Convective turbulent viscosity acting on equilibrium tidal flows: new frequency scaling of the effective viscosity

Abstract

Turbulent convection is thought to act as an effective viscosity (${\nu }_E$) in damping tidal flows in stars and giant planets. However, the efficiency of this mechanism has long been debated, particularly in the regime of fast tides, when the tidal frequency ($\omega$) exceeds the turnover frequency of the dominant convective eddies (${\omega }_c$). We present the results of hydrodynamical simulations to study the interaction between tidal flows and convection in a small patch of a convection zone. These simulations build upon our prior work by simulating more turbulent convection in larger horizontal boxes, and here we explore a wider range of parameters. We obtain several new results: 1) ${\nu }_E$ is frequency-dependent, scaling as ${\omega }^{-0.5}$ when $\omega /{\omega }_c\lesssim 1$, and appears to attain its maximum constant value only for very small frequencies ($\omega /{\omega }_c\lesssim 10^{-2}$). This frequency-reduction for low frequency tidal forcing has never been observed previously. 2) The frequency-dependence of ${\nu }_E$ appears to follow the same scaling as the frequency spectrum of the energy (or Reynolds stress) for low and intermediate frequencies. 3) For high frequencies ($\omega /{\omega }_c\gtrsim 1-5$), ${\nu }_E\propto {\omega }^{-2}$. 4) The energetically-dominant convective modes always appear to contribute the most to ${\nu }_E$, rather than the resonant eddies in a Kolmogorov cascade. These results have important implications for tidal dissipation in convection zones of stars and planets, and indicate that the classical tidal theory of the equilibrium tide in stars and giant planets should be revisited. We briefly touch upon the implications for planetary orbital decay around evolving stars.

arXiv comment20 pages, 10 figures, 3 tables, accepted for publication in MNRAS on 23rd July 2020

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