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All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Baroclinic instability in the presence of barotropic jets

A formalism is developed for the calculation of baroclinic instability for barotropically stable jets. The formalism is applied to jet versions of both the Eady and Charney problems. It is found that jets act to confine instabilities meridionally, thus internally determining meridional wave scales. Once this internally determined meridional scale is taken into account, results correspond plausibly to classical results without a jet. Consideration of the effect of such instabilities on the mean flow shows that they act to concentrate the jet barotropically while simultaneously reducing baroclinicity.

Ioannou, Petros↗
NASA NTRS1986

Ion heating in the cusp

Data from satellite observations and theoretical simulations of ion heating in the magnetospheric cusp region are compiled in tables, graphs, and diagrams and discussed. Consideration is given to the mixing of ionospheric and magnetosheath plasmas, the instability of downward-flowing ring distributions of H(+) and He(2+) to lower-hybrid waves, and oxygen and hydrogen heating at finite k(parallel). A range of unstable propagation angles of + or - 20 deg about the perpendicular is estimated for M(H)/M(e) = 50, including superthermal and background electron dynamics.

Hudson, M. K.↗
NASA NTRS1987

Atmospheric dynamics

The paper presents an overview of U.S. contributions in the period of 1983-1986, to the analysis and study of the atmosphere's motion and of the role of this motion in the thermodynamics and chemistry of the atmosphere. Special attention is given to the linear and quasi-linear studies of the dynamics of atmospheric disturbances; studies of the wave breaking phenomenon; and investigations of various long-lived features, ranging from climatological stationary waves to long-period oscillations in the tropics. Consideration is also given to diagnostics for the interactions among disturbances and the mean flow, nontraditional approaches to instability and disturbance growth, and dynamical systems and multiple equilibria.

Lindzen, Richard S.↗
NASA NTRS1987

Petrov-Galerkin methods for natural convection in directional solidification of binary alloys

A Petrov-Galerkin finite element method is presented for calculation of the steady, axisymmetric thermosolutal convection and interface morphology in a model for vertical Bridgman crystal growth of nondilute binary alloys. The Petrov-Galerkin method is based on the formulation for biquadratic elements developed by Heinrich and Zienkiewicz and is introduced into the calculation of the velocity, temperature and concentration fields. The algebraic system is solved simultaneously for the field variables and interface shape by Newton's method. The results of the Petrov-Galerkin method are compared critically with those of Galerkin's method using the same finite element grids. Significant improvements in accuracy are found with the Petrov-Galerkin method only when the mesh is refined and when the formulation of the residual equations is modified to account for the mixed boundary conditions that arise at the solidification interface. Calculations for alloys with stable and unstable solute gradients show the occurrence of classical flow transitions and morphological instabilities in the solidification system.

Adornato, Peter M.↗
NASA NTRS1986

Closure in turbulence theory using stabilization principle

The approach presented is based upon the concept that the velocity fluctuations driven by the instability of the original flow grow until a new stable state is approached. The application of this stabilization principle is demonstrated by describing the smoothing out of a velocity discontinuity.

Zak, Michail↗
NASA NTRS1988

On the receptivity of free shear layers to two-dimensional external excitation

In this paper, the receptivity of a typical free shear layer to pulse-type and periodic excitation is studied. This is done by solving the initial-value problem completely and studing its long-time behavior. This leads to a wave packet for the pulse. By the superposition of many wave packets, a spatial instability mode when the flow is convectively unstable is generated. This establishes a general and simple relationship between the receptivities for pulse-type and sinusoidal excitations. It is found that a shear layer is very receptive to high-frequency disturbances that are generated near the centerline of the layer.

Balsa, Thomas F.↗
NASA NTRS1989

Numerical investigation of the control of the secondary instability process in boundary layers

A numerical model has been developed for investigating boundary layer transition control for a flat plate boundary layer. Active control of a periodically forced boundary layer in an incompressible fluid is studied using surface heating techniques. The spatially evolving boundary layer is simulated. The Navier-Stokes and energy equations are integrated using a fully implicit finite difference/spectral method. Temperature perturbations are introduced locally along finite heater strips to directly attenuate the instability waves in the flow. A feedback control loop is employed in which a downstream sensor is used to monitor wall shear stress fluctuations. Active control of small amplitude two-dimensional and three-dimensional disturbances is numerically simulated. With proper phase control, in-phase reinforcement and out-of-phase attenuation are demonstrated. A receptivity study of the localized temperature perturbations is made. It is shown that narrow heater strips are more receptive in that they maximize the amplitude level of the disturbances in the flow. Active control of the early stages of the fundamental breakdown process is also numerically simulated. Control is achieved with either two-dimensional or three-dimensional control inputs.

