TIME-ACCURATE SIMULATION OF THE FLOW AROUND THE COMPLETE BO105 WIND TUNNEL MODEL

TIME-ACCURATE SIMULATION OF THE FLOW AROUND

THE COMPLETE BO105 WIND TUNNEL MODEL

Walid Khier, Thorsten Schwarz, Jochen Raddatz presented by

Andreas Schütte

Outline

• Motivation

• Aerodynamics of the helicopter

• Flow solver

• Wind tunnel experiment

• Results

• Code performance

• Conclusion

Motivation

hDemonstration of the capability of DLR’s block structured flow solver FLOWer to simulate the flow around a complete helicopter

hFLOWer is already validated for fixed wing applications and for isolated helicopter fuselages and rotors

hDifficulty: complex geometry and unsteady flow

hWork is part of the French-German CHANCE project Partners: Eurocopter, ONERA, IAG (Uni Stuttgart), DLR

Aerodynamics of the helicopter - a challenge for CFD solvers

htransonic flow hdynamic stall

hblade vortex interaction

htail rotor

interactions with rotor and fuselage

htransonic flow hdynamic stall

hblade vortex interaction

hrotor fuselage interactions

hflow separation at bluff bodies

Phenomena affect: • loads

• performance • vibration

• noise

DLR flow solver FLOWer (1)

hfinite volume discretization of RANS equations on structured, multi block grids

hspace discretization

- cell centered or cell vertex discretization

- central scheme with scalar dissipation or various upwind schemes

htime discretization

- flow equations: explicit multi-stage schemes (Runge-Kutta) with multigrid acceleration

- turbulence equations: explicit multi-stage scheme or implicit DDADI-method

hturbulence modeling: various 0-, 1-, 2-, 7-equation models, e.g. Spalart-Almaras, kω, kω-SST, EARSM, RSM

Numerics for unsteady computations

himplicit time integration with dual-time stepping

hoverlapping grid technique (Chimera)

hmoving / deforming meshes High performance computing

hparallelization based on MPI

hoptimized for vector computers

Wind tunnel experiment

(1)

hBO105 wind tunnel model

hexperimental data were obtained during the

HELINOVI campaign at the DNW in 2003 hinflow data: α_fuselage= -5.2 M_∞ = 0.1766 M_MR = 0.652 M_TR = 0.63 Θ_MR = 10.5° - 6.3° sin(Ψ) + 1.9° cos(Ψ) Θ_TR = 8.0°

wind tunnel model CFD model

Near field grids

hhorizontal stabilizer

hmain rotor and tail rotor

hfuselage

hspoiler and strut

hskids

Far field grid

Number of blocks and grid cells hfuselage+spoiler+ stabilizer+skids+strut 48 blocks, 6.0 M cells hmain rotor 4*3 blocks, 4*0.8 M cells htail rotor 2*3 blocks, 2*0.3 M cells hbackground grid 414 blocks, 1.9 M cells htotal 480 blocks, 11.8 M cells

Component grids are embedded in Cartesian background grid with hanging nodes

Unsteady flow computation

Parameters of the computations:

h Central discretization with scalar dissipation (JST-scheme)

h Flow variables located at cell centers

h CFL = 10.0 , 3 level multigrid

h k-ω turbulence model

h time integration with dual time stepping - 50 inner iterations

- one physical time step equals a 2° rotation of the tail rotor - one physical time step equals a 0.4° rotation of the main rotor

h computation required four weeks using eight processors of NEC SX6

h 2.3 revolutions of main rotor were computed

Variation of pressure on tail fin

Pressure distribution on main rotor

r/R = 80%

h time integration with dual time stepping method, within one physical time step execute:

1. move grids to new positions

2. cut holes and search for donor cells for interpolation (Chimera)

3. perform 50 iterations to converge the implicit time integration

h separate performance analysis for Chimera and one inner iteration

Chimera

• hole cutting • search

one inner iteration of dual-time stepping

50 x

one physical time step

t = 0

t = t_end t = t + Δt

position grids

Execution time for Chimera (hole cutting and search procedure) and flow solver on eight Processors of NEC SX 6

Chimera flow solver one time step starting point on NEC SX 6 750 s 50 * 9.3 s 1215 s

improved state on NEC SX 6 69 s 50 * 9.3 s 534 s

Early tests on NEC SX 8 48 s 50 * 5.5 s 323 s Expected on NEC SX8 13 s 50 * 3.3 s 180 s

Performance improvement of Chimera algorithms

Improvement of chimera performance

Chimera hole cutting and search procedure

flow computation

(time for one inner iteration)

t (physical time step) = t (Chimera) + 50 * t (one inner iteration)

⇒ seq: 3037 s , 8 proc: 532 s

Parallel performance on NEC SX6

hsimulation of complete helicopter wind tunnel model successful

hcomputation took four weeks on eight processors of NEC SX6

hpostprocessing of CFD results is very time consuming due to time dependent flow and large amount of data (0.4 TB)

hunsteady pressure distributions and vortices in flow field analyzed

hgood agreement for fuselage and main rotor, differences for tail rotor to be clarified

hexperimental data not optimum for code validation, many uncertainties

hexecution time per physical time step halved by optimizing Chimera algorithms

hfurther improvement of vectorization and parallelization and use of NEC SX8 will increase execution speed by factor 3

EU-Project TILTAERO

hnumerical and experimental investigation of tilt-rotor configurations

hrequires similar capabilities of flow solver as for BO105

hCalculations performed on NEC SX8

EU-Project GOAHEAD

hproject lead by DLR-AS (Dr. K. Pahlke)

hwind tunnel experiments with generic configuration in order to create a CFD validation database for helicopters

hnumerical flow simulations including

- elastic blade deformation by fluid-structure coupling - coupling with flight mechanics code to compute trim of

helicopter

- a converged solution will require approximately 15 revolutions of the main rotor

⇒

strong need for high performance computers

Other applications for HPC (2)