AERODYNAMICS AND AEROELASTICITY METHODOLOGIES FOR FUTURE CONCEPTS IN VERTICAL LIFTMarilyn J. Smith, PhDProfessorDirector, Vertical Lift Research CenterGeorgia Institute of TechnologyAtlanta, GA US

Oct 15, 2019Aerodynamics Tools and Methods in Aircraft Design

The Evolution of Modern Vertical Lift Design

https://www.lockheedmartin.com/en-us/products/sikorsky-black-hawk-helicopter.html

Main Rotor

Tail Rotor

Fuselage

Empennage

Aeromechanics of a Traditional Helicopter

V∞

W

FXC

Fuselage Flow• Drag• Component Loads

Dynamic Stall

• Loads• Performance

Rotor Fuselage Interactions• Vibration

Rotor-Wake Interactions

• Vibration• Noise• Loads• Performance

Transonic Flow

M < 1• Loads• Noise• Performance

M > 1

Tail-rotorInteractions With:

• Empennage• Main Rotor• Main-Rotor Wake

Performance, Handling-Qualities

The Evolution of Modern Vertical Lift Design

By FOX 52 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35908368

Main Rotor

Fuselage

Empennage

RecirculationEffects

Conversion

Multiple RotorsProprotors

Bell FlightJaunt Air Mobility

Urban Air Mobility

FLRAA and FARA Unmanned Vehicles/DronesLow Reynolds Numbers Coaxial/Corotating Rotors

Distributed Electric Propulsion

Shrouded/Ducted Rotors

Vertical Lift in the 21st Century

Aeromechanics Prediction Requirements• Vehicle performance• Blade loads• Airframe & drive train loads• Vibration (rotor and fuselage)• Aeroelastic stability • Flight dynamics • Handling qualities• Noise

UH-60F Hover

UH-60A forward flight

TRAM proprotor

Figures courtesy of US Army

Aerodynamic Tools for 21st Century VL

Conceptual and Preliminary Design- NDARC- GTABB/COMPASS

Detailed Design and Engineering Analysis- Comprehensive Codes: CAMRAD, RCAS, HOST- Panel-Based Methods- Dual Solver Hybrid Methods- Adjoint Design: FUNtoFEM

Physics and High Fidelity Analysis- Helios, FUN3D, OVERFLOW- FLOWer, ELSA, TAU- Academic Codes: Liverpool/Glasgow, UMD

21st Century Aerodynamic Prediction Goals

Johnson et al., “Requirements for Next Generation Comprehensive Analysis of Rotorcraft,” AHS Specialist's Conference on Aeromechanics, San Francisco, CA, January 23-25, 2008.

Multiple Core/Processor CapabilitiesAbility to model or include the effects of

New Technology

Active Flow Control

Active Surfaces/DampersMultiple Rotors/Propellers

New Designs

What Capabilities are Needed?

Multiple Core/Processor CapabilitiesAbility to model or include the effects of

New TechnologyNew Designs

What Capabilities are Needed?

Incorporate the effects ofEnvironment Urban and Nap of the EarthNear-ground OperationsIn-Flight Operations

Slung Loads

Robotic Landing Gear

Large Gusts in Urban Canyons

Gust Responses: Urban Canyon Typical

What Capabilities are Needed?

Reduce the scope of wind tunnel testing during design and flight testing for certification

• Digital Threads• Uncertainty Quantification• Removal of User-generated Errors

Quantitative Data Analysis Techniques

• Coherence: Linearity between response of two data sets, illuminates missed harmonics. Coh < 0.6 →missed harmonic

• Cost Function: Scalar measure of difference between response of two data sets

J < 100 → Very little differenceJ < 50 → Virtually identical

where !",!$, %&' !( are weighting factors for coherence, magnitude, and phase, respectively

) = 20& -

./

.0!" !$ 123 − 2 5 +!( ∠123 − ∠2

5

J= 10 40 1000

CIFER• Developed for system identification (USRA)• Computes difference between frequency response of two datasets

Cho et al. "System Identification and Controller Optimization of a Coaxial Quadrotor UAV in Hover," AIAA, 2019Smith et al., “Towards Certification of CFD as Numerical Experiments for Rotorcraft Applications,” Aeronautical Journal,122(1247), 104-130.

