Tip Surface elevation in time
1 Introduction
Working with free surface flow, one might want to track a point at the interface in time, possibly as some kind of buoy.
A common method to achieve this (e.g. in interFoam), is to sample the value of alpha1 along a line (x,y,z(q)) and averaging z(q) based on the sampled value alpha1(z(q)) (with q some space parameter). Approaches following this procedure can be found at:
- http://www.cfd-online.com/Forums/openfoam-post-processing/64462-how-monitor-free-surface-elevation-vs-time.html
- http://www.cfd-online.com/Forums/openfoam/64888-help-modified-probe-file-wave-elevation-vs-time.html
- http://www.cfd-online.com/Forums/openfoam/104673-how-polt-surface-elevation-over-time-fixed-point.html#post371193
This wiki-page will describe a different method I used, based on first sampling the iso-surface of alpha1=0.5 (needed anyhow) and subsequentlly sampling points from this iso-surface using Python.
2 Iso-surface
Sampling the iso-surface of alpha1 is straight-forward to perform during runtime. It can be obtained by putting the following sampling function at the end of the controlDict of case.
functions ( freeSurface { type surfaces; functionObjectLibs ( "libsampling.so" ); outputControl outputTime; outputInterval 1; surfaceFormat vtk; fields ( alpha1 ); surfaces ( freeSurface { type isoSurfaceCell; isoField alpha1; isoValue 0.5; interpolate false; regularise false; } ); interpolationScheme cell; } );
3 Probing the buoy
Use the file below to sample coordinates from the stored iso-surfaces File:ElevationVsTime.tar.gz
Some notes to use this python script
- Python and Numpy should be properly installed
- The script assumes the vtk-files to be in freeSurface/<time-step>/
- The script demands a freeSurface/0/ directory to read an initial (flat) surface
- Deviations from this initial surface should not be too large
- Gravity is assumed to act in the y-direction
- A file 'coords' with two columns (x, z) is expected as probing coordinates
4 Disclaimer
This is probably a disk-space demanding operation if you don't necessarily need to sample the alpha1=0.5 isosurface. Feel free to implement this in the FOAM framework that operates during run-time. This code can easily be used to calculate a time-average free surface.