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Shear/rotation competition during the roll-up of acoustically excited shear layers

Published online by Cambridge University Press:  12 April 2018

Abbas Ghasemi Open the ORCID record for Abbas Ghasemi [Opens in a new window] ,
Burak Ahmet Tuna  and
Xianguo Li Open the ORCID record for Xianguo Li [Opens in a new window]
Abbas Ghasemi
Affiliation:
Department of Mechanical and Mechatronics Engineering, University of Waterloo 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
Burak Ahmet Tuna
Affiliation:
Department of Mechanical and Mechatronics Engineering, University of Waterloo 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
Xianguo Li*
Affiliation:
Department of Mechanical and Mechatronics Engineering, University of Waterloo 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
*
†Email address for correspondence: [email protected]
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Abstract

Naturally developing and acoustically excited shear layers at the Reynolds numbers $Re_{\unicode[STIX]{x1D703}_{0}}=U\unicode[STIX]{x1D703}_{0}/\unicode[STIX]{x1D708}=85{-}945$ are studied using the hot-wire (HW) anemometry and particle image velocimetry (PIV), with a focus on the shear/rotation competition during the initial Kelvin–Helmholtz (KH) roll-up. Velocity spectra and the spatial linear stability (LST) analysis characterize the fundamental ($f_{n}$) and its subharmonic ($f_{n}/2$) mode interacting due to the vortex pairing. For $276\leqslant Re_{\unicode[STIX]{x1D703}_{0}}\leqslant 780$, the root-mean-square (r.m.s.) of the streamwise turbulence intensity shows a double-peaking phenomenon, i.e. major and minor peaks of the $u_{rms}$ coexist towards the high-speed (HS) and the low-speed (LS) sides, respectively. The single/double-peaked $u_{rms}$ profiles are found to be correlated with the scattered/organized distribution of the shear/rotation, demonstrating a transitioning character with the downstream distance, $Re_{\unicode[STIX]{x1D703}_{0}}$ and the upstream turbulence levels. The rotating vortex cores and the corresponding peripheral shear regions, demonstrate the phase reversal of the velocity fluctuations with respect to the HS and the LS sides. Excitation at $f_{n}$ increases the vortex count by 21 %, advances the location of the first KH roll-up and hence also the minor peak formation location. Due to the enhanced pairing at the $f_{n}/2$ forcing, the vortex count reduces by 23 %. Before merging into the downstream rotation core, the upstream vortex is shifted towards the HS side and the major peak is accordingly augmented. Actuation advances the transition to the nonlinear state, as well as the saturation of the amplification factor. The volumetric topologies of the shear/rotation loops tracked in consecutive phases during the period of the acoustic excitation, separate from the edge and grow in time–space due to the viscous diffusion. The shearing and rotating loops are found to be associated with the thinning (elongation) and expansion (accumulation) of the vorticity, respectively.

JFM classification

Flow Control: Instability control Turbulent Flows: Shear layer turbulence Vortex Flows: Vortex Flows
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 844 , 10 June 2018 , pp. 831 - 854
Copyright
© 2018 Cambridge University Press 

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Ghasemi et al. supplementary movie

Movie 1. Shear/rotation loops obtained from the planar phase-locked PIV measurements interpolated into volumetric topologies. The phases ϕ=00, 600, 1200, 1800, 2400, 3000 during the acoustic actuation of the fundamental mode animate the shear (blue) and rotation (red) loops of the <Q>p-criterion passed through the vorticity cut-planes (green). The shear loops are found correlated with the thinning in the stretched vorticity zones while the rotating loops intersect with the expansion/accumulation of the vorticity.

Download Ghasemi et al. supplementary movie(Video)
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