Search NASA⌕ Search

SEARCH · Search NASA

Results for “edge 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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

On eigensolutions for discontinuous liners in a duct containing uniform mean flow

Sound attenuation in a rectangular acoustically lined duct containing uniform mean flow is analytically investigated using the generalized Wiener-Hopf technique. Uniqueness of the solution is enforced for lined sections of the finite axial extent by imposing edge conditions at the liner interface. Possible edge conditions are considered, including the Kutta condition, and the causal solution corresponding to edge conditions is considered the best choice. Solution methods such as the mode matching and singularity methods imply differing edge conditions, and results show that power attenuation is insensitive to the imposed edge conditions, although significant differences are observed for the reflection coefficient. The amplitude of the exponentially increasing instability mode in the lined section must be set to zero as a first approximation to the nonlinear situation, and results indicate that measurements of the reflection factor can be used to make a more definite decision about physically appropriate edge conditions.

Koch, W.↗
NASA NTRS1994

The FU Orionis Outburst as a Thermal Disk Accretion Event: Detailed Calculations and Comparison to Observations

FU Orionis outbursts are temporary large increases in luminosity: x (40 - 250) thought to occur repeatedly in all low mass young stellar systems. We discuss detailed calculations of viscous accretion disks suggesting that FU Ori events signify the existence of a protostellar disk transporting mass at a rate of (1 - 10) x 10(exp 6) solar mass / yr, in agreement with theoretical and observational estimates of molecular cloud core collapse rates. Accretion through the inner edge of disks subject to outburst is self-regulated through the thermal ionization instability such that long periods (approx. 1000 yrs) of low mass flux: (1 - 10) x 10(exp -5) solar mass / yr, are punctuated by short periods (approx. 100 yrs) of high mass flux: (1-10) x 10(exp -5) solar mass / yr. The unstable region of the disk extends radially only to a distance of approx. = 1/4 AU. Beyond this region matter is transported stably at the infall rate. In systems for which M(sum *) = 1 solar mass with an inner disk edge of 3 solar radius, the critical rate for outbursts is 5 x 10(exp -7) solar mass / yr independent of the magnitude of the viscous ce parameter consistent with estimates of boundary layer mass flux in T Tauri stars. We use timescales of observed outbursts to constrain the magnitude of the alpha parameter to be 10(exp -4) where hydrogen is neutral and 10(exp -3) where ionized. Light curves of V1515 Cyg, FU Ori, and V1057 Cya are reproduced; the latter two require application of a small perturbation in surface density to produce observed rapid rise times. Detailed reply is made to objections to the accretion disk model for outbursts. Comparison to observations are made of time dependent spectral energy distributions, colors, and line-width velocity evolution.

Bell, K. R.↗
NASA NTRS1992

In-flight detection of flow separation, stagnation, and transition

Flight tests were conducted to demonstrate the feasibility of simultaneous determination of: (1) boundary-layer instability and transition characteristics; (2) flow separation region; and (3) the location of the leading-edge stagnation point. Surface mounted, multi-element, micro-thin, hot-film sensors were used in conjunction with a bank of constant-temperature anemometers and a PC-based 8-channel simultaneous data acquisition system. Conventional techniques were used to determine the boundary-layer instability and transition characteristics while the flow separation and the stagnation points were determined through the presence of phase reversal signatures.

Mangalam, S. M.↗
NASA NTRS1975

Trailing edge noise

Shadowgraphic observations of the flow structure of a wall jet downstream of the trailing edge of a flat plate were carried out. Shadowgraphs obtained at jet exit Mach number 0.3 to 0.8 consistently showed an orderly large oscillatory flow structure. It is believed that these large scale disturbances are the result of flow instabilities. It is also believed that these orderly disturbances are responsible for generating the dominant part of trailing edge noise either directly or indirectly. The interaction of sound generated by these coherent oscillatory flow disturbances and the plate was investigated theoretically. It is found that the farfield noise directivity is strongly influenced by diffraction of sound at the leading edge of the plate.

Tam, C. K. W.↗
NASA NTRS1992

The stability of the near-Earth magnetotail against the ballooning instability

In an extension to a previous study, conditions for the ballooning instability in the magnetohydrodynamic approximation are derived. It is necessary for exciting the ballooning instability that the plasma pressure is higher where plasma is convected outward. However, the expansion of plasma, which results from the divergence of outward flows in the curved geometry of field lines, may overcompensate for the convective change in the plasma pressure. Conditions for the instability are derived from the equation describing the linear coupling between the Alfven and slow magnetosonic waves. The result indicates that excessive curvature stabilizes, rather than destabilizes, perturbations, as well as the field aligned current and the field aligned flow. That is the field line curvature should be smaller than the pressure gradient for exciting the ballooning instability. It is inferred that this condition is not satisfied during quiet times in the near Earth tail. The plasma sheet becomes more stable to the instability in accordance with the thinning of the plasma sheets during the growth phase of substorms, while in the region nearer to the Earth (the inner edge of the plasma sheet or the ring current region) the plasma pressure gradient, which is possibly localized during the growth phase, may drive the instability.

