2D Flow simulation over a backward facing step using Converge studio
Objective : To simulate flow over a backward facing step, observing the flow characterisitics such as pressure, velcoity, cell count and mass flow rate for different mesh sizes. Mesh sizes are as follows : Case 1 : dx = dy = dz = 2e-03, Case 2 : dx = dy = dz = 1.5e-03, Case 3 : dx = dy = dz = 1.2e-03.
Principles :
Fig 1 : Flow over a backward facing step
The backward facing step is an interesting case for studying the performance and solution strategy of a turbulence model. In this case, the flow is subjected to a sudden increase of cross-sectional area, resulting in a separation of flow starting at the point of expansion.
Spatial variations in the velocity field cause production of turbulence outside the wall region and its interaction with the mean flow influences the size of the separation bubble. The size of the bubble, or the re-attachment length, is one of the quantities that must be predicted accurately by a turbulence model.
1.Preprocessing :
A. Geometry setup
The geometry was prepared in converge and searched for intersections, errors like non-manifold, open edges, and assigned with the boundaries.
2D flow is simulated , hence only front and back walls are assigned differently, and top and bottom-walls are assigned in a same boundary, as ultimately we simulate the flow ignoring the z-axis.
Fig 2 Geometry model with flagged boundaries
B. Case setup :
1. Applications
General flow (Air)
2. Materials
Predefined mixture will be Air
Turbulent Prandtl number: 0.9
Turbulent Schmidt number: 0.78
Available elements: N (Nitrogen), O (Oxygen)
3. Simulation Parameters
Pressure-based steady solver
Full hydrodynamic as simulation mode
Compressible gas flow solver
Start time: 0 cycles
End time: 15000 cycles
4. Boundary Conditions
For INLET :
INFLOW boundary type
Total Pressure specified value: 110325.0
Specified boundary condition: AIR
O2: 0.23
N2: 0.77
For OUTLET :
OUTFLOW boundary type
Total Pressure specified value: 101325.0
Specified boundary condition: AIR
O2: 0.23
N2: 0.77
For walls :
WALL boundary type
FRONT-2D
Two-D boundary type.
BACK-2D
Two-D boundary type.
5. Initial Conditions & Events
Regions and initialization: volumetric-region
Species: Air
O2: 0.23
N2: 0.77
6. Physical Models
Turbulence model as: RNG K-ε
7. Grid Control
Base grid - depends for each case,
Case 1 : dx = dy = dz = 2e-03,
Case 2 : dx = dy = dz = 1.5e-03,
Case 3 : dx = dy = dz = 1.2e-03
8. Output / Post Processing
Post variable selection
Density
Pressure
Temperature
Velocity
2. Procesing : The simulation is processed using CYGWIN for all the three cases. Output is viewed using Paraview.
3. Post- processing :
1. Velocity and pressure contours :
Case 1
Fig 3 : Velocity contour for 1st base mesh
Fig 4 : Pressure contour for 1st base mesh
Case 2 :
Fig 5 : Velocity contour for 2nd base mesh
Fig 6 : Velocity contour for2nd base mesh
Case 3 :
Fig 7 : Velocity contour for 3rd base mesh
Fig 8 : Velocity contour for 2nd base mesh
2. Cell count
Case 1
Case 2
Case 3
3. Mass flow rate :
Case 1
Case 2
Case 3
4. Velocity :
Case 1
Case 2
Case 3
5. Pressure :
Case 1
Case 2
Case 3
Conclusion :
1. Simulating flow through backward facing step helps in understanding how flow takes place whenever there is sudden change in geometry along the flow.
2. Plots of velocity and pressures near the flow seperation region is shown.
3. Flow vectors helps in understanding how the recirculation/wake takes place near the step.