## Fluids

#### How do I get the pressure force vector which is normal to the surface?

• sathesj
Subscriber

Hello,

I am simulating an airflow in the wavy channel. I am trying to find the pressure (from) drag along the wall. As seen from the image attached, I am interested in finding Fp ( pressure force along the wall) Which is normal to the wave surface. Further, I want to decompose this force and get its x-component(Fpx).

I am doing a 3D simulation in Ansys fluent. How do I get this pressure force, which is a normal pressure force vector to the wavy surface? Is there a way to get this directly in the fluent? Can someone help me with this?

Shubham

• Danica
Ansys Employee

Hi, You can use a force report to investigate the pressure and pressure coefficients for a specified vector.

Have a look at: 36.5. Forces on Boundaries (ansys.com)

However, these are for a whole surface not a particular facet

• sathesj
Subscriber

Thank you so much for the response. Maybe I framed the question wrong. I am looking to get pressure force at multiple locations on the wavy wall. I have created an iso-surface; at half the height of the wavy wall. I need to plot the scalar value of the pressure force along this iso-surface.

So far, I have tried to plot this by using : Pressure* (x-facet-area/((x-facet-a ^2)+(y-facet-a^2)^0.5)). Further, I decomposed this to get the x component.  The profile of this plot is similar to the one I am comparing it with, but the scale is way off. The excel graph in the image attached is my plot and I am comparing it with the dark black line in the other plot.

• Danica
Ansys Employee

The profile looks very close, is the experimental data definitely pressure force or just pressure? Also might be worth looking at your mesh for issues and double checking boundary conditions.

• sathesj
Subscriber

Thank you for the response. I have validated the model and all results match the results that I am comparing with, except this plot. So, I am pretty confident that my boundary conditions and mesh are correct. Also, this is computational data, the person used Ansys 17 to get the results. It's the pressure force, as I need its x-component and pressure being a scalar quantity, it cannot be decomposed.

Is there any other way to get the pressure force along the wave wall? what am I doing wrong?

• Rob
Ansys Employee

I assume by Ansys 17 you mean Fluent? Otherwise check you're comparing like with like.

• sathesj
Subscriber

Thank you for the response. Yes, I meant Ansys Fluent 17.

• Rob
Ansys Employee

If it's the same model, with the same post processing I'd expect about the same answer. Are you sure you're creating the same report?

• sathesj
Subscriber

I am trying to validate the plot. I am not sure how the original graph was plotted. I am sure the model is the same. I managed to get the profile by: Pressure* (x-facet-area/((x-facet-a ^2)+(y-facet-a^2)^0.5)). Is there any other way to get the pressure force? Also, is this the approach right?

• DrAmine
Ansys Employee

You have the face area vector at the wall then it is easy to create a custom vector for the pressure vector pointing out of the wall :)

• sathesj
Subscriber

Hello,

Thank you so much for the response. How do I get this face area vector? Do I have to create a UDF for it? Or is there any other way to get it? Once, I get this area vector, multiplying it with pressure will give me the pressure force right?

Also, because we are multiplying pressure and facet area to get the pressure force, won't this pressure force be mesh-dependent? Looking forward to hearing from you.

Thanks,

Shubham

• DrAmine
Ansys Employee

The face area vector components are available on externa boundaries for post-processing: you do just need to use them.

The force will for sure depend on your discreization of the are but I assume you have done a mesh sensitvity and the wall forces are not variying anymore (a smaller area will have have a smaller pressure force contribution compared to a larger "meshed" area).