Coherent Flow Structures at Earth's Surface by Jeremy G. Venditti & James L. Best & Michael Church & Richard J. Hardy

Author:Jeremy G. Venditti & James L. Best & Michael Church & Richard J. Hardy
Language: eng
Format: epub
ISBN: 9781118527191
Publisher: Wiley
Published: 2013-08-21T04:00:00+00:00

The existence of hairpin vortices, and others, is supported by the three-dimensional visualization of these vortical structures, through an iso-surface of a Galilean-invariant Q (Hunt et al., 1988), which is defined as:

(12.4)

where is the rate of the strain tensor and is the vorticity tensor. Hunt et al. (1988) defined a vortex as a region with Q > 0, in which the pressure attains a local minimum and hence as a local region in which vorticity dominates shear strain, allowing rotation and shear be to distinguished.

In the preliminary studies of Bommiyanuni (2010), the dominant coherent structures of flow over a single hemisphere were educed using the Q-criterion (Figure 12.7). The oncoming laminar boundary layer flow was free of large-scale flow structures and thus provided a clear look at the structures that evolve as a result of the obstacle. Figure 12.7 reveals a number of structures. Upstream of the hemisphere, a horseshoe vortex (HSV) forms as the boundary layer flow separates in front of the hemisphere, leading to a multi-component horseshoe vortex system that features several secondary vortices (sHSV). Behind the hemisphere, hairpin vortices (HP-1, HP-2 and HP-3) evolve as a result of shear between the outer flow and the recirculation zone. The hairpin vortices grow as they travel downstream. The legs of the horseshoe vortex contain significant streamwise vorticity, very similar to the vortex tubes in high- and low-speed streaks, which also results in the generation of hairpin vortices. These secondary hairpin vortices (sHP) are considerably smaller than those evolving in the lee of the hemisphere.

Figure 12.7 Turbulent flow structures over a hemisphere visualized by the Q-criterion.

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