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some basic questions about BCs and pressure

Posted 14 mag 2013, 04:24 GMT-4 Version 4.3a 1 Reply

Leilei Hu
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Dear sir or madam,

I am making models about laminar flow and thin-film flow. I saw the velocity boundary condition
in thin-film flow is called "normal inflow velocity". What is the difference between "normal inflow velocity"
in thin-film flow physics and the "velocity" in laminar flow physics?

By the way, in model inputs, I saw two options to set the absolute pressure: user defined PA and PA=P+Pref.
Can I get the same results if I set PA =1[atm] in the first option and Pref=1[atm] in the second one?

Thank you very much.
Best wishes
1 Reply Last Post 7 nov 2014, 13:24 GMT-5
COMSOL Moderator

Hello Leilei Hu

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DW
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Posted: 1 decade ago 7 nov 2014, 13:24 GMT-5
Regarding the setting for "Absolute pressure" (e.g. under "Fluid Properties" node):
(1) To model an incompressible fluid, set "Absolute pressure," pA, to "User defined" and enter the desired pressure value in the edit field. The default value is 1[atm]. In this case, any pressure-dependent material properties in the model will be evaluated using pA = user-defined value.
(2) To model flow with small pressure change, the variable "p" in the flow calculation will represent the gauge pressure. Set "Absolute pressure" to "Pressure" and enable check box "Reference pressure". Absolute pressure, equal to pA=p + p_ref, will be used to calculate pressure-dependent material properties.
(3) If you want pressure "p" in flow calculation to represent absolute pressure, then clear the check box "Reference pressure." In this case, absolute pressure will be equal to pA = p, and again pA will be used to calculate pressure-dependent material properties.
-DW
Regarding the setting for "Absolute pressure" (e.g. under "Fluid Properties" node): (1) To model an incompressible fluid, set "Absolute pressure," pA, to "User defined" and enter the desired pressure value in the edit field. The default value is 1[atm]. In this case, any pressure-dependent material properties in the model will be evaluated using pA = user-defined value. (2) To model flow with small pressure change, the variable "p" in the flow calculation will represent the gauge pressure. Set "Absolute pressure" to "Pressure" and enable check box "Reference pressure". Absolute pressure, equal to pA=p + p_ref, will be used to calculate pressure-dependent material properties. (3) If you want pressure "p" in flow calculation to represent absolute pressure, then clear the check box "Reference pressure." In this case, absolute pressure will be equal to pA = p, and again pA will be used to calculate pressure-dependent material properties. -DW

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