Week 3: Flow over a backward facing step
Aim:- To run the simulation of Flow over a Backward facing step with three different base mesh sizes with help of using Converge Studio, Cygwin and Paraview and camparing the parameters.
Objective:-
1. Running simulations with three different base mesh sizes
- dx = dy = dz =2e-3m
- dx = dy = dz =1.5e-3m
- dx = dy = dz =1.2e-3m
2. Showing the Mesh (Surface with Edges)
3. Showing Pressure and Velocity Contours
4. Showing the plots of velocity, pressure, mass flow rate and total cell count for all the 3 base mesh sizes
5. Explaining the flow separation occured in the channel
Introduction:- Backward facing step is a study about the turbulence occured in internal flow because of sudden change in geometry and due to this sudden change,flow seperation occured which leads to Pressure and Velocity parameters drop and create recirculation and reattachment zone.
Procedure:-
1. Geometry- Download the geometry of backward facing step (bfs.cvg) or create the geomety
2. Normal toggles:- Normals Orientated inside the geometry
3. Diagonsis Check:- Diagnosis check is carried out for any error occured in triangles
4. Boundary Flagging- Assigning triangles for creating boundaries (Inlet, Outlet, Top and Bottom, Front and Back)
5. Case setup:- Steps are followed for setting up a case are given below
A. Application type- Time based
B. Material- Air is used as a fluid, Gas simulation and Reaction mechanism are clicked and steps are given below
a. Gas simulation:-
b. Globle transport parameter:-
c. Reaction mechanism
C. Simulation Parameter- Run a Steady state simulation
For Setting up the Simulation parameters, Following steps are given below
a. Run Parameter:-
b. Steady state monitoring:-
c. Simulation time parameters:-
d. Solver parameters:- Navier Stokes solver type is based on pressure.
D. Boundary Conditions:-
a. Inlet boundary- Pressure -110325 pa
b. Outlet:- Pressure- 101325 pa
c. Top and Bottom:-
d. Front and Back are two dimensional
E. Initial conditions and Events:- Region 0 created and initiated
F. Physical Methods:- Turbulence model- RNG k-ε is selected
G. Grid control:
a. Base grid:- 1. dx = dy = dz =2e-3m
2. dx = dy = dz =1.5e-3m
3. dx = dy = dz =1.2e-3m
b. Fixed embedding:- Embedding setup
H. Output/post processing:-
a. Post variable selection
b. Output files:-
I. After Case setup done, click on Validate all to check any error occurs if not then export all files to particular folder
J. After exporting all files to the folder, copy the mpiexec.exe application and paste in the folder and run the simulation by using cygwin with a command of
mpiexec.exe -n 4 "C:\ProgramFiles\Convergent_Science\CONVERGE\3.0.16\bin\intelmpi\converge.exe" restricted logfile & after reaching to the folder directory.
K. After Simulations done, all post files converted into binary ouput file by using Converge or Cygwin (post.convert.exe) because Paraview reads only binary files to create 3D output.
Experiment:-
Case 1 :- dx = dy = dz =2e-3m
a. Velocity Contour
<iframe src="//www.youtube.com/embed/LCWb59iQuZI" allowfullscreen="allowfullscreen"></iframe>
b. Pressure Contour
<iframe src="//www.youtube.com/embed/ZRNnWmxO3P4" allowfullscreen="allowfullscreen"></iframe>
c. Mesh (Surface with edges)
d. Plot of Mass flow rate
e. Plot of Velocity
f. Plot of Pressure
g. Plot of Total Cell Count
Case 2 :- dx = dy = dz =1.5e-3m
a. Velocity Contour
<iframe src="//www.youtube.com/embed/OBBMwiyQMEw" allowfullscreen="allowfullscreen"></iframe>
b. Pressure Contour
<iframe src="//www.youtube.com/embed/RwtWH-feHNg" allowfullscreen="allowfullscreen"></iframe>
c. Mesh (Surface with edges)
d. Plot of Mass flow rate
e. Plot of Velocity
f. Plot of Pressure
g. Plot of Total Cell Count
Case 3 :- dx = dy = dz =1.2e-3m
a. Velocity Contour
<iframe src="//www.youtube.com/embed/Z0D81xNELiU" allowfullscreen="allowfullscreen"></iframe>
b. Pressure Contour
<iframe src="//www.youtube.com/embed/BFCLLB2G0yU" allowfullscreen="allowfullscreen"></iframe>
c. Mesh (Surface with edges)
d. Plot of Mass flow rate
e. Plot of Velocity
f. Plot of Pressure
g. Plot of Total Cell Count
Flow Separation:- As per the flow analysis in backward facing step, Flow is laminar form at starting stage after sudden expansion in geometry, flow changes into turbulence form because of pressure and velocity of flow drops and that creates the flow separation. This flow separation creates Recirculation and Reattachment zones. After reattachment, flow changes back to laminar form.
Observation:-
1. In a Flow over a backward facing step simulation, Flow separation from the boundary layer occured after the sudden expansion in geometry due to this recirculation and reattachment zones created and this leads to Velocity drop and Pressure drop.
2. At inlet stage, flow is laminar form after the expansion in middle stage flow change into turbulence form and after reattachment flow turn back to laminar form at outlet stage.
3. Fixed embedding is used to fining the mesh of boundary layer, it helps to better clarity of flow separation, recirculation and reattachment zone.
4. Turbulence in flow is calculated by RNG k-ε model.
5. Navier Stokes solver type for steady state is pressure based.
6. When mesh is getting finner it leads to increasing in total cell counts.
Conclusion:- Flow over a backward facing step with three different base mesh sizes are carried out successfully by using Converge CFD, Cygwin and Paraview and results found satisfactory.