Steady-State Simulation of Flow over a backward Facing Step Using Converge Studio
Aim:
The major objective of the project was to set up CFD simulation of the flow over a backward-facing step for 3 progressively refining mesh setting using converge Studio and result visualization using ParaView. To generate and analyze plot and contours for pressure, velocity, mass flow rate, and cell count.
Introduction:
The topic "Flow over a backward-facing step" is very important for those who want to check and validate their newly written cfd code so a lot amount of physical validation is available on this problem in order to validate the code. The report contains a detailed description of the simulation of Flow over a backward-facing step using converge CFD software, solving it using CYGWIN, and post-processing the result using ParaView. Various steps are discussed in this report while making a case set up for each case study. the report contains a detailed description of 3 case study with different grid sizing, line plots, and contours.
The three different mesh grids size are:
| S.No. | Dx(m) | Dy(m) | Dz(m) |
| 1 | 2.0e-3 | 2.0e-3 | 2.0e-3 |
| 2 | 1.5e-3 | 1.5e-3 | 1.5e-3 |
| 3 | 1.0e-3 | 1.0e-3 | 1.0e-3 |
The backward facing step is an intriguing case study for examining a turbulence model's performance and solution technique. The flow is subjected to a sudden increase in cross-sectional area in this situation, resulting in flow separation beginning at the point of expansion.
Turbulence outside the wall zone is caused by spatial changes in the velocity field, and its interaction with the mean flow determines the size of the separation bubble. One of the characteristics that a turbulence model must precisely forecast is the size of the bubble, also known as the re-attachment length.
STEADY STATE SIMULATION:
These are the simulation we run if are just interested in the end result and not in how those results are getting formed with time. The properties do not change with respect to time or there are no time derivative terms in the equations of motion that govern the fluid flow.
If we want to see how results are changing w.r.t time and how the solution is getting formed over time then we need to run the TRANSIENT STATE SIMULATION.
The time is in Cycles for STEADY STATE SIMULATION whereas time is in Seconds for TRANSIENT STATE SIMULATION.
1.Geometry Creation
First of all, the backward step geometry was setup using converge CFD software. The geometry was separately prepared using different CAD package ie Solidworks, then .stl file was imported to Converge Studio to set up the case study.
Image 1: Box geometry creation using Converge CFD.
*Each face of the step was split into triangles.
The Box Dimension is :
| Dx(m) | Dy(m) | Dz(m) |
| 0.272009 | 0.0199757 | 0.025 |
The Geometry can be downloaded from this link: Geometry
To create the geometry, the geometry was uploaded from the different CAD software package. All the normal vector were pointing out for this geometry, so we wanted it to point inside and normal to each surface. Normals point the direction of the fluid region generally. The command "Transform" was used to make the normal inside pointing the direction of the fluid region.
2.Boundary Creation
After creating the box Geometry, The Boundry tag was created along with different boundary id and coloring. A total of 5 different boundaries tag was created for 20 different split triangles of the box with each face split into two triangles. To create the boundaries naming, the "Boundary" command in the "Geometry" dock was used, and then the faces were appointed to appropriate tags along with id.
Image 2: Boundary creation representing the different id and color.
The five different boundaries were Inlet, Outlet, Top-Bottom, Front, and Back.
3.Case-Study Setup
The various parameter was considered while setup this case study. The case study setup dock was used to set up the problem which appears on the right-hand side of the converge studio.
Image 3: Case Setup Dock.
The steady state simulation monitor was used to study the steady state parameter i.e. Outlet mass flow rate.
3.1 Application Type:
In order to run a steady-state simulation, firstly, under the Application type in the setup tree, a "Time-based" simulation must be selected.
3.2 Materials:
Under the material tab "Gas simulation" was selected and "Air" as a predefined mixture.
Gas: Air
Global Transport Parameters: Fixed value
by default, the following value is considered.
