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Results for “convective instability”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Convective instabilities in traveling fronts of addition polymerization

An autocatalytic reaction in an unstirred vessel can support a constant velocity wavefront resulting from the coupling of diffusion to the chemical reaction. A flare front is a common example in which heat is the autocatalytic species that diffuses into unreacted regions stimulating a reaction that produces more heat. Traveling fronts were studied in synthetic polymerization reactions under high pressure by workers in the former USSR. More recently, propagating fronts of methacrylic acid polymerization were studied under ambient conditions, both with video techniques and by NMR.

Pojman, John A.↗
NASA NTRS1998

Convective Instability of a Gravity Modulated Fluid Layer with Surface Tension Variation

Gravity modulation of an unbounded fluid layer with surface tension variations along its free surface is investigated. In parameter space of (wavenumber, Marangoni number) modulation has a destabilizing effect on the unmodulated neutral stability curve for large Prandtl number, Pr, and small modulation frequency, Omega, while a stabilizing effect is observed for small Pr and large Omega. As Omega yields infinity, the modulated neutral stability curves approach the unmodulated neutral stability curve. At certain values of Pr and L2 multiple minima are observed and the neutral stability curves become highly distorted. Closed regions of subharmonic instability are also observed. Alternating regions of synchronous and subharmonic instability separated by very thin stable regions are observed in (1/Omega,g(sub 1)) space for the singly diffusive cases. Quasiperiodic behavior in addition to the synchronous and subharmonic responses, are observed for the case of a double diffusive fluid layer. Minimum acceleration amplitudes were observed to closely correspond with a subharmonic response, Lambda(sub im) = Omega/2 .

Skarda, J. Raymond Lee↗
NASA NTRS2001

Absolute And Convective Instability and Splitting of a Liquid Jet at Microgravity

The objective is to establish a definitive role of the capillary, viscous, and inertial forces at a liquid-gas interface in the absence of gravity by using the fluid dynamics problem of the stability of a liquid jet as a vehicle. The objective is achieved by reexamining known theories and new theories that can be verified completely only in microgravity. The experiments performed in the microgravity facility at NASA Glenn Research Center enable the verification of the theory with experimental data. Of particular interest are (1) to capture for the first time the image of absolute instability, (2) to elucidate the fundamental difference in the physical mechanism of the drop and spray formation from a liquid jet, and (3) to find the origin of the newly discovered phenomenon of jet splitting on earth and in space.

Lin, S. P.↗
NASA NTRS1984

Convective thermocapillary instabilities in liquid bridges

A steady shear flow is generated in an axisymmetric liquid bridge surrounded by a passive gas through the imposition of a temperature gradient along the bridge and the driving of the motion by thermocapillarity. This dynamic state is susceptible to convective instabilities, which can lead to propagating hydrothermal waves that feed on the underlying temperature gradients. The convective instabilities of axisymmetric return flow are presently given as functions of the liquid's Prandtl number and the surface Biot number of the interface.

Xu, J.-J.↗
NASA NTRS1985

The nature of symmetric instability and its similarity to convective and inertial instability

It is shown that there exists a local similarity among SI (Symmetric Instability), BI (Buoyancy or Convective Instability), and II (Inertial Instability) even for fully nonlinear viscous motion. The most unstable slope angles for SI and Moist SI motions are analyzed based on parcel energetics. These considerations also suggest qualitatively that CSI (Conditional SI) circulations will be slantwise and lie between the moist most unstable slope and dry least stable slope of the basic state.

