Flow over Backward facing step in CONVERGE CFD
Aim:
To simulate flow over backward steping face for differenet mesh size in CONVERGE CFD.
Theory:
Boundary Layer Separation:
Boundary layer separation is the detachment of a boundary layer from a surface into a wake. Separation occurs in flow that is slowing down, with pressure increasing, after passing the thickest part of a streamline body or passing through a widening passage. In Backward stepping face, separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculating flow and the flow through the central region of the duct is called the dividing streamline. The point where the dividing streamline attaches to the wall again is called the reattachment point. As the flow goes farther downstream it eventually achieves an equilibrium state and has no reverse flow.Boundary layer separation can be greatly reduced by making fluid flow turbulent.
Adverse Preassure Gradient:
An adverse preassure gradient occurs when the static preassure increases in the direction of the flow. This is important for boundary layer, increasing the fluid pressure is akin to increasing the potential energy of the fluid, leading to a reduced kinetic enrgy and a deceleration of the fluid. Since the fluid in the inner part of the boundary layer is slower, it is more greatly affected by the increasing pressure gradient. For a large enough pressure increase, this fluid may slow to zero velocity or even become reversed causing a flow separation.
Problem Discription:
Geometry:
Geometry is made in converge studio using vertex feature in create tab.
Material: Air
Initial Condition:
Tempraure:300 k
Preassure: 101325 Pa
Boundary Condition:
1.Inlet
Type: INFLOW
pressure: 110325 Pa (Total Preassure)
2.Outlet
Type: OUTFLOW
pressure: 101325 Pa (Static Preassure)
3. Top and Bottom Walls
Type : Walls
4.Front wall
Type :2D
5.Back wall
Type :2D
Run Parameters:
Solver: Preasure based steady state solver.
Simulation Time Parameters:
The simulation is run for 15000 cycle but we have enabled the steady state monitor.Hence the simulation will stop when variation is within tolerance.
Turbulence Modeling:
Turbulence Model: RNG k-ϵ
Mesh Size:
Simulation is run for 3 different mesh size as follow:
1. dx = 2e-3 m, dy = 2e-3 m, dz = 2e-3 m
2. dx = 1.5e-3 m, dy = 1.5e-3 m, dz = 1.5e-3 m
3. dx = 1.0e-3 m, dy = 1.0e-3m, dz = 1.0e-3 m
Fixed Embedding:
This feature helps to refine grain size in particular area.
Boundary ID: Top and Bottom Wall
scale: 1
Embed layer : 1
Result:
1.Mesh size: 2e-3 in X,Y and Z direction.
Mass Flow Rate:
Average Velocity:
Total Preassure:
Static Preassure:
Total cell Count:
Velocity Contour:
Preassure Contour:
2.Mesh size: 1.5e-3 in X,Y and Z direction.
Mass Flow Rate:
Average Velocity:
Total Preassure:
Static Preassure:
Total Cell Count:
Velocity Contour:
Preassure Contour:
3.Mesh size: 1e-3 in X,Y and Z direction.
Mass Flow Rate:
Average Velocity:
Total Preassure:
Static Preassure:
Total Cell Count:
Velocity Contour:
Preassure Contour:
Conclusion:
1.Total number of cell for mesh size 2e-3,1.5e-3 and 1e-3 is approximately 2000,3000 and 5500 respectivaly.
2. Fluid flow near step tends to reverse back forming recirculation zone,it happens due to increase in preassure in the direction of flow leads to formation of wake region(region of low preassure) causing detachment of flow from streamlines.
3.Total preassure increased at outlet and static preassure decreased at outlet because of sudden enlargement and recirculation zone.
4.As the mesh size decreases, plots obtained are getting more smoother and time required for simulation is also increase.