Transport equation for the mean turbulent energy dissipation rate on the centreline of a fully developed channel flow
Résumé
The transport equation for the mean turbulent energy dissipation rate (epsilon) over bar along the centreline of a fully developed channel flow is derived by applying the limit at small separations to the two-point budget equation. Since the ratio of the isotropic energy dissipation rate to the mean turbulent energy dissipation rate (epsilon) over bar (iso)/(epsilon) over bar is sufficiently close to 1 on the centreline, our main focus is on the isotropic form of the transport equation. It is found that the imbalance between the production of (epsilon) over bar due to vortex stretching and the destruction of (epsilon) over bar caused by the action of viscosity is governed by the diffusion of (epsilon) over bar by the wall-normal velocity fluctuation. This imbalance is intrinsically different from the advection-driven imbalance in decaying-type flows, such as grid turbulence, jets and wakes. In effect, the different types of imbalance represent different constraints on the relation between the skewness of the longitudinal velocity derivative S-1,S-1 and the destruction coefficient G of enstrophy in different flows, thus resulting in non-universal approaches of S-1,S-1 towards a constant value as the Taylor microscale Reynolds number, R-lambda, increases. For example, the approach is slower for the measured values of S-1,S-1 along either the channel or pipe centreline than along the axis in the self-preserving region of a round jet. The data for S-1,S-1 collected in different flows strongly suggest that, in each flow, the magnitude of S-1,S-1 is bounded, the value being slightly larger than 0.5.