# [waves2Foam] Modelling the vertical motion of a completely submerged plate

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July 27, 2016, 17:42
Modelling the vertical motion of a completely submerged plate
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Hi all,

First of all thank you to everyone who is a part of this community and constantly provides support for others. I really appreciate it and have learned so much from you guys. I have been stuck on something for some time now and I have tried absolutely everything that I can think of, but I have seen no success.

My case involves modelling the vertical motion of a completely submerged plate as a result of wave induced forces. I am solving the motion using sixDoFRigidBodyMotion solver and have constructed the case to be very similar to the floatingObject tutorial. My problem seems to lie with the boundary conditions. For the velocity condition, I am able to converge on a stable solution by using a zeroGradient condition; however, I am unsure if this is the correct one I should be using. My logic was that due to no-slip, the zeroGradient condition has to hold whether the object is stationary (a wall) or moving. However, I feel as though I should be using movingWallVelocity, but I am unable to obtain a solution -- the simulation quickly diverges.

I am also quite unsure about the pressure. I have tried both zeroGradient and fixedFluxPressure, but I don't exactly know if either of these are correct. It seems to me that if velocities are known throughout the domain and on the wall, then pressure should just be calculated from the momentum equation, so why do we even need a BC? I am not sure if this logic is correct, so if anyone has any insights on this, I would greatly appreciate it.

When using the zeroGradient for both velocity and pressure, the simulation converges; however, it is obvious that something is wrong. The motion is drastically overestimated, and I am sure that it is because the added mass and drag effects are being neglected because the pressures on the plate are not being calculated based on a plate with a nonzero velocity. It seems that even though my plate appears to be moving, each iteration calculates the pressure as if the plate was static. Basically, it seems that the fluid-structure interaction is uncoupled. The fluid affects the structure, but the structure does not affect the fluid, which is a big problem!

I am going to upload my case file here if any of you guys want to give it a try. Thank you so much for anyone willing to help!
Attached Files
 OscillatingPlate.zip (78.5 KB, 12 views)

Last edited by toodles; July 28, 2016 at 09:24.