Flow over a backward facing step using Converge CFD

 Objectives:

  • To create a 3D Geometry of Backward facing step using CONVERGE studio.
  • To setup the case in CONVERGE and run the Backward facing step flow simulation for three different mesh sizes using Cygwin.
  • To post Process the results using Paraview
  • To compare the results of three simulation

Description: 

The Backward facing step is a representative model, depicts the separation flows and it is widely used in Aerodynamics, IC Engine simualtions, condensers, and flow across the architectural models. The Backward facing step is also called as sudden expansion flows, circular expanding flows, backward flow or diverging channel, back step flow.

Creating the Geometry:

  1. Creating the Vertices     
  • Click on the Geometry -> Create -> Vertex -> Assign the value of vertex by coordinate -> Apply
  • Continue the process to generate all vertices.

             

     2. Creating the Triangular faces

  • Click on the Geometry -> Create -> Triangle -> From three vertices -> select the vertices to generate the triangular face
  • Continue the process to generate all triangular face on one side(i.e., Front/Back).

       

     3. Copying the triangular faces (i.e to offset the triangular faces)

  • Click on the Geometry -> Create -> Copy -> select all the triangular face entities -> Assign the offset coordinates in (x,y,z direction) -> Apply.

         

     Note: For selecting the triangular faces, select cursor pick and triangle. 

     4. Enclose the two planar surfaces with other planar surfaces.

  •  Click on the Geometry -> Create -> Triangle -> Loft Edges -> tick the box of select first set of edges -> select the first open edge ->tick the box of select second set of edges ->select the second open edge -> Apply

                           

Note: For selecting the openedges, choose Edge and in edge choose By open Edge option.

  • After creating the Geometry, to check the errors for 3D Geometry, click on Diagnosis, check for Intersections, open edges, Nonmanifold edges, Normal Orientation, Isolated triangles 

                        

  • Click on Find for identifying the errors, find out the errors and remove the errors. In this case there are no errors.

                      

     5. Creating the bounding box around the Geometry.

  • Click on Geometry -> options -> tick Geometry bounding box

  • Click on Normal Toggle, Perpendiculars to the triangular faces are coming out.

                            

  • Because of internal flow the normals direction is to be changed, To change the direction of Normals, Click on Transform from Geometry -> Normal and select any triangular face -> Apply

                        

     6. Assigning the Boundaries

  • From Geometry, select

            Boundary -> Flag -> 'Click on +' for adding the required boundaries 

  • Select the 'Cursor pick' and 'triangle' for selecting the faces.
  • Select the boundary, select the faces and click on Apply
  • Right click on the Boundary for assigning the name for the selection.

                             

 

 

Case 1: (Mesh,  dx = 2e-3 m, dy = 2e-3 m, dz = 2e-3m)

Setting up the Case:

  • Click on Case Setup -> Being Case Setup -> tick General flow -> Done

                              

   

  • Click on Materials, set the Predefined mixtures as Air, tick the boxes of Gas simulation and Reaction mechanism then click on Apply -> Yes(to overwrite gas.dat, mech.dat and therm dat) -> Done.

                         

                            

  • Click on Global transport parameters -> Ok
  • Click on Simulation Parameters -> Done                                                                                                 
  • From Simulation Parameters, click on Run parameters, set the solver to Pressure-based steady solver then click on OK -> Yes(to load default tolerance/relaxation/PISO -> Yes(load the default of Non-Engine application.                                                                                                                     

             

           

  • From Simulation Parameters,Click on Simulation time Parameters, assign the Start time, End time, Initial time-step, Minimum time-step, Maximum time-step and Maximum convection CFL limit 

          

  • Click on OK.
  • From Simulation Parameters, Click on Solver parameters -> Ok.

                

  • From Initial conditions & Events, click on  Regions and initialization, click on Add.
  • From species, click on +Air

     

  • Double click on Region Name, change the name to Volumetric_region
  • Click on Ok.
  • Click on Turbulence Modeling , then click Ok. To set the Default RNG k - `epsilon` model.

  • From Grid Control, Click on  Base grid, change the Base grid size dx, dy, dz to 2e-3m then Click on Ok.

                         

  • Click on Grid control -> tick the fixed embedding ->Done.

