Formulation• 2D Simulation Results and Discussion

• Steady Blowing Results • Pulse Blowing Results

• 3D Simulation Results and Discussion• Tangential Blowing on a Wing-flap Configuration • Spanwise Blowing over a Rounded Wing-tip

• Conclusions from the 2D Simulations• Conclusions from the 3D Simulations• Recommendations

Motivation and Objectives

• Noise pollution from the large aircraft has become a major problem that needs to be solved. NASA proposed a plan to reduce the noise by a factor of four (20dB) by 2025.

• A major source of large aircraft airframe noise during take-off and landing is the high-lift system - namely flaps, slats, associated with flap-edges and gaps.

• The high-lift system also contains many moving parts, which add to the weight of the aircraft, and are costly to build and maintain.

• These devices for generating high lift are necessary for large aircraft that use existing runways.

Boeing 737 Wing/Flap System (Paper by Robert Englar)

• An alternative to conventional high-lift systems is the Circulation Control Wing (CCW) technology.

• The CC wing can generate the same high lift with much less complexity compared to the high-lift system, and many noise sources such as flaps and slats, can also be eliminated by the CC wing.

• For example, as shown in previous figure, there are just 0-3 moving elements per wing for a Circulation Control wing with leading edge blowing, compared to 15 moving parts of a conventional Boeing 737 wing with high-lift systems.

Circulation Control Wing Concept

• Circulation Control Aerodynamics: In this approach a tangential jet is blown over a highly curved aerodynamic surface (the Coanda surface) to increase or modify the aerodynamic forces and moment with few or no moving surfaces.

• Figure (Taken from paper by Englar) shows a traditional Circulation Control Airfoil with a rounded trailing edge.

• At very low momentum coefficients, the tangential blowing will add energy to the slow moving flow near the surface. This will delay or eliminate the separation, and is called Boundary Layer Control.

• When the momentum coefficient is high, the lift of the wing will be significantly increased. This is called Circulation Control.

• The lift augmentation, which is defined as CL / C, can exceed 80 as shown in previous figure.

Circulation Control Wing Concept

• In general, the driving parameter of Circulation Control is the jet momentum coefficient, C, which is defined as:

qS

VmC jetjet

• The advanced CC airfoil, i.e. a circulation hinged flap, was developed by Englar et al to replace the traditional CC airfoil.

• This advanced CC airfoil use a small trailing edge flap with a large-radius arc upper surface and a flat low surface. The flap can be deflected 00 < f < 900.

• During take-off / landing, the flap is deflected, thus generating very high lift like a traditional rounded trailing edge CC airfoil.

• During cruise, f = 00, leading to a conventional airfoil shape with a sharp trailing edge that significantly reduces the drag.

Advanced CC Airfoil

Some Applications of the CCW Technology

• STOL (short take-off and landing) aircraft: Englar et al (1979)

• Advanced Subsonic Aircraft and High Speed Civil Transport (HSCT): Englar et al (1994, 1999)

• Circulation Control Rotor (CCR): Wilkerson et al (1973, 1979)

• X-wing stopped rotor aircraft: Williams et al (1976)

• Ground heavy vehicles, such as large tractors and trailing trucks: Englar (2000)

• There are many other potential applications for Circulation Control or Pneumatic Aerodynamic technology, which are summarized in the paper by Englar (2000).

Previous Research Work• The early research work about Circulation Control was done in England by Cheeseman (1966) and Kind (1967) et al.

• This concept was introduced into United States by Navy researchers in the 1970s. The David Taylor Naval Ship Research and Development Center (DTNSRDC) became a major center for the CC study.

• Experiments by Williams and Howe (1970), Englar (1970, 1975), Abramson (1975), Abramson and Rogers (1983) and others in DTNSRDC examined the effect of a wide range of parameters on CC airfoils performance, including the geometric factors such as the thickness, camber, angle of attack, and free-stream conditions such as Mach number.

• Englar and Applegate (1984) gave a very good summary of this research work for the years 1969 through 1983.

