Simulating flow through Backward Facing step using Converge studio software

Objective:

• To simulate flow over backward facing step simulation for three different grid sizes.
• To analyse and plot velocity, pressure contours and understand how flow seperation takes place

Theory of Backward Facing step: (Fig Reference: Rajasekaran, J. (2011))

• Many complex turbulent flow occurring in many engineering and industrial conditions that involve heat transfer, sudden expansion causing
  recirculating, flow separation, large pressure gradient, and mixing, recirculating flow. These can be represented in BFS geometry, both
  experimentally and numerically.
• Above figure shows in detail, the flow over backward facing step.

Outline of project flow:

• Geometry Set up
• Case setup
  • Boundary flagging
  • Boundary conditions
  • Define turbulence model
  • Define base grid size
• Running of simulation.
• Post processing of results
• Plotting and analysis of Results

Mesh sizes used:

Mesh 1. dx = 2e-3m, dy = 2e-3m, dz = 2e-3m

Mesh 2. dx = 1.5e-3m, dy = 1.5e-3m, dz = 1.5e-3m

Mesh 3. dx = 1.0e-3m, dy = 1.0e-3m, dz = 1.0e-3m

Geometry Set-up:

• Following figure shows the geometry that was modelled in converge software.
• After modelling, the geometry was diagnosed for any errors like intersections, Non-manifold edges, open edges etc.

Case set-up:

1. It is a general flow case.
2. Oxygen (O2) and Nitrogen (N2) are the two main species.
3. Predefined mixture is air
4. Solver: Pressure based steady solver and simulation mode: Full hydrodynamic.
5. Simulation time parameters:
   • Number of cycles: 15000
   • Initial and minimum time step: 1e-6
   • Maximum time step: 2
6. Boundary Conditions:
   • Inlet: It is inflow boundary with pressure of 110325 pa.
   • Outlet: It is outflow boundary type with pressure of 100000 pa.
   • Wall: It includes top and bottom wall.
   • Front and Back 2D: It is a 2D boundary layer.
7. Regions and Initializations: region 0 is the volumetric flow volume
8. Turbulence model: RNG k-ɛ
9. Fixed embedding is used with a scale size of 1mm for top and bottom wall.

Boundary Flagging:

• Each of the surface is assigned to a particular boundary and all the boundaries are assigned to a volumetric region.

Simulation:

• Input files were exported to a folder and using cygwin the path to these input files was assigned and simulation was run using a executable command converge-2.3.exe file.
• The simulation was run for 15000 cycles for each mesh size using Cygwin and post-processed in paraview software

Post Processing/ Results:

Surface Grid: Three different grid sizes are as shown in the figure. Fixed embedding is used near the wall surface

Mesh 1: dx = 2e-3m, dy = 2e-3m, dz = 2e-3m

Mesh 2. dx = 1.5e-3m, dy = 1.5e-3m, dz = 1.5e-3m

Mesh 3. dx = 1.0e-3m, dy = 1.0e-3m, dz = 1.0e-3m

Total cell count:

Mesh 1: Total cells - 2000

Mesh 2: Total cells - 3200

Mesh 3: Total cells - 5900

Flow Rates:

• Average mass flow rates at the inlet (ID 1) and the outlet (ID 2) for different grid sizes is as shown in the figure.

For mesh 1:

For mesh 2:

For mesh 3:

Velocity contours:

• We can observe that there is flow seperation near the step.
• After flow seperation, velocity of the flow reduces.

Mesh 1:

Mesh 2:

Mesh 3:

Pressure contours:

Mesh 1:

Mesh 2:

Mesh 3:

• When the flow seperation takes place, a low pressure region is created which causes the flow to recirculate.
• This recirculation zone is known as wake region.
• The recirculation zone is shown by the flow vectors in the figure below.

Pressure drop across the step:

Below figure shows drop in pressure after flow seperation.

Conclusion:

• Simulating flow through backward facing step helps in understanding how flow takes place whenever there is sudden change in geometry along the flow.
• Plots of velocity and pressures near the flow seperation region is shown.
• Flow vectors helps in understanding how the recirculation/wake takes place near the step.

References:

• Rajasekaran, J. (2011). On The FLow Characteristics Behind a Backward-Facing Step and The Design of a New Axisymetric 11 Model For Their Study. Toronto: Department of Aerospace Engineering, University of Toronto.