Karman Vortex Street

cavity_flow_image.png

This tutorial will cover the basics of setting up and running the fluid problem of the flow past a sphere to generate a Karman vortex street. Files for running the case yourself can be found here: LOCI-STREAM RUN FILES.

The Karman vortex street is the laminar oscillation of vortices that is shown in the animation above. It is a fluid instability problem for viscous flows. The Strouhal number is a non-dimensional number that is used in when considering vortex induced vibrations. A Strouhal number of 0.2 corresponds to the formation of the steady shedding of vortices from the cylinder. This example sets up a flow condition such that the Strouhal number is around 0.2.

The flow is incompressible, viscous, and bounded within a sealed 2D box. A steady uniform flow moves across the cylinder. 


ComputatIOnal Domain

 
Simulation_domain.png

The domain is a rectangular with a length of of 200 cm and a width of 100cm. The grid is a 3D extrusion of a 2D mesh with a single cell in the extruded direction. 


To see the Loci-Stream control file click HERE.


Running the Code

Loci-Stream is an MPI based parallel code, so execution of the code is started with a call to mpirun. A template call is shown below where the terms in brackets(<>) are replaced by the specifics of your case.
mpirun -np <np> <path_to_stream_exec> --scheduleoutput -q solution <casename> <restart> > /dev/null >run.out 2>&1 &

For this example the call is:
mpirun -np 4 /home/user/Loci-Stream/bin/stream --scheduleoutput -q solution karman  > /dev/null >run.out 2>&1 &


Results

The 'extract' utility is used to view the results of a simulation. The simulation is viewed at the 500th time step. In the run directory extract the solution using: extract -vtk karman 500 P v . This will generate a solution directory with files in the Paraview VTK format which can be opened by Paraview. The case name is 'karman' and the timestep is '500', and the variables to extract are the pressure(P) and velocity(v). Below is a contour plot of the vorticity of the flow that was created using Paraview.

For users of Paraview who may want to reproduce this image, just apply the 'GradientOfUnstructuredDataSet' filter (Filter->GradientOfUnstructuredDataSet), enable the advanced options on the filter (click the little gear icon at the top right, and check the 'compute vorticity' box.