CFD Online Discussion Forums

CFD Online Discussion Forums (
-   OpenFOAM Post-Processing (
-   -   why does 'sample' do this? wallShearStress question (

CHARLES August 7, 2013 19:30

why does 'sample' do this? wallShearStress question
1 Attachment(s)
Hello all,

I've been trying to use the 'sample' utility in OpenFOAM with hopes to get field variables at the mesh vertices (flat plate on a wall resolved mesh). For some reason 'sample' is not outputting the field values at the points corresponding to node locations...
My domain is 1m tall. If I define the 'end' of the line in the y-direction to be at 1m, the y values I get in the output file start from 0 and the next value is some value much greater than the first node point (located at y=5e-7). However, if I define the 'end' as 0.25m (as I did below) 'sample' gives the y coordinates corresponding to the node values. :confused:

I am using the finest mesh (545X385) obtained from NASA's website:

About wallShearStress problem:
I'm getting values that are almost half of what they should be when I plot the friction coefficient, Cf, as a function of x (distance along the plate), where Cf =tau_wall/(0.5*U_inf ^2 * rho). To make the plot, I am using the data from the wallShearStress output file obtained via 'sample'.

I understand that in OpenFOAM wallShearStress calculates tau_wall/rho but in this case rho=1... so wallShearStress should give me tau_wall.

What is interesting is that when I calculate Cf from the wallShearStress folder obtained via 'wallShearStress' command, the values appear as they are supposed to for the wall resolved mesh. When I look at a coarse mesh that isn't wall resolved, the plot results (obtained via 'sample') for Cf are what they are supposed to be... I've double checked the plotting commands and they're correct. The actual output file (from 'sample') for the wall resolved mesh shows tau_wall=3.XXX but the wallShearStress folder shows tau_wall=6.XXX... so the data that OpenFOAM is outputting via 'sample' is different from the one it has calculated via 'wallShearStress' command... why?!

The plot I've attached shows:
Unresolved='sample' results for coarse mesh
Calculated=results from wallShearStress file divided by 0.5*Uinf^2*rho for resolved mesh
Sampled='sample' results for resolved mesh

Thanks for taking the time to look through this!


/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  2.1.x                                |
|  \\  /    A nd          | Web:                      |
|    \\/    M anipulation  |                                                |
    version        2.0;
    format          ascii;
    class          dictionary;
    location        "system";
    object          sampleDict;

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

interpolationScheme cellPointFace;

setFormat raw;

        type            face;
        axis            y;
        start          (0.97 0.0 0.0);
        end            (0.97 0.25 0.0);
        type            face;
        axis            x;
        start          (0.0 0.0 0.0);
        end            (2.0 0.0 0.0);


// ************************************************************************* //

All times are GMT -4. The time now is 19:53.