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Results for “Flow instabilities”

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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Generation of instability waves in flows separating from smooth surfaces

This paper analyses the coupling between an imposed disturbance and an instability wave that propagates downstream on a shear layer which emanates from a separation point on a smooth surface. Since the wavelengths of the most-amplified instability waves will generally be small compared with the streamwise body dimensions, the analysis is restricted to this 'high-frequency' limit and the solution is obtained by using matched asymptotic expansions. An 'inner' solution, valid near the separation point, is matched onto an outer solution, which represents an instability wave on a slowly diverging mean flow. The analysis relates the amplitude of this instability to that of the imposed disturbance.

Goldstein, M. E.↗
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 NTRS1990

On the Goertler instability in hypersonic flows: Sutherland law fluids and real gas effects

The Goertler vortex instability mechanism in a hypersonic boundary layer on a curved wall is investigated. The precise roles of the effects of boundary layer growth, wall cooling, and gas dissociation is clarified in the determination of stability properties. It is first assumed that the fluid is an ideal gas with viscosity given by Sutherland's law. It is shown that when the free stream Mach number M is large, the boundary layer divides into two sublayers: a wall layer of O(M sup 3/2) thickness over which the basic state temperature is O(M squared) and a temperature adjustment layer of O(1) thickness over which the basic state temperature decreases monotonically to its free stream value. Goertler vortices which have wavelengths comparable with the boundary layer thickness are referred to as wall modes. It is shown that their downstream evolution is governed by a set of parabolic partial differential equations and that they have the usual features of Goertler vortices in incompressible boundary layers. As the local wavenumber increases, the neutral Goertler number decreases and the center of vortex activity moves towards the temperature adjustment layer. Goertler vortices with wavenumbers of order one or larger must necessarily be trapped in the temperature adjustment layer and it is this mode which is most dangerous. For this mode, it was found that the leading order term in the Goertler number expansion is independent of the wavenumber and is due to the curvature of the basic state. This term is also the asymptotic limit of the neutral Goertler numbers of the wall mode. To determine the higher order corrections terms in the Goertler number expansion, two wall curvature cases are distinguished. Real gas effects were investigated by assuming that the fluid is an ideal dissociating gas. It was found that both gas dissociation and wall cooling are destabilizing for the mode trapped in the temperature adjustment layer, but for the wall mode trapped near the wall the effect of gas dissociation can be either destabilizing or stabilizing.

Fu, Yibin B.↗
NASA NTRS1991

On magnetothermal instability in cluster cooling flows

Lagrangian techniques appropriate to a local calculation are used to show that a weak ordered magnetic field can result in a generic condensational mode in cluster cooling flows. However, thermal instability appears possible only if the conductivity is well below its Spitzer value, for all nonradial wavenumbers. Wavenumbers not subject to conductive damping are subject to buoyant oscillations. It is shown that when instability is present, lateral magnetic confinement of high thermal pressure regions in the plasma by radial magnetic field lines is responsible in at least equal measure with radially directed magnetic tension for the suppression of oscillations and the reappearance of local condensational modes. The general importance of even very modest magnetic fields for destabilizing thermal time scale perturbations is emphasized.

Balbus, Steven A.↗
NASA NTRS1993

The Kelvin-Helmholtz instability in photospheric flows - Effects of coronal heating and structure

A series of hydrodynamic numerical simulations has been used to investigate the nonlinear evolution of driven, subsonic velocity shears under a range of typical photospheric conditions. These calculations show that typical photospheric flows are susceptible to the Kelvin-Helmholtz instability (KHI), with rapid nonlinear growth times that are approximately half of a typical granule lifetime. The KHI produces vortical structures in intergranule lanes comparable to a typical fluxule radius; this is precisely the correct scale for maximum power transfer to the corona.

Karpen, Judith T.↗
NASA NTRS2003

Effects of Mean Flow Profiles on Instability of a Low-Density Gas Jet Injected into a High-Density Gas

The objective of this study was to investigate the effects of the mean flow profiles on the instability characteristics in the near-injector region of low-density gas jets injected into high-density ambient gas mediums. To achieve this, a linear temporal stability analysis and a spatio-temporal stability analysis of a low-density round gas jet injected vertically upwards into a high-density ambient gas were performed by assuming three different sets of mean velocity and density profiles. The flow was assumed to be isothermal and locally parallel. Viscous and diffusive effects were ignored. The mean flow parameters were represented as the sum of the mean value and a small normal-mode fluctuation. A second order differential equation governing the pressure disturbance amplitude was derived from the basic conservation equations. The first set of mean velocity and density profiles assumed were those used by Monkewitz and Sohn for investigating absolute instability in hot jets. The second set of velocity and density profiles assumed for this study were the ones used by Lawson. And the third set of mean profiles included a parabolic velocity profile and a hyperbolic tangent density profile. The effects of the inhomogeneous shear layer and the Froude number (signifying the effects of gravity) on the temporal and spatio-temporal results for each set of mean profiles were delineated. Additional information is included in the original extended abstract.

