The evolution of co-rotating vortices in a canonical boundary layer with inclined jets

Abstract

The evolution of co-rotating streamwise vortices in a canonical flat plate turbulent boundary layer (thickness of the boundary layer δ_0.99 = 25 mm) is studied. The vortices are produced by an array of inclined jets (diameter D = 14 mm) with the same orientation (skew angle β and pitch angle α of 45°). The focus of the investigation is on the immediate vicinity of the jet exit and downstream locations up to 40 D. The Reynolds number based on the diameter of the jet nozzle ranges from 9700 to 29000, at various jet speed ratios. The main method of investigation is laser Doppler anemometry. Both mean and statistic data are collected and analyzed. The streamwise vortices are a product of complex fluid flow process, featuring horseshoe vortices in front of the nozzle exit, recirculating flow to the lee side of the nozzle, contra-rotating vortices from the rolling up of vortex sheet around the jet, strong and induced spanwise flow. Two types of streamwise vortices are produced: (a) weak vortices at a jet speed ratio λ of 0.5 located close to the wall and featuring diametrically opposed, secondary, near-wall flows in between the vortices, (b) strong vortices at higher jet speed ratio featuring significant spanwise movement. The vortices are accompanied by high levels of turbulence, with distinct normal and shear stress distributions. Both turbulence production and convection play important roles in defining the normal stress but only the turbulence production is important in determining the shear stress.

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