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Lidar data of mixed layer height are used to compare the model of process partitioning to that of Eulerian partitioning. It is found that the Eulerian partitioning model more closely follows the data and that the process partitioning model underpredicts the mixed layer depth. The results suggest that shear production and consumption of turbulent kinetic energy at the mixed layer top are local processes.
A mathematical model, which incorporates heat of mixing in the energy balance, has been developed to analyze the morphological stability of a planar solid-liquid interface during the directional solidification of a binary alloy. It is observed that the stability behavior is almost that predicted by the analysis of Mullins and Sekerka (1963) at low growth velocities, while deviations in the critical concentration of about 20-25 percent are observed under rapid solidification conditions for certain systems. The calculations indicate that a positive heat of mixing makes the planar interface more unstable, whereas a negative heat of mixing makes it more stable, in terms of the critical concentration.
Line mixing effects have been observed in a CO2 Q branch recorded in 0.01/cm-resolution IR solar occultation spectra of the lower stratosphere. The spectral data were obtained by the Atmospheric Trace Molecule Spectroscopy Fourier transform spectrometer during the Spacelab 3 mission in the spring of 1985. Analysis of the 1932.47/cm Q branch of (C-12)(O-16)2 shows absorption coefficients below the band origin about 0.62 times those calculated using a standard Voigt line-shape function. Calculations of line mixing using the Lorentz halfwidths of the lines and a simple energy-gap scaling law to parameterize rotational energy transfer reproduce the observed absorption coefficients to about 10 percent. The present results provide the first quantitative information on air-broadened line mixing effects in a Q branch at low temperatures (about 210 K) and show that these effects are significant even at the low pressures of the lower stratosphere (about 100 mbar).
Three-dimensional structures in spatially developing mixing layers are examined using direct numerical simulations. The growth of streamwise vortices and their interaction with the main two-dimensional rollers as well as the effects of the observed hydrodynamic structures on asymmetric entrainment and mixing are studied. It is found that, at low Reynolds number, the streamwise vortices reduce the entrainment asymmetry of the two-dimensional flow. Passive scalar mixing is quantified by calculating the amount of combustion product produced by a fast chemical reaction. It is shown that the temporally developing case is easier to simulate, but contains an extra symmetry not present in spatially developing flows.
Two aspects of the stability of a compressible mixing layer: Absolute/Convective instability and the convective Mach number were considered. It was shown that, for Mach numbers less than one, the compressible mixing layer is convectively unstable unless there is an appreciable amount of backflow. Also presented was a rigorous derivation of a convective Mach number based on linear stability theory for the flow of a multi-species gas in a mixing layer. The result is compared with the heuristic definitions of others and to selected experimental results.
A two-equation turbulence model was extended to be applicable for compressible flows. A compressibility correction based on modelling the dilational terms in the Reynolds stress equations were included in the model. The model is used in conjunction with the SPARK code for the computation of high speed mixing layers. The observed trend of decreasing growth rate with increasing convective Mach number in compressible mixing layers is well predicted by the model. The predictions agree well with the experimental data and the results from a compressible Reynolds stress model. The present model appears to be well suited for the study of compressible free shear flows. Preliminary results obtained for the reacting mixing layers are included.
Researchers considered three-neutrino Mikheyev-Smirnov-Wolfenstein (MSW) mixing, assuming m sub 3 is much greater than m sub 2 is greater than m sub 1 as expected from theoretical consideration if neutrinos have mass. They calculated the corresponding mixing parameter space allowed by the Cl-37 and Kamiokande 2 experiments. They also calculated the expected depletion for the Ga-71 experiment. They explored a range of theoretical uncertainty due to possible astrophysical effects by varying the B-8 neutrino flux and redoing the MSW mixing calculation.
