Flow over a backward facing step
Aim :
To simulate flow over a backward facing step with 3 different mesh size and compare their velocity and pressure plots.
Given :
- dx = dy = dz =2e-3m
- dx = dy = dz =1.5e-3m
- dx = dy = dz =1.2e-3m
Geometry:
Geometry can be created in any cad software and imported to converge in stl format .In our case geometry was provided
for which scientific data is already available so that after our simulation we could validate the results .
Mesh:
Case 1:
- dx = dy = dz =2e-3m
case 2:
- dx = dy = dz =1.5e-3m
case 3:
- dx = dy = dz =1.2e-3m
The refined mesh that we see at the boundary is a result of permanent fixed embedding at boundry with value of 1mm cell size
No of cell counts for each case :
Here we can observe the cell count of each case which is
- 2k for case 1
- 3.2k for case 2
- 4.7k for case 3
Velocity and Pressure plots:
case 1: dx = dy = dz =2e-3m
velocity
pressure
case 2:
- dx = dy = dz =1.5e-3m
velocity
pressure
case 3:
- dx = dy = dz =1.2e-3m
velocity
pressure
We observe that as we refine the mesh we get better distribution of contours ,which in turn would allow us to analyse the results better.
The only drawback is it would increase the computation time.
Plots for velocity , pressure and massflow rate at the outlet:
Average velocity
Average pressure
Massflow rate
Looking at the plots we observe that the colution converges at around 3500 cycles .
RED : mesh size of 0.002
Blue : mesh size of 0.0015
yellow : mesh size of 0.0012
Average velocity at outlet was found to be around 58m/s
Average pressure at outlet was found to be 103714pa
Average mass flow rate was found to be 1.4kg/s
Observation:
In the above vector plot we see that some vectors are not pointing in the direction of flow .This happens because of sudden pressure drop,
which happens due to sudden change in area . This leads to seperation of flow.We observe there are many unstabilities behind backward facing step.This phenomina also occurs in airoplane wings knows as vortex shedding.