Xu, Q.↗
DOE OSTI2023

Atmosphere–Ocean Coupled Energy Budgets of Tropical Convective Discharge–Recharge Cycles

An energy budget combining atmospheric moist static energy (MSE) and upper ocean heat content (OHC) is used to examine the processes impacting day-to-day convective variability in the tropical Indian and western Pacific Oceans. Feedbacks arising from atmospheric and oceanic transport processes, surface fluxes, and radiation drive the cyclical amplification and decay of convection around suppressed and enhanced convective equilibrium states, referred to as shallow and deep convective discharge–recharge (D–R) cycles, respectively. The shallow convective D–R cycle is characterized by alternating enhancements of shallow cumulus and stratocumulus, often in the presence of extensive cirrus clouds. The deep convective D–R cycle is characterized by sequential increases in shallow cumulus, congestus, narrow deep precipitation, wide deep precipitation, a mix of detached anvil and altostratus and altocumulus, and once again shallow cumulus cloud types. Transitions from the shallow to deep D–R cycle are favored by a positive “column process” feedback, while discharge of convective instability and OHC by mesoscale convective systems (MCSs) contributes to transitions from the deep to shallow D–R cycle. Variability in the processes impacting MSE is comparable in magnitude to, but considerably more balanced than, variability in the processes impacting OHC. Variations in the quantity of atmosphere–ocean coupled static energy (MSE + OHC) result primarily from atmospheric and oceanic transport processes, but are mainly realized as changes in OHC. MCSs are unique in their ability to rapidly discharge both lower-tropospheric convective instability and OHC.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1992

Marangoni instability induced convection in an evaporating liquid droplet

The processes occurring when a liquid drop undergoes evaporation are described. When a liquid drop undergoes evaporation, its surface temperature decreases. If the droplet is free floating in a microgravity environment, the heat transfer process inside the droplet is initially condition controlled. As the process continues, a radial temperature gradient builds up at the free surface until the critical Marangoni number is exceeded. Then the onset of instability induces thermocapillary convective flows, which in turn speed up the evaporation. The convective flows will subside when the interior of the droplet reaches a certain equilibrium temperature, and the process will return to the diffusion controlled mode. Both preliminary modeling and recent laboratory data have confirmed that Marangoni instability induced convection can and does occur in the droplet evaporation process. Mathematical models representing Marangoni instability in an evaporating liquid drop are presented. An ideal space experiment to study and characterize the onset of Marangoni instability in an evaporating liquid droplet and to establish the effect of Marangoni instability induced convection on the droplet evaporation rate is outlined and the need for conducting such experiments in space is highlighted. However, before an opportunity to conduct experiments in space arises, ground based experiments have to be conducted to study feasibility issues and proof of concept. A ground based experiment of this type is outlined.

Chai, An-Ti↗
NASA NTRSDate not supplied

Separation and Transition on a Cone-Cylinder-Flare: Computational Investigations

Base flow computation and stability analysis were conducted for hypersonic flow over a cone-cylinder-flare (CCF) geometry for conditions that correspond to the experimental runs carried out in three wind tunnels. Owing to the presence of an attached boundary layer, a separation bubble induced by a shock-boundary layer interaction, and a reattachment region, the chosen flow configuration is physically rich. The complexity of this flowfield encompasses a combination of convective instabilities developing on the cone, global instabilities in the separation bubble, and shear-layer modes and streaks in the reattachment region. Thus, the selected CCF configuration provides the opportunity for a comprehensive comparison of the currently available methodologies for analyzing boundary layer instabilities. Various tools are used for the analysis, including global stability codes as well as convective instability analyses based on a local theory, a weakly non-parallel analysis, and tools that are applicable to strongly non-parallel flows. The paper presents a comparison of the convective instability characteristics based on different methodologies, such as linear stability theory (LST), the harmonic form of linearized Navier-Stokes equations (HLNSE), and resolvent analysis. The CCF configuration provided an effective framework for conducting a detailed cross-validation of this type, which had not yet been addressed in existing literature. This document is accompanied by a companion paper that is focused on the experimental aspects of the CCF configuration. Both papers are being presented in a dedicated session that highlights the research activities of the NATO STO Research Task Group AVT-346.

