Week 8 - Simulation of a backward facing step in OpenFOAM

Aim: Simulation of a backwards-facing step in OpenFOAM

Objectives: to simulate an incompressible-laminar-viscous flow through the backwards-facing step geometry

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

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

Mesh specification

• Number of cells along the x direction (longer dimension) = 200
• Number of cells along the longer y direction = 2

Theory:

• Incompressible flow

Incompressible flow refers to a flow in which the density remains constant in any fluid parcel, i.e. any infinitesimal volume of fluid moving in the flow. This type of flow is also referred to as isochoric flow.

• Laminar flow

Laminar flow also known as streamlined flow in which velocity, pressure and other flow properties at each point of the fluid remain constant.

• Viscous flow

Viscous flow is where the velocity at a given point does not change in magnitude and direction. The water is flowing in a steady state. It is also referred to as Laminar flow.

Procedure:

1. Copy the cavity folder from Openfoam/Tutorials to OpenFoam/Run folder.
2. Make a duplicate without a grading factor and with a grading factor of 0.2 as Cavity_ng & Cavity_g.
3. gedit blockMeshDict file from //cavity/system/.
4. Change convertToMeters to 0.01
5. Change vertices as per the figure given in the project.
6. Edit controlDict, p, U and physicalProperties as per the program and objectives.
7. check for blockMesh and checkMesh for the Mesh information and quality of the mesh.
8. Finally, run the icoFoam to start the simulation.
9. Open ParaFoam to view the result.

Scripts:

• blockMeshDict

• controlDict

• p

• U

• physicalProperties

Output:

1. Without Mesh grading

2. With Mesh grading

Conclusion:

Mesh grading is required to get more accurate results. By changing Mesh grading from 1 to 0.2, Mesh is more fine and accurate data transfer from one grid to another, hence maintaining less grading is required will simulating the solution.