Ohtani, Shin-Ichi↗
NASA NTRS2003

Mechanisms of Rotating Instability in Axial Compressors Investigated

Rotating instability is a phenomenon that occurs in the tip flow region of axial compressor stages during stable operation. It can be observed in highly staggered rotors with significant tip clearance and is strongest at high-load operating points where the characteristic levels off. In this condition, the single-stage fan under investigation radiates an audible, whistling tone, and wall pressure spectra in the vicinity of the rotor disk exhibit nonrotational components. The graph shows the spectrum of static pressure at a point on the endwall near the leading edge. A hump appears at roughly half of the blade passing frequency (BPF) and is characteristic of rotating instability. A computational model was developed at the NASA Glenn Research Center to investigate the mechanism behind this phenomenon. A three-dimensional steady Navier-Stokes code that has been successfully tested for a wide range of turbomachinery flows was modified to execute a time-accurate simulation of the full annulus of the compressor. At the inlet of the computational domain, the total pressure, total temperature, and two velocity components are specified. Since no unsteady measurements of static pressure or other flow variables were available downstream of the rotor, circumferentially averaged static pressure was specified on the shroud at the outlet of the computational domain. A three-dimensional view of the vortex from the numerical model is shown. Particle traces released near the leading edge tip have rolled up to illustrate the tip clearance vortex. Flow near the trailing edge is pushed forward by the axially reversed flow. It then interacts with the tip clearance flow and the incoming flow and results in the rotating instability vortex, the core of which is illustrated by total pressure shading on planes located successively downstream. The rotating instability vortex is formed periodically midway between the blades and moves toward the pressure side of the passage. The unsteady behavior of this vortex structure is the main mechanism of the rotating instability is shown. The numerical model can be used to detect any possible occurrence of rotating instability when the tip clearance increases during engine service.

Hah, Chunill↗
NASA NTRSDate not supplied

The Generation of Mixing Across the Stratospheric Polarvortex Edge By Breaking Gravity Waves

Modern high resolution global atmospheric analyses are capable of resolving larger gravity waves, including their generation, propagation, and breaking. Orographically generated gravity waves that break in the middle stratosphere often appear as disturbances in maps of potential vorticity, a key stratospheric dynamical quantity. Using analyses from the NASA Global Modeling and Assimilation Office, we show how gravity waves not only disturb the local flow above them but can generate flow instabilities leading to eddies that propagate around the stratospheric polar vortex, creating significant mixing across the vortex edge and into the polar region. These disturbances and mixing events are especially noticeable during NH winter months with low planetary scale wave activity such as in January 2022. During January 2022, the initially circular flow around the polar vortex was disrupted by breaking gravity waves, leading to unstable flow that created a chain of eddies around the vortex edge. The mixing associated with this process led to increased ozone in the upper stratosphere polar region.

Polar Vortex↗
NASA NTRS1980

Transmitted sound field due to an impulsive line acoustic source bounded by a plate followed by a vortex sheet

The propagation of sound due to a line acoustic source in the moving stream across a semiinfinite vortex sheet which trails from a rigid plate is examined in a linear theory for the subsonic case. A solution for the transmitted sound field is obtained with the aid of multiple integral transforms and the Wiener-Hopf technique for both the steady state (time harmonic) and initial value (impulsive source) situations. The contour of inverse transform and hence the decomposition of the functions are determined through causality and radiation conditions. The solution obtained satisfies causality and the full Kutta conditions. The transmitted sound field is composed of two waves in both the stady state and initial value problems. One is the wave scattered from the edge of the plate which is associated with the bow wave and the instability wave. These waves exist in the downstream sectors. The other is the wave transmitted through the vortex sheet which is also associated with the instability wave. Regional divisions of the transmitted sound field are identified.

Miura, T.↗
NASA NTRS1987

Experimental studies on Goertler vortices

Goertler vortices arise in laminar boundary layers along concave walls due to an imbalance between pressure and centrifugal forces. In advanced laminar-flow control (LFC) supercritical airfoil designs, boundary-layer suction is primarily used to control Tollmien-Schlichting instability and cross-flow vortices in the concave region near the leading edge of the airfoil lower surface. The concave region itself is comprised of a number of linear segments positioned to limit the total growth of Goertler vortices. Such an approach is based on physical reasonings but rigorous theoretical justification or experimental evidence to support such an approach does not exist. An experimental project was initiated at NASA Langley to verify this concept. In the first phase of the project an experiment was conducted on an airfoil whose concave region has a continuous curvature distribution. Some results of this experiment were previously reported and significant features are summarized.

