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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

On the generation of magnetohydrodynamic waves in a stratified and magnetized fluid. II - Magnetohydrodynamic energy fluxes for late-type stars

Magnetohydrodynamic (MHD) wave energy fluxes for late-type stars are calculated, using previously obtained formulae for the source functions for the generation of MHD waves in a stratified, but otherwise uniform, turbulent atmosphere; the magnetic fields in the wave generation region are assumed to be homogeneous. In contradiction to previous results, it is shown that in this uniform magnetic field case there is no significant increase in the efficiency of MHD wave generation, at least within the theory's limits of applicability. The major results are that the MHD energy fluxes calculated for late-type stars are less than those obtained for compressible modes in the magnetic field-free case, and that these MHD energy fluxes do not vary enough for a given spectral type to explain the observed range of UV and X-ray fluxes from such stars. It is therefore concluded that MHD waves in stellar atmospheres with homogeneous magnetic fields in the wave generation region cannot explain the observed stellar coronal emissions; if such MHD waves are responsible for a significant component of stellar coronal heating, then nonuniform fields within the generation region must be appealed to.

Musielak, Z. E.↗
NASA NTRS1989

Accuracy assessment, using stratified plurality sampling, of portions of a LANDSAT classification of the Arctic National Wildlife Refuge Coastal Plain

An application of a classification accuracy assessment procedure is described for a vegetation and land cover map prepared by digital image processing of LANDSAT multispectral scanner data. A statistical sampling procedure called Stratified Plurality Sampling was used to assess the accuracy of portions of a map of the Arctic National Wildlife Refuge coastal plain. Results are tabulated as percent correct classification overall as well as per category with associated confidence intervals. Although values of percent correct were disappointingly low for most categories, the study was useful in highlighting sources of classification error and demonstrating shortcomings of the plurality sampling method.

Card, Don H.↗
NASA NTRS1988

Regressed relations for forced convection heat transfer in a direct injection stratified charge rotary engine

Currently, the heat transfer equation used in the rotary combustion engine (RCE) simulation model is taken from piston engine studies. These relations have been empirically developed by the experimental input coming from piston engines whose geometry differs considerably from that of the RCE. The objective of this work was to derive equations to estimate heat transfer coefficients in the combustion chamber of an RCE. This was accomplished by making detailed temperature and pressure measurements in a direct injection stratified charge (DISC) RCE under a range of conditions. For each specific measurement point, the local gas velocity was assumed equal to the local rotor tip speed. Local physical properties of the fluids were then calculated. Two types of correlation equations were derived and are described in this paper. The first correlation expresses the Nusselt number as a function of the Prandtl number, Reynolds number, and characteristic temperature ratio; the second correlation expresses the forced convection heat transfer coefficient as a function of fluid temperature, pressure and velocity.

Lee, Chi M.↗
NASA NTRS1989

Fuel-air mixing and distribution in a direct-injection stratified-charge rotary engine

A three-dimensional model for flows and combustion in reciprocating and rotary engines is applied to a direct-injection stratified-charge rotary engine to identify the main parameters that control its burning rate. It is concluded that the orientation of the six sprays of the main injector with respect to the air stream is important to enhance vaporization and the production of flammable mixture. In particular, no spray should be in the wake of any other spray. It was predicted that if such a condition is respected, the indicated efficiency would increase by some 6 percent at higher loads and 2 percent at lower loads. The computations led to the design of a new injector tip that has since yielded slightly better efficiency gains than predicted.

Abraham, J.↗
NASA NTRS1989

Vortex paradigm for shock-accelerated density-stratified interfaces

Numerical simulations are reported that capture the observations of Haas and Sturtevant (1985) when a planar shock accelerates an inclined planar interface between two media. A vortex paradigm is presented for interpreting the evolution of shock-accelerated density-stratified interfaces beyond early times. The simulations illustrate the paradigm and are in good agreement with the results of Haas and Sturtevant. Color images of space-time diagrams of the one-space integrated vorticity function are given to show the primary and secondary features of the emerging vortex structures.

