Turbulence, waves and mixing at shear-free density interfaces. Part 2. Laboratory experiments

Abstract

A laboratory experimental study was performed to investigate turbulence, waves and mixing at a sharp density interface (with a jump in buoyancy Δb), subjected to shear-free turbulence induced by oscillating grids with typical velocity and length scales of uH and LH, respectively. The cases where turbulence is present on one side (single-sided stirring) or on both sides (double-sided stirring) of the interface were considered. Extensive flow visualization studies and quantitative measurements were performed on the motion field and mixing characteristics at the interface. It was found that, rather than any one mechanism controlling the mixing process, different mechanisms (namely engulfment, generation of waves and their breaking, eddy impingement and Kelvin–Helmholtz billows) play dominant roles over different ranges of the bulk Richardson number Ri(Ri =ΔbLH/u2H). For the Ri range where wave generation is significant, certain hypotheses and predictions of the companion paper by Fernando & Hunt (1997) were tested in detail, by flow visualization studies of the qualitative properties of interfacial motions and quantitative measurements of the r.m.s. fluctuations of interfacial velocity and displacement, the local gradient Richardson number within the stratified layer, the frequency spectra and the related fractal properties of the interface. The results are consistent with the hypothesis that, at high values of Ri(>35), the density interface consists of linear internal waves driven by turbulence at high frequencies and breaking waves with sharp horizontal gradients of density at low frequencies.

Convective Boundary-Layer Entrainment: Short Review and Progress using Doppler Lidar

“…Carruthers and Hunt 1986;McGrath et al 1997;Sullivan et al 1998). First attempts to visualize processes in the atmospheric boundary layer were done by aircraft measurements, e.g.…”

“…These different definitions have to be considered comparing results concerning the occurrence of the different entrainment mechanisms described in the previous section, because the magnitude of Ri depends on the used scales (for quantitative values see e.g. Fernando and Hunt 1997;McGrath et al 1997;Sullivan et al 1998). For small Richardson numbers, most authors agree that the large-scale engulfment (mechanism i) dominates; for increasing Ri the mechanisms are successively Kelvin-Helmholtz waves (ii) and impinging eddies (iii), and finally breaking of internal waves (iv).…”

Hydrodynamic Processes in Massive Stars

“…This conservation law for buoyancy describes the exchange between the kinetic energy in turbulence and the potential energy of stratification. A fundamental theory of mixing at convective boundaries will model these terms (some progress is being made; see , McGrath et al (1997)). While the time and horizontally averaged profiles of the buoyancy flux is smooth, a spatio-temporal decomposition reveals that the smooth profile arises from a highly dynamic underlying behavior (Figure 3).…”

The penetrative mixing in the Laptev Sea coastal polynya pycnocline layer

“…These ridges form obstacles which considerably increase the parameter of roughness length in the surface boundary layer (Shaw et al, 2008). Combined with reversible tidal currents across the fast-ice edge (Dmitrenko et al, 2010b), ice keels can produce strong turbulent mixing even in a shear-free stable density interface (McGrath et al, 1997a(McGrath et al, , 1997b.…”