Local models of stellar convection
III. The Strouhal number
1
Astronomy Division, Department of Physical Sciences, PO Box 3000, 90014 University of Oulu, Finland e-mail: petri.kapyla@oulu.fi
2
Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany
3
Observatory, PO Box 14, 00014 University of Helsinki, Finland
Received:
25
October
2004
Accepted:
9
November
2005
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 
.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 a function
of the Rayleigh number for the parameter range explored in the
present study.
© ESO, 2006