# SonicFoam - Forces, viscous force too big?

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 September 15, 2012, 12:27 SonicFoam - Forces, viscous force too big? #1 New Member   Join Date: Sep 2012 Posts: 8 Rep Power: 4 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>>>>; 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.

 September 15, 2012, 14:33 #2 New Member   Join Date: Sep 2012 Posts: 8 Rep Power: 4 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?

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