Discussion Forum

Posted: 1 decade ago 21.10.2014, 03:01 GMT-4

Hi forcing a prescribed displacement to a surface corresponds to applying forces on it to get the desired displacement. So if you add some pressure, the forces COMSOL applies will change to get the SAME displacement. This is normal behaviour, but it should also tell you why it happens, and suggest that you change your BC's accordingly. What is giving this prescribed displacement finally, and why apply it before you add the pressure, and how can both be applied together ? -- Good luck Ivar

Posted: 1 decade ago 23.10.2014, 09:44 GMT-4

My guess is that the 0.1 um is still applied so the plane will not move further regardless of how much extra pressure you apply. You have to be careful when you switch from applying displacement AND force/pressure to the same boundary. If possible avoid it and reformulate your problem such that you only apply pressure or displacement, and in many problems that is doable. If not, then make sure you disable the prescribed displacement at the same time you apply the pressure. Nagi Elabbasi Veryst Engineering

Posted: 9 years ago 07.04.2016, 04:13 GMT-4

Hi Nagi Elabbasi Can you have a look at my problem ? I have a thin beam simulated in Electromechanics multiphysics. Before adding any voltage to the beam, I fix one head and add a Prescribed Displacement boundary condition at another head of the beam. What I want is to pull down the beam as low as I can. The result is Comsol stops at a certain displacement z0, i.e Comsol works well from 0 to z0 but not lower than z0. I also test the problem with linear or nonliner solver. Besides, when I test the same beam in Solid Mechanic physic, Comsol works very well, i.e I can put any displacement as I want with linear or non-linear solver. So, do you have any idea about this problem ? Please tell me if you have. Thank you very much Best regards

Posted: 9 years ago 27.04.2016, 14:50 GMT-4

[QUOTE] My guess is that the 0.1 um is still applied so the plane will not move further regardless of how much extra pressure you apply. You have to be careful when you switch from applying displacement AND force/pressure to the same boundary. If possible avoid it and reformulate your problem such that you only apply pressure or displacement, and in many problems that is doable. If not, then make sure you disable the prescribed displacement at the same time you apply the pressure. Nagi Elabbasi Veryst Engineering [/QUOTE] Hello, I think I try to simulate exactly what you described ;) I model a contact between a deformable and a rigid body in a pressurized chamber (2D model). A load results from the contact of both bodies. Where there is no physical contact between the bodies the chamber pressure must be applied. I do not want to use Contact Pairs. Therefore, I just model the deformable body and I use BC to account for the contact with the rigid body. I see two approaches: a) Prescribed Displacement BC: This works like a charm however I am using both Prescribed and Load BC. One contributes to each other. b) Pressure Load BC: I define a piecewise pressure function dependent on the resultant gap between the bodies (H). I assume that contact between bodies occurs when the H1 micrometer I apply the pressure of the chamber. 2. When H< 1 micrometer I define a Contact Pressure function also dependent on H. The smaller the H, the higher the pressure to be applied. Would you suggest another way to do it? Would you suggest modeling both bodies and using contact pairs? Thanks in advance ;)