Use of HPC in Advanced Rotorcraft Systems

Matt Floros

US Army Research Laboratory

April 15, 2008

Rotorcraft Aeromechanics—Never a Dull Moment

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

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 wake

All Lift, Propulsion, and Control From Main Rotor Fixed Wing Rotorcraft

Lift Wing Main Rotor

Propulsion Jet Engine/Prop Main Rotor

Pitch Control Elevator Main Rotor

Roll Control Ailerons Main Rotor

Yaw Control Rudder Tail Rotor

Yaw control on multi-rotor aircraft?- Main rotors

Local Velocities from Static to Transonic Speeds

Advancing Tip: ≈ 0, M~0.85-1

Retreating Tip: near stall, Low speed subsonic, dynamic stall

Retreating Root:≈ 180, Low speed subsonic

Local Velocities from Static to Transonic Speeds

Tangential velocity V = r + sin(), for traditional helicopter 0 < < 0.4 (0 < m < 0.2 for tilt rotor), some new concepts much higher

Flow approaching airfoil from trailing edge for 0 < r < 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!

Near-Body Grids for Blades Deform 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

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 everyTime step

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

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

Boundary Conditions Are Not Straightforward

Induced inflow large in hover, diminishes with forward speed

wakefar in

disk rotor at

AT

h

AT

h

vv

v

2

2

2

~ 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

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

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

Velocity Gradient in Tip Vortex Important for Vibration, Noise

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-300-200-1000100200

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

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 extremelyComplex flowfield for validation

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

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

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 sideElastic + inertial = CFD blows up—damping wrongElastic + inertial + simple aero = CFD – simple aero

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 stepElastic + inertial = CFD ok for tight coupling

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.

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.