Week 3: Flow over a backward facing step

Flow over a backward-facing step using CONVERGE CFD

Aim:

Simulating the flow over Backward facing step using CONVERGE CFD.

Objectives:

1. Able to simulate the flow over Backward facing step for three different base mesh sizes
   • different base mesh sizes are,
     • dx = 2e-4m, dy = 2e-4m, dz = 2e-4m
     • dx = 1.5e-4m, dy = 1.5e-4m, dz = 1.5e-4m
     • dx = 1.0e-4m, dy = 1.0e-4m, dz = 1.0e-4m
2. Explain Results & plots for velocity, pressure, mass flow rate, and total cell count for all 3 base mesh sizes.

Flow simulation in CONVERGE done in 3 steps:

1. Geometry preparation and Case setup :
   • We use Converge GUI(Graphical User Interface) to set up the case and export input files.
2. Solving the problem using input files:
   • These input files are solved by Converge in the Terminal (Command-line interface). We USed Cygwin for the command line terminal.
3. Post-processing results:
   • We convert our solution into Paraview binary format to post-process the results in Paraview

Geometry preparation in CONVERGE CFD:

• Geometry preparation:
  • Import the ".STL" for the backward-facing step.
    
    • 
• Boundary setting:
  • We have to set Inlet, Outlet, Top and Bottom walls, Front & Backface boundaries for the above-created geometry.
  • By default In CONVERGE, all the surfaces are made up of triangles we have to select respective triangle faces for the boundary.
    • 
    • 
• Normal Transformation:
  • All the face normals directions should be facing towards the flow region.
  • In this fluid is flowing through the channel but normals are facing outwards. We have to transform the normal's direction.
  • 
  • 
• Diagnosis:
  • Checking for any interactions, open edges, etc.
  • 

Case Setup in CONVERGE CFD:

1. Application Type :
   • 
2. Materials:
   • Set default values in Gas Simulation & Global transport parameters.
   • Species: Add Gas species O2 & N2.
     • 
3. Initial Conditions and Events:
   • Add Volumetric region in regions
   • Add species by modifying their concentrations so that we can clearly observe the difference in post-processing.
   • 
4. Boundary conditions:
   • Defining boundary type and conditions:
     • 

   • Boundary Name	Type	Boundary Condition	Value	Species
     Inlet	INFLOW	Total Pressure	111325 Pa	O2 : 0.23 & N2 : 0.77
     Outlet	OUTFLOW	Pressure	101325 Pa	O2 : 0.23 & N2 : 0.77
     Top_Bottom_walls	WALL	No-Slip
     Front_2D	TWO_D
     Back_2D	TWO_D

     • INLET:
       • 
     • OUTLET:
       • 
     • Top_Bottom_walls
       • 
     • FRONT_2D & BACK_2D
       • 
5. Simulation Parameters:
   • Run Parameters:
     • Selecting the Solver & solver type. informing additional things to solve (Energy & Momentum equations) and monitoring steady-state.
     • 
   • Steady-state monitor:
     
     • In this, we have to Add monitor variables to be monitored during the simulation at different boundaries.
     • 
   • Simulation Parameter:
     • defining initial, Minimum & Maximum Time step sizes.
     • 
   • Solver Parameters:
     • Solver Scheme to PISO and Density-based solver.
     • under Equations window set precondition to None for all the variables.
     • 
     • 
6. Physical Models:
   • usingRenoulds Average Navier stroke equation
     • 
7. Grid Control:
   • This is the section where we define meshing size(Grid size).
   • Enable the Fixed embedding to refine the mesh at walls.
     • 
       
       1. dx = 2e-4m, dy = 2e-4m, dz = 2e-4m
          • 
       2. dx = 1.5e-4m, dy = 1.5e-4m, dz = 1.5e-4m
          • 
       3. dx = 1.0e-4m, dy = 1.0e-4m, dz = 1.0e-4m
          • 
8. Output/Post-Processing
   • Post Variable selection: select the variables to be monitored for post-processing.
     • 
     • 
9. Exporting Input files:
   • Export the input files
     • 

Solving Channel Flow in Cygwin:

1. Go to the directory where input files are exported
2. Make mpiexec.exe & converge.exe files globally available.
3. "mpiexec.exe -n 4 converge.exe restricted </dev/null> logfile &"
   
4. Use above mentioned line to start calculation in the background
5. mpiexec.exe & converge.exe are executable applications to solve the problem
6. -n 4:- number of processors
7. restricted:- my license is a restricted one.
8. </dev/null> logfile:- creates temporary memory to save data with name as logfile at a particular iteration
9. & :- To run Calculation in Background
10. "tail -10 logfile" command is used to view the last 10 lines of the logfile.
11. "mpiexec.exe -n 1 post_convert_30_serial_64" command used to convert output files to paraview binary format, so that they can be processed in the paraview.

Post Processing and Results from Paraview:-

• open "..vtm" group file in paraview. This group contains all the solution files.
• use the slice tool to create a contour section plane and plot the contours
• use Glyph to create velocity vectors
• Case 1: dx = 2e-4m, dy = 2e-4m, dz = 2e-4m
  • Mesh
    • 
    • 
  • Velocity Contour
    • 
  • Velocity Vector
    • 
  • Pressure Contour
    • 
  • Temperature Coutour
    • 
  • Plots:
    • bound_id_1 = inlet
    • bound_id_2 = outlet
    • 
    • 
    • 
    • 
• Case 2: dx = 1.5e-4m, dy = 1.5e-4m, dz = 1.5e-4m
  • Mesh
    • 
    • 
  • Velocity Contour
    • 
  • Velocity Vector
    • 
  • Pressure Contour
    • 
  • Temperature Coutour
    • 
  • Plots
    • bound_id_1 = inlet
    • bound_id_2 = outlet
    • 
    • 
    • 
    • 
• Case 3: dx = 1.0e-4m, dy = 1.0e-4m, dz = 1.0e-4m
  • Mesh
    • 
    • 
  • Velocity Contour
    • 
  • Velocity Vector
    • 
  • Pressure Contour
    • 
  • Temperature Coutour
    • 
  • Plots
    • bound_id_1 = inlet
    • bound_id_2 = outlet
    • 
    • 
    • 
    • 

Result Table:

	mesh size dx=dy=dz=	Total cells	Simulation time (seconds)	Mass flow rate(Kg/s)		Velocity(m/s)		Pressure (Pa)
	(m)			Inlet	Outlet	Inlet	Outlet	Inlet(Static)	Outlet(Total)
Case_1	0.002	1943	103.27	1.00115	1.00117	94.3839	44.0573	105694	102505
Case_2	0.0015	2994	141.76	1.07700	1.07600	102.178	47.0424	104749	102669
Case_3	0.001	5516	211.72	1.06101	1.06103	100.524	46.3512	104955	102624

Conclusion:

• As the number of cell count increases solution accuracy and simulation time increase.
• From velocity vector contours we can observe detailed turbulence as the mesh side is getting decreased.