Turbulence memory in self-preserving wakes

Abstract

The persistence of the large vortices formed at the origin of wakes and mixing layers constitutes a kind of memory of initial conditions by the turbulence. In order to study the fading of this turbulence memory, and its effect on the rate of approach to the fully developed state, two wakes with different initial conditions have been examined experimentally. The wake of a sphere was compared with the wake of a porous disk which had the same drag, but did not exhibit vortex shedding. Measurements were made of the mean and fluctuating velocities, the anisotropy of the turbulence, and the intermittency. It was found that the wake of the sphere developed self-preserving behaviour more rapidly than the wake of the disk, and that even after both wakes became self-preserving there were differences between them in the structure of the turbulence and the scale of the mean flow. From this it is concluded that the behaviour of self-preserving wakes does not depend on the drag alone, but also on the structure of the dominant eddies. Generalizing these results, it is suggested that reported differences in the value of the entrainment constant of jets, wakes, and mixing layers are due to differences in the structure of the dominant eddies, rather than differences in the type of flow.

Interfaces and inhomogeneous turbulence

“…But the forms of their profiles and their instantaneous structure converge to distinctly different states with either a continuous or a lumpy interface with larger initial disturbances (as in [35]). This demonstrates the permanent dependence on initial conditions first observed experimentally by Bevilaqua & Lykoudis [84] and explained in terms of memory (T L /T D ) by Hunt [85] and self-similarity by Johansson et al [86]. In a stably stratified flow, there is a large change in the structure of the wake; its thickness d decreases as it develops into sheared sheets and laminae.…”

Internal Waves Radiation by a Turbulent Jet Flow in a Stratified Fluid

“…The initial amplitude of the random velocity component is prescribed to be about 40% of the mean velocity maximum. This value is close to the turbulence intensity measured by Bevilaqua and Lykoudis (1978) in a homogeneous turbulent wake at the distance / 6 xD  behind the sphere. The initial velocity field is further windowed with the reference profile ) , ( z y U ref (2.1) and made divergence-free , similarly to the initialization procedure employed by Gourlay et al (2001).…”

Non-equilibrium scaling laws in axisymmetric turbulent wakes

“…A basic problem of particular interest is the scaling with streamwise distance x of the wake width δ and the centreline velocity deficit u 0 (Tennekes & Lumley 1972;Bevilaqua & Lykoudis 1978;Townsend 1976;George 1989). As stated in Johansson et al (2003), this problem "has puzzled researchers for more than a half-century since measured results have been either inconclusive or contradictive".…”