Mangalam, Siva M.↗
NASA NTRSDate not supplied

Supersonic Traveling Crossflow Wave Characteristics in Ground and Flight Tests

This paper continues analysis of data acquired in a series of supersonic crossflow transition experiments on a 65° swept- wing model tested in both wind tunnel and flight. Flow visualization data are summarized to provide context for the swept-wing instability mechanisms of interest. Unsteady pressure measurements obtained near the wing model leading edge (LE) are studied using 2nd- and 3rd-order statistical methods to extract traveling crossflow wave characteristics from these data. Comparisons between tunnel and flight data with boundary-layer (BL) Linear Stability Theory (LST) predictions provide insights into BL transition phenomena similarities and differences observed in tests using the same swept-wing model configurations. Comparisons of unsteady pressure data results for wing LE configurations with and without distributed-roughness-element (DRE) patterns installed, suggest traveling crossflow disturbance growth is reduced when this type of flow control is applied. A brief stud y of different background surface roughness levels on the wing LE suggests the potential for another flow control approach to promote laminar flow on swept wings using streamwise-biased surface finishes.

Lewis R Owens↗
NASA NTRS1994

Spatial distribution of plasma wave activity in the nightside ionosphere of Venus

In this study we use 14 years of Pioneer Venus Orbiter Electric Field Detector (OEFD) data to define the characteristics of (VLF) burst activity in the nightside ionosphere of Venus. Our statistical results show that there are essentially four types of VLF signals. The first type of signal is only observed in the 100 Hz channel and not in any of the higher frequency channels (730 Hz, 5.4 kHz or 30 kHz). Occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence of these waves is controlled by the magnetic field with a weaker dependence on electron density. The occurrence rate decreases with increasing altitude to a height of 600 km. For higher altitudes beyond 600 km the occurrence rate remains roughly constant. The statistics of these signals are what one would expect for whistler mode waves from a subionospheric source. The second type of signal is broadline wave activity appearing below 300 km in the low altitude ionosphere. These signals often occur in all four channels of the OEFD. These signals are also thought to come from a subionospheric source. The third type of signal appearing near the edge of the planetary optical shadow. They are probably ion acoustic waves generated by a current driven instability associated with plasma clouds in the wake. The fourth type of signal is a narrow band wave. It occurs in either of the two high frequency channels in the high altitude tail region, and is attributed to locally generated Langmuir waves. In addition, we also observe spacecraft interference noise in both the 100 and 730 Hz channels. These signals mainly occur near the edge of the planetary optical shadow and have an inbound and outbound asymmetry in activity.

Ho, C.-M.↗
NASA NTRS1991

Leading-edge receptivity for blunt-nose bodies

This research program investigates boundary-layer receptivity in the leading-edge region for bodies with blunt leading edges. Receptivity theory provides the link between the unsteady distrubance environment in the free stream and the initial amplitudes of the instability waves in the boundary layer. This is a critical problem which must be addressed in order to develop more accurate prediction methods for boundary-layer transition. The first phase of this project examines the effects of leading-edge bluntness and aerodynamic loading for low Mach number flows. In the second phase of the project, the investigation is extended to supersonic Mach numbers. Singular perturbation techniques are utilized to develop an asymptotic theory for high Reynolds numbers. In the first year, the asymptotic theory was developed for leading-edge receptivity in low Mach number flows. The case of a parabolic nose is considered. Substantial progress was made on the Navier-Sotkes computations. Analytical solutions for the steady and unsteady potential flow fields were incorporated into the code, greatly expanding the types of free-stream disturbances that can be considered while also significantly reducing the the computational requirements. The time-stepping algorithm was modified so that the potential flow perturbations induced by the unsteady pressure field are directly introduced throughout the computational domain, avoiding an artificial 'numerical diffusion' of these from the outer boundary. In addition, the start-up process was modified by introducing the transient Stokes wave solution into the downstream boundary conditions.

Kerschen, Edward J.↗
NASA NTRS1990

Wave interactions in a three-dimensional attachment-line boundary layer

The three-dimensional boundary layer on a swept wing can support different types of hydrodynamic instability. Here attention is focused on the so-called 'spanwise instability' problem which occurs when the attachment-line boundary layer on the leading edge becomes unstable to Tollmien-Schlichting waves. In order to gain insight into the interactions that are important in that problem a simplified basic state is considered. This simplified flow corresponds to the swept attachemnt-line boundary layer on an infinite flat plate. The basic flow here is an exact solution of the Navier-Stokes equations and its stability to two-dimensional waves propagating along the attachment line can be considered exactly at finite Reynolds number. The corresponding problem is studied for oblique waves, and their interaction with two-dimensional waves is investigated. It is shown that there are two types of oblique wave which, if excited, cause the destabilization of the two-dimensional mode and the breakdown of the disturbed flow at a finite distance from the leading edge.

