Learning Forum

[vivek.vidyadharan]

Hello CFD community,

I am working on a project which involves water coming out of the nozzle and basically I want to check the pressure drop across the nozzle and the mass flow rate of the fluid coming out of the nozzle. 

volume extracted geometry with the flow field (air where the multiphase flow takes place) :

I used Fluent meshing and used poly hexcore mesh with a total of 9 million cells. My boundary conditions were : 20 bar pressure inlet, and atmospheric pressure at the outlet. I am using Method-SIMPLEC for this approach with VOF model for the multiphase flow. While running on a steady state for about 2000 iterations I am getting the following volume fraction image. Air-0, water-1

 

And while running it on transient with a time step of 700, and 20 iterations per time step and with a step size of 1e-4. After running it the solution for about 25% I am getting the following result 

 

 

The image of transient is making more sense but then the questions that I have are :

1. Why is the steady state image showing a different spray pattern
2. and the length of the spray in the transient one is quite small. I was expecting it to go and touch the walls. Is it because I didnt run it for 100% and just stopped it at 25%. I stopped it basically because it was taking too long and after 15% the image was quite constant and there was not any change in the pattern.

I hope you could answer my queries.

Thankyou,

Vivek

[DrAmine]

I think you will get hard times running this with steady-state VOF solver. Pleae transient solver. Probably the steady state solver did go into a "stable" modi / flow-regime which has nothing to do with the reality. Multiphase Flows are inherent unsteady: we sometimes prefer to steady state solver (using sometimes implicit pseudo-relaxation) because it is more efficient but the results might deviate from reality.