Kral, L. D.↗
NASA NTRS1991

Circumstellar material around the massive close binary DH Cephei

The expanding atmosphere of the massive close binary DH Cephei is analyzed on the basis of data from ground-based polarimetry and spaceborne UV observations obtained with the IUE satellite. The mass loss from the system is estimated to be about 2 x 10 to the -6th solar mass/yr. This material can be divided into three classes: (1) wind material, which is hot gas driven from the stars via radiation pressure and reaching terminal velocity at a distance of 5a or greater; (2) scattering material, which may be the result of colliding winds or instabilities in the wind flow; and (3) small (0.1 micron) dust grains. The latter may originate in the cold high-density boundary formed as the wind from the binary plows into the ambient interstellar medium or, alternatively, may be the residue of the formation of stellar components.

Corcoran, M. F.↗
NASA NTRS1991

Asymmetric turbulent vortical flows over slender bodies

Time-accurate numerical solutions have been obtained of equations modeling turbulent subsonic flows over a slender ogive-cylinder body of revolution in the high-angle-of-attack regime where a large asymmetry in the mean flow has been observed experimentally. A modified algebraic eddy-viscosity turbulence model was utilized to correctly compute the effects of the asymmetric vortices on the underlying viscous layers. In order to reproduce any one of the experimentally observed asymmetric flow-fields, it was found necessary to add a small geometrical disturbance near the body apex. By determining an appropriate size of the disturbance, it was possible to obtain excellent agreement between numerical results and experimental data for angles of attack of 30 and 40 deg, Reynolds numbers of 3.0 x 10 to the 6th and = 4.0 x 10 to the 6th, and several roll angles. When the disturbance was removed, the flow field returned to its original symmetric shape. These results are similar in behavior to solutions obtained previously for laminar flows. Just as in the laminar case, results suggest that the origin of the asymmetry is a convective-type instability of an originally symmetric flow.

Degani, David↗
NASA NTRS1990

Physics of the outer heliosphere; Proceedings of the 1st COSPAR Colloquium, Warsaw, Poland, Sept. 19-22, 1989

This volume includes chapters on spectroscopic data on the local interstellar medium and the related XUV radiation background; solar UV backscatter on neutral Galactic gases; entry and dynamics of Galactic and anomalous cosmic rays in the heliosphere; distant solar wind plasma, magnetic field, and solar energetic particles; the interaction of solar wind with the interstellar medium; and deep-space missions. Papers are on the characteristics and the velocity structure of the local interstellar medium, the scattering of solar UV on local neutral gases, solar effects on underground muons at 570 hg/sq cm, and cosmic rays and magnetosonic instabilities of solar wind flow near the heliospheric shock wave. Consideration is also given to the shock heating of the solar wind plasma, solar wind vortex flow in the outer heliosphere, the signature of a viscous interaction at the heliopause, the expected beams of energetic neutral atoms in the outer heliosphere, and the Pioneers 10 and 11 deep-space missions.

Grzedzielski, S.↗
NASA NTRS1992

Colliding winds from early-type stars in binary systems

The dynamics of the wind and shock structure formed by the wind collision in early-type binary systems is examined by means of a 2D hydrodynamics code, which self-consistently accounts for radiative cooling, and represents a significant improvement over previous attempts to model these systems. The X-ray luminosity and spectra of the shock-heated region, accounting for wind attenuation and the influence of different abundances on the resultant level and spectra of X-ray emission are calculated. A variety of dynamical instabilities that are found to dominate the intershock region is examined. These instabilities are found to be particularly important when postshock material is able to cool. These instabilities disrupt the postshock flow and add a time variability of order 10 percent to the X-ray luminosity. The X-ray spectrum of these systems is found to vary with the nuclear abundances of winds. These theoretical models are used to study several massive binary systems, in particular V444 Cyg and HD 193793.

Stevens, Ian R.↗
NASA NTRS1993

High order finite difference and multigrid methods for spatially evolving instability in a planar channel

The fourth-order finite-difference scheme with fully implicit time-marching presently used to computationally study the spatial instability of planar Poiseuille flow incorporates a novel treatment for outflow boundary conditions that renders the buffer area as short as one wavelength. A semicoarsening multigrid method accelerates convergence for the implicit scheme at each time step; a line-distributive relaxation is developed as a robust fast solver that is efficient for anisotropic grids. Computational cost is no greater than that of explicit schemes, and excellent agreement with linear theory is obtained.

