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-   -   About the pressure coefficients (http://www.cfd-online.com/Forums/phoenics/51615-about-pressure-coefficients.html)

Wong May 30, 2003 14:37

About the pressure coefficients
 
Hi all, For the pressure coefficients

Pressure Coefficient The mass flow through the opening is: M" = C * (Pext - Pint)

when different values are used the CFD computation will definetely different,say,why not use 2 or 3 instead of 1 for the coefficients?

if pressure BC is used for the outlets the mass flow rate is not known beforehand how do you determine this pressure coefficient?when 1 is used what happens then? does that mean the mass=1.0(Pext-Pint) always?

would somebody gives me some details about how to determine the coefficents for pressure boundary conditions?


Guoqing June 11, 2003 10:33

Re: About the pressure coefficients
 
if you go back, you may find several threads on this issue. I don't know how to choose the coeff. But one thing, this coefficient does not affect the mass flow rate. mass=co*(Pext-Pint) is right. however, somehow, the mass is not determined this way. Mass has to follow the mass balance. so a small Co would cause a large Pext-Pint, and vice verse.... You may not see big difference if you change co from 1 to 2. But you mignt see some difference of the airflow in the vincity of outlet if you change it to a large coefficient, see 100. Here is my way to choose this coefficient (hope someone can confirm): if the incell pressure is close to the outlet, use a large coefficient. If there is significant pressure difference, use a small one. I am doing simulation in ventilation. Normally a fan is installed to extract the air from the room. So I use a small coefficient (1.0) assuming there is some pressure difference in the outlet.

hope this helps

Guoqing

Guoqing June 11, 2003 10:34

Re: About the pressure coefficients
 
if you go back, you may find several threads on this issue. I don't know how to choose the coeff. But one thing, this coefficient does not affect the mass flow rate. mass=co*(Pext-Pint) is right. however, somehow, the mass is not determined this way. Mass has to follow the mass balance. so a small Co would cause a large Pext-Pint, and vice verse.... You may not see big difference if you change co from 1 to 2. But you mignt see some difference of the airflow in the vincity of outlet if you change it to a large coefficient, say 100. Here is my way to choose this coefficient (hope someone can confirm): if the incell pressure is close to the outlet, use a large coefficient. If there is significant pressure difference, use a small one. I am doing simulation in ventilation. Normally a fan is installed to extract the air from the room. So I use a small coefficient (1.0) assuming there is some pressure difference in the outlet.

hope this helps

Guoqing

Leon June 11, 2003 16:15

Re: About the pressure coefficients
 
Yeah,Guoqing, I agree with you. Problem does happen when multi pressure outlets are involved. Then these coefficients are important which will dramatically determine the airflow in the domain.(natural ventilation etc.) When pressure is not the concern of the problem the coefficicient can be unimportant. Now I know the coefficients must be characterised based on the flow resistance of the opening,I was told nonlinear coefficients maybe needed then the pressure underelaxation may be large. hope I am right.Some more details are required. Phoenics supports,any more details? Thanks,


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