# Transient analysis of laminar subsonic flow of two gases with high density difference

Hello all,

I am trying to simulate the flow mentioned in the title. It is a **2D** analysis. I am using 2019 **Student version** of the ANSYS. I am using pressure based solver as it is **low Mach number** flow. There is **no temperature difference** between two gases. One of the gas is stationary(or ambient) and other one is flowing and mixing with the stationary one. Due to high density difference I have included the effect of gravity. I have four major doubts regarding the simulation, which are as follows

1. Should I turn on the Energy equation?

2. Which of the solver from SIMPLE, SIMPLEC, PISO and COUPLED will be more suitable to this particular problem?

3. As one of the gas is considered stationary, all the outlet are pressure_outlets. I am having problem in understanding the settings of Backflow condition. I have read the manual but couldn't understand it much. **For the backflow should I specify static pressure or stagnation pressure?**

4. In the real situation both the gases can come out or go into the mesh domain. ANSYS gives option to specify mole/mass fraction of both the gases. **How to specify backflow species for this situation? **

I have crammed all the four questions into this one post because I thought those are inter related. **Sorry if you find this post too long.**

Thanks in advance for any kind of help.

## Comments

Hello,

Here are the answers to your questions.

1. If your gases are likely to be compressible, then you will need to turn on the energy equation.

2. Solver settings depend on the nature of your problem. If you are solving a stiff problem, it might be useful to run a coupled solver. Otherwise, a segregated solver (like SIMPLE or SIMPLEC) might suffice. The primary difference is that the flow equations are solved either in a coupled manner or one after the other. You will find additional details in section

'28.4.3 - Pressure Velocity Coupling'in the Fluent Theory Guide.3.For backflow, you can either choose 'Static' or 'Total' pressure from the drop dowm. If you use 'Static', then Fluent will use the specified gauge pressure. If you use 'Total', then Fluent will calculate the dynamic pressure from the velocity in the adjacent cell zone and calculate the pressure based on this and the specified gauge pressure value.

4. The general idea is to pose the problem in such a way that you are sure of the mass fraction of your species in case of backflows. For example, you could have an outlet where only one species is dominant (and the mass fraction of the other species is 0 at this boundary). Please choose your computational domain in such a way that you are able to make that call appropriately.

Since we don't have anymore information about the problem you are attempting to model, I'm not able to answer some of the questions more specifically. I hope this helps.

Thanks.

Best,

Karthik

I am solving a transient problem. Therefore I have used the PISO solver.

Thanks for better clarification on point 3 and 4.

Thanks for the answer.