Heavy inertial particles in turbulent flows gain energy slowly but lose it rapidly

We present an extensive numerical study of the time irreversibility of the dynamics of heavy inertial particles in three-dimensional, statistically homogeneous and isotropic turbulent flows. We show that the probability density function (PDF) of the increment, $W(tau)$, of a particles energy over a time-scale $tau$ is non-Gaussian, and skewed towards negative values. This implies that, on average, particles gain energy over a period of time that is longer than the duration over which they lose energy. We call this $textit{slow gain}$ and $textit{fast loss}$. We find that the third moment of $W(tau)$ scales as $tau^3$, for small values of $tau$. We show that the PDF of power-input $p$ is negatively skewed too; we use this skewness ${rm Ir}$ as a measure of the time-irreversibility and we demonstrate that it increases sharply with the Stokes number ${rm St}$, for small ${rm St}$; this increase slows down at ${rm St} simeq 1$. Furthermore, we obtain the PDFs of $t^+$ and $t^-$, the times over which $p$ has, respectively, positive or negative signs, i.e., the particle gains or loses energy. We obtain from these PDFs a direct and natural quantification of the the slow-gain and fast-loss of the particles, because these PDFs possess exponential tails, whence we infer the characteristic loss and gain times $t_{rm loss}$ and $t_{rm gain}$, respectively; and we obtain $t_{rm loss} < t_{rm gain}$, for all the cases we have considered. Finally, we show that the slow-gain in energy of the particles is equally likely in vortical or strain-dominated regions of the flow; in contrast, the fast-loss of energy occurs with greater probability in the latter than in the former.