# Pressure parameter in paraview

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 April 17, 2014, 02:51 Pressure parameter in paraview #1 New Member   Sachchit Vekaria Join Date: Dec 2013 Posts: 18 Rep Power: 4 In SU2 v3.0.0 in the paraview output file how the parameter 'pressure' has been calculated? When I calculate pressure from pressure coefficient it is coming out different than the parameter 'pressure'. Is it p/(rho*U^2) or something else?

April 18, 2014, 09:28
#2
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Francisco Palacios
Join Date: Jan 2013
Location: Long Beach, CA
Posts: 342
Rep Power: 6
Quote:
 Originally Posted by Sachchit In SU2 v3.0.0 in the paraview output file how the parameter 'pressure' has been calculated? When I calculate pressure from pressure coefficient it is coming out different than the parameter 'pressure'. Is it p/(rho*U^2) or something else?

Formula to link pressure and Cp in surface_flow.csv?

for more details,

Best,
Francisco

April 19, 2014, 14:25
Still some problem
#3
New Member

Sachchit Vekaria
Join Date: Dec 2013
Posts: 18
Rep Power: 4
Hi,

I have gone through the file but the formula seems to be the normal formula of Cp but the problem can be seen in the Images. Pressure is varying from 0.155 to 0.536 and Cp is varying from -1.23 to 1.29. Reference pressure is 101325 Pa. So ideally Cp should be -ve by that formula. By the way Cp is coming out accurate, verified by other other software suites. But I can't understand the value of pressure.
The formula of Pressure in that file is
Pressure = Gamma_Minus_One*Density*(node[iPoint]->GetSolution(nDim+1)/Density-0.5*Velocity2);
Can you please explain this formula?

Attached Images
 cp.jpg (17.2 KB, 16 views) pressure.jpg (16.9 KB, 16 views)

 April 20, 2014, 08:42 #4 New Member   Join Date: Mar 2009 Posts: 13 Rep Power: 9 Sachchit, i don't understand your problem. The formula in your post is simply equation of state for perfect gas: Pressure = (Gamma - 1)*Denstity*Internal_Energy. node[iPoint]->GetSolution(nDim+1) is the last conservative variable, which equals to Density*Total_Energy, Total_Energy = Internal_Energy - 0.5*Velocity*Velocity, where Velocity is velocity magnitude (in your formula Velocity2 is squared velocity magnitude). So, node[iPoint]->GetSolution(nDim+1)/Density is total energy in specified point and (node[iPoint]->GetSolution(nDim+1)/Density-0.5*Velocity2) is the internal energy.

 April 20, 2014, 09:04 #5 New Member   Sachchit Vekaria Join Date: Dec 2013 Posts: 18 Rep Power: 4 Hi Zef, Thanks for explaining formula. I am confused with Velocity2 being y-component of velocity. I have found out later that Velocity2 is square of velocity. But can you please explain the images that I have attached earlier? I am not able to understand the numbers which are coming up in Pressure. The Coefficient of Pressure is coming accurate and pressure calculated from that coefficient is also giving good results. But the problem lies in the Pressure shown by default from the SU2. I just want to know what it is? Thanks Zef.

April 20, 2014, 19:08
#6
Super Moderator

Francisco Palacios
Join Date: Jan 2013
Location: Long Beach, CA
Posts: 342
Rep Power: 6
Quote:
 Originally Posted by Sachchit Hi Zef, Thanks for explaining formula. I am confused with Velocity2 being y-component of velocity. I have found out later that Velocity2 is square of velocity. But can you please explain the images that I have attached earlier? I am not able to understand the numbers which are coming up in Pressure. The Coefficient of Pressure is coming accurate and pressure calculated from that coefficient is also giving good results. But the problem lies in the Pressure shown by default from the SU2. I just want to know what it is? Thanks Zef.
I'm not sure if this is related (I need to check the config file) but, remember that SU2 can run dimensional or non-dimensional
for example if
REF_PRESSURE= 1.0
REF_TEMPERATURE= 1.0
REF_DENSITY= 1.0
then the code is running dimensional. You will find a description of the non-dimensionalization in http://su2.stanford.edu/documents/SU2_AIAA_ASM2013.pdf

Apart from that, the non-dimensionalization is implemented in
void CConfig::SetNondimensionalization(unsigned short val_nDim, unsigned short val_iZone)

in config_structure.cpp.

Best Regards,
Francisco

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