Hawley, John F.↗
NASA NTRS1990

Performance of a supercharged direct-injection stratified-charge rotary combustion engine

A zero-dimensional thermodynamic performance computer model for direct-injection stratified-charge rotary combustion engines was modified and run for a single rotor supercharged engine. Operating conditions for the computer runs were a single boost pressure and a matrix of speeds, loads and engine materials. A representative engine map is presented showing the predicted range of efficient operation. After discussion of the engine map, a number of engine features are analyzed individually. These features are: heat transfer and the influence insulating materials have on engine performance and exhaust energy; intake manifold pressure oscillations and interactions with the combustion chamber; and performance losses and seal friction. Finally, code running times and convergence data are presented.

Bartrand, Timothy A.↗
NASA NTRS1989

Analysis of regional-scale vegetation dynamics of Mexico using stratified AVHRR NDVI data

Previous studies have shown that a good relationship exists between AVHRR Normalized Difference Vegetation Index (NDVI) measurements, and both regional-scale patterns of vegetation seasonality and productivity. Most of these studies used known samples of vegetation types. An alternative approach, and the objective was to examine the above relationships by analyzing one year of AVHRR NDVI data that was stratified using a small-scale vegetation map of Mexico. The results show that there is a good relationship between AVHRR NDVI measurements and regional-scale vegetation dynamics of Mexico.

Turcotte, Kevin M.↗
NASA NTRS1991

Nonaxisymmetric instabilities in rotating shear flows - Internal gravity modes in stratified media and analogies with plane flows

The role of the internal gravity modes in mediating the growth of nonaxisymmetric instabilities is investigated by studying the instability of stratified incompressible differentially rotating fluid cylinders to global nonaxisymmetric modes. The results indicate that, in addition to a modified version of the well-known principal branch mediated by surface modes of the system (analogous to f-modes of stars), there exist unstable branches of the dispersion relation mediated by internal gravity modes of the system (similar to the g-modes of stars). These branches arise due to the interaction between the g-modes. It is shown that the maximum growth rate on one of the new branches can sometimes equal or exceed that on the principal branch, thus modifying the principal branch.

Ghosh, P.↗
NASA NTRS1990

Direct numerical simulations of stably-stratified sheared turbulence: Implications for oceanic mixing

Direct numerical simulations of the time evolution of homogeneous stably stratified turbulent shear flows have been performed for several Richardson numbers Ri and Reynolds numbers R(sub lambda) in earlier works. The results show excellent agreement with length scale models developed from laboratory experiments to characterize oceanic turbulence. When the Richardson number Ri is less than the stationary value Ri(sub s), the turbulence intensity grows at all scales, and the growth rate appears to be a function of Ri. The size of the vertical density inversions also increases. On the other hand, when Ri is greater than or equal to Ri(sub s) the largest turbulent eddies become vertically constrained by buoyancy when the Ellison (turbulence) scale L(sub E) and the Ozmidov (buoyancy) scale L(sub O) are equal. At this point, the mixing efficiency is maximal and corresponds to a flux Richardson number R(sub f) = 0.20. The vertical mass flux becomes counter-gradient when epsilon = 19(nu)N(exp 2) and vertical density overturns are suppressed in less than half a Brunt-Vaisala period. The results of the simulations were also recast in terms of the Hydrodynamic Phase Diagram introduced in fossil turbulence models. The so-called point of fossilization occurs when epsilon = 4DCN(exp 2); Gibson proposed 13DCN(exp 2). This value is in agreement with indirect laboratory observations and field observations. Finally, the validity of the steady-state models to estimate vertical eddy diffusivities in the oceanic thermocline is discussed.

