Experimental study of oscillating-grid turbulence interacting with a solid boundary

Abstract

Abstract:The interaction between oscillating-grid turbulence and a solid, impermeable boundary (positioned below, and aligned parallel to the grid) is studied experimentally. Instantaneous velocity measurements, obtained using two-dimensional particle imaging velocimetry in the vertical plane through the centre of the (horizontal) grid, are used to study the effect of the boundary on the rms velocity components, the vertical flux of turbulent kinetic energy (TKE), and the terms in the Reynolds stress transport equation. Identified as a critical aspect of the interaction is the blocking of a vertical flux of TKE across the boundary-affected region. Terms of the Reynolds stress transport equations show that the blocking of this energy flux acts to increase the boundarytangential turbulent velocity component, relative to far-field trend, but not the boundarynormal velocity component. The results are compared with previous studies of the interaction between zero-mean-shear turbulence and a solid boundary. In particular, the data reported here is in support of viscous and 'return-to-isotropy' mechanisms governing the intercomponent energy transfer previously proposed, respectively, by Perot & Moin [J. Fluid Mech., vol. 295, 1995, pp. 199-227] and Walker et al. [J. Fluid Mech., vol. 320, 1996, pp. 19-51], although we note that these mechanisms are not independent of the blocking of energy flux and draw parallels to the related model proposed by Magnaudet [J.

Analysis of intercomponent energy transfer in the interaction of oscillating-grid turbulence with an impermeable boundary

“…In light of its statistical properties, oscillating-grid turbulence (OGT) has also been used to study the interaction of approximately zero-mean-shear turbulence at a boundary. In these studies u has typically been found to be amplified in the boundary-affected region 6,7,14,15 in qualitative agreement with the RDT predictions. 2,8 However, in a recent study 7 the current authors presented measurements of the terms of transport equation of TKE, which attributed the observed increase in u 2 primarily to the effects of turbulent transport, rather than intercomponent energy transfer.…”

“…However, the subsequent development of the flow and its structure in studies in which a statistically stationary state is reached remains unclear. Results from a range of experimental and simulation based studies, conducted in both otherwise homogeneous 3,5 and inhomogeneous turbulence, 6,7 indicate that w remains inhibited by the boundary in accordance with the RDT proposed by Hunt and Graham 2 . However, controversy has arisen regarding whether the blocking of w continues to give rise to an intercomponent energy transfer from w 2 to u 2 through a correlation between the fluctuating pressure and velocity-strain fields, described by the so-called pressure-strain term Π s ij of the Reynolds stress transport equations.…”

“…However, controversy has arisen regarding whether the blocking of w continues to give rise to an intercomponent energy transfer from w 2 to u 2 through a correlation between the fluctuating pressure and velocity-strain fields, described by the so-called pressure-strain term Π s ij of the Reynolds stress transport equations. That is, measurements of the pressure-strain term Π s ij (in both developing and statistically stationary flow) indicate that an intercomponent energy transfer from w 2 to u 2 is significant over a thin layer, typically approximately equal in thickness to the viscous sublayer, immediately adjacent to the boundary 3,4,7 but the physical interpretation of these measurements remain under dispute. That is, studies remain divided regarding whether the intercomponent energy transfer is primarily governed by dynamic viscous effects 3 or by the kinematic blocking of the boundary.…”

“…Further from the boundary (outside the viscous sublayer but within the boundary-affected region) the sign of Π s ij is reported to reverse and an intercomponent energy transfer from energy-rich u 2 components to the energy-poor w 2 component has been reported to occur. 3,4,7,13 Walker, Leighton, and Garza-Rios 13 proposed that this so-called "return-to-isotropy" intercomponent energy transfer results from anisotropy induced from the kinematic blocking condition acting on u 3 . (Note that here the term "return to isotropy" is used specifically to refer to the energy transfer from u 2 to w 2 , in a region in which u > w, and does not refer to a temporal evolution in statistics.…”

Sediment erosion in zero-mean-shear turbulence

“…In order to study thoroughly the part turbulence plays in the erosion process, we used a well-known experimental device that is the oscillating grid 7,8 . The flow produced by this device has been widely studied 1,7,[9][10][11][12] . All different studies agree in that far enough from the grid, the mean flow becomes weak enough to be negligible while turbulence appears to be almost isotropic.…”