Many flows, including those containing suspended particles, are kept turbulent by the action of the bottom stress, and this turbulence is also responsible for maintaining sedimenting particles in suspension and in some cases entraining more particles from the bed. A convenient one-dimensional analogue of these processes is provided by laboratory experiments conducted in a mixing box, where a characterizable turbulence is generated by the vertical oscillation of a horizontal grid. In the present paper we report the results of a series of experiments with a grid located close to the bottom boundary to simulate the action of stresses acting at a rough boundary, and compare the results with those obtained using the more extensively studied geometry in which a similar grid is located in the interior of a stirred fluid layer. Experiments have been conducted both with dense, particle-free fluid layers and with layers containing sufficiently high concentrations of dense particles to have a significant effect on the bulk density. In the fluid case, the interface at the top of the stirred dense layer continues to rise as lighter fluid is entrained across the interface. Sediment layers are distinctly different, because the particles responsible for the density difference between the layers can fall out of the suspension as it changes in thickness. The work done in keeping particles in suspension and the effect of this on the turbulence above the grid must be taken into account. The mechanism of resuspension of particles depends on the level of turbulence near the bottom boundary, below the grid. As the stirring rate, and thus the intensity of turbulence, are increased three possible equilibrium states can be attained sequentially: the particles eventually all precipitate; or some particles precipitate while the remainder are held indefinitely in suspension; or all the particles are suspended. In the last two cases a stable, self-limited suspension layer is produced, separated from the overlying fluid by a sharp density interface at a fixed height. Theoretical arguments are presented which provide a satisfactory scaling of the experimental data. These are compared with previous theories and numerical experiments aimed at modelling both the one-dimensional problem and the corresponding processes in turbulent gravity currents. Comparisons are also made with sediment-laden channel flows and convecting layers containing sedimenting particles. Similar results will hold for light, positively buoyant particles or non-coalescing bubbles.
A variety of fluid flow phenomena involving fluids with thermal and compositional variations are reviewed, first as they are observed in simple laboratory experiments and then as they may apply to the formation of sulfide deposits resulting from exhalation of hot saline solutions from vents in the sea floor. Of particular interest is the case where the effluent is both very salty and hot, so that the two properties have opposing effects on the density difference between the exhaled fluid and its surroundings. This can lead to a very nonlinear density behavior during mixing, which makes it possible for initially light fluid to become heavier than sea water and for an oscillating flow to develop. Even more important are the double-diffusive effects which can occur because of the different molecular diffusion rates of the two properties. An outflow can separate into two parts, a hot, less concentrated plume which rises and a warm concentrated flow which spreads as a bottom current away from the source, maintaining a sharp boundary with the overlying sea water as it does so. If the hot salty fluid is injected into a density gradient, a situation which is typical of the ocean, a stratified lateral transport of the lighter fraction can result. For the heavier fraction, the effect of the combined processes is to maintain a stable boundary between the sea water and an exhaled hydrothermal ore solution, which might thus flow with minimal mixing along the sea floor over large distances to a distant depression before dumping its contained metals. A continuing inflow of dense fluid into such a depression produces a stable stratification, so that in a steady state the outflow spilling over the edge of the depression would be at a lower temperature and salinity and higher or lower f (sub O 2 ) . This condition provides a mechanism for localizing precipitation of sulfides within a small restricted depression from very large volumes of ore solution. Evidence of density stratification in the Bushveld Complex suggests the importance of related phenomena in the formation of layered igneous complexes. Analogous behavior in porous media is also indicated.
The rate of mixing across a density interface between two layers of liquid has been measured in a laboratory experiment which allows a direct comparison between heat and salinity transports over the same range of density differences. Low Reynolds number turbulence was produced by stirring mechanically at a fixed distance from the interface, either in one or in both layers, and the results for these two sets of experiments are also compared. The measurements cover a factor of two in stirring rate and twenty in density. Over this range of conditions the ratio of entrainment velocity to stirring velocity can be expressed as functions of an overall Richardson number Ri, and in this form the results of the one and two stirred layer experiments are indistinguishable from one another. For density differences produced by heat alone, the functional dependence is close to Ri−1 except at small values of Ri where it approaches a finite limit. For experiments with a salinity difference across the interface, the mixing rate is the same as in the heat experiments at low values of Ri, but falls progressively below this as Ri is increased, with the approximate form .An interpretation of these results has been attempted, using a dimensional analysis and qualitative mechanistic arguments about the nature of the motion. The Ri−1 dependence implies a rate of change of potential energy proportional to the rate of working by the stirrer. The decreased mixing rates for salt have been attributed to a slower rate of incorporation of an entrained element into its surroundings by diffusion, which increases the tendency for it to return to the interface and dissipate energy in wave-like motions.
As recently as 20 years ago, we assumed that such oceanic parameters as temperature (T), salinity (S), and other chemical properties varied smoothly with depth. Earlier observational data came from widely spaced water bottle samples and reversing thermometers, and curves drawn through the discrete points obtained in this way were taken to represent the actual state of the ocean. Newly developed instruments have shown, however, that the vertical distributions of properties are often very far from smooth and typically consist of a series of quasihomogeneous. nearly horizontal layers, separated by regions in which the gradients are much larger. These variations, with layer scales ranging from about a meter to several hundred meters, are now called the oceanic finestructure; they are most prominent in the vicinity of fronts, across which there are large horizontal variations of T and S. Fluctuations of temperature, salinity, and velocity representing variations on a scale of about 10 centimeters and smaller have also been measured using rapidly responding sensors, and these constitute the turbulent microstructure.
Retrospectives Development of Geophysical Fluid Dynamics: The Influence of Laboratory Experiments J. Stewart Turner J. Stewart Turner Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia e-mail: Stewart.Turner@anu.edu.au Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information J. Stewart Turner Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia e-mail: Stewart.Turner@anu.edu.au Appl. Mech. Rev. Mar 2000, 53(3): R11-R22 https://doi.org/10.1115/1.3097340 Published Online: March 1, 2000 Article history Online: April 9, 2009
Abstract It is pointed out that previous laboratory experiments which have been used to obtain information about atmospheric thermals have not dealt with the important case of an element whose total buoyancy is increasing. A new technique (based on the chemical release of small gas bubbles) is described here, which allows one to simulate in a liquid the increase of buoyancy due to the release of latent heat in clouds. It has been found possible to achieve experimentally two simple kinds of motion, namely, constant velocity or constant upward acceleration. These elements spread linearly with height in agreement with the predictions of a dimensional theory, at nearly the same angle as thermals in neutral surroundings.
A simple model of the seasonal thermocline is examined theoretically and with the aid of a laboratory experiment. It is argued that all the heat and mechanical energy which affect the water column can be put in near the surface and propagated downwards, without being influenced significantly by horizontal velocities, advection or rotation. If all the kinetic energy of stirring is used to change the potential energy of the system, one can calculate the temperature and depth of the well-mixed surface layer as a function of time, given the heat input. This has been done explicitly for a saw-tooth seasonal heating function and constant stirring rate. A step-by-step heating process has been simulated by an intermittent input of buoyant fluid at the surface, both theoretically and experimentally. Many features observed in the ocean are reproduced by both the theory and the experiments. The depth and temperature dependence of the upper mixed layer as functions of time, and the relation to the heating and cooling cycle, are in good qualitative agreement. Most important, this model shows clearly that surface mixing affects only the properties down to the topmost density interface, and that deeper features laid down early in the heating season can persist until the well-mixed layer reaches them again late in the winter.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
