Flow over a backward facing step
AIM:
To simulate and study flow separation in backward flow step problem.
OBJECTIVE:
1)To setup case for backward facing step for three mesh sizes.
2)To simulate the flow and post process in paraview.
3)Plot and analysis different plots and contours in paraview and converge line plotting module.
INTRODUCTION:
The flow through backward facing step is a benchmark problem for application and validation for various turbulent models available and developed.The main reason for acceptance of this problem is it is well defined and extensively studied flow separation problem.The separation occurring near the part where area increases is not so chaotic. So results can be easily compared with benchmark experimental results obtained from wind tunnel experiment.
When pressure gradient is available for fluid between inlet and outlet flow starts taking place from smaller area portion but as soon as it comes to region where area is enlarged it encounters adverse pressure gradient and the flow separates starts circulating.The flow which get detached from surface creates vortices.This vortex gets bigger and bigger to form separation bubble below shear layer.The flow reattaches itself after reaching upward until it gets a pressure gradient which is favourable for main flow.The point where the separated streamline attaches again to streamlines of main flow is called “reattachment point”.
The flow separation , flow reattachment is significantly important because it plays very big role in internal as well as external flow design.
GEOMETRY:
The geometry dimensions are as follows with step increase in area.
Lx=0.272009 m
Ly=0.0199757 m
Lz=0.025(doesn’t matter as its 2D problem.)
Case Setup:
Boundary Flagging:
Application Type:
Selecting Time Based Simulation With air as predefined mixture.
1)Gas Simulation:Keeping default values
2)Global Transport Properties:Keeping default values
3)Run parameters:
Using Steady State pressure based solver
4)Steady state monitor
Monitoring average velocity at inlet after 500 cycles and for sample size of 500 cycles with allowed tolerance of 0.001
5)Simulation Time Parameter:
The maximum cycles are kept at 15000 cycle assuming this much will be sufficient flow convergance
The minimum time step is kept at 1e-9 and maximum at 1 sec.
6)Solver parameters:Pressure Based solver
Boundary Conditions:
1)Inlet :
Inflow boundary condition with 110325 Pa of pressure at 300 k with air as intake.
2)Outlet:
Outflow Boundary condition with atm pressure of 101325 Pa if any backflow is there it is of air.
3)Top-Botto-Walls:
For turbulent flow LAW OF WALL is to be applied.
4)Front and Back are 2D boundaries.
Region and Initialisation
Initial condtion :
Create a region inside the geometry consisting air at atm conditions.
Turbulence Modelling:
Keeping it as RNF k-ε model.
Base Grid:
For Case 1: 0.002 for dx dy dz
For Case 2: 0.0015 for dx dy dz
For Case 3: 0.001 for dx dy dz
Fixed Embedding
Boundary Embedding of Permenant Type For Top-Bottom Wall with 1 layer and scaling factor 2.
Output files:
Writing Output files for every 100 cycles.
Results:
1.Mesh:
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
1.Velocity Contours:
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
2.Pressure Contours:
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
3)Plot over line.
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
4)Plot over line:
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
5)Mass flow rate
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
6)Average velocity plot
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
7)total pressure plot:
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
8)total cell count:
CASE1:2e-3
CASE2:1.5e-3
CASE3:1e-3
Animations:
1.Velocity Contours:
2.Vectot Plot
Vectors showing flow separation and Reattachment.
Conclusion:
1.Pressure plots over a horizontal line shows drop in pressure showing creation of adverse pressure gradient.
2.Plots of average velocity,mass flow rate confirms convergence of the solution obtained.
3.Velocity plot on vertical line showing reversal of velocity henceforth forming vortex in lower portion near step.
4.Vector plots shows flow seperation and formation of recirculation zone.
5.Reattachment of flow can be seen in velocity contours plots at inclined green zone.