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Free surface over a horizontal shear layer: vorticity generation and air entrainment mechanisms

Published online by Cambridge University Press:  26 January 2017

Matthieu A. André  and
Philippe M. Bardet
Matthieu A. André*
Affiliation:
Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC 20052, USA
Philippe M. Bardet*
Affiliation:
Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC 20052, USA
*
†Email addresses for correspondence: matandre@gwu.edu, bardet@gwu.edu
†Email addresses for correspondence: matandre@gwu.edu, bardet@gwu.edu
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Abstract

Two air entrainment mechanisms driven by vortex instability are reported in the unstable relaxation of a horizontal shear layer below a free surface. This flow is experimentally investigated by means of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) coupled with surface profilometry. PLIF identifies counter-rotating vortex pairs (CRVP) emanating from the surface following the growth of high steepness two-dimensional millimetre-size waves for Reynolds and Weber numbers based on the momentum thickness of 177 to 222 and 7.59 to 13.9, respectively. High spatio-temporal resolution PIV reveals the role of surface-generated vorticity and flow separation in the highly curved trough of the waves on the injection of a CRVP. Air bubbles are entrapped in the wake of these CRVPs at Reynolds number above 190. PIV data and spanwise PLIF images show two initiation mechanisms: primary vortex instability modulating the spanwise location where the flow separates, resulting in the pinch off of an air ligament, and secondary vortex instability turning a CRVP into $\unicode[STIX]{x1D6FA}$-shaped loops pulling the surface down. Instability wavelengths agree with linear stability analysis, and models for these new air entrainment mechanisms are proposed.

JFM classification

Drops and Bubbles: Drops and Bubbles Interfacial Flows (free surface): Interfacial Flows (free surface) Vortex Flows: Vortex Flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 813 , 25 February 2017 , pp. 1007 - 1044
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© 2017 Cambridge University Press 

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