  We determine the Strouhal number (hereafter St), which is essentially a
nondimensional measure of the correlation time, from numerical calculations of
convection. We use two independent methods to estimate St. Firstly, we apply
the minimal tau-approximation (MTA) on the equation of the time derivative of
the Reynolds stress. A relaxation time is obtained from which St can be
estimated by normalising with a typical turnover time. Secondly, we calculate
the correlation and turnover times separately, the former from the
autocorrelation of velocity and the latter by following test particles embedded
in the flow. We find that the Strouhal number is in general of the order of 0.1
to 1, i.e. rather large in comparison to the typical assumption in the
mean-field theories that St << 1. However, there is a clear decreasing trend as
function of the Rayleigh number and increasing rotation. Furthermore, for the
present range of parameters the decrease of St does not show signs of
saturation, indicating that in stellar convection zones, where the Rayleigh
numbers are much larger, the Strouhal number may indeed be significantly
smaller.
