UFR 3-33 Evaluation

Turbulent flow past a smooth and rigid wall-mounted hemisphere

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Evaluation

Unsteady results

Based on the experimental and numerical unsteady data the flow field is characterized using the systematic classification map of the unsteady flow patterns given by Savory and Toy (1986) (see Fig. 1). Seven regions are highlighted:

• (1) Upstream the hemisphere the horseshoe vortex system dominates. The hemispherical bluff body acts as a barrier, leads to positive pressure gradient, so that the boundary layer separates from the ground forming this the horseshoe vortex system.

• (2) The stagnation area is located close to the lower front surface of the hemisphere, where the stagnation point is found at the surface at an angle of about $\theta _{\text{stag}}^{\text{LDA}}\approx 166^{\circ }$ (definition of $\theta$ in Fig. 1).

• (3) Then, the flow is accelerated along the hemisphere. Therfore, it is called the acceleration area. It results to a high level of vorticity near the hemispherical surface.

• (4) At an angle of $\theta _{\text{sep}}^{\text{LDA}}\approx 90^{\circ }$ the flow detaches along a separation line.

• (5) As a consequence of the flow separation a recirculation area appears.

• (6) Strong shear layer vorticity can be observed leading to the production of Kelvin-Helmholtz vortices which travel downstream in the flow field.

• (7) To close the recirculation area the flow reattaches at the reattachment point. In this region the ``splatting effect occurs, redistributing momentum from the wall-normal direction to the streamwise and spanwise directions.

Fig. 1: Visualization of flow regions and characteristic flow features of the flow past the hemisphere: (1) horseshoe vortex system, (2) stagnation area, (3) acceleration of the flow, (4) separation point, (5) dividing streamline, (6) shear layer vorticity, (7) reattachment point.

Fig. 2: Snapshot of unsteady vortical structures visualized by utilizing the iso-surfaces of the pressure fluctuations ($p'/(\rho _{\text{air}}U_{\infty }^{2})=-2.47\times 10^{-4}$) colored by the spanwise instantaneous velocity.

Fig. 3: Velocity spectra at the monitoring points $P_{1}$ and $P_{2}$ in the wake regime of the hemisphere.

Fig. 4: Vortex shedding from the top of the hemisphere visualized by the pressure fluctuations of the LES in the symmetry plane.

Fig. 5: Visualization of the two vortex shedding types present in the wake behind the hemisphere.

Data files

Numerical data

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Contributed by: Jens Nikolas Wood, Guillaume De Nayer, Stephan Schmidt, Michael Breuer — Helmut-Schmidt Universität Hamburg

Front Page

Description

Test Case Studies

Evaluation

Best Practice Advice

References

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