Quantitative Data Analysis Techniques

J= 10 40 1000

Cho et al. "System Identification and Controller Optimization of a Coaxial Quadrotor UAV in Hover," AIAA, 2019Smith et al., “Towards Certification of CFD as Numerical Experiments for Rotorcraft Applications,” Aeronautical Journal,122(1247), 104-130.

Aerodynamics for Design

and Modeling & Simulation

Aerodynamic Tools for

Designand

Modeling & Simulation

NASA Design and Analysis of Rotorcraft (NDARC)Capabilities for:

Off-design mission analysisFlight performanceVehicle sizing

Flexibility for non-conventionalvehicles through • Synthesis of component modules• Propulsion system options (electric,

turbojet, turbofan, reaction drives, fuel cells, etc.)• Surrogate models for trade-space analysis

• Silva et al, “Multidisciplinary Conceptual Design for Reduced-Emission Rotorcraft,” AHS Specialists Conf., San Francisco, California, USA, January 16-18, 2018.

• Johnson, “NDARC: A Tool for Synthesis and Assessment of Future Vertical Lift Vehicles,” Vertiflite, Nov-Dec, 2014, pg. 26-27. Prior slide figure from this work.

Optimization with NDARC

A New Approach for Design and M&S : GTABB and COMPASS

• Development of physics-based reduced order models for aerodynamic and bluff bodies

• Extensible to new configurations: Not an interpolation• New features:

Shadowing orientation effects GUIRotor and engine effects Multiple body types

• Validated with CFD, wind tunnel, and flight test• Adopted/in adoption by US Army, US Navy, Drone Racing

League, academia• Slung loads• Control law and autonomous algorithm design• Virtual reality-based training (M&S)

• Prosser, D. and Smith, M. J., “Physics-Based Aerodynamic Simulation Models Suitable for Dynamic Behavior of Complex Bluff Body Configurations,” American Helicopter Society 71st Annual Forum, May, 2015. See also JAHS.

• Koukpaizan et al. “Rapid Vehicle Aerodynamic Modeling for Use in Early Design,” in Proceedings of the AHS Aeromechanics Design for Transformative Vertical Flight Conference, San Francisco, CA, January 16–19, 2018

GT Aerodynamics of Bluff Bodies (GTABB)

17

External Control Module

Unsteady Aerodynamics in Real Time • Unsteady phase lag• Large vortex shedding

fluctuations• Three-dimensional effects

(finite bodies)

No vortex shedding

Withvortex

shedding

US Army Blackhawk Flight Test Validation

Captures Onset of Instability

COMPlex Aerodynamic Shape Simulation (COMPASS)

• Use canonical shapes with corrections to estimate the characteristics of complex shapes.

• Add corrections for shadowing (feature blockage) and shear layers

COMPASS quadrotor representation (without rotors)

Nondimensional Aerodynamic CharacteristicsFigures from Prosser and Smith (2016)

Predict the Cp distributionabout the body

Integrate to get force andmoment coefficients

Nondimensional Aerodynamic Characteristics

Prosser, D. and Smith, M. J., “Aerodynamics of Finite Bluff Bodies,” Journal of Fluid Mechanics, Vol. 799, No. 6, pp. 1–16, 2016,

Quasi-Steady Predictions

Drag Coefficient Lift Coefficient

Typical Control Law Design:• Flat plate wetted area used for forces• Moments are neglected

trim (Deg) Range (Km) Flight Time (Min)0

5

10

15

20

25

30Flat PlateUnsteady

FP

FPFP

Vehicle Performance

Flat plate aerodynamics overpredicts performance by ~10%and underpredicts rotor trim by ~15%

Split-S Trajectory predicted by GTABB

HeightAbove

Ground (m)

Flight Test Validation

https://www.youtube.com/watch?v=UNoGxq8pQGE, Courtesy Drone Racing League

Real Flight Vehicle

Simulated Flight Vehicle

Hybrid CFD Analyses

CFD + Free Wake Model

• Data is exchanged between the CFD and free wake code at fixed intervals

• CFD near-body solver calculates the sectional loads along the blades as the solution advances

• The vortex filaments which model the wake are updated based on the sectional loads

• The outer boundary conditions of the CFD domain are updated from the wake-induced flow field to reflect the influence of the wake

