Backward Facing Step Flow - CONVERGE Studio

This projects aims to simulate a backward facing step flow using CONVERGE Studio. The flow is creating when a not reacting specie, air (23% O2 and 77% N2 in mass fraction), suffers a pressure gradient between the ends of the channel. The velocity, pressure and recirulation profiles will be analyzed.

GEOMETRY AND CASE SETUP

First, the geometry is created in the geometry model of CONVERGE (as it is a simple geometry no CAD program was needed). Then, the different boundaries are selected and named. Finally, the BC, IC and Simulation parameters are selected. Below are shown the main parameters selected.

Boundary Conditions

1) Inlet : Inflow at T=300K and p=110325 Pa

2) Outlet: Outflow at p=100325 Pa

3) Front and Back :2D case setup

4) Walls: No slip BC, T=300K

Simulation Parameters:

- Pressure Based Steady Solver

- N=15.000 cycles

- RNG k-Eps Turbulence Model.

- Fixed Embedding for the walls

- Mesh size. Simulated for three different mesh sizes. 1) 2e-3 m 2) 1.5e-3 m 3) 1e-3 m.

SIMULATION AND POST-PROCESSING

As stated before, the simulations were carried out using CONVERGE, and the results were postprocessed using Paraview. This section will be splitted into the analysis of the results for the different base grid sizes. For each of those, we will be focusing on the fluid properties of interest in any backward facing step. This is, the development of the velocity and pressure profile along the pipe, but also the recirculation right after the step.

 

1) Mesh grid size = 2e-3 m

 

 Mesh.

 Velocity Profile.

 Pressure Profile.

Recirculation.

In this three pictures, it is easy to analyze that the results look good qualitatively. We can start to see the boundary layer, and appreciate the zone where the recirculation is taking place. However, the grid size does not allow to really visuallize all the phenomena occuring at the step. This is the right path to follow, as know that we know that the simulation works, the mesh grid size has just to be refined.

 

2) Mesh grid size = 1.5e-3 m

 

Mesh.

Velocity Profile.

 Pressure Profile.

Reciruclation after the step.

In these pictures it starts to be clearer where the recirculation takes place, and the actual veloicity contour after the step. However, further resolution is neeeded if a  numerical analysis wants to be done.

 

3) Mesh grid size = 1e-3 m

 

 Mesh.

Velocity Profile.

Pressure Contour.

Recirculation after the step.

With this mesh grid size, it is the first time that we can clearly visualize the boundary layer, and how the velocity transitions to zero at the wall. We can also appreciate more about the recirculation phenomena. It can be seen that the pressur is a bit higher in these spots, due to the flow recirulating. This is shown as dark blue spots. Also, we can finally see how the recirulation starts right after the step, but takes place for a while, each time closer to the bottom wall. This is because the flow starts developing again against the "new" wall. As a result of this, there is a sudden pressure raise at the end of the channel.

The only other thing worth to mention, is that in all three cases the steady solution was reached way before the 15.000 cycles, and that further refined is needed for a cuantitative analysis of the turbulence model.


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