To Simulate the flow through a Backward Facing Step - OpenFoam
PROJECT
MESH GENERATION AND ANALYSIS USING BLOCKMESH FOR FLOW OVER A BACKWARD FACING STEP
INTRODUCTION:
The purpose of this project is to get familiarize with OpenFoam and simulate the flow through a backward facing step.
This problem is going to be solved for incompressible laminar (Low Re no) flow , therefore we shall use icoFoam Solver.
PROBLEM DIAGRAM:
ASSUMPTIONS:
l Low speed forced convection flow
l Friction factors at walls have been neglected
l Low Reynolds Number
l No slip condition at the top and bottom wall surfaces.
GEOMETRY:
The geometry is divided into 5 blocks for more accuracy , 200 cells along x axis with 2 blocks with 80 cells each and 3 blocks with 120 cell each . 10 cells along y axis with 5 cells to 2 blocks each.
We define the geometry by editing the BlockMeshDict file of the tutorial case :
- Edit for VERTICES and defining BLOCKS :
If the vertices and its blocks are not defined properly then the mesh will not be created and also the ployMesh file in the constant will not be made.
There are inlet and outlet surfaces and no slip walls , the problem is solved in 2D therefore the front and the back walls are type empty .
- Edit for defining INLET , OUTLET , NO SLIP WALLS and FRONT AND BACK
Grading Factor :
It is defined as the ratio of size of the end cell to the size of the start cell.
INITIAL CONDITION AND CONTROL VARIABLES:
We have to enter the correct Initial condition and Time step and match the initial condition with the boundary condition at each surface type .
We are Choosing `deltat` as 0.00001 for this problem for cfl number to be less than 1 , considering the cells along the direction of the flow of velocity ie x axis ; cells = 200 , Uniform Velocity of 1 `ms^-1`and size of each cell.
Edit for the control of the solution is made in the controlDict file by setting the timestep as required.
- Edit in the Pressure file
- Edit in the Velocity file
RUNNING THE SOLVER AND POST PROCESSING
n For incompressible flow we use icoFoam solver .
n Post-Processing is done with the help of paraView.
1. BLOCKMESH EDIT:
a) Grading Factor 0.8
b) Grading Factor 0.5
c) Grading Factor 0.2
2. MESH
a) Grading Factor 1
b) Grading Factor 0.8
c) Grading Factor 0.5
d) Grading Factor 0.2
3. VELOCITY MESH:
a) Grading Factor 1
b) Grading Factor 0.8
c) Grading Factor 0.5
d) Grading Factor 0.2
3. VELOCITY GRAPH
a) Grading Factor 1
b) Grading Factor 0.8
c) Grading Factor 0.5
d) Grading Factor 0.2
DISCUSSION:
l As Grading Factor decreases the accuracy of the plot increases.
l Maximum value of CFL number increases with decrease in the grading factor . Mean Value of CFL remians nearly constant.
| GRADING FACTOR | CFL MEAN | CFL MAX |
| 1 | 0.00781216 | 0.0231847 |
| 0.8 | 0.00781195 | 0.025901 |
| 0.5 | 0.00781102 | 0.0331221 |
| 0.2 | 0.00780791 | 0.0563209 |
l Boundary Layer Separation and accurate visualization is possible in the case of 0.2 Grading factor as compared to other Grading factor.
CONCLUSION:
l We can conclude that for accurate results we require finer mesh.