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Linear stability of confined flow around a 180-degree sharp bend

Published online by Cambridge University Press:  09 June 2017

Azan M. Sapardi ,
Wisam K. Hussam ,
Alban Pothérat  and
Gregory J. Sheard Open the ORCID record for Gregory J. Sheard [Opens in a new window]
Azan M. Sapardi
Affiliation:
The Sheard Lab, Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia Department of Mechanical Engineering, International Islamic University Malaysia, Kuala Lumpur 53300, Malaysia
Wisam K. Hussam
Affiliation:
The Sheard Lab, Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia School of Engineering, Australian College of Kuwait, Safat 13015, Kuwait
Alban Pothérat
Affiliation:
Applied Mathematics Research Centre, Coventry University, Coventry CV1 5FB, UK
Gregory J. Sheard*
Affiliation:
The Sheard Lab, Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia
*
†Email address for correspondence: Greg.Sheard@monash.edu
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Abstract

This study seeks to characterise the breakdown of the steady two-dimensional solution in the flow around a 180-degree sharp bend to infinitesimal three-dimensional disturbances using a linear stability analysis. The stability analysis predicts that three-dimensional transition is via a synchronous instability of the steady flows. A highly accurate global linear stability analysis of the flow was conducted with Reynolds number $\mathit{Re}<1150$ and bend opening ratio (ratio of bend width to inlet height) $0.2\leqslant \unicode[STIX]{x1D6FD}\leqslant 5$. This range of $\mathit{Re}$ and $\unicode[STIX]{x1D6FD}$ captures both steady-state two-dimensional flow solutions and the inception of unsteady two-dimensional flow. For $0.2\leqslant \unicode[STIX]{x1D6FD}\leqslant 1$, the two-dimensional base flow transitions from steady to unsteady at higher Reynolds number as $\unicode[STIX]{x1D6FD}$ increases. The stability analysis shows that at the onset of instability, the base flow becomes three-dimensionally unstable in two different modes, namely a spanwise oscillating mode for $\unicode[STIX]{x1D6FD}=0.2$ and a spanwise synchronous mode for $\unicode[STIX]{x1D6FD}\geqslant 0.3$. The critical Reynolds number and the spanwise wavelength of perturbations increase as $\unicode[STIX]{x1D6FD}$ increases. For $1<\unicode[STIX]{x1D6FD}\leqslant 2$ both the critical Reynolds number for onset of unsteadiness and the spanwise wavelength decrease as $\unicode[STIX]{x1D6FD}$ increases. Finally, for $2<\unicode[STIX]{x1D6FD}\leqslant 5$, the critical Reynolds number and spanwise wavelength remain almost constant. The linear stability analysis also shows that the base flow becomes unstable to different three-dimensional modes depending on the opening ratio. The modes are found to be localised near the reattachment point of the first recirculation bubble.

JFM classification

Instability: Instability Wakes/Jets: Separated flows Waves/Free-surface Flows: Channel flow
Type
Papers
Information
Journal of Fluid Mechanics , Volume 822 , 10 July 2017 , pp. 813 - 847
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© 2017 Cambridge University Press 

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