Copyright © ESI Group, 2012 All rights reserved.Copyright © ESI Group, 2012. All rights reserved....

Copyright © ESI Group, 2012 All rights reserved.Copyright © ESI Group, 2012. All rights reserved.

A study of the Butterfly-Effect for Massively Separated Flows using OpenFOAM®

Dr. Alexander Michalski, SL-Rasch GmbH

Dr. Ries Bouwman, ESI Group

7th OpenFOAM Workshop

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Copyright © ESI Group, 2012 All rights reserved.

ArchitectsSpecialised in special buildings and lightweight structures

SL Rasch

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ESI Worldwide Operations

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Headquarters in Paris, France

Offices in more than 30 countries

Average Headcount: +900 people

Over 200 Scientists

WW Revenue 2010: ~100 M€

Paris, France

Eschborn, Germany Pilsen, Czech Republic

Farmington Hills, MI Beijing, China Tokyo, Japan


Virtual Try-Out Space

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VTOS


Setup - Model

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Setup – Inlet Profile

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A wind-type velocity profile was applied at the inlet, with a law of:

v(z) = 21.1 (z/10)^0.26, in m/s


Setup – Post-Processing

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11 points were monitored:

• 3 points 5 meters upstream of the cube, at +/-20m and 0 in Y, and at 30m in Z.

• 4 points 5m downstream of the cube, at +/-5m and +/-15m in Y, and at 30m in Z.

• 4 points upstream of umbrella at 60m in X, at +/-5m and +/-15m in Y, and at 31m in Z.


Physics:Compressible and incompressible NS solver (transient and steady flow)

Turbulence models: Smagorinsky SGS (LES), K-epsilon, Spalart-Almaras, Baldwin-Lomax

Law of the wall or no-slip walls (with semi-structured boundary layer mesh)

Moving bodies (6-DOF rigid bodies, FSI)Boundary fitted model: ALE grid and automatic remeshing

Embedded mesh

Highly accurate numerical schemes, both in time and spaceSpatial discretisation: Edge-based finite-element and explicit/implicit NS solver, Adaptive mesh refinement

Temporal discretisation: Explicit schemes (compressible flow), Implicit pressure projection explicit scheme (incompressible), Implicit matrix free LU-SGS (compressible and incompressible flow)

DMP and SMP (hybrid) parallel runs.

FSI: MPI coupling with VPS (PAM-CRASH)

Main applications: Aeroacoustic sources characterization, Fluid Structure Interaction

PAM-FLOW

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Same mesh, different number of domains

Original Study PAM-FLOW

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Different meshes


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ConclusionsFor different number of domains, very small differences at beginning of run grow progressively to state of total dissimilitude

Actual (deterministic) instantiation completely different

Mean loads remain similar

Upstream of cube no differences

Butterfly EffectEven very small (roundoff) errors can have a pronounced effect on actual deterministic instantiation of a flowfield

Important if mean flow field is less important than actual (maximum) field


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High quality mesh generated using VisCARTESI proprietary cartesian automatic mesher

snappyHexMesh not sufficient quality for umbrella structure

Simulation performed with OpenFOAM 2.1On RedHat Linux

LESSmagorinsky Subgrid Scale model

1.800 sec real time, time step of 5msecMax Courant number of 2.0

Flow field initialised from a static flow0 m/s in all directions

Simulation was done with 3 SIMPLE partitions in X-Y-Z 8 cores: coefficients 4-2-1

32 cores: coefficients 16-2-1

64 cores: coefficients 32-2-1 in X-Y-Z)

OpenFOAM

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VisCART mesh

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Total Mesh size: 910k cells


OpenFOAM Study

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OpenFOAM Study

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15 s


OpenFOAM Study

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OpenFOAM Study

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Although the instantaneous forces/moments as well as flow pictures are different, the average flow fields are very similar.


Same effect seen with OpenFOAM as with PAM-FLOW

Irrespective of wind data

Irrespective of transient or steady state run

Effect depends on number of domains

Effect depends on memory preconditioner

Reason: inner products of very long vectors that are distributed among domains are required

Order for required additions affects iterative solver

Leads to different instantiations with different number of domains

Conclusion

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FLUENT Study

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Copyright © ESI Group, 2012 All rights reserved.Copyright © ESI Group, 2012. All rights reserved....

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