Itsweire, E. C.↗
NASA NTRS1992

Response of a stably stratified atmosphere to low-level heating - An application to the heat island problem

Two-dimensional airflow characteristics past a heat island are investigated using both a linear analytic model and a nonlinear numerical model in the context of the response of a stably stratified atmosphere to specified low-level heating in a constant shear flow. Results from the steady-state, linear, analytic solutions exhibit typical flow response fields that gravity waves produce in response to the local heat source in the presence of environmental flow. The magnitude of the perturbation vertical velocity is shown to be much larger in the shear-flow case than in the uniform-flow case. Two distinct flow features are observed for larger heating amplitude (hence, larger nonlinearity factor): the gravity-wave-type response field on the upstream side of the heat island and the strong updraft circulation cell located on the downstream side. As the heating amplitude increases, the updraft circulation cell strengthens and shifts farther downwind. The strong updraft cell is believed to be partly responsible for precipitation enhancement observed on the downstream side of the heat island. It is found that the continuing downwind propagation of the updraft circulation cell is related to basic-state wind speed.

Baik, Jong-Jin↗
NASA NTRS1992

The influence of a vertical magnetic field on oscillations in an isothermal stratified atmosphere

The effect of a uniform vertical magnetic field on the modes of an isothermal stratified atmosphere is examined. The general solutions of the wave equation for an isothermal magnetized medium are given. Asymptotic expansions of these solutions are presented in the strong- and weak-field limits. For a weak field, it is found that, to lowest order of the present perturbation expansion, the oscillation spectrum can be analyzed in terms of p- and g-like modes, magnetic Lamb modes, and magnetic or slow modes. The frequency corrections for each of the modes due to coupling with the remaining modes are calculated. It is demonstrated that when the frequencies of different modes are almost identical, strong mode coupling occurs and the waves acquire a mixed character. An analysis of the solutions in the vicinity of the degenerate frequencies is carried out. The solutions reveal the presence of avoided crossings, which occur at the otherwise degenerate frequencies. Examples of different types of avoided crossings are given.

Hasan, S. S.↗
NASA NTRS1992

Nature of fluid flows in differentially heated cylindrical container filled with a stratified solution

Semiconductor crystals such as Hg(1-x)Cd(x)Te grown by unidirectional solidification Bridgmann method have shown compositional segregations in both the axial and radial directions. Due to the wide separation between the liquidus and the solidus of its pseudobinary phase diagram, there is a diffusion layer of higher HgTe content built up in the melt near the melt-solid interface which gives a solute concentration gradient in the axial direction. Because of the higher thermal conductivity in the melt than that in the crystal there is a thermal leakage through the fused silica crucible wall near the melt-solid interface. This gives a thermal gradient in the radial direction. Hart (1971), Thorpe, Hutt and Soulsby (1969) have shown that under such condition a fluid will become convectively unstable as a result of different diffusivities of temperature and solute. It is quite important to understand the effects of this thermosolute convection on the compositional segregation in the unidirectionally solidified crystals. To reach this goal, we start with a simplified problem. We study the nature of fluid flows of a stratified solution in a cylindrical container with a radial temperature gradient. The cylindrical container wall is considered to be maintained at a higher temperature than that at the center of the solution and the solution in the lower gravitational direction has higher solute concentration which decrease linearly to a lower concentration and then remain constant to the top of the solution. The sample solution is taken to be salt water.

Wang, Jai-Ching↗
NASA NTRS1992

An off-center point explosion in a radially stratified medium - Kompaneets approximation

The Kompaneets (1960) approximation is applied to a blast in a medium that is radially stratified according to a power law. The basic equation is derived that gives the location of the shock front as a function of time. This equation is transformed by means of a conformal coordinate transformation into an equation in which the density variation of the medium does not appear. In the new coordinates, and the problem is reduced to that of a blast in a uniform medium, the solution of which is a set of concentric circles about the initial explosion point; in the transformed coordinates, the paths followed by points on the shock front are radial, making it possible to find the paths in the original coordinates. Using the new solution, the effect of the explosion is considered for a solid core embedded at the origin, and the extent of the core's surface that is affected by the blast is calculated.

