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 George March 18, 2005 21:23

pressure correction and absolure pressure

Hello to all! In my code I use the pressure correction method to solve a compressible unsteady flow.At the inflow the velocity is prescribed.When I correct the pressure: P(i,j,k)=P(i,j,k)+a*P'(i,j,k) , I also correct the density: Density(i,j,k)=Density(i,j,k)+a*P(i,j,k)/R/T(i,j,k). But when I compute the density from the equation of state (density=Pressure/R/Ttemperature) I have totally different values.From the first method the density is about 1.5 kg/m^3 and from the equation of state is about 0.8 kg/m^3.Can you please tell me a safe way to compute density and the absolute pressure? Thank you in advance!!!

 versi March 19, 2005 05:18

Re: pressure correction and absolure pressure

For compressible flow, the independent primitive variables are (rho,u,v,w,E), or (p,u.v,w,h). You can not use density=density +a*P/R/T if you already use (p,u,v,w,h) as independent variables.

 George March 20, 2005 21:03

Re: pressure correction and absolure pressure

I use the set (p,u,v,w,h).But how can I compute the density then.From the equation of state I find one value and from: density=density +a*P/R/T one other.I thought that these vakues should be the same.One more question:In incompressible flow,the pressure is corrected with this formula: P=P+a*(P'-P'reference).This should happen also in the compressible flow? Thank you

 versi March 21, 2005 05:05

Re: pressure correction and absolure pressure

with (p,u,v,w,h), you have rho=EOS(p,h), but use density=density +a*P/R/T to update density is nonsense now. If your really wishes to update density by incremetal formula, then differentiate d rho= d EOS(p,h), you will find numerical difference between rho=EOS(p,h) and rho=rho+d EOS. In compressible flow, you need not use a P_ref. In incompressible flow, since it is gradient p not p that matters, adding any reference value to pressure does not matter. This value can be zero.

 George March 21, 2005 20:04

Re: pressure correction and absolure pressure

Hello again. Thank you for your response.My question is that I have read in many papers, that the proper way to correct density is with the formula: density=density+a*P/R/T. If this is not the proper,can you please tell me how to implement this. One more question.I prescribe the velocity in inflow.Can I also prescribe pressure,or is this too much? Thank you