A general formulation of the equivalent domain integral (EDI) method for mixed mode fracture problems in cracked solids is presented. The method is discussed in the context of a 3-D finite element analysis. The J integral consists of two parts: the volume integral of the crack front potential over a torus enclosing the crack front and the crack surface integral due to the crack front potential plus the crack face loading. In mixed mode crack problems the total J integral is split into J sub I, J sub II, and J sub III representing the severity of the crack front in three modes of deformations. The direct and decomposition methods are used to separate the modes. These two methods were applied to several mixed mode fracture problems, were analyzed, and results were found to agree well with those available in the literature. The method lends itself to be used as a post-processing subroutine in a general purpose finite element program.
The NASA scatterometer (NSCAT) is an instrument designed to measure the radar backscatter of the ocean's surface for estimating the near-surface wind velocity. A given resolution element is observed from several different azimuth angles. From these measurements the near-surface vector wind over the ocean may be inferred using a geophysical model function relating the normalized radar backscatter coefficient (sigma0) to the near-surface wind. The results of a study to select a polarization mix for NSCAT using an end-to-end simulation of the NSCAT scatterometer and ground processing of the sigma0 measurements into unambiguous wind fields using a median-filter-based ambiguity-removal algorithm are presented. The system simulation was used to compare the wind measurement accuracy and ambiguity removal skill over a set of realistic mesoscale wind fields for various polarization mixes. Considerations in the analysis and simulation are discussed, and a recommended polarization mix is given.
This paper describes the evaluation of mixed implicit-explicit finite element formulations for hyperbolic heat conduction problems involving non-Fourier effects. In particular, mixed implicit-explicit formulations employing the alpha method proposed by Hughes et al. (1987, 1990) are described for the numerical simulation of hyperbolic heat conduction models, which involves time-dependent relaxation effects. Existing analytical approaches for modeling/analysis of such models involve complex mathematical formulations for obtaining closed-form solutions, while in certain numerical formulations the difficulties include severe oscillatory solution behavior (which often disguises the true response) in the vicinity of the thermal disturbances, which propagate with finite velocities. In view of these factors, the alpha method is evaluated to assess the control of the amount of numerical dissipation for predicting the transient propagating thermal disturbances. Numerical test models are presented, and pertinent conclusions are drawn for the mixed-time integration simulation of hyperbolic heat conduction models involving non-Fourier effects.
Cross isopycnal (diapycnal) and lateral mixing and stirring below the sill of Santa Monica Basin were studied by releasing two tracers, sulfur hexafluoride and perfluorodecalin, as close as possible to an isopycnal surface and measuring their subsequent dispersion. The target for the release was a potential temperature surface at about 790 m depth, roughly 100 m above the bottom and 50 m below the sill. Three surveys, performed immediately after, about 7 weeks after, and about 6 months after the release, showed that the time scales for lateral stirring and mixing in the basin were between 2 and 5 months. The diapycnal diffusivity for the whole period was found to be 0.29 + or - 0.06 sq cm/s near the injection surface, where the buoyancy frequency was about 1.1 cph. This estimate may include some mixing in the turbulent boundary layer near the walls of the basin. The best estimate for the diapycnal diffusivity in the basin interior is 0.25 + or - 0.08 sq cm/s.
Consideration is given to the effect of compressibility on a plane mixing layer that is a prototype free shear layer, amenable to study by numerical simulation and experiment. The full time-dependent compressible Navier-Stokes equations are solved numerically for a temporally evolving mixing layer employing a mixed spectral and high-order finite difference method. Simulations with random initial conditions confirm the prediction of linear stability theory that at high Mach numbers oblique waves grow faster than two-dimensional waves. Simulations are presented of the nonlinear temporal evolution of the most rapidly amplified linear instability waves.
A planar Mie scattering technique is described which allows for the direct visualization of fluid mixing in supersonic flows. The mixed fluid is visualized by laser light sheet scattering from small alcohol droplets which condense as a result of the mixing of a vapor laden subsonic stream with a cold supersonic stream. Issues related to the formation, growth and size of the droplets are addressed. The technique reveals details of the turbulent structure which are masked by the spatial integration of schlieren and shadowgraph methods. Comparative visualizations using the vapor screen method to uniformly mark the high-speed fluid are also shown.
