Transcript From Biplanes to Reusable Launch Vehicles: 75 Years of

Comparison of Numerical
Predictions and Wind Tunnel
Results for a Pitching Uninhabited
Combat Air Vehicle
Russell M. Cummings, Scott A. Morton,
and Stefan G. Siegel
Department of Aeronautics
United States Air Force Academy
USAF Academy, CO
Outline
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Introduction
Dynamic Stall/Lift
UCAV Configuration
Experimental Results
Numerical Method
Static Results
Pitching Results
Conclusions
Introduction
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UCAV’s are playing an
important role in current
military tactics
Predator and Global
Hawk are becoming
essential elements of
current operations
X-45A represents future
configurations
Introduction
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Some issues to explore in order to take advantage of a
UCAV’s uninhabited state:
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High g maneuvering
Compact configurations
Novel control actuation
Morphing wings
MEMS-based control systems
Semi-autonomous flight
Increased use of composites
Novel propulsion systems
Dynamic stall/lift
Dynamic Stall/Lift
a—separation begins
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b—leading-edge vortex forms
c—full stall
d—reattachment and return to static state
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From http://hodgson.pi.tu-berlin.de/~schatz/PIZIALI/osc.html
Utilizes rapid pitch-up
and hysteresis to
produce increased lift
A great deal of work has
been done on airfoils
and simple wings
Very little work has been
done on UCAVs
UCAV Configuration
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30°
27.38 sq ft
30°
30°
21.02 sq ft
62.43 276.15 46.19 ft
50°
24.19
MAC 241.93
BL 92.4
78.78
33.27
MAC/4
FS 170.63
33.27
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32.58 ft
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Boeing 1301 UCAV
Straight, swept leading
edge with 50o sweep
Aspect ratio of 3.1
Round leading edges
Blended wing & body
Top/front engine inlet
B-2-like wing planform
Low observable shaping
Experimental Results
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1:46.2 scale model
Academy 3 ft × 3 ft open
return low-speed wind
tunnel
Less than 0.05%
freestream turbulence
levels at all speeds
Freestream velocity of
20 m/s (65.4 ft/s)
Chord-based Reynolds
number of 1.42 × 105
Static Testing
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1.4
Lift Coefficient
Drag Coefficient
1.2
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Force Coefficient
1
0.8
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0.6
0.4
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0.2
0
0
10
20
30
40
Angle of Attack, 
50
60
70
Linear lift characteristics
up to 10o to 12o
Stall occurring at about
20o
Lift re-established up to
32o, where an abrupt
loss of lift takes place
Effect of leading-edge
vortices and vortex
breakdown?
Dynamic Testing
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The configuration was pitched at 0.5, 1.0, and
2.0 Hz (k = 0.01, 0.02, and 0.04)
Center of rotation at the nose, 35% MAC, and
the tail
The pitch cycles were completed for three
ranges of angle of attack:
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0o < α < 20o
16o < α < 35o
25o < α < 45o
 (t)   m(1cos(t))
Pitching About 35% MAC @ 2 Hz
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1.5
1.25
Force Coefficient
(t) = 25 o to 45 o
(t) = 16 o to 35 o
1
(t) = 0 to 20
o
0.75
o
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0.5
0.25
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0
Static Lift
Static Drag
-0.25
-0.5
0
5
10
15
20
25
30
Angle of Attack,  (deg)
35
40
45
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Dynamic lift is greater
than static lift during
pitchup
Pitchup lift is also
greater post stall
Dynamic lift is less than
static lift during
pitchdown
Little impact on drag
Pitching About Nose @ 2 Hz
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1.5
1.25
Force Coefficient
(t) = 25 o to 45 o
(t) = 16 o to 35 o
1
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(t) = 0 o to 20 o
0.75
0.5
0.25
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0
Static Lift
Static Drag
-0.25
-0.5
0
5
10
15
20
25
30
Angle of Attack,  (deg)
35
40
45
Similar results to pitching
about 35% MAC
Slightly less effective at
producing lift during
pitchup
Essentially identical
results in post-stall
region
Pitching About Tail @ 2 Hz
1.5
(t) = 16 o to 35 o
1.25
(t) = 25 o to 40 o
(t) = 0 o to 20 o
1
Force Coefficient
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0.75
0.5
0.25
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0
Static Lift
Static Drag
-0.25
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-0.5
0
5
10
15
20
25
30
Angle of Attack,  (deg)
35
40
45
Drastically different than
previous results
Much more lift at higher
angles of attack in
pitching cycle
