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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Impinging jets atomization

An analysis of the characteristics of the spray produced by an impinging-jet injector is presented. Predictions of the spray droplet size and distribution are obtained through studying the formation and disintegration of the liquid sheet formed by the impact of two cylindrical jets of the same diameter and momentum. Two breakup regimes of the sheet are considered depending on Weber number, with transition occurring at Weber numbers between 500 and 2000. In the lower Weber number regime, the breakup is due to Taylor cardioidal waves, while at Weber number higher than 2000, the sheet disintegration is by the growth of Kelvin-Helmholtz instability waves. Theoretical expressions to predict the sheet thickness and shape are derived for the low Weber number breakup regime. An existing mathematical analysis of Kelvin-Helmholtz instability of radially moving liquid sheets is adopted in the predictions of resultant drop sizes by sheet breakup at Weber numbers greater than 2000. Comparisons of present theoretical results with experimental measurements and empirical correlations reported in the literature reveal favorable agreement.

Ibrahim, E. A.↗
NASA NTRS1994

The effects of profiles on supersonic jet noise

The effect of velocity profiles on supersonic jet noise are studied by using stability calculations made for a shock-free coannular jet, with both the inner and outer flows supersonic. The Mach wave emission process is modeled as the noise generated by the large scale turbulent structures or the instability waves in the mixing region. Both the vortex-sheet and the realistic finite thickness shear layer models are considered. The stability calculations were performed for both inverted and normal velocity profiles. Comparisons are made with the results for an equivalent single jet, based on equal thrust, mass flow rate and exit area to that of the coannular jet. The advantages and disadvantages of these velocity profiles as far as noise radiation is concerned are discussed. It is shown that the Rayleigh's model prediction of the merits and demerits of different velocity profiles are in good agreement with the experimental data.

Tiwari, S. N.↗
NASA NTRS2010

Analysis of Instabilities in Non-Axisymmetric Hypersonic Boundary Layers Over Cones

Hypersonic flows over circular cones constitute one of the most important generic configurations for fundamental aerodynamic and aerothermodynamic studies. In this paper, numerical computations are carried out for Mach 6 flows over a 7-degree half-angle cone with two different flow incidence angles and a compression cone with a large concave curvature. Instability wave and transition-related flow physics are investigated using a series of advanced stability methods ranging from conventional linear stability theory (LST) and a higher-fidelity linear and nonlinear parabolized stability equations (PSE), to the 2D eigenvalue analysis based on partial differential equations. Computed N factor distribution pertinent to various instability mechanisms over the cone surface provides initial assessments of possible transition fronts and a guide to corresponding disturbance characteristics such as frequency and azimuthal wave numbers. It is also shown that strong secondary instability that eventually leads to transition to turbulence can be simulated very efficiently using a combination of advanced stability methods described above.

Li, Fei↗
NASA NTRS1990

Coronal proton transport in the presence of Alfven waves and implications for impact line linear polarization

The transport of fast (about 1 MeV) protons in the corona is considered when their density and ansiotropy are sufficiently large to excite Alfven waves. The Alfven waves scatter the protons and make them almost isotropic. The Alfven wave instability remains in a marginally stable state in which the proton anisotropy is just large enough to excite Alfven waves to a level which would eliminate any greater anisotropy. The protons propagate diffusively down a coronal loop and have a path length of order 100 times their length in the absence of Alfven waves. This increases their Coulomb losses, and much higher energy protons (2-20 MeV) are required initially to produce H-alpha impact line linear polarization in the chromosphere as compared to the case in the absence of Alfven waves (about 200 keV) for loop lengths about 10 to the 9th cm. The interpretation of the observed polarization becomes less direct because it depends on more poorly known parameters than in the case of the absence of Alfven waves.

Smith, Dean F.↗
NASA NTRS1991

On the viscous modes of instability of a trailing line vortex

A viscous linear stability analysis of a trailing line (Batchelor) vortex is presented. Employing a staggered Chebyshev spectral collocation technique, very accurate results were obtained. The destabilizing role of viscous forces has been shown to produce two types of viscous instability modes. These viscous disturbances consist of an axisymmetric mode and an asymmetric mode. Both disturbances are long-wave instabilities with maximum growth rates which are orders of magnitude smaller than the inviscid modes which have been found by others. Comparison with experimental results and condensation trail observations are found to be in good qualitative agreement with the present study.