Vedantam, Nanda Kishore↗
DOE OSTI2024

Saturation of fishbone instability through zonal flows driven by energetic particle transport in tokamak plasmas

Gyrokinetic and kinetic-MHD simulations are performed for the fishbone instability in the DIII-D discharge #178631, chosen for validation of first-principles simulations to predict the energetic particle (EP) transport in an ITER prefusion baseline scenario. Fishbone modes are found to generate zonal flows, which dominate the fishbone saturation. The underlying mechanisms of the two-way fishbone-zonal flows nonlinear interplay are discussed in details. Numerical and analytical analyses identify the fishbone-induced EP redistribution as the dominant generation mechanism for zonal flows. The zonal flows modify the nonlinear dynamics of phase space zonal structures, which reduces the amount of EPs able to resonate with the mode, leading to an early fishbone saturation. Simulation results including zonal flows agree quantitatively with DIII-D experimental measurements of the fishbone saturation amplitude and EP transport, supporting this novel saturation mechanism by self- generated zonal flows. Moreover, the wave-particle mode-locking mechanism is shown to determine quantitatively the fishbone frequency down-chirping, as evident in GTC simulation results in agreement with predictions from analytical theory. Finally, the
fishbone-induced zonal flows are possibly responsible for the formation of an ion-ITB in the DIII-D discharge. Based on the low EP transport and the large zonal flow shearing rates associated with the fishbone instability in gyrokinetic simulations of the ITER scenario, it is conjectured that high performance scenarios could be designed in ITER burning plasmas through fishbone-induced ITBs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1988

The growth of thermal instabilities in cooling flows

Numerical hydrodynamic simulations are used to determine the evolution of the condensation modes for thermal instabilities in 10 to the 6th to 10 to the 8th K gas. These simulations properly take into account the effects of nonequilibrium cooling below 10 to the 6th K. It is found that only perturbations greater than about 3 kpc develop shocks in the hot environment of cooling flows. These large perturbations evolve through three distinct phases: (1) the formation of a cold core, (2) a supersonic accretion phase onto the cold core which lasts for 10 to the 5th to 10 to the 6th yr with peak shock velocities about 120 km/s, and (3) a subsonic accretion phase. The X-ray emissivity and the optical-ultraviolet line emission associated with thermal instabilities are calculated. These calculations demonstrate that the optical emission from cooling flows probably does not arise from shocked condensations. The calculated optical emission lines are of a higher ionization level and a lower luminosity than the observations. Ultraviolet lines produced in the shocks may be detectable. The core must cool significantly after the shock has dissipated before star formation can commence.

David, Laurence P.↗
NASA NTRS1990

Flight-measured streamwise disturbance instabilities in laminar flow

NASA is presently conducting laminar-flow flight research to explore the limits of practical applications of laminar-flow drag reduction technology. An important aspect of this research involves studies of the dominant instability or instabilities responsible for initiating the transition process from laminar to turbulent flow. Recent subsonic flight experiments using an instrumented wing glove on a Lear 28/29 airplane measured the growth of the two-dimensional, viscous Tollmien-Schlichting (T-S) instability. The gloved wing section incorporated closely-spaced, flush-mounted, streamwise-located instrumentation for measuring the instability frequencies, surface temperatures, and pressure distributions. Flight conditions of the experiment includeed Mach numbers from 0.70 to 0.79 and chord Reynolds numbers from 10 to 20 million. An onboard engineer's station allowed for real-time analysis of the instability frequency data. In addition to a sample of results on the streamwise growth of instabilities, details of the glove and sensor installation and the onboard real-time data analysis system are given.

Lee, Cynthia C.↗
NASA NTRS1993

Measuring Tollmien-Schlichting Instabilities In Laminar Flow

NASA flight experiment conducted to evaluate surface-mounted hot-film sensors designed to detect Tollmien-Schlichting (T-S) instability in boundary layer in flight. New technique has considerable value in applied research aimed at extending regions of natural laminar flow on aircraft. Aids research into mechanisms leading to transition from laminar to turbulent boundary layers. Makes it practical to take measurements in many locations on wing surface for tests in flight. Useful in studying transitions in such flow as those in pipes, ducts, channels, compressors, and diffusers.

Lee, Cynthia C.↗
NASA NTRS1991

Combustion wave instabilities in turbulent flows

Navier-Stokes solutions are used to calculate oscillatory components of pressure, velocity, and density, which in turn provide necessary data to compute energy growth factors to determine combustion instability. It is shown that wave instabilities are associated with changes in entropy and the space and time averages of oscillatory components of pressure, velocity and density, together with the mean flow field in the energy equation. Compressible laminar and turbulent flows and reacting flows with hydrogen/oxygen combustion are considered in this paper. The space shuttle main engine combustion/thrust chamber is used for illustration of the theory. The analysis shows that the increase of mean pressure and disturbances consistently results in the increase in instability. It is shown that adequate combustion instability analysis requires at least third order nonlinearity in energy growth or decay.