Reduced lift at lower
angles
Significant impact on
drag
Numerical Method
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Cobalt Navier-Stokes solver
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Unstructured mesh
Finite volume formulation
Implicit
Parallelized
Second-order spatial accuracy
Second-order time accuracy with Newton sub-iterations
Run on Academy 64 processor Beowulf cluster, Origin
2000, and USAF HPC computers
Laminar flow with freestream conditions set to match
Reynolds number of wind tunnel experiment
Mesh and Boundary Conditions
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Three unstructured meshes:
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Coarse (1.3 million cells)
Medium (2 million cells)
Fine (4 million cells)
Half-plane model
No-slip on surface
Symmetry plane
Freestream inflow
Inlet covered to match model
No sting modeled
Mesh Convergence
4
  20
Normal Force
3
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
2
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1
0
-2
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Fine Mesh (4 million cells)
Medium Mesh (2 million cells)
Coarse Mesh (1.3 million cells)
-1
250
500
750
Number of Iterations
1000
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Steady results from all
three meshes yield
identical forces
High angle of attack
flowfields will be
unsteady
Use 2 million cell mesh
for following calculations
Detailed time step study
to follow for unsteady
flow
Steady-State Static Results
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1.4
1.2
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Force Coefficient
1
0.8
0.6
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0.4
Exp. Lift Coefficient
Exp. Drag Coefficient
CFD Lift Coefficient
CFD Drag Coefficient
0.2
0
0
10
20
30
40
Angle of Attack, 
50
60
70
Good results in linear
angle of attack range
Qualitatively similar
results in post-stall
region
Lift and drag are
significantly overpredicted in post-stall
region
Steady-State Static Results
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  20
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Wide shallow vortices
Vortex breakdown fairly
far back on configuration
Vortical structures
behind breakdown
maintain lift on aft of
vehicle
Rounded leading edge
creates weaker vortices
that breakdown sooner
Time-Accurate Static Results
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1.4
Exp. Lift Coefficient
Exp. Drag Coefficient
CFD Lift Coefficient
CFD Drag Coefficient
1.2
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Force Coefficient
1
0.8
0.6
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0.4
0.2
0
0
10
20
30
40
50
Angle of Attack,  (deg)
60
70
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Flowfields in post-stall
region are unsteady
Time-accurate results
match experiment much
more closely
Fairly good modeling of
flowfield, including drag,
up to α=45o
Differences in lift from
α=20o to α=30o (sting,
surface roughness,
transition ?)
Time-Accurate Static Results
 5
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 15
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 10
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  20
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Time-Accurate Static Results
  25
  35
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  30
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  40
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Dynamic Pitching Results
1.25
Exp. Lift Coefficient Up
Exp. Drag Coefficient Up
Exp. Lift Coefficient Down
Exp. Drag Coefficient Down
Exp. Static Lift Coefficient
Exp. Static Drag Coefficient
CFD Lift Coefficient
CFD Drag Coefficient
Force Coefficient
1
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Δt*=0.0075, nsub=5
0.75
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0.5
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0.25
0
-0.25
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0
5
10
15
Angle of Attack,  (deg)
20
25
Pitchup doesn’t
capture full lift
increase at low α
Overprediction of lift
also seen in pitchup
case
Drag is fairly well
modeled during
pitchup
More cycles
required
Dynamic Pitching Results
 15
Static Pressure
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Pitching Pressure
Conclusions
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A generic UCAV configuration has been wind tunnel tested
both statically and pitching
The configuration generates increased dynamic lift during
pitchup maneuver
Numerical simulation helps to understand causes of wind
tunnel results
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Stronger leading-edge vortex during pitchup
Leading-edge vortex persists to very high angles of attack
Vortex breakdown causes the non-linearities in lift
Collaboration between experimentalists and
computationalists leads to greater understanding of
aerodynamics
Questions?