Outer Boundary CFD

Wake Vortex Filaments

Hybrid CFD Methods

• Promise of CFD-level accuracy without the costs, up to 90% savings in some cases• Many approaches through the decades, but with

limitations:• Single blade• No fuselage (except DLR)• Very mixed results with significant errors in pitching

moment, structural bending, and hub forces• Some have known numerical formulation errors• Many are “academic” codes without formal version

controlPast (US) engineering experience not positive to

adoption in work flow

CDI-GT Hybrid CFD Approach• CFD/CFD-CSD provide highest accuracy at highest cost• Hybrid CFD methods mitigate costs by 50%-90% while maintaining accuracy

• Increase CFD-based applications to earlier design and additional analysis

Cost Accuracy

• Carefree: Couple CFD solvers with commercial wake solvers

• Flexible: Takes advantage of near-body capabilities

• Physics: Multiple level of physics

• Cost-effective: Can use same meshes

• Expanded Capabilities: Multiple components, full vehicles

Non-contiguous Methodology

Contiguous vs non-continguous

• Remove the inertial background grids• Allow boundary motion and arbitrary boundary shapes• Velocity at all unblanked outer boundaries determined by

the free wake code

Blade Vortex Interaction

Conglomeration of vortex filaments modeling the tip vortex

Illustration of tip vortex passing between domains

• Mtip = 0.65, trimmed to CT/σ=0.09• 3.9M nodes per blade (15.4M total)• O-grid topology

S-76 Hover

Helios Predictions with OVERFLOW as near-body solver (Full CFD, 448M nodes) Mark Potsdam, Rohit Jain (Army ADD)

Noncontiguous Meshes

Hover is complex computation with significant wake interactions

S-76 Integrated Loads

Torque coefficient vs. Thrust coefficient Figure of Merit

Figure of Merit

Exp. uncertainty ~0.6 countsHelios ~ 1 count OF-Charm ~ 2 countsOF-alone >2 counts

Comparable or better to all US and International participants

Lateral Wake

VerticalWake

Non-contiguous wake predictions (lines)comparable to those in Contiguous CFD domain(symbols)

Computational cost

The full grid non-contiguous simulations cost about 7.6% of the number of core-hours as the stand-alone

engineering OVERFLOW simulations

Simulation Number of grid nodes (millions)

Core-hours Cost ratio

OVERFLOW-Helios (Jain 2015) 448 122,880 57.5

OVERFLOW (Narducci 2015) 63.4 28,080 13.14

OVERFLOW-CHARM with background grids

28 14,400 6.74

OVERFLOW-CHARM non-contiguous grids (5 revs)

15.4 2,140 1

Jacobson, K. and Smith, M. J., “Performance and Physics of a S-76 Rotor in Hover With Non-Contiguous Hybrid Methodologies,” AIAA SciTech, AIAA-2016-0302, San Diego, CA, January, 2016

Evaluation in Forward Flight• Baseline mesh was provided by Boeing-PHL • Represents a typical near-body mid-size mesh• 3.7M point mesh on each of four blades• 32.9 M background Cartesian mesh (orig)• Menter kw-SST turbulence model with full viscosity in

the near-body region and Euler terms in the off-body region

• Sensitivity to mesh sizes, coupling updates, turbulence model

• Aeroelastic coupling• Examination of different rotor blade structural

properties on same rotor (experimental set-up)• Evaluation of fuselage and wind tunnel floor effects

• Wilbur et al., “Complex Vehicle Design and Analysis with Hybrid Methodologies,” AHS Aeromechanics Conference, San Francisco, CA, January 16–19, 2018. Coming soon: Journal of Aircraft

• Min, B.-Y. et al., “Toward Improved UH-60A Blade Structural Loads Correlation,” AHS 74th Annual Forum, Phoenix, AZ, May 14-17, 2018

