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

1987 ◽  
Vol 175 (-1) ◽  
pp. 1 ◽  
Author(s):  
M. S. Acarlar ◽  
C. R. Smith
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Hairpin Vortices

Substructures in a turbulent spot

1988 ◽  
Vol 197 ◽  
pp. 389-414 ◽  
Author(s):  
R. Sankaran ◽  
M. Sokolov ◽  
R. A. Antonia
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Leading Edge ◽  
Simple Relation ◽  
Sufficient Evidence ◽  
Streamwise Direction ◽  
Geometrical Construction ◽  
Turbulent Spot ◽  
Hairpin Vortices ◽  
Streamwise Distance

Substructures within a turbulent spot which develops in a slightly heated laminar boundary layer have been identified using arrays of cold wires aligned in either a streamwise direction or in a direction normal to the wall. At any given streamwise distance from the spot origin, histograms of the number of detected substructures exhibit a peak, defining the most probable spot or the spot with the most likely number of substructures. The number of substructures in the most probable spot increases with streamwise distance but all substructures are convected at approximately the same velocity for any given distance from the wall. This velocity is approximately equal to that of the leading edge of the spot and increases slightly with distance from the wall. The increase in the number of substructures accounts for the streamwise growth of the spot. A simple relation is derived for determining the number of substructures at a particular streamwise station and a geometrical construction is proposed for identifying the origin of a new substructure. There is sufficient evidence for suggesting that the new substructures are formed near the trailing edge of the spot. The convection velocity, inclination and lengthscales of the substructures compare favourably with the corresponding characteristics of hairpin vortices.

scholarly journals Control of Synthetic Hairpin Vortices in Laminar Boundary Layer for Skin-Friction Reduction

2020 ◽  
Vol 8 (1) ◽  
pp. 45
Author(s):  
Bonguk Koo ◽  
Yong-Duck Kang
Keyword(s):  
Boundary Layer ◽  
Flow Visualization ◽  
Skin Friction ◽  
Laminar Boundary Layer ◽  
Water Channel ◽  
Friction Reduction ◽  
Control Technique ◽  
Hairpin Vortices ◽  
Local Skin ◽  
Hot Film

The results of flow visualization and hot-film measurement in a water channel are presented in this paper, in which the effectiveness of controlling synthetic hairpin vortices in the laminar boundary layer is examined to reduce skin friction. In this study, hairpin vortices were generated by periodically injecting vortex rings into a cross flow through a hole on a flat plate. To control the hairpin vortices, jets were issued from a nozzle directly onto the head of the hairpins. The results of the flow visualization demonstrated that the jets destroyed the hairpins by disconnecting the heads from their legs, after which the weakened hairpin vortices could not develop. Therefore, the circulation around the legs was reduced, which suggests that the direct intervention on the hairpin heads resulted in the reduction of streamwise stretching. Data obtained by a hot-film sensor showed that the high-speed regions outside the hairpin legs were reduced in speed by this control technique, leading to a decrease in the associated local skin friction.

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