Both experimental and numerical studies have been performed aimed at the description of the decay of swirl in turbulent pipe flows. Emphasis is put on the effect of the initial velocity distribution on the rate of decay. The experiments show that, even far downstream of the swirl generator, the decay of the integral amount of angular momentum depends on the initial velocity distribution. This suggests that the description of the decay in terms of the widely suggested single exponential, function, is not sufficient. The calculations are based on (i) a standard k – ε model and (ii) models based on an algebraic transport model for the turbulent stresses. It appears that in a weakly swirling pipe flow, second-order models reduce to simple modifications of the standard k – ε model. While the standard k – ε model predicts a decay largely insensitive to the initial velocity distribution, the modified versions of the k – ε model, the ASM and the RSM, predict a strong sensitivity to the initial velocity distribution. Nevertheless, the standard k – ε model seems to predict the rate of decay of the swirl better than the second-order models. It is concluded that the corrections for the streamline curvature introduced by the second-order closures, largely overestimate the effect of rotation on the radial exchange of angular momentum.
Do extreme events trigger turbulence decay? – a numerical study of turbulence decay time in pipe flows
