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Material spike formation in highly unsteady separated flows

Published online by Cambridge University Press:  26 November 2019

Mattia Serra Open the ORCID record for Mattia Serra [Opens in a new window] ,
Seán Crouzat ,
Gaël Simon ,
Jérôme Vétel Open the ORCID record for Jérôme Vétel [Opens in a new window]  and
George Haller Open the ORCID record for George Haller [Opens in a new window]
Mattia Serra*
Affiliation:
School of Engineering and Applied Sciences, Harvard University, Cambridge, MA02138, USA
Seán Crouzat
Affiliation:
Department of Mechanical Engineering, LADYF, Polytechnique Montréal, Montréal, QC,H3C 3A7, Canada
Gaël Simon
Affiliation:
Department of Mechanical Engineering, LADYF, Polytechnique Montréal, Montréal, QC,H3C 3A7, Canada
Jérôme Vétel
Affiliation:
Department of Mechanical Engineering, LADYF, Polytechnique Montréal, Montréal, QC,H3C 3A7, Canada
George Haller
Affiliation:
Institute for Mechanical Systems, ETH Zürich, Leonhardstrasse 21, 8092Zürich, Switzerland
*
†Email address for correspondence: serram@seas.harvard.edu
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Abstract

We apply the recent frame-invariant theory of separation spike formation to complex unsteady flows, including a turbulent separation bubble, an impinging jet, and flows around a freely moving cylinder and a freely rotating ellipse. We show how the theory captures the onset of material spike formation, without any assumption on the flow type (steady, periodic, unsteady) or separation type (on- or off-wall, fixed or moving boundaries). We uncover new phenomena, such as the transition from on-wall to off-wall separation, the merger of initially distinct spikes, and the presence of severe material spikes that remain hidden to previous approaches. Remarkably, even in steady flows around curved boundaries, we detect material spikes in the absence of flow reversal, the main ingredient to existing separation criteria. Together, our results unveil how an involved network of spikes arises, interacts and merges dynamically, leading to the final ejection of particles from the wall in highly transient flow separation processes.

JFM classification

Boundary Layers: Boundary layer separation Mathematical Foundations: Topological fluid dynamics Nonlinear Dynamical Systems: Pattern formation
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 883 , 25 January 2020 , A30
Copyright
© 2019 Cambridge University Press

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