Coupling SIMPACK with Helicopter Aerodynamics€¦ · Coupling SIMPACK with Helicopter Aerodynamics...

Coupling SIMPACK with Helicopter Aerodynamics

Ludwig Krause,

Johannes Hofmann,

Stefan Surrey,

Maximilian Graser

SIMPACK User Meeting,

Oct. 9th 2014, Augsburg

• Motivation

• DLRs Rotor Code S4

• Coupled SIMPACK-S4 Environment

• SIMPACK Model

• Flexible Rotor Blade

• Cross Code Verification and Differences in Modelling

• Conclusion and Outlook

> SIMPACK UM `14 > Ludwig Krause• Coupling S4 and SIMPACK > 09.10 2014 DLR.de • Folie 2

Outline

• 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)

DLR.de • Folie 3

Motivation

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

DLR.de • Folie 4

Rotor Modelling: Modules of S4


• 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

DLR.de • Folie 5

S4 –Assets


• 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

DLR.de • Folie 6

Coupled S4-SIMPACK Environment


• 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

DLR.de • Folie 7

SIMPACK Model - 1


• 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

DLR.de • Folie 8

SIMPACK Model - 2


• 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:

DLR.de • Folie 9

The Test Blade - 1

1. Eigenvalue analysis of elastic beam

2. Static solution under initial tension to account

for geometric stiffening

0

1

2

3

4

5

6

7

8

9

10

11

12

13

0 0,2 0,4 0,6 0,8 1 1,2

ω/Ω

ref

Ω/Ωref

S4

SIMPACK

1st lag

1st torsion

3rd lag

2nd torsion

1/rev

5/rev

6/rev

4/rev

3/rev

2/rev

9/rev

8/rev

7/rev

10/rev

11/rev


DLR.de • Folie 10

The Test Blade - 2

-1,0

-0,5

0,0

0,5

1,0

0,0 0,4 0,8 1,2 1,6 2,0ΦFl

ap

r [m]

Simpack

S4

-1,0

-0,5

0,0

0,5

1,0

0,0 0,4 0,8 1,2 1,6 2,0

ΦLa

g

r [m]

SimpackS4

-1,0

-0,5

0,0

0,5

1,0

0,0 0,4 0,8 1,2 1,6 2,0ΦTo

rsio

n

r [m]

Simpack

S4

Lag

Flap

Torsion • SIMPACK beam imported from Nastran compared to

beam from S4-FEM-preprocessor by means of

eigenfrequencies and eigenmodes

• Eigenfrequencies and eigenmodes calculated at 100 %

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

• Max. deviation for eigenfrequencies at 1. Torsion (6.55 %)

• Deviation in eigenmodes negligible


• Test Case: o 1 Blade

o Hover flight

o Thrust 1200 N

Still considerable deviations in lag and

torsion!

Still differences in modelling between S4

and SIMPACK:

DLR.de • Folie 11

First Results for Hover


flap

lag

torsion

SIMPACK S4 Verification

Fully coupled motions Structurally uncoupled motions Coupled motions implemented in S4 (simplified)

Blade is structurally pitched Only forces are transformed Fixed structural pitch in SIMPACK

Centrifugal terms depend on deformation Centrifugal terms depend only on RPM No compensation (deviation <2%)

No propeller moment in SIMPACK Formulation for a flat plate Additional force-element in SIMPACK

DLR.de • Folie 12

Differences in Modelling – Secondary Components


flap

lag

torsion

• Offsets between mass and elastic axes + pretwist lead

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

Secondary components added to dynamic response

calculation in S4 (simplified)

Discrepancy in lag deflection almost completely

disappeared

DLR.de • Folie 13

Differences in Modelling – Propeller Moment

SIMPACK S4 Verification

Fully coupled motions Structurally uncoupled motions Coupled motions implemented in S4 (simplified)

Blade is structurally pitched Only forces are transformed Fixed structural pitch in SIMPACK

Centrifugal terms depend on deformation Centrifugal terms depend only on RPM No compensation (deviation <2%)

No propeller moment in SIMPACK Formulation for a flat plate Additional force-element in SIMPACK

With Propellermoment Without Propellermoment

Including propeller moment into SIMPACK using an additional force element

deviation in torsion eigenmodes disappears


DLR.de • Folie 14

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

• 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

DLR.de • Folie 15

Conclusion and outlook


DLR.de • Folie 16

Thank you for your attention!


Coupling SIMPACK with Helicopter Aerodynamics€¦ · Coupling SIMPACK with Helicopter Aerodynamics...

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