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small density change for isothermal flows

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Old   April 6, 2013, 10:02
Default small density change for isothermal flows
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Hi All,

When I simulate the isothermal flows, I used the rhoPimeplFoam. I also made some modification on that solver (1) comment the statements for solving the enthalpy equation but retain therm.correct() (2) use Low Ma number assumption, pressure for the EoS is fixed to be 101325 Pa. However, when I use this solver to simulate the isothermal flows, the density shows very small change.

I attached an example: an isothermal air jet, uj=7m/s, temperature is always 294K in the initial field and during the simulation. However, as I said, the density in the domain varies between 1.197 and 1.188. This is the density for air. In Openfoam, the density is updated through rho=p*psi, where psi=1/(R*T). Along this line, it seems that the density change should be from R because in this case p and T are fixed. But I do not know why R is changed since I use pureMixture , which should be also fixed. The complete options for thermalphysical models I used are:
thermoType hPsiThermo<pureMixture<sutherlandTransport<specieT hermo<hConstThermo<perfectGas>>>>>;

Another possible explanation is that this small variation of density is completely caused by numerical error, which should be tolerable.

I am a little confused about this problem. Thank you in advance if someone can give me comments on this.

Thanks.
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Old   April 6, 2013, 10:05
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Attach a contour of density here. Thanks.

Quote:
Originally Posted by hz283 View Post
Hi All,

When I simulate the isothermal flows, I used the rhoPimeplFoam. I also made some modification on that solver (1) comment the statements for solving the enthalpy equation but retain therm.correct() (2) use Low Ma number assumption, pressure for the EoS is fixed to be 101325 Pa. However, when I use this solver to simulate the isothermal flows, the density shows very small change.

I attached an example: an isothermal air jet, uj=7m/s, temperature is always 294K in the initial field and during the simulation. However, as I said, the density in the domain varies between 1.197 and 1.188. This is the density for air. In Openfoam, the density is updated through rho=p*psi, where psi=1/(R*T). Along this line, it seems that the density change should be from R because in this case p and T are fixed. But I do not know why R is changed since I use pureMixture , which should be also fixed. The complete options for thermalphysical models I used are:
thermoType hPsiThermo<pureMixture<sutherlandTransport<specieT hermo<hConstThermo<perfectGas>>>>>;

Another possible explanation is that this small variation of density is completely caused by numerical error, which should be tolerable.

I am a little confused about this problem. Thank you in advance if someone can give me comments on this.

Thanks.
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