Prandtl Number : 0.9
Schmidt Number : 0.78
Species: Oxygen-O2 & Nitrogen-N2
Reaction Mechanism (mech.dat) was selected for this problem.
3.3 Simulation Parameters
The Steady-state solver was selected for running this case study along with a "Pressure based" solver.
STEADY STATE SIMULATION:
These are the simulation we run if are just interested in the end result and not in how those results are getting formed with time. The time is in CYCLES for STEADY STATE SIMULATION.
The Following Simulation parameters were selected for running each simulation.
3.4: Boundary Conditions
In this tab, The volumetric region and physics for each boundary were defined.
| Boundary_Name | Boundary_Type | Paramter_Defined | Boundary ID |
| 1.Inlet | Inflow | 110325.0 Pa | 1 |
| 2.Outlet | OutFlow | 101325.0 Pa | 2 |
| 3.Top-Bottom | Wall |
Specified Value(300k) Law of wall |
3 |
| 4.Front | Two_D | 4 | |
| 5.Back | Two_D | 5 |
3.5: Initial Conditions and Events
Volumetric Region was created in order to assign this to all the boundary tag.
Species:- O2 (0.23) & N2 (0.77)
Pressure :101325Pa
Temperature:300K
3.6: Physical Models
A turbulent model was selected out of so many models.
3.7: Grid Control
The case study was run using 3 different mesh grid size. So different case study was set up in order to study the effect of value when the mesh is more refined.
3 different base mesh sizes are,
| S.No. | Dx(m) | Dy(m) | Dz(m) |
| 1 | 2.0e-3 | 2.0e-3 | 2.0e-3 |
| 2 | 1.5e-3 | 1.5e-3 | 1.5e-3 |
| 3 | 1.0e-3 | 1.0e-3 | 1.0e-3 |
3.7 Output/Post-Processing
Post-Variable Selection- Existing value
Output Files- Time interval for writing 3D output files-100.0
The time interval for writing restart files -1000.0
The case study was solved using CYGWIN and the post-processing was done in ParaView.
For post converting we used the following command
SIMULATION RESULTS
Mesh Size
Case Study 1: dx=dy=dz=2e-3 m
Mesh
Velocity Contour
Pressure Contour
Mass Average Flow Rate
Total Average Pressure
Total Average Velocity
Total Cell counts
Pressure Animation
Velocity Animation
Mesh Size
Case Study 2: dx=dy=dz=1.5e-3 m
Mesh
Velocity Contour
Pressure Contour
Mass Average Flow Rate
Total Average Pressure
Total Average Velocity
Total Cell counts
Pressure Animation
Velocity Animation
Case Study 3: dx=dy=dz=1 e-3 m
Mesh
Velocity Contour
Pressure Contour
Mass Average Flow Rate
Total Average Pressure
Total Average Velocity
Total Cell counts
Pressure Animation
Velocity Animation
Conclusion
Graph for the average mass flow rate through both inlet and outlet for all the three cases is compared below:- The mass flow rate for all the 3 cases are the same after 2500 cycles approx. the outflow and inflow mass rate are equal to 1.5 kg/s approximately.
Graph for the average velocity through both inlet and outlet for all the three cases is compared below:- The mass flow rate for all the 3 cases are the same after 2500 cycles approx. the outflow average flow velocity equal to 60m/s approximately.
Graph for the Total pressure through both inlet and outlet for all the three cases is compared below:- The mass flow rate for all the 3 cases are the same after 2500 cycles approx. the outflow total pressure equal to 104000 Pa approximately.
Image showing Different Zones of Turbulence
From this study it can be concluded that mesh study is very important in order to understand the sentivness of the results which are highly dependent on mesh. the medium mesh quality was appropriate as after that very little variation was observed. As the number of cells increses the computational power increses that why coarse mesh took less computational time as compared with fine mesh. The cross section is different due to that flow sepration can be seen and a recirculation zone is created which has low pressue zone.