Simulating 2D backward Facing step using Converge CFD for different mesh sizes
Objective
Setting up 2D Backward Facing Step using converge CFD. Here we are using different mesh sizes to observe the variations of different attributes such as mass flow rate,Pressure,velocity and total cells generated .here we are using mesh sizes of
dx=dy=dz=0.0012
dx=dy=dz=0.0009
dx=dy=dz=0.0005
Steps involved in running Channel flow simulation
1)Geomentry Creation
2)Boundary flagging
3)Diagnosing the geomentry
4)Case Setup
5)Solve
5)Post processing the results
Backward Facing Step Theory
The backward facing step is an interesting case for studying the performance and solution strategy of a turbulence model. In this case, the flow is subjected to a sudden increase of cross-sectional area, resulting in a separation of flow starting at the point of expansion.
Spatial variations in the velocity field cause production of turbulence outside the wall region and its interaction with the mean flow influences the size of the separation bubble. The size of the bubble, or the re-attachment length, is one of the quantities that must be predicted accurately by a turbulence model.
Geomentry
Here a rectangular geomentry with 0.1m x 0.01m x 0.0125m is created to perform a analysis and then boundary flagging and Diagnosing the geomenty to check any intersections or any anomoly related to geomentry check is done then lastly checkinh the nornals directions whether inward or outward as the flow taking place inside the domains normals need to point inside.then case setup begins.
Case Setup
=>In case setup we select General flow and then we leave the input parameters under material are kept default the only thing we choose air as our fluid.
=>Going to simulation parameter we are going to choose steady state(Pressure-Based) and hydrosimulation(The main difference between Hydro and non-hydro is that no navier stokes equtions are solved in non-hydro it is only used to check our mesh changes while running simulation).then rest of then are set to default. Under Simulation time parameter cycles to run the simulation is set to 10000 with Initial and minimum time step of 1e-9 and maximum time step to 1 and rest are set to defaults
=>Coming to boundary there are 5 boundaries are flagged with different Boundary ID, thhey are
Inlet:Set to inflow Here are we are specifying pressure of 110325
Outlet:Set to outflow Specifying Pressure as 101325
Front wall and Back wall: Set to 2D because here we are running 2D simulation
top & bottom wall:Set to walls
All the boundaries are set to region0 with air at 101325 pa and 300K
=>Region 0 is created at initial condition and events.Here were are not performing turbulance simulation so we can turn off turbulance we are only concentrating on Laminar flow.
=>Mesh is created with base grid and fixed embedding with scale=2 and layers= 2 at the top and bottom wall to capture the velocity at walls more accuratly.then postprossesing output files are set to defaults because the data which we are looking is already in default selections
Then we need to export the input files to a folder to start running a simulation.then by using CYgwin we are directing to the input file and simulation is stared by using an executable file and simulation is runned by using a single processor. After successfully running the simulation we can post process the reults and look for the attributes variation with the mesh
Case-I
Mesh
0.0012
Velocity Contour
Pressure Contour
Vector Plot (at Step Region)
Velocity Plot at 0.55 from Inlet
Pressure Plot along X axis
Mass Flow Rate(Kg/s)
Total Cells
Case-II
Mesh
0.0009
Velocity Contour
Pressure Contour
Vector Plot(at step Region)
Velocity Plot at 0.55 from Inlet
Pressure Plot along X axis
Mass Flow Rate(Kg/s)
Total Cells
Case-III
Mesh
0.0005
Velocity Contour
Pressure Contour
Vector Plot(at step Region)
Velocity Plot at 0.55 from Inlet
Pressure Plot along X axis
Mass Flow Rate(Kg/s)
Total Cells
Conclusion:
By performing mesh refinement we got to know the time taken for the solution to get converged is increased as the grid size reduces because solver need to solve for more cells.But if we try to fine the mesh we can capture results more accuratly here by refining the mesh we can see vortex more clearly.And the plots are getting more smoother and accurately. Here we used fixed embedding to capture results more accuratly at the top and bottom walls so that we can clearly understant what was going on the the wall of the domain