Getting forces on fixations from a pressure field

Santos-Dumont · July 24, 2012, 17:27

Hello Fluent guys,

I'm working on a aircraft project and I have a tricky analysis to do.
The aircraft have a pod mounted under it's fuselage. The pod have the shape on a very long bathtub and it is used to protect some electronic equipments.

The pod is held by something like 40 screws along it's perimeter. The force is indeed not equally distributed on each screws.

I would like to know if it's possible to get the actual stress (due to aerodynamic forces) on each screws ?
Calculating the pressure field is easy but I don't think that Fluent itself is capable of computing the stress on the screws. I was thinking of an export of the pressure field and then a static analysis with the pressure field as an input force. Using the Ansys Workbench may be helpful.

So I would like to have your opinion on that problem. Did some of you guys experiences a similar simulation ?

Thanks
Charles

Santos-Dumont · July 25, 2012, 16:19

This thread needs a little lift

NormalVector · July 25, 2012, 20:37

I don't think you can output mechanical stress, it is a CFD program after all.

. You might be able to find a way to export the pressure field and bring it into Abaqus or something.

flotus1 · July 26, 2012, 02:02

In the workbench environment, it is actually straightforward to export the pressure on a surface you get from a CFD simulation and use it as a boundary condition for stress calculations in one of the other tools.
I rather see difficulties concerning computational cost solving the stress analysis with 40 screws.

Santos-Dumont · July 26, 2012, 02:37

Hi Alex, thanks for the answer.

I tried to do a simple export and surprizingly, it worked pretty well. On the Static Structural I now have a pressure load given by Fluent. For the screws analysis, I was thinking of adding 40 points on my geometry and then use then to create Fixed Support. Then I shall be able to get the reaction on each support and therefore the stress on each screws by calculating Stress = Force / Surface

I'll try that on a simple model to see if it works

flotus1 · July 26, 2012, 02:56

Well your approach (using fixed support instead of screws) will give you results.
But calculating Stress=Force/Area for a screw doesn't work. Since the screw and the material are already under stress before any load is applied (due to the pre-tightening of the screw), the calculation is more complex than that.

You could create a proper model of one screw connection and apply the loads you get from the structural analysis here.

July 24, 2012, 17:27	Getting forces on fixations from a pressure field	#1
Santos-Dumont Member Charles Join Date: May 2011 Location: France Posts: 77 Rep Power: 14	Hello Fluent guys, I'm working on a aircraft project and I have a tricky analysis to do. The aircraft have a pod mounted under it's fuselage. The pod have the shape on a very long bathtub and it is used to protect some electronic equipments. The pod is held by something like 40 screws along it's perimeter. The force is indeed not equally distributed on each screws. I would like to know if it's possible to get the actual stress (due to aerodynamic forces) on each screws ? Calculating the pressure field is easy but I don't think that Fluent itself is capable of computing the stress on the screws. I was thinking of an export of the pressure field and then a static analysis with the pressure field as an input force. Using the Ansys Workbench may be helpful. So I would like to have your opinion on that problem. Did some of you guys experiences a similar simulation ? Thanks Charles

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July 25, 2012, 16:19		#2
Santos-Dumont Member Charles Join Date: May 2011 Location: France Posts: 77 Rep Power: 14	This thread needs a little lift

July 25, 2012, 20:37		#3
NormalVector Member NormalVector Join Date: Oct 2010 Posts: 71 Rep Power: 15	I don't think you can output mechanical stress, it is a CFD program after all. . You might be able to find a way to export the pressure field and bring it into Abaqus or something.

July 26, 2012, 02:02		#4
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46	In the workbench environment, it is actually straightforward to export the pressure on a surface you get from a CFD simulation and use it as a boundary condition for stress calculations in one of the other tools. I rather see difficulties concerning computational cost solving the stress analysis with 40 screws.

July 26, 2012, 02:37		#5
Santos-Dumont Member Charles Join Date: May 2011 Location: France Posts: 77 Rep Power: 14	Hi Alex, thanks for the answer. I tried to do a simple export and surprizingly, it worked pretty well. On the Static Structural I now have a pressure load given by Fluent. For the screws analysis, I was thinking of adding 40 points on my geometry and then use then to create Fixed Support. Then I shall be able to get the reaction on each support and therefore the stress on each screws by calculating Stress = Force / Surface I'll try that on a simple model to see if it works

July 26, 2012, 02:56		#6
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46	Well your approach (using fixed support instead of screws) will give you results. But calculating Stress=Force/Area for a screw doesn't work. Since the screw and the material are already under stress before any load is applied (due to the pre-tightening of the screw), the calculation is more complex than that. You could create a proper model of one screw connection and apply the loads you get from the structural analysis here.