Week 3: Flow over a backward facing step

Aim - Flow-over Backward facing step in Converge 3.0

Introduction - Observing the flow-over Backward facing step with 3 different mesh sizes.

Theory - Pressure - Velocity relation, Flow Separation

Geometry -

• Geometry creation -
  • Geometry was Imported. Drag-Drop
  • 
• Boundary Flagging
  • Boundaries were defined using the Boundary function
  • 5 different boundaries were defined as follows -
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Case Setup -

• Application type > Time-based (For 2D flow)
• Materials > Select Predefined mixture "Air"
  • O2 and N2 Gas species added
• Simulation parameters
  • Run Parameters - Select steady-state monitor > Add variables that are needed to be monitored(Velocity, Pressure, etc)
  • Simulation time parameters - End time > 15000 cycles
  • Solver parameters(SS) - Density-based NS solver
• Boundary conditions - parameters assigned as follows
  • Inlet - Inflow Type - 110325.0Pa, 300K, Air Species
  • Outlet - Outflow Type - 101325.0Pa, 300K, Air Species
  • Top Bottom - Wall - Law of wall
  • Front & Back - 2D
• Initial Conditions and Events
  • Region and initialization - Add Volumetric Region with Air species, Initial pressure = Inlet pressure = 101325.0Pa, 300K
• Physical Models - Default
• Grid Control -
  • Base Grid
    • Case I - Base Grid - dx = 2e-3m, dy = 2e-3m, dz = 2e-3m
    • Case II - Base Grid - dx = 1.5e-3m, dy = 1.5e-3m, dz = 1.5e-3m
    • Case III - Base Grid - dx = 1.2e-3m, dy = 1.2e-3m, dz = 1.2e-3m
  • Fixed Embedding
    • Boundary - Top and Bottom - Permanant - Scale - 1, Embed Layers - 2
• Output/Post-processing
  • Time Interval 3D O/p files - 200cycs
• Advance Parameters
• 

Observations - After running the calculation on Cygwin following are the results observed in Paraview and Converge Line plotting tool. The calculations were done with 3 different mesh sizes as mentioned below. At the start, we observed the pressure difference between Inlet and Outlet leading to a unidirectional flow through the channel. Flow separation due to the step is also clearly visible in all the cases. As the size of the base grind reduces the no cells increase and the flow separation effect can be seen more clearly in the output.

• Case I - Base Grid - dx = 2e-3m, dy = 2e-3m, dz = 2e-3m
  
  • Mesh & Total cell count
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  • Velocity across Inlet and Outlet
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  • Pressure across Inlet and Outlet
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  • Mass flow rate at Inlet and Outlet
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• Case II - Base Grid - dx = 1.5e-3m, dy = 1.5e-3m, dz = 1.5e-3m
  
  • Mesh & Total cell count
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  • 
  • Velocity at Inlet and Outlet
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  • Pressure at Inlet and Outlet
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  • Mass flow rate at Inlet and Outlet
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• Case III - Base Grid - dx = 1.2e-3m, dy = 1.2e-3m, dz = 1.2e-3m
  
  • Mesh & Total cell count
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  • 
  • Velocity at Inlet and Outlet
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  • 
  • Pressure at Inlet and Outlet
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  • 
  • Mass flow rate at Inlet and Outlet
  • 

Animation Link - https://youtu.be/ELXV0ac3oyI

Conclusion -

The flow separation due to sudden change in area is clearly visible in all the cases. Pressure drop and flow recirculation can be observed near the step. The velocity profiles at the inlet and outlet seem stable.

In the velocity and pressure plots, we observed that Pressure and velocity are inversely proportional to each other

Flow can be observed from the High-pressure inlet to the Low-pressure outlet.

The mass flow rate plot indicates that mass flow through the inlet equals mass flow through the outlet. No accumulation mass inside the channel.