• Recently, many experimental studies have been focused on the CCW applications for the rotary and fixed wing aircraft.

Previous Research Work

• Acoustic studies for CC wings are very limited. Salikuddin, Brown and Ahuja (1987), Howe (2002) and Munro(2002) are the only known work on CCW.

• Early numerical research by Davork et al (1979, 1983), based on potential methods did not achieve enough accuracy for CC airfoil design purpose.

• Recently numerical studies based on the Navier-Stokes equations, such as Berman (1985), Pulliam (1985), Viegas et al (1986), and Shrewbury (1985, 1986, 1989) etc, have demonstrated that Navier-stokes simulations can provide good estimates of the lift, pressure distribution, and pitch moments of CC airfoils provided the turbulence model is accurate enough to give a reasonable good estimate of the jet separation point from the Coanda surface.

Previous Research Work• Other characteristics of CC airfoils, such as dynamic stall (Shrewbury 1990), jet stall (Linton 1994), and unsteady effects (Liu and Sun 1996) etc, have also been studied by Navier-Stokes methods.

• A limited number of numerical studies have also been done for the advanced hinged flap CC airfoil by Englar and Smith et al (1993).

• Studies by Wygnansky et al (1996,2000), Lorber et al (2000), Wake et al (2001), and Hassan (1998) etc, have been done on the use of synthetic jets (massless jets) to control the boundary layer and eliminate flow separation. However, studies on using pulsed jets to achieve high lift with relative less mass flow rate compared to a steady jet are very limited (Olyer 1972).

• The use of advanced turbulence models (Slomski et al 2002), Large-eddy Simulation (Yang and Voke 2001) and Direct Numerical Simulation (Li and Liu 2003) to model the CC airfoil numerically have also been reported in last two years.

Research Objectives

• Computational modeling of advanced dual radius CCW configuration

• Assessment of the use of pulse jets to achieve desired high lift values, at lower mass flow rates

• Evaluation of Circulation Control for the elimination or modification of flap edge vortices and tip vortices

• Three-dimensional compressible unsteady Reynolds Averaged Navier-Stokes equations are solved in a strong conservation form on curvilinear coordinates.

• This solver can be used in both a 2D mode and a 3D mode in this study for different applications.

• The scheme is second or fourth order accurate in space and first order accurate in time.

• Baldwin-Lomax and Spalart-Allmaras one-equation turbulence models have been used.

• The jet slot location, slot size, blowing velocity and direction of blowing can easily be varied in the analysis.

Mathematical and Numerical Formulation

Initial and Boundary Conditions

• Initial flow conditions are set to free stream values inside the flow field.

• Boundary Conditions

• Outer Boundary

• Solid Surface Boundary

• Wake Cut Boundary

• Jet Slot Exit Boundary

• The driving parameter for jet blowing is the momentum coefficient, Cdefined as follows:

Jet Slot Boundary Conditions

SV21

VmC

2

jet

Where jetjetjet AVm is the mass flow rate of jet flow

• The C orientation of the jet and the total temperature of jet are specified in the analysis.

• Other quantities such as pressure and density are found by extrapolation and /or Ideal Gas Law.

• The total jet pressure can also be specified as the boundary condition instead of the momentum coefficient.

Code Validation

• The figures are the Cp distribution at two span locations of a small aspect-ratio wing made of NACA 0012 airfoil sections.• The results are in good agreement with the measured data (from Bragg and Spring 1987) except near the tip region where increased grid resolution is needed.

50% SPAN

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Chord

Cp

Exp CFD

85% SPAN

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CHORD

Cp

Exp CFD

• Lift distribution along span for NACA 0012 wing.

