Rapid distortion theory as a means of exploring the structure of turbulence

Abstract

Turbulence structure is discussed in terms of the different length scales of a turbulent flow—the large scale motions characteristic of the boundary conditions and forcing of the particular flow and, for high Reynolds numbers, the universal small scale motions. Evidence is presented that in shear flows and flows near boundaries the large scale structure of many different turbulent flows is similar. The analysis and understanding of different types and different regions of turbulent flows, and in particular their sensitivity to boundary and initial conditions, is clarified by using the classification of “rapidly changing turbulence” (RCT) and “slowly changing turbulence” (SCT), according to whether the time (T D) over which fluid particles pass through (or near) changes in the mean flow or boundary conditions is much less or much greater than the characteristic time of the large scales of the turbulence (T L). It is noted that in all unconfined turbulent flows, the turbulence structure adjusts so that, T D ~ T L, which implies that some features of the initial conditions, and boundary conditions can persist throughout the flow. A possible physical explanation is suggested.