Coupling SIMPACK with Helicopter Aerodynamics

Ludwig Krause, Johannes Hofmann, Stefan Surrey, Maximilian Graser SIMPACK User Meeting, Oct. 9th 2014, Augsburg

DLR.de • Folie 2

> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Outline • • • • • • •

Motivation DLRs Rotor Code S4 Coupled SIMPACK-S4 Environment SIMPACK Model Flexible Rotor Blade Cross Code Verification and Differences in Modelling Conclusion and Outlook

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Motivation • Complex helicopter aerodynamics requires appropriate modelling  S4 rotor code developed at DLR Institute of Flight Systems since 1980s

• Modeling of structural dynamics in S4 doesn‘t fulfill modern requirements o Non-moving reference frame (isolated Rotors) o Advanced Blade design requires complex, flexible blade models

• Constrained rotor movement most naturally handled within MBS-formalism • MBS Software like SIMPACK enables integration of flexible bodies through

interface to commercial FEM-Codes (ANSYSY, NASTRAN, ABAQUS etc.) • Goal: Develop fully coupled aeroelastic simulation environment between SIMPACK (structural dynamics) and S4 (aerodynamics)

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Rotor Modelling: Modules of S4

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

S4 –Assets • Comprehensive code  very fast and accurate • Interface for loose coupling to DLR CFD Codes Flower and Tau • BVI (Blade-Vortex Interaction) Prediction (Noise) Exact knowledge of blade position needed! • Validated with wind tunnel data in several international test campaigns

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Coupled S4-SIMPACK Environment • Strong coupling: Data exchanged between both codes at every time step • Data transferred through SIMPACK IPC Co-Simulation Interface • Strictly speaking: No Co-Simulation! • Equation of Motion solely solved by SODASRT 2  S4 solver discarded • Interface programmed in C and integrated in S4 FORTRAN code

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

SIMPACK Model - 1 • 4-bladed rotor, each blade imported from NASTRAN as a beam element • Per blade: o 20 aerodynamic marker along the c/4 line o 20 sensors to measure blade element translational and rotational position/orientation, velocity and acceleration o 18x20=360 y-outputs to transfer sensor output to S4 (6 DOF per node) o 6x20=120 u-vectors and u-inputs to import aerodynamic forces and moments o 20 force elements at respective aerodynamic markers to apply these forces

• Further u-vectors/u-inputs for rotational speed and pitch regulation  Modell creation fully automated through QtScript

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

SIMPACK Model - 2 • Additional elements/bodies incorporated to account for: • Lag/Flap, torsion joint • Control rods, hub, clamp, cell • For verification purpose: • Fixed rotor hub • Rigid control rod

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

The Test Blade - 1 • Beam-Element generated in NASTRAN • Verified in NASTRAN to 3D- Volume Element w.r.t. Eigenfrequencies and –modes

• Further validated through modal testing • To incorporate into SIMPACK, 2 calculations in NASTRAN were conducted: 13

11/rev 2nd torsion

12

10/rev

3rd lag

9/rev

11 10

8/rev

9

7/rev

S4

8

ω/ Ωref

SIMPACK

6/rev

7 6

5/rev

5

4/rev

4

3/rev

1st torsion 3

2/rev 2

1/rev

1

1st lag 0 0

0,2

0,4

0,6

Ω/ Ωref

0,8

1

1,2

1. Eigenvalue analysis of elastic beam 2. Static solution under initial tension to account for geometric stiffening

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

The Test Blade - 2 •

1,0

Simpack S4

SIMPACK beam imported from Nastran compared to

beam from S4-FEM-preprocessor by means of

•

0,0

eigenfrequencies and eigenmodes

-0,5

Eigenfrequencies and eigenmodes calculated at 100 %

-1,0

rotational speed (=109 rad/s)  geometrical stiffening !

1,0

Max. deviation for eigenfrequencies at 1. Torsion (6.55 %) Deviation in eigenmodes negligible

0,4

0,8

1,2

1,6

2,0

1,2

1,6

2,0

1,2

1,6

2,0

r [m] Simpack S4

Lag

0,5

ΦLag

•

Torsion

0,0

0,0 0,0

0,4

0,8

-0,5

-1,0

r [m]

1,0

Simpack S4

Flap

0,5

ΦFlap

•

ΦTorsion

0,5

0,0

0,0

0,4

0,8

-0,5

-1,0

r [m]

DLR.de • Folie 11

> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

First Results for Hover flap

• Test Case: o 1 Blade o Hover flight o Thrust 1200 N lag

Still considerable deviations in lag and torsion!  Still differences in modelling between S4 and SIMPACK:

torsion

SIMPACK

S4

Verification

Fully coupled motions Blade is structurally pitched Centrifugal terms depend on deformation No propeller moment in SIMPACK

Structurally uncoupled motions Only forces are transformed Centrifugal terms depend only on RPM Formulation for a flat plate

Coupled motions implemented in S4 (simplified) Fixed structural pitch in SIMPACK No compensation (deviation SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Differences in Modelling – Secondary Components

•

Offsets between mass and elastic axes + pretwist lead flap

to coupled motions  No pure flap, lag or torsion motion

•

S4 used only dominant mode shapes so far  Sufficient assumption for most cases so far

lag

 Secondary components added to dynamic response calculation in S4 (simplified)  Discrepancy in lag deflection almost completely disappeared

torsion

DLR.de • Folie 13

> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Differences in Modelling – Propeller Moment SIMPACK

S4

Verification

Fully coupled motions Blade is structurally pitched Centrifugal terms depend on deformation No propeller moment in SIMPACK

Structurally uncoupled motions Only forces are transformed Centrifugal terms depend only on RPM Formulation for a flat plate

Coupled motions implemented in S4 (simplified) Fixed structural pitch in SIMPACK No compensation (deviation SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Verification in Hover • After implementing secondary components and propeller moment deviations negligible • 2 explanations for slight deviation in flapping: 1. Centrifugal terms in SIMPACK depend on

deformation 2. Simplified approach for secondary mode components in S4

•

Accuracy of results largely independent of

load case •

Coupling chain verified for hover flight

DLR.de • Folie 15

> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Conclusion and outlook • Tight fluid-structure coupling between comprehensive rotor code S4 and SIMPACK has been established • Using 1D-beam elements for the flexible rotor blades leads to underestimation of torsional deflection  additional force elements needed • Further complex test cases have to be investigated o Forward flight o Advanced blade design

• Still a lot of verification needed until SIMPACK-S4 can be used to simulate the complex aeroelastics of helicopter rotors • Coupling environment can be extended to overall helicopter simulation to take full advantage of SIMPACKs MBS formalism

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> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014

Thank you for your attention!