Flow Simulation in a backward facing step
Introduction to Backward Facing step:
The Backward-Facing Step (BFS) flows, also called as “sudden expansion flows”, “circular expanding flow”, “back-step flow”, “diverging channel” or “backward flow” under some other occasions, involves the basic features of general separation flows.
Backward-Facing Step (BFS) flow is one representative model for separation flows, which can be widely seen in aerodynamic flows (airfoil, spoiler, high attack angle process), engine flows, condensers, vehicles (cars, boat), heat transfer systems, and even the flow around buildings, etc. The flow separation after a simple stage will introduce separation bubble formation, evolution and re-attachment process, which is dependent on the BFS geometric design, the inlet and outlet conditions, turbulent intensity, as well as heat transfer conditions.
Geometry
Geometry of the backward facing step along with the geometry bouding box
Inlet Boundary condition
outlet Boundary condition
Walls Boundary condition
Size of each computational cell along each of the three directions is specified here.
The fixed embedding feature is used to set different mesh sizes to specific surfaces or specific boundaries. This is helpful to set small element sizes near boundaries. This is used to capture the boundary physics properly and hence obtain accurate results at the boundary. The boundary capturing is extremely important as critical phenomenon occurs at the boundaries.
Case1:
Mesh size dx=dy=dz=2e-3m
Mesh for dx=dy=dz=2e-3m
Velocity contour for mesh size 2e-3m
pressure contour for mesh size 2e-3m
mass flow rate of air at inlet
mass flow rate of air at outlet
total pressure of air at inlet
static pressure of air at outlet
average velocity of air at inlet
average velocity of air at outlet
Cell Count
The animations of pressure and velocity are as above.
Case2:
Mesh size dx=dy=dz= 1.5e-3m.
Mesh
Pressure contour
velocity contour
mass flow rate at inlet
mass flow rate at outlet
Total pressure inlet
static pressure outlet
velocity inlet
velocity outlet
Cell count
The above links are the animations for pressure and velocity with respect to time.
Case3:
Mesh size dx=dy=dz=1.2e-3m
Mesh
pressure contour
velocity contour
inlet mass flow rate
outlet mass flow rate
Total pressure inlet
Static pressure outlet
Inlet average velocity
outlet average velocity
Cell Count
The above links are the animations of pressure and velocity for mesh size of 1.2e-3.
Recirculation zones in the step of lower height.
Conclusions:
The backward facing step is a classical benchmark investiigative CFD problem to look into the physics of boundary layers, recirculation zones. The recirculation of the fluid occurs between the bottom wall of the upper step and the lower step area after there is a sudden increase in the area of the fluid flow. The results obtained in the three cases of meshes are very similar to each other indicating the problem is grid independent. The boundaries have to be captured accurately and for this less element sizes are used as compared to the other regions of the geometry. The inlet and outlet mass flow rates are the same indicating the mass flow of fluid entering is equal to the mass of the fluid exiting. The cell count is increasing as the size of a cell is decreasing.
The time taken for cell size of 2e-3 is 700 seconds and for 1.5e-3 is 1020 seconds and for 1,2e-3 is 1200 seconds indicating that as the cell count increases, the time taken for the simulation to complete also increases.