Khorrami, Mehdi R.↗
NASA NTRS1986

The laser interferometer skin-friction meter - A numerical and experimental study

Limits to the applicability of thin-film lubrication theory are established. The following two problems are considered: (1) the response of the oil film to a time-varying skin friction such as is encountered in turbulent boundary layers, and (2) a 'surface-wave instability' encountered at high skin-friction levels. Results corresponding to the first problem reveal that the laser interferometer skin-friction meter may, in principle, be applied to the measurement of instantaneous skin friction. In addressing the second problem, it is shown that the observed surface waves are not the result of a hydrodynamic instability.

Murphy, J. D.↗
NASA NTRS2000

Generation and Radiation of Acoustic Waves from a 2-D Shear Layer using the CE/SE Method

In the present work, the generation and radiation of acoustic waves from a 2-D shear layer problem is considered. An acoustic source inside of a 2-D jet excites an instability wave in the shear layer, resulting in sound Mach radiation. The numerical solution is obtained by solving the Euler equations using the space time conservation element and solution element (CE/SE) method. Linearization is achieved through choosing a small acoustic source amplitude. The Euler equations are nondimensionalized as instructed in the problem statement. All other conditions are the same except that the Crocco's relation has a slightly different form. In the following, after a brief sketch of the CE/SE method, the numerical results for this problem are presented.

Loh, Ching Y.↗
NASA NTRS1971

Ion-acoustic instability of a two-temperature, collisional, fully-ionized plasma.

From a perturbation analysis of the fluid equations for a homogeneous unmagnetized plasma, it is shown that long wavelength ion waves are unstable when the electron temperature exceeds the ion temperature. Thus, the temperature difference can drive a resistive-type ion wave instability in a plasma which on a collisionless basis is stable. The additional destabilizing effect of a current is also investigated.

Rognlien, T. D.↗
NASA NTRS2000

Onset of Convection Due to Surface Tension Variations in Multicomponent and Binary Fluid Layers

Under certain conditions, such as in thin liquid films or microgravity, surface tension variations along a free surface can induce convection. Convection onset due to surface tension variation is important to many terrestrial technological processes in addition to microgravity materials processing applications. Examples include coating, drying crystallization, solidification, liquid surface contamination, and containerless processing. In double-diffusive and multicomponent systems, the spatial variations of surface tension are associated with two or more stratifying agencies, respectively. For example, both temperature and species (concentration) gradients are associated with convection in the solidification of binary alloys or salt ponds. The direction of the two (or more) gradients has a profound effect on the nature of the flow at or slightly beyond the onset of convection. Our recent work at the NASA Lewis Research Center focused on characterizing surface-tension-induced onset of convection, often referred to as Marangoni-Benard convection. Exact solutions for the stationary neutral stability of multicomponent fluid layers with interfacial deformation were derived. These solutions also permit the computation of a boundary curve that separates the long and finite wavelength instabilities. Computing points along this boundary using the exact solution (when possible) is more efficient than the typical numerical approaches, such as finite difference or spectral methods. Above the curve, a long wavelength instability was predicted, suggesting that convection would occur principally through one large flow cell in the layer, whereas below the curve, finite wavelength instabilities occur which suggest multiple finite-sized circulation cells. For many common liquids with layer depths greater than 100 mm, finite wave instability is predicted under terrestrial conditions; however, with little exception, long wavelength instability is predicted in microgravity for the identical fluid systems.