Yoon, W. S.↗
DOE OSTI2024

Soft interface instability and gas flow channeling in low-permeability deformable media

Understanding gas percolation through a clay layer or a shale formation is of great importance for the development of a geologic repository for nuclear waste disposal, a subsurface system for gas storage, and an engineering approach for hydrocarbon extraction from unconventional reservoirs. Gas injection experiments have revealed complex dynamic behaviours of gas percolation through water saturated compacted bentonite, characterized by a high breakthrough pressure, rapid breakthrough, a pressure/stress decay after the breakthrough, a relatively high migration rate, high-frequency periodic/nonperiodic variations in flow rate, stepwise rate reductions during relaxation, and low gas saturation over the whole process, all indicating channelling nature of the processes. Using linear stability analyses, we show that this channelling can autonomously emerge from the instability of the deformable interface between the injected gas and the compacted bentonite matrix driven by local stress concentration, pore dilation, and hydrologic gradient. Channel patterns formed would possess a fractal geometry. We further show that, once a percolating channel is established, the gas injected would percolate through the channel in a chain of gas bubbles, also due to the interface instability, resulting in periodic/chaotic variations in gas flow rate. Our work provides a unified explanation for key features observed for gas percolation in low-permeability deformable media. The work also suggests a possibility of designing an engineered barrier system for a nuclear waste repository that can have controllable gas release while limit water transport.

Bentonite↗
NASA NTRS1978

Experimental study of the laminar-turbulent transition of a concave wall in a parallel flow

The instability of the laminar boundary layer flow along a concave wall was studied. Observations of these three-dimensional boundary layer phenomena were made using the hydrogen-bubble visualization technique. With the application of stereo-photogrammetric methods in the air-water system it was possible to investigate the flow processes qualitatively and quantitatively. In the case of a concave wall of sufficient curvature, a primary instability occurs first in the form of Goertler vortices with wave lengths depending upon the boundary layer thickness and the wall curvature. At the onset the amplification rate is in agreement with the linear theory. Later, during the non-linear amplification stage, periodic spanwise vorticity concentrations develop in the low velocity region between the longitudinal vortices. Then a meandering motion of the longitudinal vortex streets subsequently ensues, leading to turbulence.

Bippes, H.↗
NASA NTRS1987

In-flight detection of Tollmien-Schlichting instabilities in laminar flow

This paper presents the results of a recent flight experiment conducted to evaluate surface-mounted hot-film sensors designed to detect Tollmien-Schlichting instability in the boundary layer in flight. Flight data are presented to illustrate the design and operation of the sensors and to provide information on disturbance growth and transition mode. The results of calculations using a boundary-layer analysis code and boundary-layer linear stability analysis methods are included.

Croom, Cynthia C.↗
NASA NTRS1990

Assessment of shock capturing schemes for resonant flows in nonlinear instability analysis

The paper presents computational assessment of advanced numerical schemes for nonlinear acoustic problems related to combustion instabilities in liquid rocket engines. Several time-accurate, shock capturing schemes have been evaluated on a benchmark, closed-end resonant pipe flow problem. It involves the numerical solution of inviscid, compressible gas dynamics equations to predict acoustic wave propagation, wave steepening, formation of shocks, acoustic energy dissipation and wave-wall reflection for several hundred wave cycles. It was demonstrated that high accuracy TVD type schemes can be used for direct, exact nonlinear analysis of combustion instability problems, preserving high harmonic energy content for long periods of time. The selected scheme was then applied to analyze the acoustic responses of resonant pipe-resonator, radial acoustic modes and hub-baffle configurations. Interesting observations of wave shape and damping characteristics have been drawn from presented computational studies.

Przekwas, A. J.↗
NASA NTRS2002

Gravitational Instability in Suspension Flows

The gravity-driven flow of non-neutrally buoyant suspensions is shown to be unstable to spanwise perturbations when the shearing motion generates a density profile that increases with height. The instability is simply due to having heavier material over light. The wavelength of the perturbation is found to be on the order of the thickness of the suspension layer. The parameters important to the problem are the angle of inclination of the layer relative to gravity, the relative density difference between the particles and fluid, the ratio of the particle size to the suspension layer, and the bulk volume fraction of particles. An example showing the growth rate as a function of wave number is shown.

Carpen, Ileana C.↗
NASA NTRS1988

Flow-induced morphological instabilities - The rotating disc

The morphological stability of a rotating and solidifying disk is investigated under the assumption that delta, the thickness of the viscous boundary layer, is much larger than delta(c), the thickness of the solute boundary layer. It is found that axisymmetric disturbances with wavelengths comparable to delta respond to nonparallel flow effects and have stability characteristics quite different from disturbances in a parallel flow. These long waves are unstable because of the nonparallel flow and would decay without it. This analysis thus identifies a new mechanism of morphological change induced by flow.

Brattkus, K.↗