CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

SonicFoam - Forces, viscous force too big?

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   September 15, 2012, 12:27
Default SonicFoam - Forces, viscous force too big?
  #1
New Member
 
Join Date: Sep 2012
Posts: 8
Rep Power: 4
Kryo is on a distinguished road
Im just running a simulation of a rocket flying with Ma=1.7 through air. I use sonicFoam for this, and everything looks quite well when i view it with paraview. (compression, expansion and so on).

But then I try to calculate forces and coefficients and imo I get too high drag forces. Drag is directed to the bottom, therefore negative Z-axis is the drag.
Have a look at the output:

Code:
forces output:
    forces(pressure, viscous)((9.821624 11.33696 -398.8834) (0.6874167 -0.04157073 -1979.892))
    moment(pressure, viscous)((8.025379 -7.056551 0.2301483) (-0.1572154 -0.5736852 -0.04660123))

forceCoeffs output:
    Cd = 1.790804
    Cl = 0.008503463
    Cm = 0.003948905
    Cl(f) = 0.0003028264
    Cl(r) = 0.008200637
I expected the Cd value to be between 0.5 and 1 but not that high! Im not sure yet, but is it possible that the viscous force is too big? its 5x bigger than the pressure force.

The following settings might be connected to this, but as far as I know they are set correctly for my case :/

Code:
 forces
    {
        type            forces;
        functionObjectLibs ( "libforces.so" );
        outputControl   timeStep;
        outputInterval  1;

        patches
        (
            rocketbody
        );

        pName       p;
        UName       U;
        rhoName     rho;
        log         true;
        rhoInf      1.16;
        CofR        ( 0 0 0 );
        liftDir     ( 0 1 0 );
        dragDir     ( 0 0 -.1 );
        pitchAxis   ( 1 0 0 );
        magUInf     600;
        lRef        1.500;
        Aref        0.00636172512351933130788685285114;
    }
forceCoeffs
    {
        type            forceCoeffs;
        functionObjectLibs ( "libforces.so" );
        outputControl   timeStep;
        outputInterval  1;

        patches
        (
            rocketbody
        );

        pName       p;
        UName       U;
        rhoName     rho;
        log         true;
        rhoInf      1.16;
        CofR        ( 0 0 0 );
        liftDir     ( 0 1 0 );
        dragDir     ( 0 0 -1 );
        pitchAxis   ( 1 0 0 );
        magUInf     600;
        lRef        1.500;
        Aref        0.00636172512351933130788685285114;
this is in my controldict. As far as I see it, its all correct :O


thermophysicalProperties:
Code:
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "constant";
    object      thermophysicalProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

thermoType      ePsiThermo<pureMixture<constTransport<specieThermo<eConstThermo<perfectGas>>>>>;

mixture
{
    specie
    {
        nMoles          1;
        molWeight       28.9;
    }
    thermodynamics
    {
        Cv              717.5;
        Hf              0;
    }
    transport
    {
        mu              14.74e-06; mu=eta/rho ; eta= 17.1e-6, rho=1.16
        Pr              0.7;
    }
}
can anyone find a mistake? or does somebody know of another setting that might interfer here?

EDIT: Ok I found out, viscous force is bigger than the pressure force for rockets, but still, its far too high in total. What could be connected to: I am using LaunderSharmaKE as RAS turbulence model.

Last edited by Kryo; September 15, 2012 at 13:02.
Kryo is offline   Reply With Quote

Old   September 15, 2012, 14:33
Default
  #2
New Member
 
Join Date: Sep 2012
Posts: 8
Rep Power: 4
Kryo is on a distinguished road
sorry for double post, but I found the reason for the high values: Random k-epsilon- boundaries for the turbulence model...

Any idea what to use here? are there any formulas to calculate the correct k and epsilon boundaries?
Kryo is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
ActuatorDiskExplicitForce in OF2.1. Help be_inspired OpenFOAM Programming & Development 8 July 3, 2014 10:54
Force can not converge colopolo CFX 13 October 4, 2011 22:03
Viscous force and coefficient viscous force tobino Main CFD Forum 0 February 20, 2011 22:06
how does Fluent calculate the pressure and viscous force vector? Ralf Schmidt FLUENT 2 May 17, 2010 15:49
pressure force and viscous force CFD FLUENT 0 March 7, 2006 02:03


All times are GMT -4. The time now is 10:14.