Learning Forum

[andres20992025]

I'm creating an example of water flow over a fully submerged structure in "Fluent". I'm interested in understanding the reactions at the base of the structure (for which I'll use "Mechanical").

 

First, I created some small models, testing with VoF (with a very small air height above the water flow) and others without VoF (trying to mimic the model with VoF). I made several different attempts in Fluent, trying to obtain results similar to the model with VoF. I managed to get similar reaction results in some directions, but in the direction perpendicular to the flow and for the moment in the flow direction, I obtained very different results. In all cases, I considered a "Velocity Inlet" for the inlet section, a "Wall" for the flow walls, and a "Pressure Outlet" for both the free surface and the flow outlet section (which is not a discharge, but an outlet from the control volume of a flow that continues beyond). In all cases, I defined the "Reference Pressure Location" at the free surface.

 

When I don't use VoF, I don't get the "Open Channel" options that I had with VoF, so I have to add a "Gauge Pressure" for the free surface and the outlet section. In the outlet section, I tried using a Gauge Pressure of 0 Pa and a hydrostatic pressure function, but in both cases, I got a significant difference between the reactions perpendicular to the flow direction and the moment in the flow direction compared to the case where I used VoF.

 

Is there any way to simulate VoF without using it? (I want to create a larger model later, and if I try to use VoF for this, I run into problems with file size [GB], my PC's capacity, and convergence in Fluent.)

[Rob]

If you don't need to see free surface effects you don't need a multiphase model. 

Did you set the operating density to be precisely that of the liquid? If so just set a 0 Pa pressure on the outlet and it should be fine. With gravity off (fine for constant density) you can omit the operating density part. 

[andres20992025]

For an FSI case, where there is flow around a solid (single-phase model in Fluent), should I set the operating density to zero in Fluent so that the hydrostatic pressure is considered within the total pressure on the solid?

[Rob]

Does the hydrostatic pressure effect the flow or force? Ie hydrostatic head will compress something, but won't it still wobble based purely on the dynamic flow force?