Flow over back step: parametric study

This is a classical case in Fluid Dynamics which has been investigated widely, experimentally as well as numerically. This flow configuration is sometimes used as a validation case for any numerical methods in Computational Fluid Dynamics.
The computational domain used for this parametric study is described in following sketch. The two parameters related to this flow configurations are (a) Expansion Ratio (ratio of the height downstream the expansion to the height before expansion) and "Aspect Ratio" (ratio of width of the channel to that of height of channel before step expansion).
Back-Step: Computational Domain
The mesh generated using free-meshing in ICEM CFD is depicted below. The Tcl/Tk script used to create the geometry, generate high quality all-quad mesh and extrude in normal direction can be accessed here. This script can be made fully parametric by defining appropriate variables. It is at the moment left to interested visitor.
Mesh - Backward Facing Step

The boundary condition and conditions is shown in next image.
Back-Step: Boundary Conditions

The material property and other features of the simulation are:
  • Solver: CFX V16
  • Incompressible air
  • Constant transport properties (no dependence on temperature)
    Back-Step: Material Properties
  • Smooth walls with no-slip boundary conditions

The results with k-ε model is presented in following plots.

The Y+ plot on the walls of the back-step, velocity contour and wall shear are shown here.
Y-plus Contour
Figure: Y-plus contour with k-ε turbulence model

Velocity Contour
Figure: Velocity contour with k-ε turbulence model
Shear Stress
Figure: Wall shear stress contour with k-ε turbulence model

The results with SST model is presented in following plots.

Velocity Contour:SST
Figure: Y+ contour with SST turbulence model
Velocity Vector - SST
Figure: Velocity vector plot with SST turbulence model
Total Pressure - SST
Figure: Contour of Total Pressure with SST turbulence model
Static Pressure - SST
Figure: Contour of Static Pressure with SST turbulence model
From the previous two figures, contour plots of static and total pressures, one may observe that the static pressure is higher towards the exit, even after expansion losses in the sudden expansion at the back step.


  • Backward-Facing Step Flows for Various Expansion Ratios at Low and Moderate Reynolds Numbers: G. Biswas (Mechanical Engineering Department, Indian Institute of Technology, Kanpur-208016, India), M. Breuer, F. Durst (Lehrstuhl fuer Stroemungsmechanik, Universitaet Erlangen-Nuernberg Cauerstr. 4, D-91058 Erlangen, Germany e-mail: breuer@lstm.uni-erlangen.de

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