NFAC Tunnel Test

Dual-Solver Vortex Interactions

Correlation with Industry CFD-aloneNormalized, means removed

r/R=0.4

r/R=.92

• Not a correlation with experiment – correlation with “best industry mesh/practices”• Reduction in background mesh by 40% nodes – no difference in loads• Convergence occurs at 75% of full CFD convergence requirements• None of the poor correlations with structural, hub or pitching moments as

observed with other hybrid approaches

Full CFD AnalysesFull CFD Analysis

Rotorcraft simulation and adjoint-based derivative evaluation framework

FUNtoFEM: Adjoint-enabled aeroelastic analysisFUN3D

Surface integration Flow solver

Mesh warping

FUNtoFEM

Load transferLoad adjoint

Displ. transferDispl. adjoint

FEM/TACS

Forward pathAdjoint path

I General interface for aeroelastic load and displacement transferbetween FUN3D and a finite-element code

I E�cient hand-coded partial derivatives terms and transposeJacobian-vector products that enable adjoint-based derivativeevaluation

I Di↵erent transfer schemes enabled through an abstract interfacelayer: MELD and RBF (for surface-to-surface transfer), beam-specifictransfer

I Di↵erent bodies can be assigned di↵erent transfer schemes

3 / 29

Time-accurate Aeroelastic Rotorcraft Simulation

I On-going work to verify against HART-II case

23 / 29

• FUNtoFEM: A general interface for aeroelastic simulation and adjoint-based gradient evaluation

• Enables use of either loosely or tightly coupled simulation• Separation of time integration schemes for CSD/CFD• Different bodies can employ their own load/displacement

transfer methods• Efficient hand-coded derivative terms enable efficient

computation of coupled Jacobian-vector products

Comparison of natural frequencies with DYMORE

I Fan plot for HART-II type model

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2�/�ref

123456789

10111213141516

�/�

ref

TACS flap TACS lag TACS torsionTACS flap TACS lag TACS torsion

25 / 29

(Graeme Kennedy at Georgia Tech)

Helios Overview

Steady forward flight

Sikorsky X2™

Unsteady maneuver

Wake resolution

Interactional aerodynamics

Elastic bending

• Rotary-wing product of CREATE™-AV− Jointly developed by CREATE and Army ADD

• Multi-mesh/Multi-solver- Strand, unstructured, structured curvilinear

- Cartesian high-order with AMR

- Automated domain interpolation

• Interfaces to rotorcraft comprehensive analysis codes for CSD and trim

• Scalable execution on HPC systems

• 100+ active user licenses- DoD organizations (Army, Navy)

- Various US rotorcraft companies

- Various academic institutions

• Helios integrates simulation codes from CREATE, Army, and outside organizations

• Software integrated through an extensible Python infrastructure

UH-60 main/tail simulationCourtesy U.S.Army AED

H-47 tandem simulationCourtesy Boeing

JMR “Defiant”Sikorsky/Boeing

JMR “Valor” Bell Helicopter

CH47 – Block II Boeing/AED/ADD

AH-64 Apache Courtesy Boeing

Cap

abili

ty

V2

AMR

V6V5

SIF, PUNDIT, SAMARC, NSU3D, RCAS, SAMRAI

V3

Rotor-Fuselage

V4

Multi-rotor

PARAVIEWOVERFLOW

V1

Dualsolver

CAMRADIICSI MELODI

SAMCartFUN3D

V7

mStrandKCFD

Developed under CREATE

Third-party packages

2010 2011 2012 2013 2014 2015 2016

Propellers/Propulsers

Maneuver/Strand slvr

Annual releases

COVIZ

DES Wake

V8

Automated Strands

20182017

V9

3D Structures

US Army Helios Development

AH-64 E Apache, Hover:

Narducci & Tadghighi, BoeingAIAA-2016-0564

• High-fidelity, time-accurate rotor and fuselage combinations

• Boeing Mesa

Apache INSTALLED ROTOR PERFORMANCE

V2?B1? V4

V3B2 B3

B4

B1T3iT1i

T4i W4T1o W1

Blade 4

Blade 3

Blade 2

UH-60A Rotor Modeling & Validation§ Blade tip vortex (B)§ Twist/planform vortex (V)

§ Trim tab vortex (T)§ Wake sheet (W)

Blade 1CFD

Helios wake velocities, vortex locations and strength predictions are within experimental errors

PIV

NASA/Army UH-60A NFAC test

Sikorsky X-2™ TD

Helios models the interactional aerodynamics between lift-offset coaxial compound rotors, fuselage, and propulsor configuration

Main rotor unsteady lift

Reed, Egolf, “Coaxial Rotor Wake and Prop Induction Impact on a Horizontal Tail using Helios”, AIAA-2015-0554, 53rd AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Jan 2015

Nor

mal

ized

Am

plitu

de Unsteady fuselage lift

Unsteady propeller thrust

82 kts

1 rev

1 rev

6 revs

Validation with Bell XV-15 Tiltrotor

Reingestion

Reingestion

Wake impingement

Fountain flow

!"/$Download/Thrust (%)

Wing Fuselage Nacelle Total

CFD 0.1195 10.31 5.41 1.07 16.79

Flight test (Arrington et al.)

