dimanche 21 octobre 2012

Fun with OpenFOAM and turbulence models

Creation of these movies was a part of sudden teaching assistance duties. In the beginning the idea was to just reproduce Kármán vortex street. And then it evolved to the comparison of vortex street simulation results with different turbulence models. Case files for the LES simulations can be found at https://bitbucket.org/mrklein/openfoam-ravings.

Channels and meshes

We have simple straight channel with two bodies of different shape put inside the channels. Simple schemes of the channels are shown on the figures below.


"Circle" channel scheme

"Rectangle" channel scheme

At the inlet fluid velocity changes linearly from 0 to 1 m/s during time interval 0-5 s and then it stays constant for the rest of the simulation time (20 s). So the flow inside the channels goes from laminar to turbulent as Reynolds number grows.

For all cases rather simple uniform mesh was used. Surely it was possible to use mesh grading to reduce computation time but cases were 2D and I've decided not to mess with all these gradings. Meshes are shown on figures below.

Mesh for the "circle" channel

Mesh for the "rectangle" channel

Models and simulation results

Two different approaches were selected for description of turbulence - RANS and LES. In case of RANS different variations of k-epsilon model were used: RNG k-epsilon and realizable k-epsilon. Kinetic energy at the inlet was estimated from turbulence intensity using standard 1.5(uI)^2 and dissipation rate was calculated from kinetic energy value and channel characteristic length (7% of Dh). For LES smooth delta with Smagorinsky sub-grid viscosity model were used.

Flow around rectangle

LES velocity movie
LES vorticity movie
RANS (realizable k-epsilon) velocity movie
RANS (realizable k-epsilon) vorticity movie
RANS (RNG k-epsilon) velocity movie. In this case there's no "vortex street".

Flow around circle

LES velocity movie
LES vorticity movie
RANS (realizable k-epsilon) velocity movie
RANS (realizable k-epsilon) vorticity movie
RANS (RNG k-epsilon) velocity movie. In this case we have "vortex street".
RANS (RNG k-epsilon) vorticity movie

4 commentaires:

  1. Could you please upload case files for RAS turbulent model cases also ? thanks a ton for this post...

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    1. Unfortunately I don't have it but basically you need to create constant/RASProperties file, put there the name of the model you'd like to use, create additional files in 0 folder to correspond to the RAS model, and modify fvSchemes and fvSolution to include variables from RAS model (k, epsilon/omega).

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