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
Introduction Dynamic Stall/Lift UCAV Configuration Experimental Results Numerical Method Static Results Pitching Results Conclusions
Introduction
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
Some issues to explore in order to take advantage of a UCAV’s uninhabited state:
– 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
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
From http://hodgson.pi.tu-berlin.de/~schatz/PIZIALI/osc.html
a—separation beginsb—leading-edge vortex formsc—full stalld—reattachment and return to static state
UCAV Configuration
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
Boeing 1301 UCAV
33.27
62.43 276.15 46.19 ft
78.78
33.27
32.58 ft
MAC 241.93BL 92.4
MAC/4FS 170.63
24.1950°
30°
30°
30°
27.38 sq ft
21.02 sq ft
Experimental Results
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
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? Angle of Attack,
Fo
rce
Co
eff
icie
nt
0 10 20 30 40 50 60 700
0.2
0.4
0.6
0.8
1
1.2
1.4
Lift Coefficient
Drag Coefficient
Dynamic Testing
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:– 0o < α < 20o
– 16o < α < 35o
– 25o < α < 45o
))cos(1()( tmt
Pitching About 35% MAC @ 2 Hz
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 dragAngle of Attack, (deg)
Fo
rce
Co
eff
icie
nt
0 5 10 15 20 25 30 35 40 45-0.5
-0.25
0
0.25
0.5
0.75
1
1.25
1.5
(t) = 16o to 35o (t) = 25o to 45o
(t) = 0o to 20o
Static Drag
Static Lift
Pitching About Nose @ 2 Hz
Similar results to pitching about 35% MAC
Slightly less effective at producing lift during pitchup
Essentially identical results in post-stall region
Angle of Attack, (deg)
Fo
rce
Co
eff
icie
nt
0 5 10 15 20 25 30 35 40 45-0.5
-0.25
0
0.25
0.5
0.75
1
1.25
1.5
(t) = 25o to 45o
(t) = 0o to 20o
(t) = 16o to 35o
Static Drag
Static Lift
Pitching About Tail @ 2 Hz
Drastically different than previous results
Much more lift at higher angles of attack in pitching cycle
Reduced lift at lower angles
Significant impact on dragAngle of Attack, (deg)
Fo
rce
Co
eff
icie
nt
0 5 10 15 20 25 30 35 40 45-0.5
-0.25
0
0.25
0.5
0.75
1
1.25
1.5
(t) = 25o to 40o
(t) = 0o to 20o
(t) = 16o to 35o
Static Drag
Static Lift
Numerical Method
Cobalt Navier-Stokes solver– 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
Three unstructured meshes:– 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
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
Number of Iterations
No
rma
lFo
rce
250 500 750 1000-2
-1
0
1
2
3
4
Fine Mesh (4 million cells)Medium Mesh (2 million cells)Coarse Mesh (1.3 million cells)
20
Steady-State Static Results
Good results in linear angle of attack range
Qualitatively similar results in post-stall region
Lift and drag are significantly over-predicted in post-stall regionAngle of Attack,
Fo
rce
Co
eff
icie
nt
0 10 20 30 40 50 60 700
0.2
0.4
0.6
0.8
1
1.2
1.4
Exp. Lift Coefficient
Exp. Drag Coefficient
CFD Lift Coefficient
CFD Drag Coefficient
Steady-State Static Results
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
20
Time-Accurate Static Results
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 ?)
Angle of Attack, (deg)
Fo
rce
Co
eff
icie
nt
0 10 20 30 40 50 60 700
0.2
0.4
0.6
0.8
1
1.2
1.4
Exp. Lift Coefficient
Exp. Drag Coefficient
CFD Lift Coefficient
CFD Drag Coefficient
Time-Accurate Static Results
5
15
10
20
Time-Accurate Static Results
25
35
30
40
Dynamic Pitching Results
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 requiredAngle of Attack, (deg)
Fo
rce
Co
eff
icie
nt
0 5 10 15 20 25-0.25
0
0.25
0.5
0.75
1
1.25
Exp. Lift Coefficient UpExp. Drag Coefficient UpExp. Lift Coefficient DownExp. Drag Coefficient DownExp. Static Lift CoefficientExp. Static Drag CoefficientCFD Lift CoefficientCFD Drag Coefficient
Δt*=0.0075, nsub=5
Dynamic Pitching Results
Static Pressure Pitching Pressure
15
Conclusions
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
– 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?
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