Skarda, J. Raymond Lee↗
NASA NTRS1974

On the generation of the sequence of diffuse resonances observed on top side ionograms

Two different plasma wave instabilities have been invoked in earlier models for the generation of top side ionogram diffuse resonances. Arguments are presented that support the model based on the Harris instability, i.e., the instability resulting from a sounder-stimulated anisotropic electron velocity distribution. Two modifications to this model are also presented. First, it is only necessary for the instability mechanism to operate for a period of the order of milliseconds (rather than tens of milliseconds) in order to explain the observations. Second, the wave associated with the second harmonic of the electron cyclotron frequency, which enters into the nonlinear wave-wave interaction process included in the model, comes from the side bands of the transmitted sounder pulse rather than from the turbulent state of the plasma caused by the pulse. The second modification provides an explanation for the variation of the time duration observed among the members of the sequence of diffuse resonances.

Benson, R. F.↗
NASA NTRS1993

On the interaction of isotropic pickup ions with oblique Alfven waves

We consider the oblique Alfven wave instability for isotropic shells of pickup ions which was introduced by Wu and Yoon. We provide some further physical insight into this instability but show that the results published to date have not been correct. Our analysis indicates that the instability disappears when the thickness of the shell in velocity space becomes comparable to the average radius of the shell. Thus the generation of these wave, will not lead to a filling-in of the shell.

Isenberg, Philip A.↗
NASA NTRS1994

Five years' central Pacific sea level from in situ array, satellite altimeter and numerical model

Temporal and spatial fetures of central equatorial Pacific Ocean sea-level variation appear similar, in measurements from two very different systems (one in the ocean and one carried on a satellite), and in results from a numerical model of the region. In particular, there is an interannual cycle: during El Nino, Kelvin waves appear at the equator, and the sea-surface ridge associated with the equatorial current system shifts southward; in non-El Nino years, instability waves appear at 6 deg N (strongest around the end of each calendar year), and the ridge shifts to the north. This three-way comparison gives support to both measurement systems and to the numerical model.

Donohue, Kathleen A.↗
NASA NTRS1998

Effects of Nose Radius and Aerodynamic Loading on Leading Edge Receptivity

An analysis is presented of the effects of airfoil thickness and mean aerodynamic loading on boundary-layer receptivity in the leading-edge region. The case of acoustic free-stream disturbances, incident on a thin cambered airfoil with a parabolic leading edge in a low Mach number flow, is considered. An asymptotic analysis based on large Reynolds number is developed, supplemented by numerical results. The airfoil thickness distribution enters the theory through a Strouhal number based on the nose radius of the airfoil, S = (omega)tau(sub n)/U, where omega is the frequency of the acoustic wave and U is the mean flow speed. The influence of mean aerodynamic loading enters through an effective angle-of-attack parameter ti, related to flow around the leading edge from the lower surface to the upper. The variation of the receptivity level is analyzed as a function of S, mu, and characteristics of the free-stream acoustic wave. For an unloaded leading edge, a finite nose radius dramatically reduces the receptivity level compared to that for a flat plate, the amplitude of the instability waves in the boundary layer being decreased by an order of magnitude when S = 0.3. Modest levels of aerodynamic loading are found to further decrease the receptivity level for the upper surface of the airfoil, while an increase in receptivity level occurs for the lower surface. For larger angles of attack close to the critical angle for boundary layer separation, a local rise in the receptivity level occurs for the upper surface, while for the lower surface the receptivity decreases. The effects of aerodynamic loading are more pronounced at larger values of S. Oblique acoustic waves produce much higher receptivity levels than acoustic waves propagating downstream parallel to the airfoil chord.

Hammerton, P. W.↗
NASA NTRS1992

Formation and stability of polarization sheaths of a cross-field beam

A simple description is presented for a pure ion sheath formed on one side and a pure electron sheath formed on the other side of a neutral plasma beam. Both the ion and the electron sheath are found to be one-dimensionally stable and two-dimensionally unstable in a mode known as magnetron, slipping stream, and diocotron. This mode connects with the Kelvin-Helmholtz instability in neutral matter. The analytical results are confirmed both qualitatively and quantitatively by numerical simulations. The simulations help define the linear behavior of the long-wave instability generated in the electron (or ion) sheath, including a strong shear.

