CFD - What is pressure force?

[BF_10136207]

Let me first try to explain the issue. The below graphs are the pressure forces observed in Creo flow analysis (Figures 1 and 2). When I compare these results to the hand calculations  (Figure 3) I made, there is a stark difference. But when I compare the hand calculations to the reaction force (Figure 4) I see in ANSYS (which uses the pressure distribution from Creo flow analysis), the values have a smaller percent difference. I also have ensured that the unit system is correct (Figure 5). So I guess my question is: Are pressure force and reaction force the same?

[tbraxton]

A free body diagram would go a long way in describing what you are solving for. Describe the geometry the boundary conditions (picture) so that others have an understanding of the problem.

[BF_10136207]

Please let me know if more clarification is needed.

[cfdgeek]

Hello,

We need to know your geometry and boundary conditions/settings. But briefly, How do you compute the pressure force on flat plate by hands?  your equation just computes the dynamic pressure head times the area, which is totally wrong. There is no way you can compute a flat plate in a turbulent flow by hands. For laminar boundary layer flow over a plate, there is Blasius solution which is a fitting to the experiments. 

Anyway, in CFA, you can select all boundaries and  combine the pressure force from them to compute the total force applied directly and plot it out. 

How do you export CFA pressure distribution to ansys? I don't think I implement that output to couple with Ansys.  

For Ansys products, we don't know for sure what they are doing there. 

Thanks.

[BF_10136207]

Hi there,

The equation I used is just for a force caused by a fluid jet onto a plate which is found in any basic fluid dynamics textbook. So I suppose my understanding is limited by the definition of pressure force. I looked up the definition according to PTCs user manual. Is there some literature that I could reference?

The Pressure distribution can be exported under output options (See figure) to a text file. This point cloud can then be imported into Ansys.

 

[cfdgeek]

Hi,

I see what you trying to compute. So the plate is not fully immersed in the liquids? Please share your creo flow settings.

You need to setup your CFD simulation matching the same assumption of the equation you are trying to apply. Probably you are trying to simulate the following 

So there should be no flow after the plate and for the jet. In your hand calculation, you still have back pressure on the back of the plate which means you set your plate immersed in the flow. also you need to apply correct boundary conditions around it, you may need multiphase to simulate this, which is a premium package of creo flow.