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NASA NTRS1984

Thermohaline circulation below the Ross Ice Shelf - A consequence of tidally induced vertical mixing and basal melting

The warmest water below parts of the Ross Ice Shelf resides in the lowest portion of the water column because of its high salinity. Vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf. A numerical tidal simulation indicates that vertically well-mixed conditions predominate in the southeastern part of the sub-ice shelf cavity, where the water column thickness is small. Basal melting in this region is expected to be between 0.05 and 0.5 m/yr and will drive a thermohaline circulation having the following characteristics: high salinity shelf water (at - 1.8 C), formed by winter sea ice production in the open Ross Sea, flows along the seabed toward the tidal mixing fronts below the ice shelf; and meltwater (at -2.2 C), produced in the well-mixed region, flows out of the sub-ice shelf cavity along the ice shelf bottom. Sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature.

Macayeal, D. R.↗

NASA NTRS1984

Interactive phenomena in supersonic jet mixing problems. I Phenomenology and numerical modeling techniques

The interactive phenomena that occur in supersonic jet mixing flowfields, and numerical modeling techniques developed to analyze such phenomena are discussed. A spatial marching procedure based on solving the parabolized Navier-Stokes jet mixing equations is presented. This procedure combines shock-capturing methodology for the analysis of supersonic mixing regions with pressure-split methodology for the analysis of subsonic mixing regions. The two regions are coupled at viscous sonic lines utilizing a viscous-characteristic coupling procedure. Specialized techniques for the treatment of jet boundary growth, strong discontinuties (Mach disks), and small embedded subsonic zones (behind Mach disks) are presented. Turbulent processes are represented by two-equation turbulence model formulations. In Part II of this article, numerical studies are presented for a variety of supersonic jet interactive phenomena.

Dash, S. M.↗

NASA NTRS1985

An experimental study of plane mixing layer development

Mean flow and turbulence was measured in the near field of two plane mixing layers in air with a maximum velocity of 21 m/sec. The experimental rig enabled mixing layers of velocity ratios 0 and 0.46 to be generated simultaneously. Gases with both tripped and untripped initial boundary layers were studied. It was found that the two stream layer developed to the self preserving state in a distance much shorter than did the single stream layer which followed accepted criteria for the development distance. The asymptotic levels of the turbulence quantities in the two stream layer and the development of the single stream layer showed agreement with existing data. It is suggested that the two stream mixing layer should provide a better test case for the development of turbulence models and calculation methods than the single stream mixing layer.

Mehta, R. D.↗

NASA NTRS1984

Transverse ageostrophic circulations associated with elevated mixed layers

The nature of the frontogenetically forced transverse ageostrophic circulations connected with elevated mixed layer structure is investigated as a first step toward diagnosing the complex vertical circulation patterns occurring in the vicinity of elevated mixed layers within a severe storm environment. The Sawyer-Eliassen ageostrophic circulation equation is reviewed and applied to the elevated mixed layer detected in the SESAME IV data set at 2100 GMT of May 9, 1979. The results of the ageostrophic circulation diagnosis are confirmed and refined by considering an analytic specification for the elevated mixed layer structure.

Keyser, D.↗

NASA NTRS1985

Coherent structures in a turbulent mixing layer - A comparison between direct numerical simulations and experiments

An eduction scheme has been developed in an attempt to determine the characteristics of large-scale vortical structures in a turbulent mixing layer. This analysis scheme has been applied to a set of experimental data taken in a new, larger mixing layer facility designed to minimize boundary and resonance effects. A similar scheme has been developed to apply to the results of a direct numerical simulation of a temporally growing mixing layer. A comparison of the two approaches shows important similarities in the coherent structures. The numerical simulations indicate that low levels of coherent forcing can dramatically change the evolution of the mixing layer. In the absence of such forcing, the numerical simulations and experiments show a lack of regularity in the transverse position, spacing, amplitude, shape and spanwise coherence of the large-scale vortical structures.

Metcalfe, R. W.↗

NASA NTRS1986

Progress in the development of parabolized Navier-Stokes (PNS) methodology for analyzing propulsive jet mixing problems

A brief review of 2D PNS methodology is first presented which describes the specialized features of supersonic shock-capturing and subsonic pressure-split models required for the analysis of aircraft, rocket and scramjet jet mixing problems. These features include techniques for dealing with various types of embedded and interfacing subsonic regions, the inclusion of finite-rate chemistry and the direct-coupling with potential flow solutions. Preliminary 3D extensions of this PNS methodology geared to supersonic and subsonic rectangular free jet mixing problems are also reviewed. New 3D PNS work will be described which includes the development of a hybrid supersonic/subsonic free jet mixing model, and, a supersonic model geared to the analysis of turbulent mixing and combustion processes occurring in scramjet combustor/nozzle flowfields.

