A study of hairpin vortices in a laminar boundary layer. Part 2. Hairpin vortices generated by fluid injection

Abstract

It has been suggested that hairpin vortices are a major sustaining flow structure involved in the perpetuation of turbulent boundary layers, although their origin within the boundary layer is unclear. One hypothesis is that hairpin structures are formed by the breakdown of the low-speed streak structures which develop adjacent to the surface beneath turbulent boundary layers. To examine this hypothesis, a water-channel study has been done which utilizes injection through surface slots in a flat plate to create artificial low-speed streak-type regions beneath a laminar boundary layer. Under appropriate conditions, these synthesized low-speed streaks develop a three-dimensional, shear-layer instability which breaks down to form a hairpin-vortex street. Employing both flow visualization and anemometry measurements, the characteristics of these hairpin structures and the parameters influencing their generation have been examined. The hairpin streets were determined to develop in a very periodic and repeatable manner within a definite range of flow parameters. Detailed flow patterns obtained using dye and hydrogen bubbles, both individually and collectively, indicate a remarkable similarity with previously observed patterns in the near-wall region of turbulent boundary layers. In addition, the development of the hairpin structures is observed to be quite sensitive to external forcing, as well as exhibiting a tendency for organized development of larger, more complex structures through a pairing-type process. Velocity measurements indicate the initial presence of strong inflexional profiles which evolve rapidly to velocity and turbulence-intensity profiles commensurate with those associated with turbulent boundary layers, but which do not exhibit the marked spreading associated with turbulence.

The near wall effect of synthetic jets in a boundary layer

“…Similar observations were made by Acarlar and Smith (1987) formed out of the orifice. The vortical structure produced by the synthetic jet is seen to experience an asymmetric roll up, similar to the findings of Zhong et al (2005).…”

Numerical Simulation of Roughness-Induced Transient Growth in a Laminar Boundary Layer

“…Further calculations would also help delineate the critical range of roughness heights beyond which the flow becomes unsteady and vortex shedding occurs as the precursor to the formation of turbulent spots (Acarlar andSmith 1987, Klebanoffet al 1992). An additional.…”

Self-Sustained Localized Structures in a Boundary-Layer Flow

“…The root of the instability responsible for the regeneration of these secondary structures is not local but has to be sought upstream, precisely where hairpin vortices are generated. Isosurfaces of negative 2 show that neighboring streamwise vortices located near the pinching region can reconnect to form hairpin-shaped vortices lifted up above the low-speed streaks [22,23]. Hairpin vortices form as the finite-amplitude outcome of streak instabilities of the subharmonic-sinusoidal or varicose types [24].…”