Flow over a backward facing step
AIM:- To successfully simulate the flow inside a backward face step.
INTRODUCTION:- Objective of this simulation is to study the flow separation phenomenon of the fluids either externally or internally. In this we took backwards step face, this is because of its simple geometry. Many geometries can give the flow separation, but here we have only one separated flow region. This is the reason behind it to select the BFS.
Geometry:- All dimensions shown in the figure are in SI units
Boundary Flagging:-
1. Inlet and outlet are in lying on the left and right-hand sides of the geometry.
2. Top & bottom walls are not moving hence those are just walls.
3. Front and back walls of the geometry are given as 2-D.
4. Air is taken as the medium or fluid passing through the channel.
Boundary Conditions:-
1. At the inlet pressure is given as boundary condition: 110325 Pa (Total as B.C but Total=static because of stationary fluid).
2. Outlet pressure is static and 101325 Pa or 1 bar
3. Temperature is 300k
4. Walls are in no-slip condition ( The velocity of the fluid layer attached to a solid surface is zero and thereafter it increases slowly).
CASE-SETUP:-
1. We start with the application type.
2. After selecting the appropriate choice, whether its a transient or steady-state we move on to materials.
3. Materials are nothing but the fluids we work with the present simulation, here we have pre-defined mixtures. You can select any of these.
4. In the subsections of materials, we have species, which is N2 and O2 from the air.
5. In the next step, simulations parameters are the choice.
6. As we can see in the illustrated picture above, type of solver, type of flow is of main interest in this step.
7. This is a steady-state simulation hence we need to give the time parameters, such as no. of cycles, initial time step, min. time step and max. time step.
8. In the subsection of this, we have solver parameters such as density-based or pressure-based solver, the algorithm either PISO or SIMPLE.
9. In the next step, the region of operation is going to be selected, the air is the region and it is assigned to every other boundary.
10.
1. We start with the application type.
2. After selecting the appropriate choice, whether its a transient or steady-state we move on to materials.
3. Materials are nothing but the fluids we work with the present simulation, here we have pre-defined mixtures. You can select any of these.
4. In the subsections of materials, we have species, which is N2 and O2 from the air.
5. In the next step, simulations parameters are the choice.
6. As we can see in the illustrated picture above, type of solver, type of flow is of main interest in this step.
7. This is a steady-state simulation hence we need to give the time parameters, such as no. of cycles, initial time step, min. time step and max. time step.
8. In the subsection of this, we have solver parameters such as density-based or pressure-based solver, the algorithm either PISO or SIMPLE.
9. In the next step, the region of operation is going to be selected, the air is the region and it is assigned to every other boundary.
10. In the next step, the physical models are of main interest, whether it's a combustion modelling, spray modelling, VOF or Turbulence.
11. In this, we are dealing with the turbulent flows, hence turbulence modelling is required.
12. We can go to physical models to enable turbulence modelling.
13. In turbulence modelling, we have many methods to model the flow. Mainly we have RANS and DNS.
14. In RANS modelling again there are many different methods, here we chose the RNG k-epsilon model.
15. Description of these briefed at just before the Conclusion section.
16. In the next step, the base grid is to be mentioned, as given in the challenge we have three different step sizes: 1e-3, 1.5e-3 and 2e-3.
12. In the last step, the variables which we are interested in monitoring during the simulation, those can be checked.
13. Before we proceed further, another important step is to validate the inputs, to do that right-click on any of the case setup options and click on validating all.
14. If there are no errors it's good to go.
15. Before we start with simulation, we need to export all the input case data to the working directory.
16. To export the data, click on the file, go to export, select export input files.
Simulation using Cygwin:-
1. Open Cygwin and navigate to the exported files in the above step.
2. To change the directory, use command "cd drive(name: c,d,e)" : Cygwin is Case sensitive.
3. After changing the directory, then type " mpiexec.exe -n 8 converge-mpi.exe restricted </dev/null> logfile & " to start the simulation.
4. A log file will be submitted, in the directory in the name of the output folder.
5. Repeat the same steps with different base grid sizes as mentioned in the challenge.
6. To change the base grid size every time, no need to open the Converge Studio again and again. In the files, we exported, data that needed for simulation to run are stored.
7. Go to the folder where these files are stored, look for .in type of files, click on the file named solver.in and edit it with a text editor.
8. In that search for the input that you gave in the first simulation. If the base grid is given as 2e-3 for x,y and z look for those and carefully change to the required size.
9. This procedure can be repeated for the other base grid size too.
Post-Processing (Conversion):-
1. After the simulation is done, we have files that are in the .h5 format, these files need to be converted into other formats to start post-processing.
2. To convert these files into Para View readable format, we can utilise the converge converter given with converge studio.
3. Open Cygwin, change the directory to the folder that contains output files.
4. Write the command " mpiexec.exe -n 8 post_converter_mpi.exe " and hit enter.
5. On the Cygwin command interface, a number of options will be given for you to chose the format of the output files.
6. Chose the Para View vtk binary format, because it is the format Para View can read.
7. After choosing the file format, a number of variables appear on the screen. Choose the variables that you need to post-process the data.
8. You can choose all, to choose all type "all" hit enter or if you want particular files only then type the number before the variable you need to convert and hit enter.
9. After hitting the enter, you can see the post-processing is done by the converter in Cygwin.
10. You have the files that can be accessible by Para View.
Post-Processing (Analyzing and Plotting):-
1. First, open the Para View load the .vtm files.
2. We don't need to analyze the data along the z-axis, because we assumed the flow t be 2-D in the operating region.
3. Cut the geometry using a plane normal to the z-axis, click on slice option on the left-hand top in the common controls bar.
4. Click on that to get a 2-d geometry.
Results:-
2e-3 base grid plots
Velocity contour:-
Pressure Contour:-
MESH:-
PRESSURE _Vs_Cycles:-
MASS FLOW_Vs_Cycles:-
Velocity_Vs_Cycles:-
Total Cells:-
1.5e-3 base grid plots
Velocity contour:-
Pressure contour:-
MESH:-
PRESSURE _Vs_Cycles:-
MASS FLOW_Vs_Cycles:-
Velocity_Vs_Cycles:-
Total Cells:-
1.0e-3 base grid plots
1.5e-3 base grid plots
Velocity contour-:
Pressure Contour:-
MESH:-
Pressure_Plot:-
Velocity plot-:
Mass_Flow_Plot:-
Total Cells:-
Observations:-
1. The first and foremost thing in this simulation is about flow separation.
2. Flow is being separated due to the adverse pressure gradient, but why is this adverse pressure gradient?
3. Let's start from the other point of view, as we can see from the above picture, there's definitely vorticity is being involved in the flow.
4. Vorticity is defined as the curl of a velocity field. For 2-D flows, this is going to be reduced to a simple scalar, vorticity exists only around the z-direction .i.e, perpendicular to the XY plane.
5. To have positive vorticity in the flow there must be some negative vorticity acting, but this comes from the eqns below.
6. From the above explanation, we can say that the adverse pressure gradient in flow is because of the negative vortex gradient on the wall and hence positive vortex gradient in the flow.