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Competition between the centrifugal and strato-rotational instabilities in the stratified Taylor–Couette flow

Published online by Cambridge University Press:  06 February 2018

Junho Park Open the ORCID record for Junho Park [Opens in a new window] ,
Paul Billant ,
Jong-Jin Baik  and
Jaemyeong Mango Seo
Junho Park*
Affiliation:
School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea
Paul Billant
Affiliation:
LadHyX, CNRS, Ecole Polytechnique, F-91128 Palaiseau CEDEX, France
Jong-Jin Baik
Affiliation:
School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea
Jaemyeong Mango Seo
Affiliation:
School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea
*
†Email address for correspondence: jhsise@gmail.com
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Abstract

The stably stratified Taylor–Couette flow is investigated experimentally and numerically through linear stability analysis. In the experiments, the stability threshold and flow regimes have been mapped over the ranges of outer and inner Reynolds numbers: $-2000<Re_{o}<2000$ and $0<Re_{i}<3000$, for the radius ratio $r_{i}/r_{o}=0.9$ and the Brunt–Väisälä frequency $N\approx 3.2~\text{rad}~\text{s}^{-1}$. The corresponding Froude numbers $F_{o}$ and $F_{i}$ are always much smaller than unity. Depending on $Re_{o}$ (or equivalently on the angular velocity ratio $\unicode[STIX]{x1D707}=\unicode[STIX]{x1D6FA}_{o}/\unicode[STIX]{x1D6FA}_{i}$), three different regimes have been identified above instability onset: a weakly non-axisymmetric mode with low azimuthal wavenumber $m=O(1)$ is observed for $Re_{o}<0$ ($\unicode[STIX]{x1D707}<0$), a highly non-axisymmetric mode with $m\sim 12$ occurs for $Re_{o}>840$ ($\unicode[STIX]{x1D707}>0.57$) while both modes are present simultaneously in the lower and upper parts of the flow for $0\leqslant Re_{o}\leqslant 840$ ($0\leqslant \unicode[STIX]{x1D707}\leqslant 0.57$). The destabilization of these primary modes and the transition to turbulence as $Re_{i}$ increases have been also studied. The linear stability analysis proves that the weakly non-axisymmetric mode is due to the centrifugal instability while the highly non-axisymmetric mode comes from the strato-rotational instability. These two instabilities can be clearly distinguished because of their distinct dominant azimuthal wavenumber and frequency, in agreement with the recent results of Park et al. (J. Fluid Mech., vol. 822, 2017, pp. 80–108). The stability threshold and the characteristics of the primary modes observed in the experiments are in very good agreement with the numerical predictions. Moreover, we show that the centrifugal and strato-rotational instabilities are observed simultaneously for $0\leqslant Re_{o}\leqslant 840$ in the lower and upper parts of the flow, respectively, because of the variations of the local Reynolds numbers along the vertical due to the salinity gradient.

JFM classification

Instability: Instability Geophysical and Geological Flows: Stratified flows
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 840 , 10 April 2018 , pp. 5 - 24
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Copyright
© 2018 Cambridge University Press 

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