Calculating Loss Coefficients for a porous media

CFX_Beginner · October 22, 2015, 19:22

Hello,

I need help on calculating the right Loss Coefficient for a porous media.

In the grafic down below you can see the setup and the equation that's causing the pressure loss in the porous media.

I prescribe dp/dx and with the help of the equation I want to calculate the right Loss Coefficient, so the porous media is performing exactly this dp/dx.

I get all the other variables I need to solve the equation from CFD Post, loading a finished simulation with the same parameters as the one I want to do except for the Loss Coefficient and I set the Permeability (Kperm = 1 m²).

The problem I have is that when I calculate the Loss Coefficient with that equation and run the simulation, it's not achieving the dp/dx that I calculated the Loss Coefficient for.
That means I'm making some small mistake during the calculation process...

For example:
The Loss Coefficient I calculated is 250.
The correct one that achieves the dp/dx I prescribed is 540, which I found out by manual testing.

Taking into account that the range of the result is nearly the same, I thought that the mistake is that I didn't take the right value for some variable in the equation.
Sadly there is no documentation from Ansys Help on where those variables are taken exactly.

At the moment I get the Density, Dynamic Viscosity, Velocity Component Ux and the Velocity |U| from a Volume Average of the porous media.

I hope i explained my problem properly and some of you are able to help me.

Opaque · October 23, 2015, 19:42

Not sure what wall boundary conditions you are applying, but I would use Free Slip on the walls so you can get a 1-D porous media model that matches the equation you are showing in a particular x_i

Hope the above helps,

CFX_Beginner · October 24, 2015, 21:16

Just tested it with Free Slip Wall, but the overall problem is still the same. The pressure loss in the simulation is not the same as the one in the calculation.

I'm sure I use the right variables for the equation so there must be a mistake in the overall understanding of the equation or the "transport" from the equation into the simulation.

October 22, 2015, 19:22	Calculating Loss Coefficients for a porous media	#1
CFX_Beginner New Member Phil Join Date: Jul 2015 Posts: 19 Rep Power: 11	Hello, I need help on calculating the right Loss Coefficient for a porous media. In the grafic down below you can see the setup and the equation that's causing the pressure loss in the porous media. I prescribe dp/dx and with the help of the equation I want to calculate the right Loss Coefficient, so the porous media is performing exactly this dp/dx. I get all the other variables I need to solve the equation from CFD Post, loading a finished simulation with the same parameters as the one I want to do except for the Loss Coefficient and I set the Permeability (Kperm = 1 m²). The problem I have is that when I calculate the Loss Coefficient with that equation and run the simulation, it's not achieving the dp/dx that I calculated the Loss Coefficient for. That means I'm making some small mistake during the calculation process... For example: The Loss Coefficient I calculated is 250. The correct one that achieves the dp/dx I prescribed is 540, which I found out by manual testing. Taking into account that the range of the result is nearly the same, I thought that the mistake is that I didn't take the right value for some variable in the equation. Sadly there is no documentation from Ansys Help on where those variables are taken exactly. At the moment I get the Density, Dynamic Viscosity, Velocity Component Ux and the Velocity \|U\| from a Volume Average of the porous media. I hope i explained my problem properly and some of you are able to help me.

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October 23, 2015, 19:42		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	Not sure what wall boundary conditions you are applying, but I would use Free Slip on the walls so you can get a 1-D porous media model that matches the equation you are showing in a particular x_i Hope the above helps,

October 24, 2015, 21:16		#3
CFX_Beginner New Member Phil Join Date: Jul 2015 Posts: 19 Rep Power: 11	Just tested it with Free Slip Wall, but the overall problem is still the same. The pressure loss in the simulation is not the same as the one in the calculation. I'm sure I use the right variables for the equation so there must be a mistake in the overall understanding of the equation or the "transport" from the equation into the simulation.