Pressure oscillations with compressible air
 

Hi all,

I am modelling the impact of a plate inclined at an angle of 10 degrees with water. The plate is fixed in space and the free surface moves with a constant upwards velocity of 1m/s/. I am measuring the pressure at different positions along the bottom of the plate.

Water is modelled incompressible. I ran cases with air as both compressible (ideal gas) and incompressible.

The problem I have is that when I model air as compressible the pressure time histories show significant oscillations during the time the plate is travelling through air/ initial stages of water entry (up to t = approx. 0.03s in figure). The magnitude of these oscillations diminish as I further lower initial position of the free surface (h). Are these results physical or not?

Thank you for your time.

Best,

Josef

First and foremost, how did you manage negative time values? Am I not understanding your graph?

To make sure that I understand your problem, you are trying to model what happens when you drop a plate angled at 10deg through air and into water but the water/air interface is what moves?

It's kind of tough to say if the oscillations are legit from the info provided, it isn't a very common type of scneario. You could try using an overset mesh and giving motion to the plate rather than the freesurface and see if the plots agree. It would be a good reality check anyway.

Actually, I did just have a thought although it probably should have happened with incompressible air too. It could be that you see the pressure oscillate because the plate has a dynamic wake as it falls (or more correctly, as the air is pushed toward it by the water). Think of von-karman vortex shedding on a sphere but to much less of a degree.

It might be useful to create a movie of the surface pressrure distribution on the plate to help visualize if what is going on.

i work on this plate in cfx...
 

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