Boundary Layer Transition↗
NASA NTRS1985

The nature of symmetric instability and its similarity to convective and inertial instability

It is presently noted that, even for the case of fully nonlinear viscous motion, there exists a local similarity among symmetric, buoyancy or convective, and inertial instabilities. The most unstable slope angles for symmetric instability are analyzed through consideration of parcel energetics; the results obtained suggest that qualitatively conditional symmetric instability circulations will be slantwise, and will lie between the moist and most unstable and the dry and least stable slopes of the basic state.

Xu, Q.↗
NASA NTRS1993

Numerical simulation of upstream disturbance on flows around a slender body

Numerical solutions of the thin-layer approximation of the compressible Navier-Stokes equations have been obtained for flows around an ogive-cylinder body with and without a small fixed disturbance placed upstream of the body tip. Locating the disturbance at positions in the flow field upstream of the tip provokes the same range of behavior of the asymmetric flow that was numerically produced earlier by use of a geometrical disturbance on the body tip. Results remain consistent with the presence of a convective instability mechanism, and demonstrate the potential for a precise mapping of the body's receptivity to fixed disturbances in the flow field. Numerical solutions were also obtained for the flow-field responses to impulsive upstream disturbances to determine whether there is a growing response to an asymmetric impulsive disturbance that is consistent with presence of a convective instability mechanism. Results for surface pressure are interpreted with the aid of a mathematical model. The model suggests that the observed growth of surface pressure gradient with time and distance along a ray in response to an asymmetnc impulsive disturbance is in accord with the solution of a Ginzberg-Landau equation, with distinguishing features of the solution being consistent with the convective instability mode of behavior.

Degani, David↗
NASA NTRS1999

Long-Wavelength Rupturing Instability in Surface-Tension-Driven Benard Convection

A liquid layer with a free upper surface and heated from below is subject to thermocapillary-induced convective instabilities. We use very thin liquid layers (0.01 cm) to significantly reduce buoyancy effects and simulate Marangoni convection in microgravity. We observe thermocapillary-driven convection in two qualitatively different modes, short-wavelength Benard hexagonal convection cells and a long-wavelength interfacial rupturing mode. We focus on the long-wavelength mode and present experimental observations and theoretical analyses of the long-wavelength instability. Depending on the depths and thermal conductivities of the liquid and the gas above it, the interface can rupture downwards and form a dry spot or rupture upwards and form a high spot. Linear stability theory gives good agreement to the experimental measurements of onset as long as sidewall effects are taken into account. Nonlinear theory correctly predicts the subcritical nature of the bifurcation and the selection between the dry spot and high spots.

Swift, J. B.↗
NASA NTRS1989

Theory of local thermal instability in spherical systems

The gasdynamical properties of local thermal instability in optically thin astrophysical plasmas as it occurs in spherical accretion and winds is investigated. In a medium characterized by both thermal and hydrostatic equilibrium, if the cooling function is not an explicit function of position and does not display isentropic thermal instability, then isobaric thermal instability by the Field criterion is present if and only if convective instability is present by the Schwarzschild criterion. In this case, thermal overstability cannot occur. Convective instability by the Schwarzschild criterion will also occur in accretion flows locally dominated by external heating or in marginally unbound, radiatively cooling outflows. A very general Lagrangian equation for the development of nonradial thermal instability in flows with spherical symmetry is derived and is solved analytically in certain regimes. The results are applied to cluster X-ray cooling flows.

Balbus, Steven A.↗
NASA NTRS1986

Laboratory experiments in a baroclinic annulus with heating and cooling on the horizontal boundaries

Experiments have been performed in a cylindrical annulus with horizontal temperature gradients imposed upon the horizontal boundaries and in which the vertical depth was smaller than the width of the annulus. Qualitative observations were made by the use of small, suspended, reflective flakes in the liquid (water). Four basic regimes of flow were observed: (1) axisymmetric flow, (2) deep cellular convection, (3) boundary layer convective rolls, and (4) baroclinic waves. In some cases there was a mix of baroclinic and convective instabilities present. As a 'mean' interior Richardson number was decreased from a value greater than unity to one less than zero, axisymmetric baroclinic instability of the Solberg type was never observed. Rather, the transition was from non-axisymmetric baroclinic waves, to a mix of baroclinic and convective instability, to irregular cellular convection.

