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 Wong May 30, 2003 14:37

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

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

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