Vortical structures and coherent motion in turbulent flow over smooth and rough boundaries

Abstract

It has been recognized for some years that the bursting phenomenon is a common feature of turbulent boundary layers irrespective of wall roughness condition. In confirmation of Theodorsen’s original conjecture, there is now a convincing body of evidence, particularly deriving from recent direct numerical simulation studies, that the dynamics of this bursting process over smooth walls, is directly linked to the presence of powerful vortical structures with a general horseshoe-type configuration, embedded in the wall flow field. The present paper describes the results of physical experiments which demonstrate that these vortex structures are also present in turbulent boundary layers over rough walls and that they are similarly linked to the bursting events. Novel velocity measurement techniques were used in the investigation which, for the first time in physical fluid mechanics research, allowed quasi-instantaneous vortex lines to be traced through a three-dimensional block of flow space to reveal the vortical structures. The preliminary results from a second investigation are also presented which demonstrate that, just as in the smooth wall case, the spanwise flow structure adjacent to a rough boundary exhibits a well defined cross-flow wavelength. The measurements indicate that this wavelength, which reflects the spanwise scale of the near-wall vortex structures, is directly proportional to the size of the boundary roughness elements under fully rough conditions. Consistent with Townsend’s ‘attached-eddy’ hypothesis, the vortical structures are observed to increase in scale with increasing wall distance while remaining attached back to the vorticity generation zone at the boundary. The paper commences with a review of the status of current knowledge relating to coherent vortical structures in turbulent boundary layers and its interpretation in the context of large scale geophysical flows.

Numerical Simulation of Flow over a Rough Bed

“…We can clearly see the absence of the viscous sublayer and the presence of a retarded fluid layer near the bed ͑Y + Ͻ 60͒ due to the penetration of the roughness elements into the logarithmic region. Comparison with the experimental results of Grass et al ͑1991͒ andDefina ͑1996͒ show that the DNS profile follows the standard pattern of velocity defect increasing with wall roughness.…”

“…However, experimental investigations indicate a decoupling of the flow field from the effect of the roughness elements further away from the roughness layer. Grass et al ͑1991͒ show that the nearwall bursting events characterized by sweeps and ejections which are present above smooth walls are also present in rough wall turbulent flows, and thus, indicate that the coherent structures should also be similar. This apparent similarity of turbulent flow over different surface types has been termed the wall similarity hypothesis by Raupach et al ͑1991͒.…”

Flow past a two– or three–dimensional steep–edged roughness

“…The influences of surface roughness in practice can be substantial: for example, within a turbulent boundary layer as in Grass et al (1991), Perry et al (1969), Krogstad et al (1992); in the transition of a laminar boundary layer as in Mochizuki (1961), Klebanoff & Tidstrom (1972), Kendall (1981), Van Dyke (1982), Corke et al (1986), Morkovin (1990), Kachanov (1995), Saric et al (1995), Zanchetta & Hillier (1995), Savin (1996), Smith (1996); or within a predominantly laminar flow, e.g. Sedney (1973), Sykes (1980) and references therein.…”

On the spatial linear growth of gravity-capillary water waves sheared by a laminar air flow

“…9,10 At present, this knowledge can only come from direct numerical simulation and advanced physical experimentation, paralleling similar studies of solid wall layer turbulence. 11,12 The complexity of the interaction process leads to uncertainties in the determination of the friction velocity on an undulating surface and even the exact shape of the turbulent velocity profile. These are, however, precisely the fundamental factors responsible for the probability of significant discrepancies in the wave growth rate prediction from linear instability theory.…”