Liu, C.↗
NASA NTRS1996

A Simulated Spectrum of Convectively Generated Gravity Waves: Propagation from the Tropopause to the Mesopause and Effects on the Middle Atmosphere

This work evaluates the interaction of a simulated spectrum of convectively generated gravity waves with realistic middle atmosphere mean winds. The wave spectrum is derived from the nonlinear convection model described by Alexander et al. that simulated a two-dimensional midlatitude squall line. This spectrum becomes input to a linear ray tracing model for evaluation of wave propagation as a function of height through climatological background wind and buoyancy frequency profiles. The energy defined by the spectrum as a function of wavenumber and frequency is distributed spatially and temporally into wave packets for the purpose of estimating wave amplitudes at the lower boundary of the ray tracing model. A wavelet analysis provides an estimate of these wave packet widths in space and time. Without this redistribution of energies into wave packets the Fourier analysis alone inaccurately assumes the energy is evenly distributed throughout the storm model domain. The growth with height of wave amplitudes is derived from wave action flux conservation coupled to a convective instability saturation condition. Mean flow accelerations and wave energy dissipation profiles are derived from this analysis and compared to parameterized estimates of gravity wave forcing, providing a measure of the importance of the storm source to global gravity wave forcing. The results suggest that a single large convective storm system like the simulated squall line could provide a significant fraction of the zonal mean gravity wave forcing at some levels, particularly in the mesosphere. The vertical distributions of mean flow acceleration and energy dissipation do not much resemble the parameterized profiles in form because of the peculiarities of the spectral properties of the waves from the storm source. The ray tracing model developed herein provides a tool for examining the role of convectively generated waves in middle atmosphere physics.

Alexander, Joan↗
NASA NTRS1996

A Simulated Spectrum of Convectively Generated Gravity Waves: Propagation from the Tropopause to the Mesopause and Effects on the Middle Atmosphere

This work evaluates the interaction of a simulated spectrum of convectively generated gravity waves with realistic middle atmosphere mean winds. The wave spectrum is derived from the nonlinear convection model described by Alexander et al. [1995] that simulated a two-dimensional midlatitude squall line. This spectrum becomes input to a linear ray tracing model for evaluation of wave propagation as a function of height through climatological background wind and buoyancy frequency profiles. The energy defined by the spectrum as a function of wavenumber and frequency is distributed spatially and temporally into wave packets for the purpose of estimating wave amplitudes at the lower boundary of the ray tracing model. A wavelet analysis provides an estimate of these wave packet widths in space and time. Without this redistribution of energies into wave packets the Fourier analysis alone inaccurately assumes the energy is evenly distributed throughout the storm model domain. The growth with height of wave amplitudes is derived from wave action flux conservation coupled to a convective instability saturation condition. Mean flow accelerations and wave energy dissipation profiles are derived from this analysis and compared to parameterized estimates of gravity wave forcing, providing a measure of the importance of the storm source to global gravity wave forcing. The results suggest that a single large convective storm system like the simulated squall line could provide a significant fraction of the zonal mean gravity wave forcing at some levels, particularly in the mesosphere. The vertical distributions of mean flow acceleration and energy dissipation do not much resemble the parameterized profiles in form because of the peculiarities of the spectral properties of the waves from the storm source. The ray tracing model developed herein provides a tool for examining the role of convectively generated waves in middle atmosphere physics.

Alexander, M. Joan↗
NASA NTRS1997

Solitary Waves in the Western Equatorial Pacific Ocean

During the spring tides of early January and February 1993, groups of solitary internal waves were observed propagating through the Intensive Flux Array of the TOGA COARE experiment. The waves appear to originate near the islands of Nugarba (3 deg S 30 deg S - 154 deg 30'E). They travel north-eastward at 2.5-3 m/s, closely coupled with the semi-diurnal baroclinic tide. Peak amplitudes exceed 60 m. Velocities are in excess of .8 m/s. Sea-surface vertical displacements of order.3 m can be inferred directly from the lateral acceleration of surface waters. The Equatorial Undercurrent is displaced by soliton passage but apparently is unaffected otherwise. The intrinsic shear of the solitary crests is small compared to ambient equatorial shears. The crests, while not themselves unstable, are effective at triggering instabilities on the background flow. The motions potentially contribute 10-15 Watts/sq m to the flux of heat into the mixed layer.

Pinkel, R.↗
NASA NTRS1995

Effect of Far-Field Boundary Conditions on Boundary-Layer Transition

The effect of far-field boundary conditions on the evolution of a finite-amplitude two-dimensional wave in the Blasius boundary layer is assessed. With the use of the parabolized stability equations (PSE) theory for the numerical computations, either asymptotic, Dirichlet, Neumann or mixed boundary conditions are imposed at various distances from the wall. The results indicate that asymptotic and mixed boundary conditions yield the most accurate mean-flow distortion and unsteady instability modes in comparison with the results obtained with either Dirichlet or Neumann conditions.

Bertolotti, Fabio P.↗
NASA NTRS1994

Effect of Far-Field Boundary Conditions on Boundary-Layer Transition

The effect of far-field boundary conditions on the evolution of a finite-amplitude two-dimensional wave in the Blasius boundary layer is assessed. With the use of the parabolized stability equations (PSE) theory for the numerical computations, either asymptotic, Dirichlet, Neumann or mixed boundary conditions are imposed at various distances from the wall. The results indicate that asymptotic and mixed boundary conditions yield the most accurate mean-flow distortion and unsteady instability modes in comparison with the results obtained with either Dirichlet or Neumann conditions.

Bertolotti, Fabio P.↗