The aim of this test case is to validate steady-state flow over a backward-facing step. The incompressible, turbulent case is validated with the experimental results of Driver and Seegmiller  as archived in the NASA Turbulence Modeling Resource . The following parameters have been analysed:
The geometry was meshed locally and a mesh upload to the platform was performed.
The geometry is constructed based on the reference case , as shown in Fig.1. The height of the step is h=12.7 cm, and the tunnel height is 8h. The origin is located at the base of the step. The face details have been given in Table 1.
The blockMesh tool was used to generate the mesh locally (see Fig.2. and Table 2.). A single-cell width was assigned in the z-direction to ensure a 2D mesh.
A typical property of the generated mesh is the y+ (“y-plus”) value, which is defined as the non-dimensionalized distance to the wall; it is given by y+=u∗y/ν. A y+ value of 1 would correspond to the upper limit of the laminar sub-layer.
- Explicit resolution of the near-wall region: The first cell lies at most at the boundary of the laminar sub-layer and no further. Here, y+ value is 1 or below.
- Use of wall-functions to resolve the near-wall region: There is no need to place cells very close to the laminar sub-layer, and typically 30⩽y+⩽300.
A y+ value of 30 was used for the inflation layer. The k−ω SST turbulence model was chosen, with wall functions for near-wall treatment of the flow.
Tool Type : OPENFOAM®
Analysis Type : simpleFoam
Mesh and Element types :
|Mesh type||Number of volumes||Type|
|blockMesh||5.5 * 105||2D hex|
Kinematic Viscocity (ν = 1.4694×10−5 m2s−1)
The boundary conditions for the simulation are shown in Table 3.
|Velocity||44.2 ms−1||Symmetry||0.0 ms−1||Zero Gradient|
|Pressure||Zero Gradient||Symmetry||Zero Gradient||0.0 Pa|
|k||5.336 m2s−2||Symmetry||Wall Function||Zero Gradient|
|ω||182.399 s−1||Symmetry||Wall Function||Zero Gradient|
Shown below in Figure 3 are comparisons of velocity profiles from SimScale simulation results with the reference  at different distances into the domain. All distances have been normalized with the step height h, and the velocity is normalized with respect to the inlet velocity vin=44.2 ms−1.
Fig.3. Vecloity profiles at different depths into the domain.
Coefficient of Pressure
Shown below in Figre 4 is the comparison of the cofficient of pressure Cp=P−P∞12ρV2∞ from SimScale simulation results with the reference  at the lower and upper walls.
Fig.4. Coefficient of Pressure at lower and upper walls
The reattachment length is the distance from the step at which the flow resumes in the positive flow direction all over the cross-section. The reattachment length was calculated to be 6.82 cm, which lies within a 12% error limit of the experimental value of 7.74 cm.
|||(1, 2, 3, 4) Driver, D. M. and Seegmiller, H. L., “Features of Reattaching Turbulent Shear Layer in Divergent Channel Flow,” AIAA Journal, Vol. 23, No. 2, Feb 1985, pp. 163-171.|
|||(1, 2) Langley Research Center: Turbulence Modeling Resource – 2D Backward Facing Step|
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