Cai, Dong S.↗
NASA NTRS2011

Effects of Cavities and Protuberances on Transition over Hypersonic Vehicles

Surface protuberances and cavities on a hypersonic vehicle are known to cause several aerodynamic or aerothermodynamic issues. Most important of all, premature transition due to these surface irregularities can lead to a significant rise in surface heating. To help understand laminar-turbulent transition induced by protuberances or cavities on a Crew Exploration Vehicle (CEV) surface, high-fidelity numerical simulations are carried out for both types of trips on a CEV wind tunnel model. Due to the large bluntness, these surface irregularities reside in an accelerating subsonic boundary layer. For the Mach 6 wind tunnel conditions with a roughness Reynolds number Re(sub kk) of 800, it was found that a protuberance with a height to boundary layer thickness ratio of 0.73 leads to strong wake instability and spontaneous vortex shedding, while a cavity with identical geometry only causes a rather weak flow unsteadiness. The same cavity with a larger Reynolds number also leads to similar spontaneous vortex shedding and wake instability. The wake development and the formation of hairpin vortices for both protuberance and cavity were found to be qualitatively similar to that observed for an isolated hemisphere submerged in a subsonic, low speed flat-plate boundary layer. However, the shed vortices and their accompanying instability waves were found to be slightly stabilized downstream by the accelerating boundary layer along the CEV surface. Despite this stabilizing influence, it was found that the wake instability spreads substantially in both wall-normal and azimuthal directions as the flow is evolving towards a transitional state. Similarities and differences between the wake instability behind a protuberance and a cavity are investigated. Computations for the Mach 6 boundary layer over a slender cylindrical roughness element with a height to the boundary layer thickness of about 1.1 also shows spontaneous vortex shedding and strong wake instability. Comparisons of detailed flow structures associated with protuberances at subsonic and supersonic edge Mach numbers indicate distinctively different instability mechanisms.

Chang, Chau-Lyan↗
NASA NTRS2007

Acoustic Receptivity of Mach 4.5 Boundary Layer with Leading- Edge Bluntness

Boundary layer receptivity to two-dimensional slow and fast acoustic waves is investigated by solving Navier-Stokes equations for Mach 4.5 flow over a flat plate with a finite-thickness leading edge. Higher order spatial and temporal schemes are employed to obtain the solution whereby the flat-plate leading edge region is resolved by providing a sufficiently refined grid. The results show that the instability waves are generated in the leading edge region and that the boundary-layer is much more receptive to slow acoustic waves (by almost a factor of 20) as compared to the fast waves. Hence, this leading-edge receptivity mechanism is expected to be more relevant in the transition process for high Mach number flows where second mode instability is dominant. Computations are performed to investigate the effect of leading-edge thickness and it is found that bluntness tends to stabilize the boundary layer. Furthermore, the relative significance of fast acoustic waves is enhanced in the presence of bluntness. The effect of acoustic wave incidence angle is also studied and it is found that the receptivity of the boundary layer on the windward side (with respect to the acoustic forcing) decreases by more than a factor of 4 when the incidence angle is increased from 0 to 45 deg. However, the receptivity coefficient for the leeward side is found to vary relatively weakly with the incidence angle.

Malik, Mujeeb R.↗
NASA NTRS1988

An estimate of equatorial wave energy flux at 9- to 90-day periods in the Central Pacific

Deep fluctuations in current along the equator in the Central Pacific are dominated by coherent structures which correspond closely to narrow-band propagating equatorial waves. Currents were measured roughly at 1500 and 3000 m depths at five moorings between 144 and 148 deg W from January 1981 to March 1983, as part of the Pacific Equatorial Ocean Dynamics program. In each frequency band resolved, a single complex empirical orthogonal function accounts for half to three quarters of the observed variance in either zonal or meridional current. Dispersion for equatorial first meridional Rossby and Rossby gravity waves is consistent with the observed vertical-zonal coherence structure. The observations indicate that energy flux is westward and downward in long first meridional mode Rossby waves at periods 45 days and longer, and eastward and downward in short first meridional mode Rossby waves and Rossby-gravity waves at periods 30 days and shorter. A local minimum in energy flux occurs at periods corresponding to a maximum in upper-ocean meridional current energy contributed by tropical instability waves. Total vertical flux across the 9- to 90-day period range is 2.5 kW/m.

Eriksen, Charles C.↗