Week 3: Flow over a backward facing step
I. Aim: CFD analysis of 2D flow over a back step type geometry using Converge CFD.
II. Problem Statement: Perform 2D steady-state flow over a back step geometry using a suitable solver available in Converge CFD. The case setup is done using ConvergeCFD and simulation is run in the CYGWIN terminal. Further, post-processing is done using Converge CFD and ParaView. The main objective of this project is to simulate a simple case of flow involving re-circulation using ConvergeCFD.
III Objectives:
1. Simulate 2D steady-state flow over a backward facing step using ConvergeCFD and CYGWIN terminal for the following mesh cases:
- dx = dy = dz =2e-3 m
- dx = dy = dz =1.5e-3 m
- dx = dy = dz =1.0e-3 m (If this is too much for your PC, use 1.2e-3 m)
2. Postprocess results in ParaView and ConvergeCFD and report the following:
- Velocity and pressure contours for all the 3 base mesh sizes
- Show mesh (i.e surface with edges) for the 3 base mesh sizes
- Plots for velocity, pressure, mass flow rate, and total cell count for all 3 base mesh sizes.
IV: Solving & Modelling approach:
2D steady-state CFD analysis of flow over a backward-facing step case is set up in ConvergeCFD and then running the solution in CYGWIN terminal. Air is used as the fluid flowing through the channel (77% N2 and 23%O2). Postprocessing is done using ConvergeCFD and ParaView. Three meshes are used to check the effect of cell size on the results.
1. Geometry/Domain:
A 3D solid geometry (.stl file) of the typical backward-facing step is imported into ConvergeCFD. The following figure shows the imported geometry. In ConvergeCFD triangles are the basic building blocks. Hence the surfaces of the channel are a combination of triangles. The dimensions of the channel are as follows:
Length = 0.272009 m
Width = 0.0199757 m
Height = 0.025 m
2. Boundary Flagging:
The boundary zones in ConvergeCFD are created by flagging boundaries. This can be done by creating boundary flags in the boundary tab as shown below:
3. Case setup:
After assigning boundary zones as mentioned above, the solution case setup is to be done. ConvergeCFD has a dedicated menu to completely set up the solution. Details are as follows:
-In the application type option, Time-based is selected.
- The materials option are selected as shown in below images:
Air is chosen as the working fluid with 77% N2 and 23% O2.
- Simulation Parameters:
The steady-state solver option is selected and respective monitors for various flow variables are selected to monitor during the solution process.
- Boundary Conditions:
The boundary conditions are assigned to the respective boundary zones in this menu. The applied boundary conditions are as follows:
| Boundary Zone | Boundary condition |
| Inlet |
Inflow -Total Pressure = 111325 Pa, -Velocity = Zero normal gradient -Temperature = 300 K - Species - O2 = 0.23, N2 - 0.77 |
| Outlet |
Outflow - Pressure = 101325 Pa -Velocity = Zero normal gradient - Temperature backflow = 300 K - Species - O2 = 0.23, N2 - 0.77 |
| Top_Bottom_walls |
Wall - Stationary, Law of wall, zero normal gradient temperature. - Turbulence K.Energy = Zero normal gradient - Turbulence K.Dissipation = Wall model, Standard wall function |
| Front-2D | TWO_D |
| Back-2D | TWO_D |
The above boundary conditions can be defined only after defining a region in the Initial conditions and Events section of the case setup as shown below:
The turbulence in the flow is modelled using RNG-K-`epsilon` RANS model with standard wall function as shown below:
- Grid control:
The mesh size for the domain is defined in this tab as shown below:
The fixed embedding option is turned on to increase the cell count near the top and bottom walls to accurately capture the boundary layer physics.
Three mesh sizes are considered in this analysis as below:
- Mesh1 - dx = dy = dz =2e-3 m
- Mesh2 - dx = dy = dz =1.5e-3 m
- Mesh3 - dx = dy = dz =1.0e-3 m
- Output and post-processing:
The flow variables of interest, which are to be post-processed after the solution are selected in this option, and also at how many intervals these are written to the output file.
