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At least 1,171 records · Page 65
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NASA NTRS2022

Instabilities in the Wake of a Pseudorandom Roughness on a Supersonic Flat Plate

A roughness patch defined by a two-dimensional Fourier series was installed in a highly-polished flat plate model immersed in a supersonic quiet flow. The NASA Langley Research Center Supersonic Low Disturbance Tunnel is capable of providing a Mach 3.5 flow with low freestream acoustic noise and low turbulence levels. Measurements made with a hot-wire probe downstream of the roughness element showed the presence of instability modes that did not cause transition in the measurement region at a freestream unit Reynolds number of Re =12.6×10^6/m. The largest amplitude mode appeared to be similar to a symmetric mode. The same roughness patch immersed in a higher freestream acoustic noise environment showed little to no evidence of the instability modes. Instead, disturbances found in the boundary layer matched closely to the freestream spectra measured in the empty tunnel under the same conditions, indicating some evidence of freestream forcing in the boundary layer.

Roughness induced transition↗
NASA NTRS1992

Fully coupled resonant-triad interaction in an adverse-pressure-gradient boundary layer

The nonlinear resonant-triad interaction, proposed by Raetz (1959), Craik (1971), and others for a Blasius boundary layer, is analyzed here for an adverse-pressure-gradient boundary layer. We assume that the adverse pressure gradient is in some sense weak and, therefore, that the instability growth rate is small. This ensures that there is a well-defined critical layer located somewhere within the flow and that the nonlinear interaction is effectively confined to that layer. The initial interaction is of the parametric resonance type, even when the modal amplitudes are all of the same order. This means that the oblique instability waves exhibit faster than exponential growth and that the growth rate of the two-dimensional mode remains linear. However, the interaction and the resulting growth rates become fully coupled, once oblique-mode amplitudes become sufficiently large, but the coupling terms are now quartic, rather than quadratic as in the Craik (1971) analysis. More importantly, however, new nonlinear interactions, which were not present in the Craik-type analyses, now come into play. These interactions eventually have a dominant effect on the instability wave development.

Goldstein, M. E.↗
NASA NTRS1978

Concerning isothermal self-similar blast waves. I - One-dimensional flow and its stability. II - Two-dimensional flow and its stability

One-dimensional self-similar isothermal flow behind a blast wave propagating in a medium whose density varies with distance is investigated for the cases of one-dimensional and two-dimensional flow. The isothermal flow model is adopted as an alternative to adiabatic models of self-similar flow, which neglect heat flux. The topology of the one-dimensional flow solutions, the singularities, and the influence of boundary conditions are discussed; the instability of the isothermal blast waves against nonself-similar perturbations is also considered. The number of critical points in the two-dimensional solutions is found to vary from the number in the one-dimensional problem.

Lerche, I.↗
NASA NTRS1985

Theoretical and experimental studies of baroclinic processes

An experimental study in a baroclinic annulus with heating and cooling on the upper and lower horizontal surfaces was completed. Four basic regimes of flow were observed: (1) axisymmetric flow, (2) cellular (deep) convection, (3) convective rolls (boundary layer), and (4) baroclinic instability. There was no symmetric (or nearly symmetric) baroclinic instability observed, although it was determined by numerical calculations that we transversed regions in parameter space where the symmetric instability was present. The non-symmetric baroclinic instability (Eady type) was observed instead. Although the vertical depth was rather small and hence viscous damping was strong, large-amplitude baroclinic waves were made possible by imposing a very weak static stability.

Miller, T.↗
NASA NTRS1986

On the interpretation of the beta(sub p) relation in interstellar clouds

Troland and Heiles (1986) have recently presented an updated compilation of observational data concerning the relationship between the interstellar magnetic field strength B and the gas density rho (or, equivalently, the particle density n). One of the main findings of their survey was that B remains constant over the density range 0.1 - approx. 100 cu. cm and shows evidence for increase only a higher densities. They compared this result with theoretical predictions based on the Parker-instability scenario for the formation and evolution of interstellar clouds in the presence of the galactic magnetic field. In this picture, low-density gas is driven by the magnetic Rayleigh-Taylor instability into magnetic valleys, where it accumulates into denser concentrations. The gas initially flows along the magnetic field lines and there is little increase of the field strength with density; B only starts to rise when n becomes large enough for self-gravity to begin competing with the magnetic stresses. For a cloud mass of approx. 1,000 sub M and the measured background field strength, the critical density for contraction in approx. 75 cu. cm. Troland and Heiles therefore concluded that this scenario is basically consistent with the observations. This conclusion is debated.

