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Notes: A turbulent region is generated by horizontal pulsed injection at the interface of a two-layer fluid. Flow visualization studies reveal the existence of three stages in the evolution of the vertical size of this region: growth, maximum height, and collapse. A scaling analysis for the height of the turbulent region is presented, which appears to be in good agreement with the measurements. Comparable results were obtained by Fernando, van Heijst, and Fonseka (submitted to J. Fluid Mech.) for similar experiments in a linearly stratified fluid. Thorpe-scale measurements of the turbulent region reveal that the ratio of the rms displacement Lt and the maximum displacement Ltmax remain constant with time. The eventual formation process of the dipolar vortices after the collapse and the influence of interfacial wave motions on these dipolar vortices are discussed.

Notes: This paper describes a laboratory study on the evolution of a point turbulent plume placed at the free surface of a homogeneous fluid layer in the presence of background rotation. It is shown that the plume initially evolves as if there is no rotation. However, the rotational effects become important after the plume descends a vertical distance hc1(approximate)3.3(B/Ω3)1/4 for a normalized time Ωtc1(approximate)2.4, whence the vertical descent rate of the plume is reduced while maintaining approximately the same lateral growth rate. Here Ω is the rate of background rotation and B is the specific buoyancy flux of the plume. The rotational effects inhibit the lateral growth of the plume at a time Ωtc2(approximate)5.5, when the maximum plume width is bc(approximate)1.4(B/Ω3)1/4. Thereafter, the vertical descent continues and the plume evolves into a cylindrical shape while developing a cyclonic circulation in and around it, except near the plume front. Upon reaching the bottom surface after traveling a fluid depth of H, the plume deflects, propagates horizontally, and becomes unstable breaking up into anticyclonic eddies. Studies carried out for the case of H〈hc1 show that this instability is initiated at a horizontal length scale proportional to the Rossby deformation radius of the deflected flow, and hence it is of baroclinic type. These eddies appear to align vertically with the cyclonic eddies formed by the barotropic instability of the surface rim current, thus producing heton-like structures. The influence of the diameter d0 of the plume on the flow evolution is also studied, and it is shown that plumes with aspect ratio h/d0〈12 (where h is the vertical extent) can be approximated as point plumes. Scaling arguments are advanced to explain the results. Some geophysical applications of the study are also discussed. © 1998 American Institute of Physics.

Notes: Turbulence generated by vertical oscillations of a horizontal monoplanar grid in homogeneous water contained in a tank was used to study certain properties of nearly isotropic turbulence. The cases of sustained oscillations and the removal of forcing after a period of oscillations were considered. The former was used to evaluate the Eulerian-frequency spectra of nearly isotropic turbulence, and the velocity–time power law and spectral behavior of decaying turbulence were studied using the latter. The experimental observations are compared with available theoretical formulations as well as previous experimental observations.

Notes: Experimental observations and associated scaling analyses are presented for the flow of a linearly stratified fluid past a sphere for conditions under which the wake is fully turbulent. It is shown that for internal Froude numbers Fi(approximately-less-than)2, the turbulent wake is suppressed by stratification in the immediate lee of the sphere. For Fi(approximately-greater-than)2, the normalized thickness δT/D grows as t1/3 before the wake height reaches its maximum (which takes place at Nt≈2.0), much the same as for wakes of spheres in homogeneous flow; here D is the sphere diameter, N is the buoyancy frequency, and t is the time. The normalized horizontal wake dimension γT/D, on the other hand, grows in the near wake as t1/3 for all Fi but, for Nt(approximately-greater-than)2.0, grows somewhat more rapidly as t1/2. The Strouhal number for the far-wake vortex street is measured and shown to be roughly a constant St≈0.18, independent of the Reynolds and Froude numbers. It is suggested that the frequency of the far-wake, quasi-two-dimensional vortices is established by the eddy shedding frequency of the near wake and thus is independent of stratification effects.

