Showing papers on "Convective mixing published in 1993"

A primitive equation ocean circulation model using a general vertical coordinate transformation: 1. Description and testing of the model

Abstract: The ocean general circulation model of the Geophysical Fluid Dynamics Laboratory has been modified to accommodate a generalized vertical coordinate transformation. The model with a slightly modified σ transformation (σ = z/H) is applied to three simple test cases. A clear improvement of barotropic and baroclinic topographic Rossby waves has been achieved compared with the z-coordinate model. A somewhat more typical time-independent problem for the overflow of dense water masses over a zonal ridge exhibits the potential benefits as well as the flaws of the new model. Vertically integrated mass transport is very sensitive to changes in bottom slope in regions where planetary and topographic β are of comparable magnitudes. This translates into large errors in the z model due to the crude approximation to the actual topography. The deep flow in the z model does not follow the bottom exactly. Horizontal overshoots of dense water lead to convective mixing on the southern side of the ridge. This can be a serious problem for modeling more realistic overflow situations with the z model. Increased diapycnal diffusion has been identified as the most serious problem with the terrain-following coordinate system for this specific application.

Diurnal variations of convective mixing and the spring bloom of phytoplankton

Abstract: Turbulent stirring of the surface mixed layer extends to shallower depths during the day than in the night because the increased buoyancy resulting from solar heating inhibits the mixing associated with wind action and surface heat loss. Calculations using a simple model in which a mixed layer of constant depth is more strongly coupled to deeper layers at night than during the day indicate that the reduction of mixing by day may be critical to the onset of the spring phytoplankton bloom, for this occurs at a time when there is increasing solar warming in the day and yet considerable heat loss and wind stirring at night. If the Kraus-Turner model of the mixed layer is used to estimate the mixing rates occurring during darkness, the values obtained agree with those that give realistic simulations of the spring bloom. Diurnal observations of chlorophyll a , p CO 2 and oxygen saturation made at 60°N during the Lagrangian experiment carried out in 1989 as part of the U.K. Biogeochemical Ocean Flux Study can be modelled more successfully if the day-night changes in vertical mixing are included in the same manner as the single layer model. The calculations indicate that these changes may shift the timing of the bloom by about 1 week and may account for the depth of penetration of some spring blooms. This process needs to be considered when modelling the coupling between climate and phytoplankton.

Tank pressure control in low gravity by jet mixing

Abstract: The Tank Pressure Control Experiment (TPCE) is a space experiment developed to help meet the need for a critical aspect of cryogenic fluid management technology: control of storage tank pressures in the absence of gravity by forced convective mixing. The experiment used a 13.7-liter tank filled to a constant 83 percent level with refrigerant 113 at near saturation conditions to simulate the fluid dynamics and thermodynamics of cryogenic fluids in space applications. The objectives of TPCE were to characterize the fluid dynamics of axial jet-induced mixing in low gravity, to evaluate the validity of empirical mixing models, and to provide data for use in developing and validating computational fluid dynamic models of mixing processes. TPCE accomplished all of its objectives in flight on Space Shuttle Mission STS-3 in August of 1991. The range of flow patterns photographed generally confirmed a prior correlation based on drop tower tests. A closed-form equation derived from a simple thermodynamic model was found to provide a first-order prediction of the pressure reduction time as a function of mixer parameters, tank size, and fluid thermophysical properties. Low energy mixing jets were found to be effective and reliable at reducing thermal non-uniformities, promoting heat and mass transfer between the phases, and reducing tank pressure.

Experimental studies of magma mixing

Abstract: The paper is devoted to experiments on mixing of natural melts of different compositions at 1300-1850° C and 1-12 kbars. Two series of experiments were carried out: one involving gravity-driven convective mixing and one involving diffusive mixing. The results demonstrate the effectiveness of mixing of contrasting magmas in the course of relative motion. Less viscous mafic melt transforms into andesitic much more easily than viscous silicic melt. The latter tends to “dissolve” into the mafic melt. Diffusive runs revealed selective behavior of alkalies and other components due to diffusion. Uphill diffusion of alkalies may cause double-diffusive convection in intercoupled melts. Diffusive interaction of two contrasting melts is explained as a multistage chemical reaction following the principle of acid-base interaction of components in silicate melts.

Mixing in explosions

Abstract: Explosions always contain embedded turbulent mixing regions, for example: boundary layers, shear layers, wall jets, and unstable interfaces. Described here is one particular example of the latter, namely, the turbulent mixing occurring in the fireball of an HE-driven blast wave. The evolution of the turbulent mixing was studied via two-dimensional numerical simulations of the convective mixing processes on an adaptive mesh. Vorticity was generated on the fireball interface by baroclinic effects. The interface was unstable, and rapidly evolved into a turbulent mixing layer. Four phases of mixing were observed: (1) a strong blast wave phase; (2) and implosion phase; (3) a reshocking phase; and (4) an asymptotic mixing phase. The flowfield was azimuthally averaged to evaluate the mean and r.m.s. fluctuation profiles across the mixing layer. The vorticity decayed due to a cascade process. This caused the corresponding enstrophy parameter to increase linearly with time -- in agreement with homogeneous turbulence calculations of G.K. Batchelor.

Baroclinic mixing in HE fireballs

Abstract: Numerical simulations of the turbulent mixing in the fireball of an HE blast wave were performed with a second-order Godunov code. Adaptive mesh refinement was used to capture the convective mixing processes on the computational grid. The calculations revealed that the interface between the shock-compressed air and the dense detonation products was unstable. Vorticity was generated in that region by baroclinic effects. This caused the interface to roll-up into a turbulent mixing layer. Four phases of mixing were identified: a strong blast wave phase, where the mixing region was swept outward by the shockinduced flow; an implosion phase, that stretched the inner boundary of the mixing region back toward the origin; a re-shocking phase, where the imploding shock expands back outward from the origin and re-energizes the mixing later by RichtmyerMeshkov effects; and an asymptotic mixing phase, where line-scale structures are continually recreated by folding effects but the overall vorticity decays through a cascade process. The flowfield was azimuthally averaged to evaluate the mean-flow profiles and r.m.s. fluctuation profiles across the mixing layer. The mean kinetic energy rapidly approached zero as the blast wave decayed, but the fluctuating kinetic energy asymptotically approached a small constant value. This represents the rotationalmore » kinetic energy driven by the vorticity field, that continued to mix the fluid at late times. It was shown that the vorticity field corresponds to a function that fluctuates between plus and minus values-with a volume-averaged mean of zero.« less