lift/drag forces for FSI modeling using a fluid-shell or fluid-membrane interface

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Dear COMSOL users,

I was trying to develop a fluid-structure interaction model in COMSOL where a fluid will be flowing over a soft shell or membrane and was trying to calculate the drag force accordingly. I have seen several tutorials in COMSOL application gallery for this that use fluid-solid interaction, fluid-shell interaction, and fluid-membrane interaction. To calculate the drag force, usually all the tutorial uses intop1 operator or any other integral method. However, I have seen that this only works with fluid-solid interaction to get the drag or lift and whenever I use it for fluid-shell interaction or fluid-memrane interaction, either the drag/lift comes as 0 or says NaN. Could you please explain why this happens and how to get the aerodynamicc forces for fluid-shell or fluid-membrane interaction? I tried in many differnt ways, but it looks like that it works only for fluid-solid interaction where the solid is rigid or deformable and never works for me for fluid-shell or fluid-membrane interaction. Thanks.


5 Replies Last Post Sep 30, 2023, 11:22 a.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 3 months ago Sep 5, 2023, 2:39 a.m. EDT

Can you provide a simple example? There should not be any difference.

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Henrik Sönnerlind
COMSOL
Can you provide a simple example? There should not be any difference.

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Posted: 3 months ago Sep 9, 2023, 11:17 p.m. EDT
Updated: 3 months ago Sep 9, 2023, 11:19 p.m. EDT

Hello Henrik,

Thanks for your reply. I am making one example and will get back to you soon.

Thanks Pratik

Hello Henrik, Thanks for your reply. I am making one example and will get back to you soon. Thanks Pratik

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Posted: 3 months ago Sep 10, 2023, 1:10 p.m. EDT
Updated: 3 months ago Sep 10, 2023, 1:13 p.m. EDT

Hi Henrik,

Here is a sample mpdel I created in COMSOL (attached) where water is flowing over a membrane. The model is created as a fluid-structure interaction model by using 'fluid-membrane interaction' module. I have created the 'intop1' function as seen in another comsol tutorial (screenshot attached) to calculate lift and drag and I used this technique for several models without any problem including fluid-solid interaction or CFD models. But somehow when you use fluid-shell interaction or fluid-membrane interaction, then it is not coming through which I have seen for every models that has shell or membrane involved. For this model attached, I am getting NaN for drag force (screenshot attached).

I was thinking about the reason behind this and one thing that came to my mind was, when we used fluid-solid interaction or CFD models, then the surfaces on which you want to measure the drag were to be specified and each of those surfaces had some sort of thickness depending on which you could select upper or lower surfaces. However, here for the membrane, since it has no thickness, therefore, when I selected the face of the membrane as the surface of interest where I need to calculate the drag, there was only one selection of face and I am not sure if it selected upper or lower surface of the membrane. Could this NaN value of drag be because of this problem (I don't think so)?

As you mentioed earlier, the technique should work similary for all models, but here for some reason it is not working. If you could put some insight on this, I will greatly appreciate.

I have cleared the solutions to reduce the file size, but if you want you can run it. It just takes 40 seconds to run.

Thanks Pratik

Hi Henrik, Here is a sample mpdel I created in COMSOL (attached) where water is flowing over a membrane. The model is created as a fluid-structure interaction model by using 'fluid-membrane interaction' module. I have created the 'intop1' function as seen in another comsol tutorial (screenshot attached) to calculate lift and drag and I used this technique for several models without any problem including fluid-solid interaction or CFD models. But somehow when you use fluid-shell interaction or fluid-membrane interaction, then it is not coming through which I have seen for every models that has shell or membrane involved. For this model attached, I am getting NaN for drag force (screenshot attached). I was thinking about the reason behind this and one thing that came to my mind was, when we used fluid-solid interaction or CFD models, then the surfaces on which you want to measure the drag were to be specified and each of those surfaces had some sort of thickness depending on which you could select upper or lower surfaces. However, here for the membrane, since it has no thickness, therefore, when I selected the face of the membrane as the surface of interest where I need to calculate the drag, there was only one selection of face and I am not sure if it selected upper or lower surface of the membrane. Could this NaN value of drag be because of this problem (I don't think so)? As you mentioed earlier, the technique should work similary for all models, but here for some reason it is not working. If you could put some insight on this, I will greatly appreciate. I have cleared the solutions to reduce the file size, but if you want you can run it. It just takes 40 seconds to run. Thanks Pratik


Henrik Sönnerlind COMSOL Employee

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Posted: 3 months ago Sep 11, 2023, 4:25 a.m. EDT

When you have a membrane that is embedded in the fluid, there will be two different tractions on the upside and the downside. The ordinary traction variable is not defined there.

The expressions you need to integrate are spf.T_stress_dx+spf.T_stress_ux etc. Here 'd' and 'u' stands for 'down' and 'up' respectively.

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Henrik Sönnerlind
COMSOL
When you have a membrane that is embedded in the fluid, there will be two different tractions on the upside and the downside. The ordinary traction variable is not defined there. The expressions you need to integrate are spf.T_stress_dx+spf.T_stress_ux etc. Here 'd' and 'u' stands for 'down' and 'up' respectively.

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Posted: 2 months ago Sep 30, 2023, 11:22 a.m. EDT
Updated: 2 months ago Sep 30, 2023, 11:21 a.m. EDT

Hello Henrik,

Thank you very much for your feedback.

Our COMSOL software was unsinstalled for the last few days due to some maintenance and we were not able to run the model as per your feedback. However, when we tried it for the earlier few days before maintenance, it was giving an error and we were not able to diagnose it further as the sytem was down. I will be running it from today and at first, I will try to fix the previous errors. If the error comes consistently, then I will get back yo you.

I appreciate your time behind this.

Thanks Pratik

Hello Henrik, Thank you very much for your feedback. Our COMSOL software was unsinstalled for the last few days due to some maintenance and we were not able to run the model as per your feedback. However, when we tried it for the earlier few days before maintenance, it was giving an error and we were not able to diagnose it further as the sytem was down. I will be running it from today and at first, I will try to fix the previous errors. If the error comes consistently, then I will get back yo you. I appreciate your time behind this. Thanks Pratik

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