After all the above setup is completed final validation option is selected to check for the case setup is free of setup errors. After validation, the case setup files are exported using the export option in the File menu to the working folder. These files will be used by the CGYWIN terminal to run the solution. The above process is repeated for all the mesh sizes and the respective setup files are exported.
Solution in CYGWIN :
The solution is run in the CYGWIN terminal. First, we have to navigate to the respective working folder in the CYGWIN terminal, where case setup files are present. Then by executing the following command, the solution can be run:
Executing the converge.exe using 4 parallel processors to run the solution:
The solution will run-up to the number of cycles set in the run setup and the output files are saved to the respective folders.
V Post-Processing Results:
After the solution is complete, post-processing of the results is done using CONVERGE CFD and ParaView.
The plots are plotted in CONVERGE CFD, whereas to visualize the flow variables using contour plots the output files need to be converted to ParaView compatible files. This can be done in the CONVERGE CFD Post-Processing 3D tab as shown below:
Visualization in ParaView:
The converted ParaView compatible output files are opened in ParaView and the grid, contour plots for Pressure, Velocity, and velocity streamlines are visualized:
1. Mesh1 (dx = 2e-3m, dy = 2e-3m, dz = 2e-3m):
Grid:
Velocity Contour:
Pressure Contour:
Velocity Streamlines:
2. Mesh2 (dx = 1.5e-3m, dy = 1.5e-3m, dz = 1.5e-3m):
Grid:
Velocity Contour:
Pressure Contour:
Velocity Streamlines:
3. Mesh3 (dx = 1e-3m, dy = 1e-3m, dz = 1e-3m):
Grid:
Velocity Contour:
Pressure Contour:
Velocity Streamlines:
From the above contour plots, it can be observed that there is a low-pressure region at the section of the step, due to which the flow recirculation happens near the step. Also, the flow is more realistic with the mesh2 and mesh3 grid size. The velocity streamlines clearly show the recirculation zone after the step.
The flow development for all three grid sizes is shown in the below animation:
Graph plots in Converge CFD:
The graphs are plotted for various flow variables using the line plotting tab in Converge CFD:
1. Mesh1 (dx = 2e-3m, dy = 2e-3m, dz = 2e-3m):
Average Velocity at inlet(bound_id_1) and outlet (bound_id_2) :
Pressure at inlet(bound_id_1) and outlet (bound_id_2) :
Mass flow rate at inlet(bound_id_1) and outlet (bound_id_2) :
Cell count:
2. Mesh2 (dx = 1.5e-3m, dy = 1.5e-3m, dz =1.5e-3m):
Average Velocity at inlet(bound_id_1) and outlet (bound_id_2) :
Pressure at inlet(bound_id_1) and outlet (bound_id_2) :
Mass flow rate at inlet(bound_id_1) and outlet (bound_id_2) :
Cell count:
3. Mesh3 (dx = 1e-3m, dy = 1e-3m, dz =1e-3m):
Average Velocity at inlet(bound_id_1) and outlet (bound_id_2) :
Pressure at inlet(bound_id_1) and outlet (bound_id_2) :
Mass flow rate at inlet(bound_id_1) and outlet (bound_id_2) :
Cell count:
The above contour plots and graph plots show the velocity, pressure, mass flow rate, streamlines, and cell count for the flow over a backward-facing step geometry for three different mesh sizes.
Conclusion:
In this project, the basic case setup in ConvergeCFD along with turbulence modeling, running the setup in CYGWIN terminal, and post-processing in ParaView, Converge CFD is carried out for flow over a backward-facing step, for three different mesh sizes. The effect of the step geometry on the low-pressure region created and re-circulation due to this low-pressure region is studied using the RANS RNG k-`epsilon` turbulence model. Further study can be done with varying inlet velocity (flow Reynolds number) and analyzing its impact on the re-circulation.