Concentration and mean flow measurements with nanoshadowgraphs and surface flow visualization have been obtained by a study of mixing phenomena during gas injection into hypersonic flow. At Mach 6, a comparison of the matched-pressure injection case with the underexpanded case indicated a greater injectant core penetration rate and a greater concentration-decay rate, leading to a shorter distance for the injectant core to reach an H2-air stoichiometric ratio. The entire injectant plume remained supersonic, and only moderate pressure losses were found. While injector yaw did not increase the mixing rates, it led to an increase in overall injectant plume cross-section, with consequent increase in the size of the mixing region.
The characteristics of internal flowfields in a Mixed Flow Vectored Thrust (MFVT) propulsion system for Short Take Off and Vertical Landing (STOVL) fighter aircraft were investigated using flow visualization techniques. A highly parametric 14 percent scale transparent model was used to simulate the MFVT propulsion system. The working medium was water. The mixing between the fan and core flow was studied using flow visualization for a variety of geometrical and flow parameters including three bypass ratio variations. The flow visualization technique involves the use of laser light sheet for illumination and fluorescent dye or air bubbles as tracers. Results indicate the existence of a strong vortex flowfield in the hover mode of operation that is responsible for good mixing between the fan and core flows.
An experimental and computational investigation of a contoured wall fuel injector is presented. The injector was aimed at enabling shock-enhanced mixing for the supersonic combustion ramjet engines currently envisioned for applications on hypersonic vehicles. Three-dimensional flow field surveys, and temporally-resolved planar Rayleigh scattering measurements are presented for Mach 1.7 helium injection into Mach 6 air. These experimental data are compared directly with a three-dimensional Navier-Stokes simulation of the flow about the injector array. Two dominant axial vorticity sources are identified and characterized. The axial vorticity produced strong convective mixing of the injectant with the freestream. Shock-impingement was particularly effective as it assured seeding of baroclinic vorticity directly on the helium/air interface. The vorticity coalesced into a counter-rotating vortex pair of a sense which produced migration of the helium away from the wall. The influences of spatial averaging on the representation of the flow field as well as the importance of the fluctuating component of the flow in producing molecularly-mixed fluid are addressed.
The chemical evolution and mixing and vortical motion of a High Speed Civil Transport's engine exhausts must be analyzed in order to track the gas and its speciation as emissions are mixed to atmospheric scales. Attention is presently given to an analytic model of the wake dynamical processes which accounts for the roll-up of the trailing vorticity, its breakup due to the Crow instability, and the subsequent evolution and motion of the reconnected vorticity. The concentrated vorticity is noted to wrap up the buoyant exhaust and suppress its continued mixing and dilution. The species tracked encompass those which could be heterogeneously reactive on the surfaces of the condensed ice particles, and those capable of reacting with exhaust soot particle surfaces to form active contrail and/or cloud condensation nuclei.
A mixed formulation is chosen to overcome deficiencies of the standard displacement-based shell model. Element development is traced from the incremental variational principle on through to the final set of equilibrium equations. Particular attention is paid to developing specific guidelines for selecting the optimal set of strain parameters. A discussion of constraint index concepts and their predictive capability related to locking is included. Performance characteristics of the elements are assessed in a wide variety of linear and nonlinear plate/shell problems. Despite limiting the study to geometric nonlinear analysis, a substantial amount of additional insight concerning the finite element modeling of thin plate/shell structures is provided. For example, in nonlinear analysis, given the same mesh and load step size, mixed elements converge in fewer iterations than equivalent displacement-based models. It is also demonstrated that, in mixed formulations, lower order elements are preferred. Additionally, meshes used to obtain accurate linear solutions do not necessarily converge to the correct nonlinear solution. Finally, a new form of locking was identified associated with employing elements designed for biaxial bending in uniaxial bending applications.