• Coarse Grid (121*21*41); Fine Grid (151*51*51)

0

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1

0 0.2 0.4 0.6 0.8 1Span, Y/C

Cl

Exp 8 DEG

CFD, BL Model, Coarse Grid

CFD, SA Model, Coarse Grid

CFD, BL Model, Fine Grid

2D Steady Blowing Results• Steady blowing performance at different C values, and at different angles of attack

• Effects of parameters that influence the momentum coefficient:

• Free-steam velocity effects with fixed C

• Jet slot height effects with fixed C

• Other considerations for the CC airfoil:

• Comparison with the unblown baseline case

• Steady blowing at a given total jet pressure

• Comparison with conventional high-lift systems

• Leading edge blowing

The CCW Airfoil

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Jet Slot Location

30 degree integral flap

The Computational Grid

Flow Conditions

• P = 14.2 psia = 0.9324 atm

• = 0.00225 slugs/ft3 = 1.1596 kg/m3

• V = 94.3 ft/sec = 28.743 m/s

• M = 0.0836, Re = 0.395 * 106

• Chord of the Airfoil : C = 8” = 0.20 m

• Jet Slot Height : h = 0.015” = 0.0004 m 0.2 % Chord

• Jet slot is located at x/c = 88.75% on the upper side of the airfoil.

• These values closely match the test conditions.

Lift Coefficient vs. C

Angle of Attack 0 degrees, Integral Flap at 30 degrees

0

1

2

3

4

5

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

C

Cl

Cl, Measured

Cl, Computed

Lift Coefficient vs. Angle of Attack

0

1

2

3

4

-4 -2 0 2 4 6 8 10 12 14 16Angle of Attack

Lif

t C

oe

ffic

ien

t, C

l

EXP, Cmu = 0.0

EXP, Cmu = 0.074

EXP, Cmu = 0.15

CFD

C=0.1657

C=0.074

C=0.0

The Stream lines over CC airfoil, C = 0.1657, = 60

Free-stream Velocity Effects with Fixed C

2

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

(Vinf in CFD) / (Vinf in Exp.)

Cl

C = 0.1657, h = 0.015 in. and V, exp = 94.3 ft/sec

Jet Slot Height Effects with Fixed C

The Efficiency vs. Jet Slot Height, V = 94.3 ft/sec

0

5

10

15

20

0.006 0.009 0.012 0.015 0.018

Jet Slot Height (inch)

Eff

icie

nc

y C

l/(C

d+

Cm

u)

Cmu = 0.04

Cmu = 0.1657

The Mass Flow Rate vs. Jet Slot Height, V = 94.3 ft/sec

0

0.0005

0.001

0.0015

0.002

0.0025

0.006 0.009 0.012 0.015 0.018

Jet Slot Height (inch)

Ma

ss

Flo

w R

ate

(s

lug

s/s

ec

)

Cmu = 0.04

Cmu = 0.1657

Jet Slot Height Effects with Fixed C

Comparison with the Unblown Case

The Stream Lines for the Blowing Case

Dominant Vortex Shedding Frequency 

Scott’s measurement =1600 Hz

Frequency (Hz)

0 500 1000 1500 2000 2500 30000

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25

30

Frequency (Hz)

Dominate Vortex at 1080 Hz

Acoustic Measurement at 1600 Hz

The FFT of the Lift Coefficient Variation with Time

Steady Blowing at Given Total Jet Pressure

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1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Pjet-total / Pinf

C

Lift Coefficient vs. Jet Momentum Coefficient

0

1

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3

4

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

C

Cl

Cl, Measured

Cl, Computed by Specified Cmu

Cl, Computed by Specified Jet TotalPressure

Comparison with a Conventional High-lift System Airfoil

• The figures show the high-lift systems configuration with a 300 fowler flap and the body-fitted grid.• The results are obtained with a 2-D multi-block version of the present method.

• For the multi-element airfoil, high lift is achieved by changing the angle of attack; For the CCW airfoil, high lift is achieved by changing the blowing coefficient while the angle of attack is fixed at 0 degrees.

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Drag Coefficient, Cd

Lif

t C

oe

ffic

ien

t, C

l

Multi-element Airfoil with 30 degrees fowler flap

CCW Airfoil with 30 degrees flap, Cd not corrected

CCW Airfoil with 30 degrees flap, Cd corrected withCd + Cmu

Leading Edge Blowing

• At high angles of attack, the leading edge separation and stall can occur for the CC airfoil, due to the large pressure gradients.