Dash, S. M.↗

NASA NTRS1987

On the coalescence-dispersion modeling of turbulent molecular mixing

The general coalescence-dispersion (C/D) closure provides phenomenological modeling of turbulent molecular mixing. The models of Curl and Dopazo and O'Brien appear as two limiting C/D models that bracket the range of results one can obtain by various models. This finding is used to investigate the sensitivtiy of the results to the choice of the model. Inert scalar mixing is found to be less model-sensitive than mixing accompanied by chemical reaction. Infinitely fast chemistry approximation is used to relate the C/D approach to Toor's earlier results. Pure mixing and infinite rate chemistry calculations are compared to study further a recent result of Hsieh and O'Brien who found that higher concentration moments are not sensitive to chemistry.

Givi, Peyman↗

NASA NTRS1986

Middle and upper tropospheric carbon monoxide mixing ratios as measured by a satellite-borne remote sensor during November 1981

Mixing ratios of tropospheric carbon monoxide measured by a gas filter radiometer carried aboard the Space Shuttle during November 1981 are reported. The data represent average mixing ratios in the middle and upper troposphere between 38 deg N and 38 deg S latitude. Approximately 10,000 individual measurements were obtained in each of the two channels of the instrument. The data are presented in the form of plots that show the individual carbon monoxide mixing ratio measurements as a function of latitude and longitude and in the form of maps that show the data averaged over 5 deg latitude by 5 deg longitude squares. The data show relatively little variation in the mixing ratio in the Southern Hemisphere; however, the data from both the tropics and the Northern Hemisphere show strong gradients with both latitude and longitude.

Reichle, H. G., Jr.↗

NASA NTRS1987

Numerical simulation of a plane turbulent mixing layer, with applications to isothermal, rapid reactions

A hybrid method has been developed for the numerical prediction of turbulent mixing in a spatially-developing, free shear layer. Most significantly, the computation incorporates the effects of large-scale structures, Schmidt number and Reynolds number on mixing, which have been overlooked in the past. In flow field prediction, large-eddy simulation was conducted by a modified 2-D vortex method with subgrid-scale modeling. The predicted mean velocities, shear layer growth rates, Reynolds stresses, and the RMS of longitudinal velocity fluctuations were found to be in good agreement with experiments, although the lateral velocity fluctuations were overpredicted. In scalar transport, the Monte Carlo method was extended to the simulation of the time-dependent pdf transport equation. For the first time, the mixing frequency in Curl's coalescence/dispersion model was estimated by using Broadwell and Breidenthal's theory of micromixing, which involves Schmidt number, Reynolds number and the local vorticity. Numerical tests were performed for a gaseous case and an aqueous case. Evidence that pure freestream fluids are entrained into the layer by large-scale motions was found in the predicted pdf. Mean concentration profiles were found to be insensitive to Schmidt number, while the unmixedness was higher for higher Schmidt number. Applications were made to mixing layers with isothermal, fast reactions. The predicted difference in product thickness of the two cases was in reasonable quantitative agreement with experimental measurements.

Lin, P.↗

NASA NTRS1988

Fully-coupled analysis of jet mixing problems. Three-dimensional PNS model, SCIP3D

Numerical procedures formulated for the analysis of 3D jet mixing problems, as incorporated in the computer model, SCIP3D, are described. The overall methodology closely parallels that developed in the earlier 2D axisymmetric jet mixing model, SCIPVIS. SCIP3D integrates the 3D parabolized Navier-Stokes (PNS) jet mixing equations, cast in mapped cartesian or cylindrical coordinates, employing the explicit MacCormack Algorithm. A pressure split variant of this algorithm is employed in subsonic regions with a sublayer approximation utilized for treating the streamwise pressure component. SCIP3D contains both the ks and kW turbulence models, and employs a two component mixture approach to treat jet exhausts of arbitrary composition. Specialized grid procedures are used to adjust the grid growth in accordance with the growth of the jet, including a hybrid cartesian/cylindrical grid procedure for rectangular jets which moves the hybrid coordinate origin towards the flow origin as the jet transitions from a rectangular to circular shape. Numerous calculations are presented for rectangular mixing problems, as well as for a variety of basic unit problems exhibiting overall capabilities of SCIP3D.

Wolf, D. E.↗

NASA NTRS1988

Direct simulations of chemically reacting turbulent mixing layers, part 2

The results of direct numerical simulations of chemically reacting turbulent mixing layers are presented. This is an extension of earlier work to a more detailed study of previous three dimensional simulations of cold reacting flows plus the development, validation, and use of codes to simulate chemically reacting shear layers with heat release. Additional analysis of earlier simulations showed good agreement with self similarity theory and laboratory data. Simulations with a two dimensional code including the effects of heat release showed that the rate of chemical product formation, the thickness of the mixing layer, and the amount of mass entrained into the layer all decrease with increasing rates of heat release. Subsequent three dimensional simulations showed similar behavior, in agreement with laboratory observations. Baroclinic torques and thermal expansion in the mixing layer were found to produce changes in the flame vortex structure that act to diffuse the pairing vortices, resulting in a net reduction in vorticity. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers were shown to result from vorticity generation by baroclinic torques.

