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SG Force / Normal Force in SW FlowSimulation

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Old   April 14, 2020, 05:14
Default SG Force / Normal Force in SW FlowSimulation
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Hello everyone!

I am Philipp and I write my masterthesis about a cfd topic.

I use SW FlowSim 2019 SP5 and I have an internal flow simulation, time-independent.

To get to the point:
I have a 4mm round channel, 50mm long which Ends in the envoirement (1bar). At the beginning there is a pressure Opening (4bar). I want to know the (normal) force on my outlet Surface (lid) in a way you can see the pressure in a cutplot or a Surface plot. the lid is set with the boundary condition "envoiremental pressure".

Therefor I created a SG Force. Here I selected both sides of the lid.
The solver Shows me invalid SG.. why? the Computational Domain includes the outer Surface..

But: When I create a flat plate directly in front of the lid an also create a SG Force with both sides of this plate I get a value for the force and no warning.

In generell: How is it possible to see the force at any Point in the fluid volume (virtual Surface?)?

Could anyone explain me how to handle this Problem? Does it makes sense to insert an equation Goal?

To see the fields of different forces over the complete outlet Surface like you can see the pressure in a Surface lot would be perfect.

Best regards,
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Old   April 15, 2020, 09:16
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Boris Marovic
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There are multiple errors in your approach.

1. It is an opening and there is no force where there is no body to press against.

2. no matter how large you make the computational domain, if it is an internal flow problem, then there is no calculation happening on the outside and therefore the surface you selected on the outside is in no contact with the fluid.
Before you consider now to have an external flow problem, then it is like a wormhole on the inside of the inlet where the fluid flows out into the wormhole (still an opening) and on the outside surface acts the pressure from the outside.

3. A force is the pressure integrated over the surface, so it is an integral value and not a distribution. Hence, there can be no scalar plot that shows the force distribution over a surface.

4. Where there is no body, there is no force as a force is a result of pressure difference on all of the bodies surfaces and if there is no body (a virtual body is no body as it is not there) then there is no force.

5. Don't mix up plotting the pressure at any location in the model vs. having a force at any location in the model. A force always needs a body to act on and if that body is not really there, then there is also nothing to act on. Pressure exists always and everywhere (maybe it is almost zero in a vacuum) but a force only exists where the flow is pushing against a body with a resistance in the flow. An opening which a "Pressure opening" is, is not body, it is maybe modeled in CAD as a body, but only because the CFD uses that surface as a reference for the boundary condition in order to know where that opening is. In the simulation, there is just a hole where the fluid comes in or leaves.

The problem is that you are thinking of it the wrong way.
What is it that you really want to find out in real life (not the virtual 3D CFD simulation)?
What does your model look like in reality?
Is it a 50mm long 4mm diameter tube that hangs in mid-air and fluid magically (through a wormhole) appears on the one side and vanishes at the other?

A good way is to start from scratch (with your thinking, not your thesis).
Where do you expect any forces and why, what is their "cause", the force is the "effect".
And consider it in real life, not the modeled virtual CFD. After all, you are trying to model the reality, but if you don't model "the reality" then you won't get any useful results.

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