A study of hairpin vortices in a laminar boundary layer. Part 1. Hairpin vortices generated by a hemisphere protuberance

Abstract

It has been suggested that hairpin vortices may play a key role in developing and sustaining the turbulence process in the near-wall region of turbulent boundary layers. To examine this suggestion, a study was done of the hairpin vortices generated by the interaction of a hemisphere protuberancee within a developing laminar boundary layer. Under the proper conditions, hairpin vortices are shed extremely periodically, which allows detailed examination of their behaviour. Shedding characteristics of the hemispheres were determined using hot-film-anemometry techniques. The flow patterns created by the presence of the hairpin vortices have been documented using flow visualization and hot-film-anemometry techniques, and cross-compared with the patterns observed in the near-wall of a fully turbulent boundary layer. In general, it has been observed that many of the visual patterns observed in the near-wall region of a turbulent boundary layer can also be observed in the wake of the hairpin-shedding hemisphere, which appears supportive of the importance of hairpin vortices in the near-wall turbulence production process. Furthermore, velocity measurements indicate the presence of strong inflexional profiles just downstream of the hairpin-vortex generation region which evolve into fuller profiles with downstream distance, eventually developing a remarkable similarity to a turbulent-boundary-layer velocity profile.

Advanced Fluid Research On Drag reduction In Turbulence Experiments –AFRODITE–

“…The amount of vorticity concentrated in the streamwise legs of the standing vortex depends on the relative height of the element with respect the boundary layer thickness k/δ. It must be pointed out that the experiments by Acarlar & Smith (1987) show a more complicated flow pattern. Indeed, depending on the top edge geometry of the elements and for sufficiently high values of the roughness Reynolds number Re k , a shedding of periodic hairpin vortices may be present.…”

“…Setting up the streamwise streaks in an experiment White (2002) investigated the transient growth of disturbances of small amplitude (stable) steady streaks generated by using a spanwise periodic array of roughness elements of circular section Bakchinov et al (1995); Kendall (1990), after a complex region of growing and decaying modes a high speed region is induced behind the roughness element. An explanation for this behavior can be found by considering the perturbation induced by a roughness element in a wall-bounded shear flow (Hunt et al, 1978;Acarlar & Smith, 1987;Dèlery, 2001). The spanwise vorticity of the incoming shear flow is wrapped around the front part of the obstacle forming a steady horseshoe-shaped vortex structure with the two streamwise legs pointing downstream.…”

Lift-up process in a heated-cylinder wake flow

“…This lift-up process is also often encountered when discussing the evolution of hairpin structures in a boundary layer flow. 22 To illustrate what happens in detail for the heated wake flow under consideration, the corresponding temperature isosurfaces are examined in Fig. 14 during this transformation process.…”

Investigating the Three-dimensional Flow Separation Induced by a Model Vocal Fold Polyp

“…While flow separation from a wall mounted hemispheroid in steady flow has been investigated [15][16][17][18][19][20][21][22][23] , surprisingly, there is little information regarding unsteady three-dimensional flow separation from a hemispheroid on a wall subject to pulsatile or unsteady flow conditions as are found in speech. The seminal work of Acarlar and Smith 15 provided an analysis of the three-dimensional coherent structures generated by steady flow over a wall mounted hemispheroid within a laminar boundary layer.…”

“…The seminal work of Acarlar and Smith 15 provided an analysis of the three-dimensional coherent structures generated by steady flow over a wall mounted hemispheroid within a laminar boundary layer. Acarlar and Smith identified two types of vortical structures.…”