Week 8 - Simulation of a backward facing step in OpenFOAM

AIM:-

1. To simulate an incompressible-laminar-viscous flow through the backward facing step geometry.
2. To perform a transient simulation.
3. To explain the entire simulation procedure. provide explanations to every change you make inside the reference case files obtained from OpenFOAM tutorials.
4. To perform 2 case studies,

Case 1 - Simulate the flow without using any grading factor (i.e., GF = 1)

Case 2 - Simulate the flow with grading factor of 0.2. The cells should be finer near the walls (includig the step wall).

PROCEDURE:-

• First of all Open the OpenFoam and find the solver for Incompressible -Laminar-Viscous Flow.
• Then next step is copy the file In to FOUM_RUN File so previous file remain same.
• Find Out blockMeshDict File for create the geometry and Provide mesh Property.
• Use BlockMesh Command for generate the mesh.
• Open the Simulation file in Paraview by using ParaFoam Command and plot the velocty Profile at point 0.085 m From inlet.
• Repeat the procedure up to grading factor 0.2 then Check the results.

Mesh Specification:-

1. Number of cell In X Direction = 200;

2. Number of Cell In Y Direction= 20.

Boundary Condition Specification:-

The Domain Specifiaction As Below Figure.

Fig.1. Domain Specifiaction

The Following Steps Used To Simulate Problem.

CASE-1 :- Simulate the flow Without Grading Factor ie.(G.F=1)

STEP-1: First of All we have to open New Terminal by using 'ctrl+Alt+T' then Type Foam to open OpenFoam and then open Tutorial by using Short cut key 'tut' to find the appropriate Tutorial then view list by using 'ls'.

STEP-2: Then we have to open Incompressible File using 'cd incompressible' in that file we have to search 'icoFoam' Solver, because we have to Solve Governing equation for Incompressible Laminar Flow.

STEP-3: Then in that icoFoam Folder we have to Search for cavity folder to create Required Domain.

STEP-4: Then To copy the file in FOAM_RUN Folder by using cp Command and create Folder as Cavity_test1. then check Folder by using 'ls'.

STEP-5: Open the System Folder by using 'cd' , we use 'gedit' command to edit the content available in folder Script file then we edit the BlockMeshDict file.

STEP-6: Then make changes for generate Block Mesh.

STEP-7: To create the geometry we have to add vertices, ie. total 21 vertices used to create geometry.

the 21 vertices created as below fig

the values for co-ordinate x,y,z used as per given Domain. but we assume 0.1 for z value means thickness.

Then next step is Define the type of mesh

hex(0 1 2 3 11 12 13 14) (80 5 1) simple grading (1 1 1) // block 1 here we uses 8 vertices and containing mesh points in x,y,z Direction and to make uniform mesh uses (nx,ny,nz) for given domain, then we can change (nx,ny,nz) Values we used z value is 1 because 2D Problem.last Bracket Containing Simple Grading for streching the mesh as application Along x,y,z. then we complete block 2, block 3, block 4 and block 5.

STEP-8: To Define the Boundary conditions as Given.

STEP-9: To set the Start and End Time for Simulation, set the end time as 0.2 and Delta(T)= 0.0001. by using editing controlDict File.

STEP-10: To Set the initial oundary condition for Pressure and velocity at each faces, for laminar flow we use velocity along x directin 1m/s for simulation, the requred changes for velocity in p and U as below.

For Velocity:-

For Pressure:-

Then we have to enter the value of Kinematic viscocity.

Then we check the block mesh by using 'blockMesh' Command and we use icoFoam Solver for simulation, then we use 'paraFoam' to view paraview.

the given geometry with uniform mesh as below CASE 1 (grading Factor is 1) :-

RESULT:-

• Velocity Distribution Over Domain for CASE 1 (grading Factor is 1):-

• Pressure Distribution Over Domain for CASE 1 (grading Factor is 1):-

• Velocity Profile at 0.085 m from the inlet of geometry for CASE 1 (grading Factor is 1)

• For Simulation of CASE 2 Following Changes made in blockMeshDict File

• Mesh Grid for CASE 2 grading Factor (0.2):-

• Velocity Distribution Over Domain for CASE 2 (grading Factor is 0.2):-

• Pressure Distribution Over Domain for CASE 2 (grading Factor is 0.2):-

• Velocity Profile at 0.085 m from the inlet of geometry for CASE 2 (grading Factor is 0.2)

CONCLUSION:-

• Grading Factor Importance :-

1. In CFD we use Finer Mesh to capture the changes near Wall, so Grading is Useful for Computational Efficiency, so by changing Simple grading from 1 to 0.2 Mesh becomes Finer.
2. If we use Large Number of elements in Mesh it may take more time to converge the solution, but it helps to capture flow at boundary wall and obstacles.

• By comparing two CASES we found that CASE 2 (grading Factor is 0.2) takes More time for Execute the Solution than CASE 1 (grading Factor is 1).