Use of HPC in Advanced Rotorcraft Systems

1. Use of HPC in Advanced Rotorcraft Systems Matt Floros US Army Research Laboratory April 15, 2008

2. Rotorcraft Aeromechanics—Never a Dull Moment Mach number range 0—1 Steady state is unsteady Large induced inflow Flexible blades Intermeshing rotors 3-D acoustic field 360-degree angle of attack range Flaps & multi-element airfoils Rotor operates in its own wake Fuselage and tail are in rotor wake Image from Bhagwat, Dimanlig, et al, CFD/CSD Coupled Trim Solution for the Dual-Rotor CH-47 Helicopter Including Fuselage Modeling, AHS Specialists’ Conference on Aeromechanics, Jan, 2008

3. All Lift, Propulsion, and Control From Main Rotor Yaw control on multi-rotor aircraft? - Main rotors

4. Local Velocities from Static to Transonic Speeds Advancing Tip: a≈ 0, M~0.85-1 Retreating Tip: anear stall, Low speed subsonic, dynamic stall Retreating Root: a ≈ 180, Low speed subsonic

5. Local Velocities from Static to Transonic Speeds • Tangential velocity V = Wr + m sin(y), for traditional helicopter 0 < m < 0.4 (0 < m < 0.2 for tilt rotor), some new concepts much higher • Flow approaching airfoil from trailing edge for 0 < r < m on retreating side • Near-body grid for blade has to encompass • Transonic flow/shocks • Separated flow • Dynamic stall • Reverse flow, radial flow • All in one revolution!

6. Near-Body Grids for Blades Deform at Every Time Step • Moment balance, propulsion, and control come from blade flapping • Most helicopters have hinge offset rotors • Rigid body rotation at flap hinge • Elastic deformation with time • Grid deforms at every • Time step Image from Bhagwat, Dimanlig, et al, CFD/CSD Coupled Trim Solution for the Dual-Rotor CH-47 Helicopter Including Fuselage Modeling, AHS Specialists’ Conference on Aeromechanics, Jan, 2008

7. What About “Advanced” Rotors • Apache, Blackhawk, Chinook, etc. ~30 years old, V-22 20 • Active rotor technologies being researched: • Active Flaps – Vibration reduction • Active Slats – Lift augmentation • Active Blowing – Stall alleviation • Active Twist – Vibration or performance improvements • Near-body grids must account for these in rotating frame • Passive technologies also actively researched • Advanced tips • Advanced airfoils

8. Flapping Wing the “Buzz” in Vertical Lift Nascent research area for UAV, MAV applications Small scale, low Reynolds number critical for flapping wing lift Physical features of flow dramatically different than traditional rotorcraft aerodynamics Ample work to be done in development and validation

9. Boundary Conditions Are Not Straightforward Induced inflow large in hover, diminishes with forward speed ~ 50 ft/sec for 20,000 lb helicopter, depending on rotor radius Cat 4 hurricane for “SoloTrek” ducted fan exoskeleton Grid must either be large enough that inflow is zero or must account for inflow at boundaries

10. Rotor Wake Critical for Hover Performance • Calculation of downwash and swirl affect figure of merit/power required • Wing download critical for tilt rotor hover performance • Special topics: • Coaxial rotors • Vortex ring state • Intermeshing rotors • Tandem rotors

11. Rotorcraft Wake Modeling • Several approaches being studied: • Grid refinement • Vortex transport method • Particle vortex transport method • Traditional free wake methods highly empirical, sensitive to parameter changes in model

12. Velocity Gradient in Tip Vortex Important for Vibration, Noise Would like to keep tip vortex organized for multiple revs Fine mesh required to resolve velocity gradient in trailed vortices Traditional RANS CFD numerically diffuses vortex within several chord lengths, does not model rigid body rotation

13. Wake Impinges on Fuselage and Tail Even in Benign Conditions Interaction between main rotor wake and tail rotor important for noise, control, and vibration Download can adversely affect performance How to measure extremely Complex flowfield for validation

14. Rotor Dynamics 101 Natural frequency of rotating blade hinged at the root: 1/rev (Hinge offset rotor < 1.05/rev) Cyclic pitch used to balance moment, control helicopter Cyclic pitch inputs applied at 1/rev => rotor being forced near or at resonance Do not get infinite response because of large flap damping ~ 50% critical Controls highly coupled

15. Don’t Forget Structural Dynamics • “A helicopter is a fatigue testing machine that also flies” • Severe vibration in rotor system • In theory, only multiples of N/rev transmitted down shaft • In reality, largest vibration comes from 1/rev • High-fidelity, nonlinear structural dynamics models would be useful but don’t exist • Multi-body dynamics models more common, often require extremely small time step, difficult to parallelize

16. Coupled CFD/CSD Analysis—Loose Coupling Couple comprehensive analysis with CFD airloads “CSD” is stick model—beam theory for blades, simple fuselage if at all, mutlibody dynamics Exchange data once per revolution for loose coupling Calculating periodic response—“trim solution” Can’t put airloads on right hand side Elastic + inertial = CFD blows up—damping wrong Elastic + inertial + simple aero = CFD – simple aero

17. Coupled CFD-CSD Analysis—Tight Coupling Couple comprehensive analysis with CFD airloads “CSD” is stick model—beam theory for blades, simple fuselage if at all, multibody dynamics Exchange data once per time step for tight coupling Integrating equations in time—“transient solution” Airloads go on right hand side at every time step Elastic + inertial = CFD ok for tight coupling

18. What Would We Do In CSD If We Did CSD Research? • Detailed 3-D model of blades • Current technology is beam theory • Reality is complex composite structures with tuning weights, large variation in sectional properties, actuators? • Rotor dynamics and fuselage dynamics run independently • Rotor dynamics code has elastic modes for fuselage • Fuselage code has forcing function to simulate rotor • Coupled rotor/airframe analysis not on the radar • Fuselage nonlinear from windows, doors, fasteners, etc.

19. Acoustics—CFD for Noise Sources Noise calculated, then propagated to observer Either calculated at source or on “permeable sphere” Noise often dissipated in CFD solution because it’s “in the noise” Large grid required for permeable sphere around entire helicopter.