[pzr1988]

Hi,All:

I'm working on a Euler Fluid Simulator, And I have applied free-slip boundary condition currently by applying the two rules(velocities are zero normal to wall and gradient are zero tangent to wall).But this settings can cause negative pressures.

Consider a rectangular block of water falling down and has just hit the ground,all the velocity are pointing downwards and the ground velocity is zero because of free-slip,when solving incompressibility,I get all positive pressures. However,if the above block of water is leaving the ground,that is,all the velocity are pointing upwards and still ground velocity is zero.In this reverse setting, negative pressure is unavoidable.

I'm considering modifying free-slip boundary condition:When the velocity is pointing away from the wall,boundary velocity is same as the fluid velocity.When the velocity is pointing to the wall,boundary velocity is zero. But I'm not sure if this will work,or negative pressure is just ok?

Could anyone provide some suggestions?

[Vasiliy]

I think that negative pressure in fluid is ok when velocity flows from the wall. I saw such behaviour in turbomachinery.
However it is not so often and can be connected with fact that you use incompressible water.
Also you need to check convergence.

[pzr1988]

Thanks for reply,in my system,convergence is always checked to be below 1E-4.

But if a block of water that Virtual Contacts the ceiling boundary wall and is starting to fall down,it will be impacted by the negative pressure and deform.Is this an expected phenomenon governed by NS equation?