RECENT studies have shown that the turbulence production process in shear flows is not entirely random, but rather quasiperiodic in nature. Since the large eddies play a decisive role in these processes in all shear flows, it is instructive to see if they bear any common characteristics. The possible universal distributions1 of time and velocity scales (T,up) in the low-frequency component of the turbulent signals—defined, respectivley, as the interval between successive zero crossings and the intervening absolute peak value of the longitudinal velocity signal—have been explored in a number of flows. These include the self-preserving regions of a turbulent boundary layer, plane jet, circular jet, and plane mixing layer and the initial mixing regions of a plane jet and circular jet. Both T and up distributions show universal trends: T distributions agree well with the log normal distribution except in their extreme excursions, whereas up distributions are intermediate between log normal and Gaussian distributions. Contents The large structure variates studied here are the zerocrossing interval T and the absolute value up of the velocity peaks or troughs between two successive zero crossings. The a.c. component in a typical unfiltered u signal of a turbulent shear flow is shown in Fig. 1. Such signals have a slowly varying component on which the high-frequency component rides. This slowly varying part can be obtained by suitably low-pass filtering the signal as shown in the figure. The lowfrequency component contains most of the kinetic energy and is considered to be the signature of the large structures. Tand up are two simple variables describing the time and velocity scales of the signal. To obtain T and up, the hot-wire w-signals were digitally filtered and processed. When up/up. is plotted against log (T/T) following Badri Narayanan,1 no clear trend becomes apparent (the overbars indicate time mean values). Noting that (T/T)~ ! is analogous to frequency, plots of up/up vs (T/T) for the nearwall point in the boundary layer and those in the plane jet are shown in Fig. 2. Distributions for other flows are similar. The data have not been averaged here in order to retain the scatter in the temporal data. The distributions appear similar to the one-dimensional power spectra of these flowfields. For example, the figure exhibits a decade where the data follow a -5/3 trend. The drop is slower at lower values of the ordinate, whereas it is faster at higher values. For the lowest