Metcalfe, Ralph W.↗

NASA NTRS1988

Mixing control in a plane shear layer

An investigation of mixing processes in a plane shear layer by a direct numerical simulation with a high-order numerical technique, the isoparametric spectral element method, is presented. The computational domain includes a region with the splitter plate that allows to start the flow as a laminar (Blasius) boundary layers with different velocities and concentrations, merging in the shear layer and undergoing transition to highly-unsteady structures with intensive mixing. The augmentation of mixing as a function of a downstream location, the intensity and type of vortical structures in the layer is investigated by an introduction of disturbance at the high-velocity inflow. The character of the dynamic mixing is displayed along with time averaged profiles and statistical characteristics.

Korczak, K. Z.↗

NASA NTRS1988

Effect of line mixing on atmospheric brightness temperatures near 15 microns

The effect of line mixing on the effective blackbody brightness temperature of the earth in the region of the 15-micron nu 2 Q branch of CO2 is calculated. The procedures used to compute line mixing follow those used to successfully model mixing observed in laboratory spectra of two near-infrared CO2 Q branches. The atmospheric radiances are calculated between 664 and 670/cm, a spectral region that is of interest for sounding the upper troposphere and the stratosphere. Mixing was found to lower the observed brightness temperatures by as much as 3 K for some temperature profiles. Ignoring this effect would significantly impact the ability of advanced sounders to produce temperature retrievals which meet the projected accuracy requirement of 1 K/km.

Strow, L. Larrabee↗

NASA NTRS1988

Phase decorrelation, streamwise vortices and acoustic radiation in mixing layers

Several direct numerical simulations were performed and analyzed to study various aspects of the early development of mixing layers. Included are the phase jitter of the large-scale eddies, which was studied using a 2-D spatially-evolving mixing layer simulation; the response of a time developing mixing layer to various spanwise disturbances; and the sound radiation from a 2-D compressible time developing mixing layer.

Ho, C. M.↗

NASA NTRS1988

Sensitivity of mixing layers to three-dimensional forcing

It is well known that turbulent mixing layers are dominated by large scale, fairly coherent structures, and that these structures are related to the stability characteristics of the flow. These facts have led researchers to attempt controlling such flows by selectively forcing certain unstable modes, which can in addition have the effect of suppressing other modes. Much of the work on controlling the mixing layer has relied on forcing 2-D instabilities. The results of forcing 3-D instabilities are addressed. The objectives of the work are twofold: to understand how a mixing layer responds to 3-D perturbations, and to test the validity of an amplitude expansion in predicting the mixing layer development. The amplitude expansion could be very useful in understanding and predicting the 3-D response of the flow to a variety of initial conditions.

Riley, J. J.↗

NASA NTRS1988

Line mixing in infrared atmospheric spectra

The phenomenon of line mixing in IR vibration-rotation spectra and its impact on atmospheric transmission and radiative transfer is discussed. Laboratory spectra exhibiting line mixing in both Q-branch and R-branch spectra of CO2 are examined. Methods for the calculation of line mixing in atmospheric spectra are discussed, including methods for dealing with temperature dependencies. Observations of line mixing in both limb and nadir viewing atmospheric spectra are presented.

Strow, L. Larrabee↗

NASA NTRS1989

An experimental study of the effect of streamwise vorticity on supersonic mixing enhancement

An initial experimental study of the effect of streamwise vorticity on supersonic turbulent mixing has been carried out. A Mach 3 streamwise vortex is generated using a strutmounted swirl injector and is injected into a Mach 3.5 freestream. The resulting flowfield is investigated using both five-hole angularity probe and total temperature probe surveys. The results are compared to identical experiments with a baseline, swirl-free Mach 3 jet. Laser Light Sheet (LLS) images are used to observe the mixing phenomena. The entrainment of energy and mass is used to evaluate the degree of mixing between the two streams for both the vortex and jet cases. The results reveal that streamwise vorticity does lead to a modest mixing enhancement of about 34 percent for the conditions tested.

Naughton, J. W.↗

NASA NTRS1989

Mixing enhancement in a supersonic combustor

Research has been conducted for a number of years at the NASA Langley Research Center to develop a supersonic combustion ramjet (scramjet) capable of propelling a vehicle at hypersonic speeds in the atmosphere or beyond. Recently, that research has been directed toward the optimization of the scramjet combustor, and in particular the efficiency of fuel-air mixing and reaction in the engine. This paper describes a numerical study of fuel-air mixing and reaction in a supersonic combustor, and discusses the analysis of a technique that was used to enhance the mixing processes and overall combustion efficiency in the flow. Based on the results of that study, conclusions are drawn regarding the applicability of the technique to enhance mixing in a scramjet combustor.

Drummond, J. Philip↗