            

  • From Grid Control, Click on Fixed embedding -> 'Entity type - Boundary' -> 'Boundary ID - Top & Bottom Walls' -> 'Mode - PERMANENT' -> 'Scale - 1' -> Ok.

  • From Output/ Post Processing, Click on Post Variable selection -> Ok.

              

  • From Output/ Post Processing, Click on Output files -> Ok.

       

  • From Boundary conditions, Click on Boundary then
  • Click on Inlet for assigning the boundary conditions
  • select the Boundary Type as INFLOW
  • From Pressure Boundary Conditions, change the total pressure to 110325 Pa.
  • From Species Boundary Condition, 'Click on +Air' to add species

  

 

  • Double click on Region name for inlet, change the region name to volumetric_region.
  •  Click on Front, select the Boundary Type as TWO_D 

  • Double click on Region name for Front, change the region name to volumetric_region.
  • Click on Back-2D, select the Boundary Type as TWO_D

  • Double click on Region name for Back, change the region name to volumetric_region.
  • Click on Top-Bottom-Walls, select the Boundary Type as WALL
  • Defaults were maintained.

  • Click on Outlet, select the Boundary Type as OUTFLOW 
  • From Pressure Boundary Conditions, change the total pressure to 101325 Pa.
  • From Species Backflow, 'Click on +Air' to add species

  • Double click on Region name for Outlet, change the region name to volumetric_region.
  • Complete Case Setup is validated, no issues found.

 

Mesh (dx = 2e-3;  dy = 2e-3;   dz = 2e-3):

Velocity Contour:

Pressure Contour:

Velocity vectors:

The recirculation takes place at the sudden increase/fall of cross-sectional area, but in this case the recirculation exists in the flow domain is at the sudden increase of area, because of sudden increase in area the reduction in Static Pressure occurs and results to recirculation, from the above image it is clear that at the recirulation area the static pressure is small.

Velocity Animation:

Plotting Results:

Average Velocity:

Static Pressure:

Total Pressure:

 Mass Flow Rate:

Total Cells:

Case 2 : (Mesh dx = 1.5e-3;  dy = 1.5e-3;   dz = 1.5e-3):

The Geometry and case setup is same as the previous case, but in the Grid Control, base grid has to be changed to 1.5e-3, and also the number of cycles from simulation time parameters has changed to 25000.

 

Velocity Contour:

 

Pressure Contour:

 

Velocity vectors:

 

The recirculation takes place at the sudden increase/fall of cross-sectional area, but in this case the recirculation exists in the flow domain is at the sudden increase of area, because of sudden increase in area the reduction in Static Pressure occurs and results to recirculation, from the above image it is clear that at the recirulation area the static pressure is small.

Velocity Animation:

 

Plotting Results:

Average Velocity:

Static Pressure:

 

Total Pressure:

 

 Mass Flow Rate:

 

Total Cells:

 

 Case 3 : (Mesh dx = 1.2e-3;  dy = 1.2e-3;   dz = 1.2e-3):

The Geometry and case setup is same as the previous case, but in the Grid Control, base grid has to be changed to 1.2e-3, and also the number of cycles from simulation time parameters has changed to 40000.

 

Velocity Contour:

 

Pressure Contour:

 

Velocity vectors:

 

The recirculation takes place at the sudden increase/fall of cross-sectional area, but in this case the recirculation exists in the flow domain is at the sudden increase of area, because of sudden increase in area the reduction in Static Pressure occurs and results to recirculation, from the above image it is clear that at the recirulation area the static pressure is small.

 Velocity Animation:

 

Plotting Results:

Average Velocity:

 

Static Pressure:

 

Total Pressure:

 

 Mass Flow Rate:

 

Total Cells:

 

Conclusion:

  • As the grid size reduces the results accuracy increases.
  • Simulation time increases with the increase in number of cycles, as the mesh becomes finer, the number of cycles increases.
  • If the recirculation come into play, then finer mesh helps to carry out the study of recirculation at micro level and also helps in more detailed visualization.
  • As the mesh becomes finer the velocity plot becomes smoother, where as the velocity increase is too small. 
  • For all the three cases the Average velocity, static Pressure, Total Pressure, Mass Flow rate becomes stable after 'n' number of cycles increase at inlet and outlet. 

Projects by Shravankumar Nagapuri

 

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