Koenigl, A.↗
NASA NTRS1988

On the preferred mode of jet instability

The preferred mode of instability was investigated in an axisymmetric air jet of moderate Reynolds number. Natural instabilities are shown to scale with local shear-layer thickness and the preferred mode is shown to be a shear-layer instability. The spatial evolution of the preferred mode was examined by exciting the flow acoustically and then mapping the phase-locked velocity fluctuations. Throughout the potential core region the phase-locked profiles are shown to agree with the eigensolutions of the Orr-Sommerfeld stability equations provided the calculations are based on measured, mean velocity profiles. The excitation intensity was varied from low levels, where the flow was merely tagged, to high levels where the mean flow was substantially distorted, and over that range of excitation there was no apparent deterioration in the agreement with stability predictions.

Petersen, R. A.↗
NASA NTRS1989

Wind-disk shocks around T Tauri stars

It is suggested that the winds of T Tauri stars are heated at distances of 50-100 AU by oblique shocks with circumstellar disks, producing the observed high-velocity forbidden-line emission. The low shock velocities characteristic of this model provide low-excitation emission and line profiles in reasonable agreement with observations. The blueshifted character of the observed emission is explained by a combination of disk occultation of the receding flow, plus a modest collimation of the observable wind by the disk envelope. Instabilities at the wind-disk interface may be important in producing the observed emission.

Hartmann, Lee↗
NASA NTRS1987

Stability theory applications to laminar-flow control

In order to design Laminar Flow Control (LFC) configurations, reliable methods are needed for boundary-layer transition predictions. Among the available methods, there are correlations based upon R sub e, shape factors, Goertler number and crossflow Reynolds number. The most advanced transition prediction method is based upon linear stability theory in the form of the e sup N method which has proven to be successful in predicting transition in two- and three-dimensional boundary layers. When transition occurs in a low disturbance environment, the e sup N method provides a viable design tool for transition prediction and LFC in both 2-D and 3-D subsonic/supersonic flows. This is true for transition dominated by either TS, crossflow, or Goertler instability. If Goertler/TS or crossflow/TS interaction is present, the e sup N will fail to predict transition. However, there is no evidence of such interaction at low amplitudes of Goertler and crossflow vortices.

Malik, Mujeeb R.↗
NASA NTRS1990

Transition to hard turbulence in thermal convection at infinite Prandtl number

Direct numerical simulations of two-dimensional high Rayleigh (Ra) number, base-heated thermal convection in large aspect-ratio boxes are presented for infinite Prandtl number fluids, as applied to the earth's mantle. A transition is characaterized in the flow structures in the neighborhood of Ra between 10 to the 7th and 10 to the 8th. These high Ra flows consist of large-scale cells with strong intermittent, boundary-layer instabilities. For Ra exceeding 10 to the 7th it is found that the heat-transfer mechanism changes from one characterized by mushroom-like plumes to one consisting of disconnected ascending instabilities, which do not carry with them all the thermal anomaly from the bottom boundary layer. Plume-plume collisions become much more prominent in high Ra situations and have a tendency of generating a pulse-like behavior in the fixed plume. This type of instability represents a distinct mode of heat transfer in the hard turbulent regime. Predictions of this model can be used to address certain issues concerning the mode of time-dependent convection in the earth's mantle.

Hansen, Ulrich↗
NASA NTRS1993

Pinching solutions of slender cylindrical jets

Simplified equations for slender jets are derived for a circular jet of one fluid flowing into an ambient second fluid, the flow being confined in a circular tank. Inviscid flows are studied which include both surface tension effects and Kelvin-Helmholtz instability. For slender jets a coupled nonlinear system of equations is found for the jet shape and the axial velocity jump across it. The equations can break down after a finite time and similarity solutions are constructed, and studied analytically and numerically. The break-ups found pertain to the jet pinching after a finite time, without violation of the slender jet ansatz. The system is conservative and admissible singular solutions are those which conserve the total energy, mass, and momentum. Such solutions are constructed analytically and numerically, and in the case of vortex sheets with no surface tension certain solutions are given in closed form.

Papageorgiou, Demetrios T.↗
NASA NTRS1994

The behavior of unsteady thermocapillary flows

The thermocapillary-driven flow of a liquid in a shallow slot provides a simple model to explore the effects of free-surface deflection and end walls on the stability of the flow. Such an investigation may be useful in understanding the complex flow seen in processes such as the float-zone refining of single crystals. A linear stability analysis of the viscously-dominated slot flow indicates stable basic states to both tow- and three-dimensional infinitesimal disturbances for capillary numbers below 43.3. Above this capillary number steady solutions do not exist. We also discuss current work involving the development of an inertially-dominated slot-flow model using singular perturbation methods. If these flows are unstable, inertial effects would be the only possible cause of the instability.