• The stall angle is decreased quickly with the increase of the jet momentum coefficient of the trailing edge blowing.

• Leading edge Coanda blowing can eliminate this and increase the stall angle.

• In reality, because CCW airfoils can achieve very high lift even at zero angle of attack with a small amount of blowing, there is no real need for operation at high angles of attack unless maneuver requires it.

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Angle of Attack (degrees)

Lif

t C

oe

ffic

ien

t, C

l LE Blowing, C = 0.08TE Blowing, C = 0.04

LE Blowing, C = 0.04TE Blowing, C = 0.08

LE Blowing, C = 0.00TE Blowing, C = 0.08

2D Pulsed Jet Results• Pulsed jet studies were done to answer:

---- Can pulsed jets be used to achieve desired increases in the lift coefficient at lower mass flow rates relative to a steady jet? ----What is the optimum wave shape for the pulsed jet, ie, how should it vary with time? ---- What are the effects of the pulsed jet frequency on the lift coefficient?

• Sinusoidal and Square wave form variations were considered. Sinusoidal forms were found ineffective.

• tfFCCtC ,0,0,

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Real Time

Mo

me

ntu

m C

oe

ffic

ien

t,C

Square Wave Pulsed Jet

Steady JetDT = 0.025 sec

Square Wave Pulsed Jet, Frequency = 40 Hz

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Time-Averaged Momentum Coefficient, C0

Cl

Steady Jet

Pulsed Jet , f = 40Hz

Pulsed Jet, f = 120 Hz

Pulsed Jet, f = 400 Hz

A

B

C

D

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Time Averaged Mass Flow Rate (slug/sec)

Cl

Steady Jet

Pulsed Jet , f = 40Hz

Pulsed Jet, f = 120 Hz

Pulsed Jet, f = 400 Hz

A

B

C

D

Average Lift Coefficient Vs. Frequency For Pulsed Jet

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0 40 80 120 160 200 240 280 320 360 400

Frequency (Hz)

Cl

Pulsed Jet, Ave. Cmu=0.04

Steady Jet, Cmu=0.04

0

Strouhal Number ( f * Chord / Vinf)

2.8281.414

Pulsed Jet at 400 Hz requires only 73% of the steady jet mass flow rate while achieves 95% of the steady jet lift.

Effect of Frequency at Fixed C

• High Frequencies were more effective.

• This is explained as follows:

• When the jet is turned off, the beneficial Coanda effect persists for several chord lengths of travel.

• If a new cycle starts soon, the Coanda effect quickly reestablishes itself.

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0.455 0.46 0.465 0.47 0.475 0.48 0.485 0.49 0.495

Real Time (sec)

Lif

t C

oe

ffic

ien

t, C

l

DT-cycle = 0.02501 sec

DT-down = 0.00335 secDT-up = 0.00137 sec

Time History of the Lift CoefficientFrequency = 40 Hz

Time History of the Lift CoefficientFrequency = 200 Hz

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Real Time (sec)

Lif

t C

oe

ffic

ien

t, C

l

DT-cycle = 0.00501 sec

DT-down = 0.00248 sec DT-up = 0.00113

Pulsed Jet Frequency = 120 Hz

Pulsed Jet Frequency = 400 Hz

Strouhal Number Effects

• The non-dimensional frequency, Strouhal number is defined as :

U

L*fStr ref

Where, Lref is the chord of airfoil, and U is the free-stream velocity.

• Three Cases have been studied:

• Case 1: Strouhal number was not fixed; U and Lref were fixed

• Case 2: Strouhal number and Lref were fixed; U was not fixed

• Case 3: Strouhal number and U were fixed; Lref was not fixed

• Strouhal number = 1.41 for Case 2 and 3

Lift Coefficient vs. Frequency

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Frequency

Lif

t C

oe

ffic

ien

t, C

l

Case 1

Case 2

Case 3

U = 47.15 ft/sec

U = 118.6 ft/secU = 94.3 ft/secLref = 16 in.

