Learning Forum

[vipul.gupta]

HI 

I am modeling multiphase flow using VOF modeL. I am doing modeling in two parts: nozzle & main component. In the first part, I am modeling nozzle with only one phase (liquid steel). At the inlet, I am specifying the mass flow rate, and at outlet, I am giving pressure outlet with constant gauge pressure. I am giving an operating density value equal to zero to include the hydrostatic pressure effect. 

In the second part, I am modeling the main component using VOF. I am specifying the velocity inlet at inlet and pressure outlet at outlet. In order to specify the velocity at the inlet, I exported the velocity and pressure profile from the previous nozzle simulation outlet boundary and specified at inlet boundary of the main component and at outlet i am specifying constant pressure. 

My query is, do I need to make operating density zero in multiphase VOF to include the hydrostatic pressure? Default FLUENT operating density method is minimum phase averaged. 

[Rob]

It depends. What orientation are the boundaries, can you use open channel? Do you need to worry about hydrostatic pressure? 

[vipul.gupta]

Thanks for your response!

Here are the details of the boundary condition and orientation. Below view is the internal center vertical plane. Yes hydrostatic pressure is important,. 

 

 

 

[Rob]

You may have a problem with the inlet depending on flow velocity and acceleration under gravity. Have a look at the flow of water out of a tap. Is the jet/column of water uniform diameter all the way down? Ignore spray or twisting effects in the flow. 

The second model may also not behave as you'd like. With a fixed flow in and out the liquid level will both determine the hydrostatic pressure on the other outlet but also becomes part of the solution: that may be unstable, and the end solution may be full or empty. Think carefully what the point of the model is, ie what you are trying to find out. 

[vipul.gupta]

I am not getting what you are trying to say. 

I am modeling a tundish multiphase involving liquid steel and slag. Liquid steel comes from the Nozzle, and at the top of the tundish slag layer is there. Nozzle is connected with tundish. For ease in modeling, i am modeling in two parts. Part 1 has only nozzle with liquid steel only. Part 2 is tundish flow involving both liquid steel & slag. 

Here are the two-phase contour plots of models. My query is on making operating density zero in the multiphase VOF model to account for the hydrostatic pressure. Fluent says that to specify the gauge pressure at the outlet, it is important to make the operating density zero (Model 1). 

I want to know, do I need to make the operating density zero in the multiphase VOF model?  I am asking because i am giving gauge pressure at one of the outlets. 

 

[kim08wexler]

Hi,

Yes, for a multiphase VOF model, setting the operating density to zero is a good way to include the hydrostatic pressure effect explicitly in your simulation. By default, FLUENT uses the minimum phase-averaged density as the operating density, which may not fully capture the hydrostatic pressure variation, especially in cases involving significant height differences or where gravity-driven flow is important.

By setting the operating density to zero, you ensure that the gravitational force is applied directly to each phase according to its actual density, which should better represent the hydrostatic pressure distribution in your multiphase flow.

Good luck with your modeling!