0.1271 – 0.1296 - - - 13.42 – 14.50

Config 1FUN3D-

OVERFLOW

Config 2mStrand

OVERFLOW

FUN3D

mStrand

mStrand

Felker et al, ‘87

Download Validation with JVX

FUN3D-OVERFLOW mStrand

0.01360.0138

0.0133

0.007410.007540.0075

0.0000

0.0025

0.0050

0.0075

0.0100

0.0125

0.0150

0.0175

0.0200

Rotor Thrust CT

Low thrustq = 6o

High thrustq = 12o

Test

FUN3D/OFLOW

mStrand

0.001350.001320.00134

0.0008460.000852

0.000813

0.00000

0.00025

0.00050

0.00075

0.00100

0.00125

0.00150

0.00175

0.00200

Wing/Flap Download CZ

10.2%9.6%10.1%

11.4%11.3%

10.8%

0% 2% 4% 6% 8% 10%12%

14%16%

18%20%

Download - CT/CZ

JVX DOWNLOAD ANALYSIS

• 40 revs, 9 sec (real-time)

• Flight path angle 3 deg at start to 35 deg at end

Manuever: UH-60A UTAAS Pull-Up

UH-60A UTTAS ManeuverSection Pitching Moment Cm M2

86.5 % R

UH-60A UTTAS Maneuver : Pushrod loads

Helios demonstrates big improvement over RCAS for pushrod load prediction

Pushrod load

Joint CFD-Experimental Validation!• Full-scale UH-60A rotor in Air Force 40- by 80-foot Wind Tunnel under

NASA/Army Airloads Wind Tunnel Test Program (2010)• Extensive database of performance, hub loads, air loads, structural loads, wake

PIV, blade deformation, RBOS on highly instrumented rotor for validating analytical tools

• PIV phase• Wake measurements at 90 deg azimuth over 50% of outer blade

• Vortex characteristics extracted: circulation, size, position

NASA/Army UH-60A NFAC test

PIV camera portslaser launch port

PIV ROI

mirror port

smoke generators

airflow

Future Research Directions• Continuous improvement in speed and efficiency on multi-core processors for

all levels of aerodynamic prediction tools• Integration of reduced-order modeling that is physics-based for earlier design

of maneuvering and operational effects• Advanced design capabilities for rapid, highly-accurate design• Using high-fidelity methods, exploration of physics to understand complex

interactional phenomena and shortcomings of current algorithms• Improved prediction of acoustics (interior and exterior) from unsteady aerodynamics• Jointly designed experimental datasets with sufficient information for CFD validation!

• International collaboration to solve complex problems

Multi-rotor trim

3D Structures

Acknowledgements• This effort was partially sponsored by the U.S. Government under

• U.S. Army/Navy/NASA Vertical Lift Research Center of Excellence under the directionof Mahendra Bhagwat of AFDD, Agreement No. W911W6-11-2-0010.

• Other Transaction number W15QKN-10-9-0003 between Vertical Lift Consortium, Inc. and the Government

• Navy STTR contract N68335-09-C-0335 with guidance from technical monitors Jennifer Abras and Mark Silva

• US DOE STTR DESC0004403

• The US Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon.

• The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the U.S. Government.

• The authors would like to thank• Andy Wissink and Roger Strawn of CCDC Aviation Command for the Helios figures

and slides• Glen Whitehouse and Dan Wachspress of Continuum Dynamics, Inc.• Rohit Jain and Mark Potsdam for their insights and figures• Ted Meadowcroft of Boeing-Philadelphia for the UH-60A computational mesh and input

decks

• All slides that require approval have been previously approved for prior presentations.

Questions?