Lref = 4 in.Lref = 8 in.

3D Streamwise Tangential Blowing

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

The Wing-Flap Configuration with Tangential Blowing

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Lift

Co

effic

ien

t, C

l

Noblowing on Main Wing

Constant Blowing on Main Wing

Gradual Blowing on Main Wing

Lift Coefficient Distribution along Span

3D Spanwise Tangential Blowing over a Rounded Wing-tip

• A Rectangular Wing with NACA0012 Section

• Aspect Ratio = 2.0

• Jet slot is located above the rounded wing tip edge.

X

Y

The Surface Grid for Rounded Wing-tip

Vorticity Contours in the Wingtip

Region (X/C = 0.81)

No-Blowing Case

Less Blowing Case(C= 0.04)

More Blowing Case (C= 0.18)

Vorticity Contours at the trailing edge

(x/c = 1.0)

No-Blowing Case

Less Blowing Case (C= 0.04)

More Blowing Case (C= 0.18)

Velocity Vectors in the Wing Tip

Region (x/c = 0.81)

No-Blowing Case

Less Blowing Case (C= 0.04)

More Blowing Case (C= 0.18)

• CCW concept is an extremely effective way of achieving high CLmax, without the drawbacks of conventional high-lift systems.

• The steady jet calculations are in good agreement with the measurements. It is seen that blowing can successfully eliminate the vortex shedding, a potential noise source.

• The stall angle of the CC airfoil is decreased quickly with the increase of the momentum coefficient. It is a leading edge stall, and can be significantly delayed by leading edge blowing

Conclusions from the 2D Simulations - I

• The momentum coefficient is increased uniquely with the jet total pressure, and the predicted lift coefficient is almost the same for both cases.

• At fixed momentum coefficient, a thin jet cost much less mass flow rate than a thick jet to get almost the same efficiency. Thus, a thin jet is more aerodynamically beneficial, although the power requirement for a thin jet is high.

• Compared to the conventional high-lift system, the CC airfoil can achieve a higher efficiency at the same lift coefficient, and it also could generate very high lift without stall.

Conclusions from the 2D Simulations - II

• The pulsed jet configuration can give larger increments in lift coefficient compared to the steady jet at the same mass flow rate.

• The sinusoidal pulsed jet is not very effective compared to the square wave pulsed jet due to the higher mass flow rate required.

• The pulsed jet performance improved at higher pulse frequencies.

• The Strouhal number has a more dominant effect on the performance of the pulsed jet than just the frequency. Thus, for a larger configuration or at a smaller free-stream velocity, the same lift can be obtained with a lower frequency pulsed jet.

Conclusions from the 2D Simulations - III

• The flap-edge vortex is generated by the suddenly increase of the bound circulation and lift along the flap-edge interface.

• Constant streamwise tangential blowing can modify the lift distribution along the span, so move the flap edge vortex toward the main wing.

• Gradual streamwise tangential blowing on the main wing can efficiently reduce the lift discontinuity on the flap edge, thereby eliminating the flap edge vortex.

Conclusions from the 3D Simulations - I

• Spanwise tangential blowing on rounded wing-tip can not cancel or eliminate the tip vortex.

• It can push down the tip vortex, and make is move away from the wingtip, thus increase the vertical clearance between the wing and the tip vortex.

• The approach has the potential of reducing the blade vortex interaction, and the BVI noise.

Conclusions from the 3D Simulations - II

• Turbulence models can play a very important role in the CC study. A systematic study of improved turbulence model is recommended for the future research work.

• Methods of improving the pulsed jet performance at low frequencies will be very useful. The method of changing the slot height dynamically while keeping a constant jet total pressure to generate a low frequency pulsed jet is recommended.

• There are other potential applications for the Circulation Control technology for practical three-dimensional configurations beyond what has been studied in this work.

Recommendations

Q & A