flow over backward facing step

Flow Over A Backward Facing Step

Introduction

The purpose of this report is to understand flow over a backward facing step using converge CFD tool. a case set up is produced in converge with a step height of 9.98 mm, when the turbulent flow encounters a abrupt backward step the flow separates and reattach after some time. in this report simulation of a specimen with three different mesh size is shown, 2 mm, 1.5 mm, 1 mm, respectively. the turbulant model used is RNG k-e.

Mesh

The images of three different mesh size is shown below:

mesh size 1mm

mesh size 1.5mm

mesh size 1mm

 The mesh is refined at the top and bottom wall Boundary with a scale of 1.

Contour

The contour of velocity and pressure are shown below. the file is produced using peraview.

velocity contour for mesh size 2mm

Pressure contour for mesh size 2mm

velocity contour for mesh size 1.5mm

Pressure Contour for mesh size 1.5mm

velocity contour for mesh size 1mm

Pressure Contour for mesh size 1mm

The velocity contour shows that their is a turbulant boundary layer in the first passage before the step, as the step is introduce the separetion takes place as the area increases. vortex shedding occours at the bottom of the wall due to a sudden step.

Plots

above are the plots for mesh size 2 mm with bound_id_1 being inlet and bound_id_2 the outlet

above are the plots for mesh size 1.5mm with bound_id_1 being inlet and bound_id_2 the outlet.

above are the plots for mesh size 1 mm with bound_id_1 being inlet and bound_id_2 the outlet.

The mass flow graph for inlet and outlet shows steady state at 1.4 Kg/s after 2000 cycles, the negetavie and posetive sign represents inflow and outflow respectively.
The plot for pressure gives a steady state static pressure of 98000 Pa at inlet and total pressure of 104200 Pa at outlet boundary.
The Avg Velocity at inlet and outlet -140 m/s and 68 m/s respectively, the drop in velocity is due to change in area.

Flow Separetion

The flow separates because of the expansion caused by the step, as the flow expands an adverse pressure gradient is formed which causes the flow reversal.

The link below shows the animation of velocity for the three mesh size.
https://www.youtube.com/watch?v=83HNmEC43ds