Korycansky, D. G.↗
NASA NTRS1992

Growth and decay of disturbances in stratified shear flow in a rotating frame

The stability of stratified plane Couette flow in a rotating frame is investigated for a case in which the gravitational force is parallel to the rotation vector. Partial differential equations describing the behavior of disturbances in the linear regime are derived. Unstratified flow is stable as long as the angular momentum gradient is positive. If the gradient is negative, nonaxisymmetric disturbances grow as a power law in time, if the gradient is sufficiently steep. In flow which is unstable to convection, all perturbations asymptotically grow at the rate given by the Brunt-Vaisala frequency. If heat diffusion is included, all nonaxisymmetric perturbations now eventually decay as t exp -2, even if the flow is unstable to convection. If heat diffusion and viscosity are weak, nonaxisymmetric disturbances in convectively unstable flow will undergo a large transient growth before their eventual decay.

Korycansky, D. G.↗
NASA NTRS1992

Direct simulation of the stably stratified turbulent Ekman layer

The Navier-Stokes equations and the Boussinesq approximation were used to compute a 3D time-dependent turbulent flow in the stably stratified Ekman layer over a smooth surface. The simulation data are found to be in very good agreement with atmospheric measurements when nondimensionalized according to Nieuwstadt's local scaling scheme. Results suggest that, when Reynolds number effects are taken into account, the 'constant Froud number' stable layer model (Brost and Wyngaard, 1978) and the 'shearing length' stable layer model (Hunt, 1985) for the dissipitation rate of turbulent kinetic energy are both valid. It is concluded that there is good agreement between the direct numerical simulation results and large-eddy simulation results obtained by Mason and Derbyshire (1990).

Coleman, G. N.↗
NASA NTRS1993

Inviscid vortex motions in weakly three-dimensional boundary layers and their relation with instabilities in stratified shear flows

We consider the inviscid instability of three-dimensional boundary-layer flows with a small crossflow over locally concave or convex walls, along with the inviscid instability of stratified shear flows. We show how these two problems are closely related through the forms of their governing equations. A proposed definition of a generalized Richardson number for the neutrally stable inviscid vortex motions is given. Implications of the similarity between the two problems are discussed.

Blackaby, Nicholas D.↗
NASA NTRS1993

Wave-turbulence dynamics in the stably stratified boundary layer

New data obtained at the Boulder Atmospheric Observatory are analyzed to obtain separation of wave, turbulence, and mean field necessary for a complete treatment of wave-turbulence interaction. The data were compared with a linear stability analysis of the background atmospheric state, showing good agreement between measured wave parameters (such as wavelength, period, and vector phase velocity) and the eigenvalues of the linear solution. The analysis of the budgets of wave heat flux and temperature variance revealed the essential role of wave-turbulence interaction in maintaining a large amplitude temperature wave and countergradient heat flux. A mechanism for the maintenance of turbulence by waves in strongly stratified boundary layers is described, which emphasizes that the time-mean Richardson number is an irrelevant parameter at such times.

Einaudi, F.↗
NASA NTRS1993

Stratified turbulence in the atmosphere and oceans - A new subgrid model

Turbulence in stably and unstably stratified media is studied, and the results are used to construct a subgrid simulation (SGS) model for use in large eddy simulation (LES). It was found that, although the assumption of inertiality of the subgrid scales is reasonable for the case of unstable stratification, it ceases to be so in the case of stable stratification, where gravity removes kinetic energy from the eddies: the generation of gravity waves becomes the dominant physical process, with dissipation relegated to higher wavenumbers. Preliminary results show that the total kinetic energy dissipation length scale increases with stability, in accordance with LES results but in disagreement with Deardorff's model that suggests a decrease of all dissipation scales in presence of stratification.

Canuto, V. M.↗