Local models of stellar convection III: The Strouhal number
  Aims: The Strouhal number (St), which is a nondimensional measure of the correlation time, is determined from numerical models of convection. The Strouhal number arises in the mean-field theories of angular momentum transport and magnetic field generation, where its value determines the validity of certain widely used approximations, such as the first order smoothing (hereafter FOSA). More specifically, the relevant transport coefficients can be calculated by means of a cumulative series expansion if St < St_crit \approx 1. Methods: We define the Strouhal number as the ratio of the correlation and turnover times, which we determine separately, the former from the autocorrelation of velocity, and the latter by following test particles embedded in the flow. Results: We find that the Strouhal numbers are, generally, of the order of 0.1 to 0.4 which is close to the critical value above which deviations from FOSA become significant. Increasing the rotational influence tends to shorten both timescales in such a manner that St decreases. However, we do not find a clear trend as function of the Rayleigh number for the parameter range explored in the present study. 
P. J. K\"apyl\"a (Oulu & KIS), M. J. Korpi (Helsinki), M. Ossendrijver   (KIS) & I. Tuominen (Helsinki)