Presentation on theme: "Use of HPC in Advanced Rotorcraft Systems"— Presentation transcript:
1 Use of HPC in Advanced Rotorcraft Systems Matt FlorosUS Army Research LaboratoryApril 15, 2008
2 Rotorcraft Aeromechanics—Never a Dull Moment Mach number range 0—1Steady state is unsteadyLarge induced inflowFlexible bladesIntermeshing rotors3-D acoustic field360-degree angle of attack rangeFlaps & multi-element airfoilsRotor operates in its own wakeFuselage and tail are in rotor wakeImage 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 Fixed WingRotorcraftLiftWingMain RotorPropulsionJet Engine/PropPitch ControlElevatorRoll ControlAileronsYaw ControlRudderTail RotorYaw control on multi-rotor aircraft?- Main rotors
4 Local Velocities from Static to Transonic Speeds Advancing Tip:a ≈ 0, M~0.85-1Retreating Tip:a near stall, Low speed subsonic, dynamic stallRetreating 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 higherFlow approaching airfoil from trailing edge for 0 < r < m on retreating sideNear-body grid for blade has to encompassTransonic flow/shocksSeparated flowDynamic stallReverse flow, radial flowAll in one revolution!
6 Near-Body Grids for Blades Deform at Every Time Step Moment balance, propulsion, and control come from blade flappingMost helicopters have hinge offset rotorsRigid body rotation at flap hingeElastic deformation with timeGrid deforms at everyTime stepImage 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 20Active rotor technologies being researched:Active Flaps – Vibration reductionActive Slats – Lift augmentationActive Blowing – Stall alleviationActive Twist – Vibration or performance improvementsNear-body grids must account for these in rotating framePassive technologies also actively researchedAdvanced tipsAdvanced airfoils
8 Flapping Wing the “Buzz” in Vertical Lift Nascent research area for UAV, MAV applicationsSmall scale, low Reynolds number critical for flapping wing liftPhysical features of flow dramatically different than traditional rotorcraft aerodynamicsAmple 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 radiusCat 4 hurricane for “SoloTrek” ducted fan exoskeletonGrid 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 requiredWing download critical for tilt rotor hover performanceSpecial topics:Coaxial rotorsVortex ring stateIntermeshing rotorsTandem rotors
11 Rotorcraft Wake Modeling Several approaches being studied:Grid refinementVortex transport methodParticle vortex transport methodTraditional 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 revsFine mesh required to resolve velocity gradient in trailed vorticesTraditional 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 vibrationDownload can adversely affect performanceHow to measure extremelyComplex flowfield for validation
14 Rotor Dynamics 101Natural frequency of rotating blade hinged at the root: 1/rev (Hinge offset rotor < 1.05/rev)Cyclic pitch used to balance moment, control helicopterCyclic pitch inputs applied at 1/rev => rotor being forced near or at resonanceDo not get infinite response because of large flap damping ~ 50% criticalControls highly coupled
15 Don’t Forget Structural Dynamics “A helicopter is a fatigue testing machine that also flies”Severe vibration in rotor systemIn theory, only multiples of N/rev transmitted down shaftIn reality, largest vibration comes from 1/revHigh-fidelity, nonlinear structural dynamics models would be useful but don’t existMulti-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 dynamicsExchange data once per revolution for loose couplingCalculating periodic response—“trim solution”Can’t put airloads on right hand sideElastic + inertial = CFD blows up—damping wrongElastic + 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 dynamicsExchange data once per time step for tight couplingIntegrating equations in time—“transient solution”Airloads go on right hand side at every time stepElastic + inertial = CFD ok for tight coupling
18 What Would We Do In CSD If We Did CSD Research? Detailed 3-D model of bladesCurrent technology is beam theoryReality is complex composite structures with tuning weights, large variation in sectional properties, actuators?Rotor dynamics and fuselage dynamics run independentlyRotor dynamics code has elastic modes for fuselageFuselage code has forcing function to simulate rotorCoupled rotor/airframe analysis not on the radarFuselage nonlinear from windows, doors, fasteners, etc.
19 Acoustics—CFD for Noise Sources Noise calculated, then propagated to observerEither 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.