Miller, T. L.↗
NASA NTRS1984

Laboratory studies of baroclinic instability at small Richardson number

As part of the support program for the Atmospheric General Circulation Experiment, laboratory studies of baroclinic and other convective instabilities were performed for a thin layer of fluid between thermally conducting horizontal discs. There were three types of modes identified. The first has a spiral-arm appearance, and exists for large enough horizontal thermal forcing, weak enough static stability, and large enough rotation. The source of this wave is shown to be the Eady mode of instability. The second mode is due to convective instability in the thermal boundary layers which exist due to the thermally conducting horizontal boundaries. Finally, for strong enough negative static stability, thermal convection of the Benard type appears. The most significant result is that the symmetric (Solberg) mode was not found, even though the infinite-plane theory predicts this mode under certain experimental conditions.

Miller, T. L.↗
NASA NTRS1989

Extension during continental convergence, with application to the Tibetan Plateau

An explanation in terms of the thermal evolution of thickened continental lithosphere is offered to explain the transition, in the late Tertiary to Quaternary, from north-south compression to east-west extension in the strain rate field of the Tibetan Plateau. The lower part of the lithosphere consists of a thermal boundary layer which, when thickened by horizontal shortening, is colder and denser than its surroundings. Convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere. The convective instability would happen in a time brief compared with the collision time scale but would only occur after there had already been substantial thickening of the lithosphere. The increase in surface height and of potential energy are sufficient for east-west extension to replace north-south compression as the dominant feature of the stress field.

England, Philip↗
NASA NTRS1981

Effect of gravity on coupled convective and interfacial instabilities during directional solidification

Instabilities occurring during the directional constant-rate solidification of a dilute solution of tin in lead are investigated as a function of gravity with particular emphasis on small constant gravitational accelerations. Two types of instabilities have been identified: (1) convective type that corresponds to long wavelengths, and (2) morphological type that corresponds to short wavelengths. These instabilities are coupled but while the convective instabilities depend strongly on gravity, the morphological instabilities are practically independent of gravity. For low velocities, the convective instabilities occur at much lower solute concentrations than the morphological instabilities, whereas at high velocities the reverse is true.

Coriell, S. R.↗
DOE OSTI2025

Relative Contributions of Surface Parcel and Atmospheric Environment to Convective Potential during Interstorm Periods

Convective instability is affected by both surface heat fluxes into near-surface air parcels and the atmospheric profile encountered as these air parcels are lifted aloft. In this study, we investigate the evolution of buoyancy experienced by surface-based parcels, taking interstorm soil moisture drydowns as fundamental time periods. We use profile observations of atmospheric temperature and humidity from ground-based passive remote sensing and in situ observations available at the U.S. Department of Energy Atmospheric Radiation Measurement Southern Great Plains site in Oklahoma, United States, and examine drydown events during the warm seasons from 2017 to 2022. Results reveal new insights on the emergence and growth of convective available potential energy (CAPE) during drydowns. We observe that the peak convective potential appears in the afternoon and grows in the midtroposphere (68% of 66 drydown events), with the largest CAPE values toward the end of the drydown event. We also find that the near-surface parcel contributes more to the evolution of CAPE than the atmospheric environment (86% of drydown events). These findings highlight the importance of surface heat fluxes and moisture availability from surface-based parcels in modulating the evolution of atmospheric conditions favorable for convection, underscoring the interplay between surface processes and atmospheric dynamics in driving convective instability.

Atmosphere-land interaction↗