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
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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.
Circulation Control Wing Concept
• In general, the driving parameter of Circulation Control is
the jet momentum coefficient, Cm, which is defined as:
Cm 
 jet Vjet
m
qS
• 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 / Cm, can
exceed 80 as shown in previous figure.
Advanced CC Airfoil
• 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.
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 Navierstokes 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), Largeeddy 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
Mathematical and Numerical
Formulation
• 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.
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
Jet Slot Boundary Conditions
• The driving parameter for jet blowing is the momentum
coefficient, Cm, defined as follows:
 Vjet
m
Cm 
1
 V2S
2
   jet Vjet A jet is the mass flow rate of jet flow
Where m
• The Cm, 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
-3
-2.5
-2.5
50% SPAN
-2
85% SPAN
-2
Exp
-1.5
CFD
-1.5
Exp
CFD
-1
Cp
Cp
-1
-0.5
-0.5
0
0
0.5
0.5
1
1
1.5
0
0.1
0.2
0.3
0.4
0.5
Chord
0.6
0.7
0.8
0.9
1
1.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
CHORD
• 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.
1
1
0.8
Cl
0.6
0.4
Exp 8 DEG
CFD, BL Model, Coarse Grid
0.2
CFD, SA Model, Coarse Grid
CFD, BL Model, Fine Grid
0
0
0.2
0.4
0.6
Span, Y/C
0.8
• Lift distribution along span for NACA 0012 wing.
• Coarse Grid (121*21*41); Fine Grid (151*51*51)
1
2D Steady Blowing Results
• Steady blowing performance at different Cm values, and at
different angles of attack
• Effects of parameters that influence the momentum
coefficient:
• Free-steam velocity effects with fixed Cm
• Jet slot height effects with fixed Cm
• 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
0.5
0.4
0.3
Jet Slot Location
0.2
0.1
0
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
-0.2
30 degree integral flap
-0.3
-0.4
-0.5
1
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. Cm
5
4
Cl
3
2
Cl, Measured
1
Cl, Computed
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Cm
Angle of Attack 0 degrees, Integral Flap at 30 degrees
Lift Coefficient vs. Angle of Attack
4
EXP, Cmu = 0.0
EXP, Cmu = 0.074
Cm=0.1657
EXP, Cmu = 0.15
3
CFD
Lift Coefficient, Cl
Cm=0.074
2
Cm=0.0
1
0
-4
-2
0
2
4
6
Angle of Attack
8
10
12
14
16
The Stream lines over CC airfoil, Cm = 0.1657,  = 60
Free-stream Velocity Effects with Fixed Cm
4
Cl
3.5
3
2.5
2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
(Vinf in CFD) / (Vinf in Exp.)
Cm = 0.1657, h = 0.015 in. and V, exp = 94.3 ft/sec
2
Jet Slot Height Effects with Fixed Cm
20
Efficiency Cl/(Cd+Cmu)
15
10
Cmu = 0.04
Cmu = 0.1657
5
0
0.006
0.009
0.012
0.015
0.018
Jet Slot Height (inch)
The Efficiency vs. Jet Slot Height, V = 94.3 ft/sec
Jet Slot Height Effects with Fixed Cm
0.0025
Cmu = 0.04
Cmu = 0.1657
Mass Flow Rate (slugs/sec)
0.002
0.0015
0.001
0.0005
0
0.006
0.009
0.012
0.015
0.018
Jet Slot Height (inch)
The Mass Flow Rate vs. Jet Slot Height, V = 94.3 ft/sec
Comparison with the Unblown Case
The Stream Lines for the Blowing Case
The FFT of the Lift Coefficient Variation with Time
30
Dominate Vortex at 1080 Hz
25
Dominant
Vortex
Shedding Frequency
20
15
Scott’s measurement =1600 Hz
Acoustic
Measurement at
1600 Hz
10
5
0
0
Frequency (Hz)
500
1000
1500
2000
Frequency (Hz)
2500
3000
Steady Blowing at Given Total Jet Pressure
0.6
Cm
0.4
0.2
0
1
1.1
1.2
1.3
1.4
Pjet-total / Pinf
1.5
1.6
1.7
1.8
Lift Coefficient vs. Jet Momentum Coefficient
5
4
Cl
3
2
Cl, Measured
Cl, Computed by Specified Cmu
1
Cl, Computed by Specified Jet Total
Pressure
0
0
0.05
0.1
0.15
0.2
0.25
Cm
0.3
0.35
0.4
0.45
0.5
Comparison with a Conventional Highlift 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.
3.5
3
Lift Coefficient, Cl
2.5
2
1.5
Multi-element Airfoil with 30 degrees fowler flap
1
CCW Airfoil with 30 degrees flap, Cd not corrected
CCW Airfoil with 30 degrees flap, Cd corrected with
Cd + Cmu
0.5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Drag Coefficient, Cd
• 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.
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.
