Week 3: Flow over a backward facing step

AIM: To simulate the flow over the Backward facing step using CONVERGE CFD.

OBJECTIVE:

• Able to simulate the flow over Backward facing step for three different base mesh sizes, different base mesh sizes are,
  • dx = 2e-4m, dy = 2e-4m, dz = 2e-4m
  • dx = 1.5e-4m, dy = 1.5e-4m, dz = 1.5e-4m
  • dx = 1.2e-4m, dy = 1.2e-4m, dz = 1.2e-4m

• The impact of three different mesh sizes will be studied Understand the development of flow with the aid of plots for velocity, pressure and mass flow rate.

For this analysis, a backwards-facing step with the below dimensions is considered for the analysis.

A.CASE SETUP:

A. Geometry Creation:

1. Creation of Boundaries: In the geometry which is created above, boundaries such as inflow, outflow, top and bottom walls, and front and back faces are labelled.

2. Checking Orientation of Normals: Orientation of Normals was checked to ensure all the normals are pointing inwards. This was done using the Normal Toggle

3. Finally a Diagnosis check was run to ensure no errors in the geometry.

B. CASE SETUP:

1. Application Type:- This was set to Time based,

2. Materials:- This was set to Air,

i) Gas Simulation: Default properties were retained,

ii) Global Transport Parameters:- Default properties retained,

3. Simulation Parameters

• Run Parameters:- Steady state solver was used. The simulation mode was set to Full Hydrodynamic. The gas flow solver is set to Compressible

- Simulation Time Parameters:- The parameters were set as displayed below. Max CFL limit set to 1.0

- Solver Parameters:- A Pressure based PISO solver was used, Maximum Convection CFL limit was set to 0.8.

4. Boundary:

- Inlet:- Boundary flow - Inflow

Inlet Pressure- 110325 Pa

- Outlet- Boundary - Outflow,

• Top and Bottom Walls:- Stationary wall with Fixed surface movement used,
• Front and Back faces set to Two D,

4. Initial condition and events

• The created geometry was selected as the Volumetric Region

6. Turbulence Modelling:- RNG-epsilon turbulence model is used

7. Grid Control:- Base Grid. Here the size of the mesh for the three cases is entered,

7. Output/Post Processing:- The variables that we intend to see during post-processing are selected. In this case, Density, Pressure, Temperature and Velocity were selected.

After setting the above, the input files were exported to a directory

To execute the simulation, the directory in which these input files were stored was accessed in Cygwin and the following command was typed "mpiexec.exe -n 4 converge.exe"

After the Simulation was executed, the results were converted into a form which would be compatible with Paraview. This was done using the post_convert.exe command.

RESULT & POST_ PROCESSING:

1. Case-1 ( dx=dy-dz=2e-4):

• View of the Mesh:

• Velocity Contour:

• Pressure Contour:

Plots for :

• Velocity (After approximately 15000 cycles)

• Pressure:

• Mass Flow Rate:

• Cell Count:

• Velocity Animation:

2. Case-2 (Mesh Size 1.5e-03):

• View of the Mesh:

• Velocity Contour:

• Pressure Contour:

Plots for :

• Velocity (After approximately 15000 cycles)

• Pressure:

• Mass Flow Rate:

• Cell Count:

• Velocity Animation:

3. Case 2 (Mesh Size 1.2e-03):

• View of the Mesh:

• Velocity Contour:

• Pressure Contour:

Plots for :

• Velocity (After approximately 15000 cycles)

• Pressure:

• Mass Flow Rate:

• Cell Count:

• Velocity Animation:

CONCLUSION:

1. The driving force for the flow is the variation in pressure between the inlet and the outlet.

2. The flow at the inlet is around 140 m/s. As it flows through the step the area increases resulting in a loss of velocity. This reduction in velocity can be expressed by the continuity equation Area*Velocity=constant.

3. From the velocity contour, we can also observe that in the step region there is an area where flow velocity drops to zero. Flow separates as a result of the adverse pressure gradient. However, The turbulent structures in the shear layer entrain irrotational fluid from the non-turbulent region outside the shear layer. This flow entrainment causes the formation of a low-velocity recirculation in the region, which is located between the shear layer and the adjacent wall. The recirculation zone is mainly comprised of a primary vortex in the centre and a secondary vortex adjacent to the corner of the step. Due to the favourable pressure gradient created by the fluid entrained, the shear layer eventually curves down towards the wall and impinges at a location known as the reattachment point.

4. From the mass, velocity and total pressure plots we observe that after about 7000 cycles it reaches a steady state and there is no variation in the values. From the animations, we observe that after 25 seconds the solution converges to its steady state value.

5. Minor variations are observed in the values of mass flow rates, average velocity etc during the simulation.

6. As the cell count increases solution accuracy and simulation time increase. From velocity vector contours we can observe detailed turbulence as the mesh side is getting decreased.