Vrane, David R.↗
NASA NTRS1994

The stability of two-phase flow over a swept-wing

We use numerical and asymptotic techniques to study the stability of a two-phase air/water flow above a flat porous plate. This flow is a model of the boundary layer which forms on a yawed cylinder and can be used as a useful approximation to the air flow over swept wings during heavy rainfall. We show that the interface between the water and air layers can significantly destabilize the flow, leading to traveling wave disturbances which move along the attachment line. This instability occurs for lower Reynolds numbers than in the case of the absence of a water layer. We also investigate the instability of inviscid stationary modes. We calculate the effective wavenumber and orientation of the stationary disturbance when the fluids have identical physical properties. Using perturbation methods we obtain corrections due to a small stratification in viscosity, thus quantifying the interfacial effects. Our analytical results are in agreement with the numerical solution which we obtain for arbitrary fluid properties.

Coward, Adrian↗
NASA NTRS1996

Investigation of Two-Phase Flows in Piping Bends and Elbows

An experimental investigation of the hydrodynamic characteristics of two-phase R-113 flow has been carried out. Straight tube pressure drop data, as a function of mass flow rate (mass flux) and flow quality has been obtained using the Two-Phase Flow Test Facility located in the Advanced Thermal Laboratories of the Crew and Thermal Systems Division at the Lyndon B. Johnson Space Center. Additionally, after successfully obtaining the straight tube pressure drop data, the test facility was modified in order to obtain pressure drop data for the flow of two-phase R-113 through 180 deg piping bends. Inherent instabilities of the test facility prevented the successful acquisition of pressure drop data through the piping bends. The experimental straight tube data will be presented and compared with existing predictive correlations in an attempt to gain insight into the utility of such correlations as the basis for developing design criteria. A discussion of the instabilities which rendered successful acquisition of the piping bend data will be presented and suggestions will be made for eliminating these system tendencies. Finally, recommendations for future investigations, based on successful reconfiguration of the test facility, will be made.

Duncan, Allen B.↗
NASA NTRS2003

Discussion of Priorities

The Microgravity Science Division identifies four priority ratings for microgravity research and technology issues: 1) Critical; 2) Severely Limiting; 3) Enhancements; 4) Communication. Reduced gravity instabilities are critical, while severely limiting issues include phase separation, phase change, and flow through components. Enhancements are listed for passive phase separation and phase change. This viewgraph presentation also classifies microgravity issues as spaceflight, ground-based, or other for the time periods 2003-2008, 2009-2015, and beyond.

Source record↗
NASA NTRS2007

Potential Fluctuations and Energetic Ion Production in Hollow Cathode Discharges

Ions with energies significantly in excess of the applied discharge voltage have been reported for many years in hollow cathode discharges. Models of dc potential hills downstream of the cathode and instabilities in postulated double layers in the cathode orifice have been proposed to explain this, but have not been substantiated. Measurements of the dc and rf plasma density and potential profiles near the exit of hollow cathodes by miniature fast-scanning probes suggests that turbulent ion acoustic fluctuations and ionization instabilities in the cathode plume significantly increase the energy of the ions that flow from this region. Increases in the discharge current and/or decreases in the cathode gas flow enhance the amplitude of the fluctuations and increase the number and energy of the energetic ions, which increases the erosion rate of the cathode electrodes. The transition from the quiescent 'spot mode' to the noisy 'plume mode' characteristic of these discharges is found to be a gradual transition of increasing fluctuation amplitudes.

keeper↗
NASA NTRS2010

The Impact of Ocean Data Assimilation on Seasonal-to-Interannual Forecasts: A Case Study of the 2006 El Nino Event

This study investigates the impact of four different ocean analyses on coupled forecasts of the 2006 El Nino event. Forecasts initialized in June 2006 using ocean analyses from an assimilation that uses flow-dependent background error covariances are compared with those using static error covariances that are not flow dependent. The flow-dependent error covariances reflect the error structures related to the background ENSO instability and are generated by the coupled breeding method. The ocean analyses used in this study result from the assimilation of temperature and salinity, with the salinity data available from Argo floats. Of the analyses, the one using information from the coupled bred vectors (BV) replicates the observed equatorial long wave propagation best and exhibits more warming features leading to the 2006 El Nino event. The forecasts initialized from the BV-based analysis agree best with the observations in terms of the growth of the warm anomaly through two warming phases. This better performance is related to the impact of the salinity analysis on the state evolution in the equatorial thermocline. The early warming is traced back to salinity differences in the upper ocean of the equatorial central Pacific, while the second warming, corresponding to the mature phase, is associated with the effect of the salinity assimilation on the depth of the thermocline in the western equatorial Pacific. The series of forecast experiments conducted here show that the structure of the salinity in the initial conditions is important to the forecasts of the extension of the warm pool and the evolution of the 2006 El Ni o event.

Yang, Shu-Chih↗