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.

New experimental results are presented that investigate the nature of the intercomponent energy transfer that occurs in the interaction between oscillating-grid turbulence and a solid impermeable boundary, using instantaneous velocity measurements obtained from two-dimensional particle imaging velocimetry (PIV). Estimates of the pressure-strain correlation term (Π s ij) of the transport equation of the Reynolds stress tensor, which represents intercomponent energy transfer, are obtained using the PIV data from a balance of the remaining terms of the transport equation. The influence of Π s ij on the flow is examined by computing the energy spectra and conditional turbulent statistics associated with events in which intercomponent energy transfer is thought to be concentrated. Data reported here is in support of viscous and 'return-to-isotropy' mechanisms governing the intercomponent energy transfer previously proposed, respectively, by Perot & Moin [B.

Oscillating-grid turbulence (OGT) is an experimental tool that has been widely used to study the role of turbulent fluctuations under conditions of small mean flow. We report experiments to investigate the structure of the turbulent flow produced by an oscillating grid, using velocity measurements obtained through the application of two-dimensional particle image velocimetry in the vertical plane through the centre of the grid. Ensemble averages of the fluid velocity measurements at specific stages of the grid's oscillation indicate that mean flow is induced in OGT by the merging of grid-induced jets close to the tank sidewalls. The installation of an open-ended 'inner box' (with its top edge positioned just below the bottom of the grid's oscillation) is shown to inhibit the merging of the jets, thereby resulting in a reduction in the magnitude of the mean flow within the interior of the inner box region. Measurements of the time-averaged root-mean-square turbulent velocity components and the time-averaged turbulent kinetic energy flux indicate that the installation of the inner box results in turbulence that is in good agreement with the well-established models of OGT across the central 50% of the inner box's width, but that distinct anisotropic regions exist adjacent to the vertical sidewalls. We anticipate that this simple amendment to reduce the mean flow present in OGT can be readily used in future work that utilises OGT to isolate the effects of turbulent fluctuations from those of the mean flow.

Results are presented from a comprehensive experimental campaign studying the aerodynamics of 3D-printed analogues of ice particles. Measurements of the drag coefficient of the analogues were acquired by using a novel experimental approach to digitally reconstruct the analogues' trajectory and orientation as they fall through a quiescent viscous liquid, using images acquired by a series of digital cameras. The data are used to evaluate commonly used parametrisations of ice particle fall speeds. We find that the accuracy of each of the parametrisations reduces at Reynolds numbers greater than approximately 100, as a result of effects associated with the onset of unsteady oscillating, fluttering or tumbling motions as the analogues fall. We propose a new fall speed parametrisation that predicts the measured drag coefficients at high Reynolds number with an accuracy consistent with that of the leading parametrisation at low Reynolds number.