[Shamoon Jamshed]

I have an ambiguity in Fluent results of dynamic pressure on the wall. Shouldn't it be zero. If I assume that it takes cell centre values, the velocity in the first cell should still be near to zero (very very small) but the value of dyn press. is very high that is around 90,000 that gives velocity of 382 m/s with air as medium. Please give me some suggestion on that. The flow is modelled laminar by the way.

[BlnPhoenix]

Hello,

i have the exact same problem in understanding what Fluent does when it calculates the dynamic pressure on walls.

On the wall velcity = 0 in all directions. Where does the velocity on the wall comes from, so that Fluent calculates a non-zero dynamic pressure, which i assume is a Force/per are acting on the wall.
Can i use the dynamic pressure profile on a wall from Fluent to calculate mechanical load on an area?

[LuckyTran]

When you select a wall on a variable that exists only in the interior (i.e. temperature), Fluent takes the value of the cell adjacent to the wall. Maybe that is what is happening here.

[Shamoon Jamshed]

Originally Posted by BlnPhoenix

Hello,

i have the exact same problem in understanding what Fluent does when it calculates the dynamic pressure on walls.

On the wall velcity = 0 in all directions. Where does the velocity on the wall comes from, so that Fluent calculates a non-zero dynamic pressure, which i assume is a Force/per are acting on the wall.
Can i use the dynamic pressure profile on a wall from Fluent to calculate mechanical load on an area?

I would recommend to use custom field function to define the dynamic pressure and used that variable.

[BlnPhoenix]

Originally Posted by LuckyTran

When you select a wall on a variable that exists only in the interior (i.e. temperature), Fluent takes the value of the cell adjacent to the wall. Maybe that is what is happening here.

Hi, yes! That's what i found elsewhere too. I suspect this is problematic, since the value will depend on how fine the Boundary Layer is meshed on the wall.

From my limited understanding of the matter i would think that the correct pressure to use for Force Calculation would be the static pressure, since total pressure would be equal to the static pressure at the stagnation point (= wall, where velocity is zero). I have to check how fluent calculates the Forces. It only says "Pressure Force" in the manual, but i suspect it's actually the static pressure on a wall...

[ali_b124]

Hi all,

I have exactly same two questions:

1. Dynamic pressure is ρV2/2. We may expect that to be zero at wall. But appearently it is not. I assume it takes the first cell center velocity value, thats why dynamic pressure at wall is non-zero. Am i correct?

2. What should we take for mechanical load? Static pressure is due to the perpendicular force. I guess dynamic pressure is related to the viscous force parallel to the surface.
Static pressure sould be included to the mechanical load in any case. But dynamic pressure should be included if only it affects our moments in mechanical case, but not as pressure values as Viscous Force value with directions. Am i correct?

What is your opinion? Thank you

[Shamoon Jamshed]

Originally Posted by ali_b124

Hi all,

I have exactly same two questions:

1. Dynamic pressure is ρV2/2. We may expect that to be zero at wall. But appearently it is not. I assume it takes the first cell center velocity value, thats why dynamic pressure at wall is non-zero. Am i correct?

2. What should we take for mechanical load? Static pressure is due to the perpendicular force. I guess dynamic pressure is related to the viscous force parallel to the surface.
Static pressure sould be included to the mechanical load in any case. But dynamic pressure should be included if only it affects our moments in mechanical case, but not as pressure values as Viscous Force value with directions. Am i correct?

What is your opinion? Thank you

Ali, this is not for the first time Fluent is doing this. It happens in the computation of heat flux as well, where it takes wall values and compute wrong heat flux. So, you are right, that it is taking cell-centered values if it is giving dynamic pressure on walls. however, you should also set your reference values by clicking compute from and set the desired values.