Hall, Philip↗
NASA NTRS1991

Assessment of meanflow solutions for instability analysis of transitioning flows

Meanflow solutions of 3D supersonic flow past a cone at incidence and a swept leading edge wing have been obtained by thre methods, viz., boundary-layer, parabolized Navier-Stokes, and thin shear-layer Navier-Stokes solvers. The smoothness and accuracy of the solution profiles are compared with a view to applying the meanflow solution to boundary-layer stability analysis.

Iyer, Venkit↗
NASA NTRS1997

Drag Prediction and Transition in Hypersonic Flow

This paper discusses progress on issues such as instability studies, nose-bluntness and angle-of-attack effects, and leading-edge-contamination problems from theoretical, computational, and experimental points of view. Also included is a review of wind-tunnel and flight data, including high-Re flight transition data, the levels of noise in flight and in wind tunnels, and how noise levels can affect parametric trends. A review of work done on drag accounting and the role of viscous drag for hypersonic vehicles is also provided.

Reed, Helen L.↗
NASA NTRS1979

Recent advances in the solution of three-dimensional flow over wings with leading edge vortex separation

Recent advances in a panel method for the solution of three-dimensional flow about wing and wing-body combinations with leading-edge vortex separation are presented. These advances were achieved as part of an ultimately successful assault on two shortcomings of the method, namely convergence failures in seemingly random cases, and overprediction of lift coefficient for high aspect-ratio wings. Advances include the implementation of improved panel numerics for the purpose of eliminating the highly non-linear effects of ring vortices around doublet panel edges, and the development of a least squares procedure for damping vortex sheet geometry update instabilities. A variety of cases generated by the computer program implementing the method are presented. These cases are of two types. The first type consists of numerical studies, which verify the underlying mathematical assumptions of the method and moreover show that the results are strongly invariant with respect to such user dependent input as wing panel layout, initial sheet shape, sheet rollup, etc. The second type consists of cases run for the purpose of comparing computed results with experimental data, and these comparisons verify the underlying physical assumptions made by the method.

Johnson, F. T.↗
NASA NTRS1980

An improved panel method for the solution of three-dimensional leading-edge vortex flows. Volume 1: Theory document

An improved panel method for the solution of three dimensional flow and wing and wing-body combinations with leading edge vortex separation is presented. The method employs a three dimensional inviscid flow model in which the configuration, the rolled-up vortex sheets, and the wake are represented by quadratic doublet distributions. The strength of the singularity distribution as well as shape and position of the vortex spirals are computed in an iterative fashion starting with an assumed initial sheet geometry. The method calculates forces and moments as well as detail surface pressure distributions. Improvements include the implementation of improved panel numerics for the purpose of elimination the highly nonlinear effects of ring vortices around double panel edges, and the development of a least squares procedure for damping vortex sheet geometry update instabilities. A complete description of the method is included. A variety of cases generated by the computer program implementing the method are presented which verify the mathematical assumptions of the method and which compare computed results with experimental data to verify the underlying physical assumptions made by the method.

Johnson, F. T.↗
NASA NTRS2013

Wind Tunnel Investigation of Passive Vortex Control and Vortex-Tail Interactions on a Slender Wing at Subsonic and Transonic Speeds

A wind tunnel experiment was conducted in the NASA Langley 8-Foot Transonic Pressure Tunnel to determine the effects of passive porosity on vortex flow interactions about a slender wing configuration at subsonic and transonic speeds. Flow-through porosity was applied in several arrangements to a leading-edge extension, or LEX, mounted to a 65-degree cropped delta wing as a longitudinal instability mitigation technique. Test data were obtained with LEX on and off in the presence of a centerline vertical tail and twin, wing-mounted vertical fins to quantify the sensitivity of the aerodynamics to tail placement and orientation. A close-coupled canard was tested as an alternative to the LEX as a passive flow control device. Wing upper surface static pressure distributions and six-component forces and moments were obtained at Mach numbers of 0.50, 0.85, and 1.20, unit Reynolds number of 2.5 million, angles of attack up to approximately 30 degrees, and angles of sideslip to +/-8 degrees. The off-surface flow field was visualized in cross planes on selected configurations using a laser vapor screen flow visualization technique. Tunnel-to-tunnel data comparisons and a Reynolds number sensitivity assessment were also performed. 15.

Erickson, Gary E.↗