4
3.5
LE Blowing, Cm = 0.04
TE Blowing, Cm = 0.08
Lift Coefficient, Cl
3
LE Blowing, Cm = 0.08
TE Blowing, Cm = 0.04
2.5
LE Blowing, Cm = 0.00
TE Blowing, Cm = 0.08
2
1.5
1
0.5
0
0
2
4
6
8
10
12
14
Angle of Attack (degrees)
16
18
20
22
24
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?
•
C m (t )  C m ,0 + C m ,0 F ( f , t )
• Sinusoidal and Square wave form variations were
considered. Sinusoidal forms were found ineffective.
Square Wave Pulsed Jet, Frequency = 40 Hz
Square Wave Pulsed Jet
0.09
Steady Jet
DT = 0.025 sec
0.08
Momentum Coefficient,C m
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0.44
0.45
0.46
0.47
Real Time
0.48
0.49
3
Steady Jet
2.5
Pulsed Jet , f = 40Hz
Pulsed Jet, f = 120 Hz
Cl
2
Pulsed Jet, f = 400 Hz
1.5
D
C
A
1
B
0.5
0
0
0.02
0.04
0.06
0.08
0.1
Time-Averaged Momentum Coefficient, Cm0
0.12
0.14
3
Steady Jet
2.5
Pulsed Jet , f = 40Hz
Pulsed Jet, f = 120 Hz
Cl
2
Pulsed Jet, f = 400 Hz
1.5
D
C
A
1
B
0.5
0
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
Time Averaged Mass Flow Rate (slug/sec)
0.0014
0.0016
Average Lift Coefficient Vs. Frequency For Pulsed Jet
2
1.6
Pulsed Jet at 400 Hz requires only
73% of the steady jet mass flow
rate while achieves 95% of the
steady jet lift.
Cl
1.2
0.8
0.4
Pulsed Jet, Ave. Cmu=0.04
Steady Jet, Cmu=0.04
0
0
40
80
120
160
200
240
280
320
360
400
Frequency (Hz)
0
1.414
Strouhal Number ( f * Chord / Vinf)
2.828
Effect of Frequency at Fixed Cm
• 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.
2.5
Time History of the Lift Coefficient
Frequency = 40 Hz
DT-cycle = 0.02501 sec
Lift Coefficient, Cl
2
1.5
1
0.5
DT-up = 0.00137 sec
DT-down = 0.00335 sec
0
0.455
0.46
0.465
0.47
0.475
Real Time (sec)
0.48
0.485
0.49
0.495
2.5
Time History of the Lift Coefficient
Frequency = 200 Hz
DT-cycle = 0.00501 sec
Lift Coefficient, Cl
2
1.5
1
DT-down = 0.00248 sec
DT-up
= 0.00113
0.5
0
0.21
0.212
0.214
0.216
Real Time (sec)
0.218
0.22
Pulsed Jet Frequency = 120 Hz
Pulsed Jet Frequency = 400 Hz
Strouhal Number Effects
• The non-dimensional frequency, Strouhal number is
defined as :
f * L ref
Str 
U
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
2
Lref = 16 in.
Lift Coefficient, Cl
1.8
U  = 118.6 ft/sec
U = 94.3 ft/sec
1.6
Lref = 8 in.
Lref = 4 in.
U  = 47.15 ft/sec
Case 1
Case 2
Case 3
1.4
1.2
50
100
150
200
250
Frequency
300
350
400
450
3D Streamwise Tangential Blowing
Symmetry BC
15 C
C
5C

5C
Small blowing to suppress vortex
shedding
This region is modeled as
shown in next figure
2-D BC
The Wing-Flap Configuration with Tangential Blowing
Lift Coefficient Distribution along Span
1.6
Noblowing on Main Wing
1.4
Constant Blowing on Main Wing
Lift Coefficient, Cl
1.2
Gradual Blowing on Main Wing
1
0.8
0.6
0.4
0.2
0
0
5
10
15
Y
20
25
3D Spanwise Tangential Blowing over
a Rounded Wing-tip
Y
X
• A Rectangular Wing with NACA0012 Section
• Aspect Ratio = 2.0
• Jet slot is located above the rounded wing tip edge.
The Surface Grid for Rounded Wing-tip
No-Blowing
Case
Less Blowing
Case(Cm= 0.04)
Vorticity Contours
in the Wingtip
Region (X/C = 0.81)
More Blowing
Case (Cm= 0.18)
No-Blowing
Case
Less Blowing
Case (Cm= 0.04)
Vorticity Contours
at the trailing edge
(x/c = 1.0)
More Blowing Case
(Cm= 0.18)
No-Blowing
Case
Less Blowing
Case (Cm= 0.04)
Velocity Vectors in
the Wing Tip
Region (x/c = 0.81)
More Blowing Case
(Cm= 0.18)
Conclusions from the 2D Simulations - I
• 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 - II
• 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 - III
• 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 3D Simulations - I
• 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 - II
• Spanwise tangential blowing on rounded wingtip 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.
Recommendations
• 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 threedimensional configurations beyond what has been
studied in this work.
Q&A