[kkpal]

Dear all,
In order to calculate the instantaneous total force, which is composed of pressure force and shear force on the surface of the body, I need to obtain the pressure(p) and friction on the surface. The former could be easily done by including a surface probe function in the controlDict. However, there seems to be no such function for obtaining the friction distribution at runtime. Only some post-processing tools like wallShearStress, wallGradU are available. Could someone give me some hints about how to obtain the runtime wallGradU ?

best,

[tomf]

Hi,

Is there any particular reason you do not use the forces functionObject? It does calculate those forces instantaneously for you.

Regards,
Tom

[kkpal]

Hi, in order to use the functionObject, I need to write down all the files at time interval that is small enough and do the postProcessing with these written files. In the case of large mesh, it would be prohibitive because writing large amount of data to disk is time and disk space consuming. That's why I need to use a runtime utility of wallGradU.
With the extensibility of OpenFOAM I'm sure one could do some programming to achieve this... Could someone give me some hints on this?

Originally Posted by tomf

Hi,

Is there any particular reason you do not use the forces functionObject? It does calculate those forces instantaneously for you.

Regards,
Tom

[tomf]

The functionObject is executed at runTime, that is the whole point of the functionObjects.

Just use it like this (in the functions part of your controlDict):

    
    forces_Total
    {
        type forces;
        functionObjectLibs ("libforces.so");
        patches
        (
           <yourPatchNames(s)>
        );
        rhoName        rho;//rhoInf; (rhoInf for incompressible)
        pName           p;
        rhoInf             1.225;
        CofR              (0 0 0);
        outputControl  timeStep; //This says to execute every timeStep!
        outputInterval  1;
        log                 false;
    }

This will give you a contiuously updated file with the pressure viscous (and porous) forces.

Regards,
Tom

[kkpal]

Hi, Tomf,
I made some mistakes in the previous post, I was a little dizzy when I was writing that yesterday

I now realize that the functionObject is the thing I needed to output the total or averaged viscous force on the wall. Thank you.

However, the problem I'm facing is one step further. I need to calculate the sectional viscous force on a circular cylinder, since this cylinder is 3 dimensional(waviness or taper in the span-wise direction). The available functionObject will only give the mean viscous force of the whole cylinder.

This is the question that is haunting me. Sorry for the vague illustration in the previous posts.

[tomf]

Well, there is actually the "bin" functionality, which gives "sectional" output, just add it to the functionObject description.

This was released in version 2.2:
http://www.openfoam.org/version2.2.0...processing.php

    binData
    {
        nBin        20;
        direction   (1 0 0);
        cumulative  yes;
    }

You may want to google for some examples/more descriptions.

[kkpal]

Wow, that's exciting! I will try that out!
thank you, Tom