Notes: Diffusive thermohaline staircases have been observed over wide regions of the Northwestern Weddell Sea in the March 1986 AMERIEZ experiments.1 The spatial distribution and temporal evolution of these staircases suggests that these layers may migrate upwards and also may bifurcate into multiple layers. These observations provided the motivation for a laboratory study of the evolution of thermohaline staircases. In the present study, a thermohaline staircase was generated by heating a stable salinity gradient from below. The thicknesses of the convecting layers, heat and salt transports across the interfaces, and migration of the interfaces due to interfacial turbulent mixing were measured. During the experiments, the interfaces and convecting layers evolve with time, ultimately assuming steady positions and thicknesses h.The experimental measurements of h, away from the bottom boundary, were found to be in good agreement with a theoretical prediction based on the assumption that a balance between the kinetic and potential energies of the turbulent eddies within the convecting layers sets the thicknesses of the convecting layers. Further, the laboratory data showed an excellent agreement with the oceanic data gathered by the previous investigators. Because of buoyancy transport across the interfaces, the stratification in the system weakens and the buoyancy jump Δb across the interfaces reduces. When the interfacial Richardson number Ri=Δbδ/w*2, where δ is the thickness of the interface and w* is the convection velocity in the layers, drops below a critical value Ric,which is about 1.5, the interfaces showed rapid migration to new quasiequilibrium levels, at which they remain stationary until Ri drops below Ric again. Flow visualization studies show that these rapid migrations are caused by engulfment of the interfacial layer fluid by the eddies of the lower, strongly convecting, layers. The experiments also show that, under certain conditions involving low heat fluxes and high density stratifications, heating of a stable salinity gradient does not lead to the formation of a thermohaline staircase structure; a criterion is developed to predict the conditions favorable for their formation. In addition, when the stratification is small and heat flux is high, the convectively mixed layer propagates upward as in a nonstratified fluid and the stratification tends to be destroyed without forming layers. The laboratory results successfully explain some of the field observations, while providing insight into the physics of thermohaline convection.

Notes: Eulerian frequency spectra were measured in zero-mean-shear (oscillating-grid induced) turbulent flows and were compared with the spectral form proposed by Tennekes [J. Fluid Mech. 67, 561 (1975)]. A satisfactory agreement between the theoretical prediction and experimental results were obtained in a limited frequency range. The empirical constants pertinent to the spectral law, obtained experimentally, were in good agreement with the numerical simulation results. © 1995 American Institute of Physics.

Notes: Oscillating-grid induced turbulence in confined geometries (tanks) is commonly used in the study of turbulence with zero-mean shear. It is demonstrated that the mean secondary circulation generated during such experiments can be reduced by selecting conditions that lessen the Reynolds-stress gradients within the fluid. A simple power law for the spatial decay of turbulent velocity fluctuations is realized only in the absence of such mean circulation.

Notes: The upward dispersion of heavy particles in suspension in turbulent flow was studied using a numerical model. The interaction between the turbulence and the particle diffusion leads to the formation of a horizontal front (or a "lutocline''), across which the diffusion of particles and the propagation of turbulent energy are inhibited. However, as the settling velocity of the particles becomes larger, or as the particle concentration becomes smaller, the interaction weakens, thus suppressing the front formation. One-dimensional model equations for the problem are solved numerically to calculate the evolution of the particle concentration. A criterion for the formation of the front is proposed and the steady depth of the suspension layer is determined.

Notes: The purpose of this paper is to present the results of a laboratory experiment dealing with one-dimensional propagation of a turbulent front induced by an oscillating grid in a homogeneous fluid. The symmetrical quadrupolar mode of the flow, induced by the grid elements, was studied in the range KC=4–45, S=1–20 (KC is the Keulegan–Carpenter number, S is the Stokes number). It is shown that the depth H of the turbulent layer increases with time t as H∝t1/2. Experimental data for different grid parameters and oscillation amplitudes and frequencies are presented in the nondimensional form h=const τ1/2, using nondimensional depth h of the turbulent layer and nondimensional time τ. A semiempirical model is presented to explain this behavior. This model is based on the properties of a flow induced in a visous fluid by localized forcing of a line force dipole (force doublet). © 1996 American Institute of Physics.

Notes: The formation of a thermocline generated by the interaction between a stabilizing buoyancy flux and shear-free turbulence was studied using a numerical model. The time evolutions of the vertical distributions of the buoyancy and turbulent kinetic energy were calculated and were used to evaluate the depth of the thermocline and the time required for its formation. The numerical results are compared with the results of previous laboratory experiments. The mechanisms responsible for the formation of the thermocline are discussed in view of the numerical results.