Learning Forum

[Aalif Mohammed]

Hello,

I am new to ANSYS and am trying to simulate air flow over still water(not as a tank but a free surface such as a lake/pond). I am using a rectangular 2D mesh as below using the multiphase VOF model with the SST k-w solver . I have enabled gravity and implicit body forces and am using an explicit solver. I am using a velocity inlet with a log profile and a pressure outlet as boundary conditions. I used constant surface tension of 0.072 for the interface and did not enable surface tension force modelling.

For the velocity inlet, I have given volume fraction 0 for the secondary phase(water).

For the pressure outlet, I have given backflow fraction as 1 for water.  

All the other boundaries, I have assumed wall conditions and have tried the no-slip conditions and specified shear(0 shear since water and air have relatively negligible shear)

.

I have used these reference values computed from inlet

I have tried 2 separate approached with this setup (for steady-state simulation), both did not work out and I would appreciate suggestions on how to proceed or what is happening in the model.

Approach 1: Patching just a water region

I patched the bottom part of the rectangle(below the inlet) after hybrid initlialisation as having a volume fraction of 1 for water. I get phase seperation but after running the calculation I get a lot of phase mixing

I suspected this was due to an already initialised velocity field in the domain before I did patching for the water region

Approach 2: Patching phase and velocities

I tried patching the x and y velocities for both phases as well(i defined both phases as different regions and applied my default velocity conditions to them(only x-direction velocity for air))

I tried running this calculation for 1000s and the simulation crashed because of high residuals and non-convergence.

 

I do not understand why water is behaving in this manner and my calculations are going haywire.

Does a patch not define cell properties like density and viscosity? If so, how do I proceed with this, should I define seperate geometries for the phases or is there some settings i can modify?

[Rob]

In 2d if you don't select surfaces you'll get a continuous contour. 

Set the backflow as zero water. Otherwise gravity will come into play and water may pour into the domain. 

Patch is used to set value of something, eg phase, velocity etc.  Properties are set in the Materials panel for each material, that material is then assigned to the phase.