Simulating flow over backward facing step using Converge CFD   (Converge CFD, Skill Lync, Easy)
Aim:
To simulate the flow over backward facing step using Converge CFD and observe some of its characteristic features.
Introduction:
When a new turbulence model setting is to be tested for its accuracy, the flow over backward facing step is simulated using the test model. The results are compared to experimental results of this flow problem. This flow problem is simulated in current report using Converge CFD software. The basic description of backward facing step problem is that fluid enters a channel and then a sudden expansion in volume leads to drop in pressure and formation of recirculation flow regions. The below image points out the different features of this flow problem.
(Image credits: https://www.semanticscholar.org/paper/A-Computational-Investigation-of-a-Turbulent-Flow-a-Al-Jelawy-Kaczmarczyk/5c1c53173a20b85a52abcaefb2743daa61625a03)
In this report, a 2D analysis is done of backward flow problem. The images below show boundary names and dimensions of the geometry used for the simulations. The simulation is performed for 3 mesh sizes.
Here are the details of simulation:
Mesh Sizes: (Element size along X, Y, Z axes) (in metres)
There is only one element along Z axis.
1) (0.0020, 0.0020, 0.025)
2) (0.0015, 0.0015, 0.025)
3) (0.0010, 0.0010, 0.025)
The feature of 'Fixed Embedding' is used on 'walls' with scale as 1.
Type of simulation: Steady state with only gas flow simulation.
Steady State monitor properties: (Used to stop simulation when difference between two monitor variable values in sample is lower than tolerance)
Monitor variable: Average Velocity at outlet calculated from mass average flow otput file.
Monitor start delay: 200 cycles
Sample size: 100 cycles
Relative Tolerance: 0.001
Boundaries:
Inlet: Inflow boundary condition with pressure inlet at 110325 Pa.
Outflow: Outflow boundary condition with pressure outlet at 101325 Pa (difference of 9000 Pa)
Top and Bottom: No Slip walls.
Front and Back: 2D faces. (It tells solver to not solve in Z direction)
Initial region pressure = 101325 pa
Turbulence model: RNG k-e model with standard wall functions.
Solver Properties:
Solver: Pressure based 'PISO' solver with 'step' as flux limiter.
Details: 2 minimum iterations and 20 max iterations per cycle. Tolerance = 0.001
Minimum Cycles = 0
Maximum Cycles = 15000
Minimum time step = 1e-6 sec
Maximum time step = 1 sec
Max Convection and diffusion CFL: 1
Max Mach CFL: 50
Results:
1) Mesh:
The mesh got refined near walls by using embedding feature.
2) Pressure Contour:
The pressure gets reduced in region where flow seperates and recirculation flow can be seen. The pressure in the region (98000 Pa) can be seen going below outlet pressure (101325 Pa)
3) Presssure Plot:
The outlet pressure is higher than 101325 Pa. It can also be seen that simulation of case 3 ran for less cycles than case 2.
4) Velocity contour & vector plot:
The velocity contours show that the flow inside channel was nearly developed. It also shows that the velocity in recirculation region is low (0 - 5 m/s). The vectors plots show the flow of air near the step region. The flow reversal can be seen clearly.
5) Streamlines: (of case 3 only)
The Streamlines show the primary and secondary recirculation regions.
6) Velocity plot:
The velocity doesn't seem to converge to a specific value suggesting that the solution is grid dependent.
7) Mass Flow Rate:
The mass flow rate of inlet nearly matches with outlet, suggesting mass conservation.
8) Total cells:
9) Velocity animations:
Conclusion:
The flow over backward facing step is simulated using Converge CFD and its flow features like recirculation region can be seen in the results.