Flow over a Backward Facing Step using CONVERGE CFD
Obective:
- Setup a base grid to simulate the flow.
- Use of three different grid sizes.
- Comparing the Pressure and Velocity contours using ParaVIEW.
- Plotting velocity, pressure, massflow rate and total cell count for all the 3 different base sizes.
Geometry:
A base setup of a backward step was built containing inlets, outlets and walls exposed to environment conditions. The base mesh size was changed to 3 different base mesh sizes to check the action of different variables acting on the Backward step. It was inspected for any surface errors and then boundaries were assigned.
Backward step dimensions (ie base setup):
Length in X-axis = 0.273m
Width in Y-axis = 0.019m
Height in Z-axis = 0.025m
Different base mesh sizes are:
Case(1): dx = 2e-3m dy = 2e-3m dz = 2e-3m
Case(2): dx = 1.5e-3m dy = 1.5e-3m dz = 1.5e-3m
Case(3): dx = 1.0e-3m dy = 1.0e-3m dz = 1.0e-3m
Case setup:
Fluid used:- Air.
Solver :- Pressure-based steady solver
Start Time:- 0 cycle
End Time :- 15000 cycles
Initial and Minimum time steps :- 1e-06
Maximum time step :- 2
Turbulence modeling :- RNG k-ε
Inlet Pressure :- 110325Pa
Outlet Pressure :- 101325Pa
Inlet and Outlet Temperatures :- 300k
Top and bottom Wall : Law of Wall
Top and bottom Wall Temperature :- 300k
At Front and back :- 2D
Geometry mesh/Surface with edges from ParaVIEW:
The view of different sizes of meshes are shown below after slicing them in ParaVIEW.
Case(1) :
Case(2) :
Case(3) :
Velocity and Pressure contours:
Velocity contours:
The velocity contours of different mesh sizes are shown below. The behaviour of velocity of air through inlet, walls and outlet are noted using different colour options.
Case(1)
Case(2)
Case(3)
Pressure contours:
The pressure contours of different mesh sizes are shown below. The behaviour of pressure exerted by the air through inlet, walls and outlet are noted using different colour options.
Case(1)
Case(2)
Case(3)
As seen in the above contours, the pressure remains almost identical in all the 3 cases.
Plots for velocity, pressure, massflow rate and total cell count:
Case(1)
Velocity plot:
Above plot shows the behaviour of velocity of air in both inlet and outlet of the Backward step during the air flow.
Velocity of air at inlet reaches steady state in and around 1900 cycles whereas at the outlet the velocity attains the steady state within 1500 cycles.
Pressure plot:
Above plot shows the pressure at both inlet and outlet of the step.
The static pressure of the inlet attains steady state at 1800 cycles but the total pressure at outlet never really completely attains the stady state. It might require more number of cycles to attain the steady state completely.
Massflow rate plot:
Above plot shows the difference in the mass of air as it passes through inlet and comes out of outlet through no slip walls.
In the inlet the steady state is attained at 1200 cycles but at outlet it is attained at 1400 cycles.
Total cell count plot:
Below plot shows the total cell count and the individual variation of cells of 4 processors used for processing the simulation.
Case(2)
Velocity plot:
Above plot shows the behaviour of velocity of air in both inlet and outlet of the Backward step during the air flow.
Velocity of air at inlet struggles to reaches steady state whereas at the outlet the velocity attains the steady state within 6000 cycles.
Pressure plot:
Above plot shows the pressure at both inlet and outlet of the step.
The static pressure of the inlet attains steady state at 10000 cycles and the total pressure at outlet attains the stady state at 12000 cycles.
Massflow rate contour:
Above plot shows the difference in the mass of air as it passes through inlet and comes out of outlet through no slip walls.
In the inlet the steady state is attained at 8000 cycles but at outlet it is attained at 5500 cycles.
Total cell counts:
Above shows the total cell count and the individual variation of cells of 4 processors used for processing the simulation.
Case(3):
Velocity plot:
Above plot shows the behaviour of velocity of air in both inlet and outlet of the Backward step during the air flow.
Velocity of air at inlet reaches steady state at 6000 cylces whereas at the outlet velocity attains the steady state at 3500 cycles.
Pressure plot :
Above plot shows the pressure at both inlet and outlet of the step.
The static pressure of the inlet attains steady state at 5500 cycles and the total pressure at outlet attains the stady state at 2800 cycles.
Massflow rate plot:
Above plot shows the difference in the mass of air as it passes through inlet and comes out of outlet through no slip walls.
In the inlet the steady state is attained at 3000 cycles but at outlet it is attained at 3800 cycles.
Total cell count:
Above shows the total cell count and the individual variation of cells of 4 processors used for processing the simulation.
Separation :
Separation happens due to higher static pressure which is caused by the creation of recirculation zone near the step.
Due to excessive momentum loss near the step, a pressure gradient called as recirculation zone is created. As a result there might be more time consumed by the system to attain steady state for velocity.