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-   -   Dynamics pressure versus velocity (http://www.cfd-online.com/Forums/fluent/109384-dynamics-pressure-versus-velocity.html)

Shamoon Jamshed November 16, 2012 06:36

Dynamics pressure versus velocity
 
I have a problem in modelling flow over a sphere. The wall has no slip boundary condition. This means that velocity is zero. While this is shown in Fluent as zero the dynamics pressure is still giving certain value on the wall. Dynamic pressure is 0.5 rho*v*v. If v is zero on the wall why FLUENT is showing dynamics pressure on the wall. I checked both by plotting node values and cell centre values.

LuckyTran November 16, 2012 21:16

You are obviously correct in your interpretation. Dynamic pressure at the wall must be zero.

The reason you are getting a non-zero dynamic pressure at the wall has to do with the fact that Fluent employs a finite-volume method and data are stored at cell centers (cell centers are also sometimes called nodes, but fluent does not use this terminology).

When you ask Fluent to calculate variables at a boundary face or wall (or node values) it must interpolate/extrapolate values from the cell centers to these locations (using the spatial discretization scheme that you specified). This interpolation process allows for non-physical events to occur (resulting in non-zero dynamic pressure at the wall). This is an inherent weakness of finite-volume methods.

This does not necessarily mean that your results are garbage. Even if your solution is perfect, it can still give rise to these results. Again, it is a result of the finite-volume representation. Any other software that uses the finite-volume method can also exhibit this effect. Using different spatial discretization schemes will also change the results slightly (1st order, 2nd order, or higher order schemes); but regardless of how accurate the spatial discretization scheme is, it will not guarantee that your dynamic pressure will be 0 at the wall.

Philipov November 17, 2012 08:38

Excellent explanation and give reasons for accurate results interpretations. Just to add something. If is good to see forces on a given surface on which dynamic pressure will give effect on. This I think can help understanding the positive and negative consequences from FVM.


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