[jlamb]

Hi,

I'm trying to set up a pressure loss interface to model a porous material in 2D. I have used the CFX-pre theory book to find the isotropic loss model I would like to use and written it into an expression as CEL as:

dp = -(viscosity/Kperm)*Velocity - Kloss*(density/2)*Velocity^2

I have defined both Kperm and Kloss in expressions with units of [m^2] and [m^-1] respectively. This is where the issue lies. When I put this expression into the 'Pressure Change' box in my interface I get an error 'Expression resolves to invalid units (kg m^-2 s^-1). Expected units ('kg m^-1 s^-2').

I understand where this error is coming from, but cannot figure out what units I should be defining here to fix the error. The units I defined were taken from CFX's own interface (if you initialise a porous domain you can see the permability and resistance loss units), and the expression is taken directly from the theory book and replicated in multiple other sources.

If anyone has any experience with using a Pressure Change interface that would be great.

[jlamb]

I have just manually adjusted the units, just to see if the rest of the setup was right (fully aware what I have changed them to are probably not correct) and a new error message: 'Non-scalar variable 'Velocity' referenced by parameter 'Pressure Change'. Non-scalars are only allowed as arguments to integrated quantity functions'.
This makes me think I'm barking up the wrong tree entirely, any help would be massively appreciated!

[evcelica]

The isotropic loss model you are referring to is a volumetric loss model, meant for use in a porous domain, not across a zero thickness interface. If you look at the units of the equation, they result to dP/dL, not just dP.

[Opaque]

Originally Posted by jlamb

I have just manually adjusted the units, just to see if the rest of the setup was right (fully aware what I have changed them to are probably not correct) and a new error message: 'Non-scalar variable 'Velocity' referenced by parameter 'Pressure Change'. Non-scalars are only allowed as arguments to integrated quantity functions'.
This makes me think I'm barking up the wrong tree entirely, any help would be massively appreciated!

Warning you that your model involves a vector, when it expects a scalar.

Which velocity do you think your "pressure interface model" (once you settled on which one to use) requires?