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At least 1,315 records · Page 73
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NASA NTRS1988

A simple model describing the nonlinear dynamics of the dusk/dawn asymmetry in the high-latitude thermospheric flow

A simple nonlinear, axisymmetric, shallow-water numerical model has been used to study the asymmetry in the neutral flow between the dusk and dawn sides of the auroral oval. The results indicate that the Coriolis force and the curvature terms are nearly in balance on the evening side and require only a small pressure gradient to effect adjustment. The result is smaller neutral velocities near dawn and larger velocities near dusk than would be the case for a linearized treatment. A consequence is that more gravity wave energy is produced on the morning side than on the evening side.

Gundlach, J. P.↗
NASA NTRS1988

Cosmic-ray-modified stellar winds. III - A numerical iterative approach

A numerical iterative method is used to determine the modification of a stellar wind flow with a termination shock by the galactic cosmic rays. A two-fluid model consisting of cosmic rays and thermal stellar wind gas is used in which the cosmic rays are coupled to the background flow via scattering with magnetohydrodynamic waves or irregularities. A polytropic model is used to describe the thermal stellar wind gas, and the cosmic-rays are modeled as a hot, low-density gas with negligible mass flux. The positive galactic cosmic-ray pressure gradient serves to brake the outflowing stellar wind gas, and the cosmic rays modify the location of the critical point of the wind, the location of the shock, the wind fluid velocity profile, and the thermal gas entropy constants on both sides of the shock. The transfer of energy to the cosmic rays results in an outward radial flux of cosmic-ray energy.

Ko, C. M.↗
NASA NTRS1988

The effects of rotational flow, viscosity, thickness, and shape on transonic flutter dip phenomena

The transonic flutter dip phenomena on thin airfoils, which are employed for propfan blades, is investigated using an integrated Euler/Navier-Stokes code and a two degrees of freedom typical section structural model. As a part of the code validation, the flutter characteristics of the NACA 64A010 airfoil are also investigated. In addition, the effects of artificial dissipation models, rotational flow, initial conditions, mean angle of attack, viscosity, airfoil thickness, and shape on flutter are investigated. The results obtained with an Euler code for the NACA 64A010 airfoil are in reasonable agreement with published results obtained by using transonic small disturbance and Euler codes. The two artifical dissipation models, one based on the local pressure gradient scaled by a common factor and the other based on the local presure gradient scaled by a spectral radius, predicted the same flutter speeds except in the recovery region for the case studied. The effects of rotation flow, initial conditions, mean angle of attack, and viscosity for the Reynolds number studied seem to be negligible or small on the minima of the flutter dip.

Reddy, T. S. R.↗
NASA NTRS1988

Theory of centrifugally driven magnetospheric diffusion

A general model of centrifugally driven diffusion is formulated that incorporates the effects of the pressure gradient of a radiation belt ion population and includes a realistic precipitation loss rate of the radiation belt ions that takes on a spatially peaked form, leading to a spatially varying ionospheric conductivity. The model is applied specifically to Jupiter. Model solutions for the flux tube content of the radiation belt ion population are found to compare favorably with reported Voyager 1 observations of ions in the energy range 0.2-30 MeV. Solution profiles for the flux-tube content of the cool Iogenic ion population typically favor a gradient near L = 7.5 of between one-half and one-third of the value originally reported by Bagenal and Sullivan (1981) and Siscoe et al. (1981). The model solutions are characterized by unexpectedly low values of the precipitation parameter.

Summers, Danny↗
NASA NTRS1988

Analytical models of helical wind-type astrophysical flows

Three classes of analytic solutions of the basic hydrodynamic equations assumed to govern plasma flow in the atmosphere of a rotating astrophysical object are presented. Attention is focused on the balance of the inertial and gravitational forces with suitable pressure gradients in the presence of rotation. The solutions are written in terms of dimensionless physical parameters in order to facilitate their direct application to specific astrophysical flows, such as winds from massive premain sequence objects and T Tauri stars, and bipolar flows in young stellar objects.

Tsinganos, K.↗
NASA NTRS1987

The generalization of mixing length theory to rotating convection zones and applications to the sun

The consequences of a balance between the Coriolis forces, pressure gradients, and buoyancy forces in a compressible medium are explored, and the generalization of mixing length theory to rotating convection zones is considered, with particular attention being given to the approximations involved. It is shown that if this balance holds, the latitudinally dependent part of the superadiabatic gradient is determined by the angular velocity. Reynolds stresses are found to transport angular momentum towards the equator and towards the deeper layers of the solar convection zone (SCZ). The behavior of the angular velocity at the boundaries of the SCZ is also studied.

Durney, Bernard R.↗
NASA NTRS1987

Direct simulation of a turbulent oscillating boundary layer

The turbulent boundary layer driven by a freestream velocity that varies sinusoidally in time around a zero mean is considered. The flow has a rich behavior including strong pressure gradients, inflection points, and reversal. A theory for the velocity and stress profiles at high Reynolds number is formulated. Well-resolved direct Navier-Stokes simulations are conducted over a narrow range of Reynolds numbers, and the results are compared with the theoretical predictions. The flow is also computed over a wide range of Reynolds numbers using a new algebraic turbulence model; the results are compared with the direct simulations and the theory.

Spalart, Philippe R.↗
NASA NTRS1987

Turbulent drag reduction research at NASA Langley - Progress and plans

Prospective research efforts planned at NASA-Langley in view of results obtained to date in passive turbulent drag reduction experiments are discussed. It has been established that conventional flow-aligned riblets are effective even in the presence of a degree of flow inclination and pressure gradients, and at transonic speeds. No increase in net drag reduction is expected from nonconventional riblet geometries. Large eddy breakup devices promise drag reductions in the 8-15 percent range. Heat transfer-augmentation, noise-reduction, turboprop/fuselage interaction noise reduction, are other advantages expected from this line of research.

Wilkinson, S. P.↗
NASA NTRS1988

Passive control plate for shock-boundary layer interactions

Computational and experimental results are presented for a passive method capable of reducing or completely eliminating the separation region in shock-boundary layer interactions of the type that can result in severe performance degradation due to flow separation in supersonic and hypersonic inlets. The method involves the placement of an embedded plate in the outer portion of the boundary layer, parallel to the wall, so that the incident shock will impinge upon, and be reflected from, its surface. This will lower the pressure gradient in the low-momentum, near-wall region. A porous control plate's effect has also been investigated.

Goodman, Wesley L.↗
NASA NTRS1988

Rotating disk transition due to isolated roughness with intense acoustic irradiation

The low transition Reynolds numbers associated with boundary-layers which exhibit crossflow instability have made them an important area of study in fluid mechanics. An excellent test bed for the study of this flow is a flat disk rotating in still fluid. An experimental investigation of stability and transition of a rotating disk boundary-layer has been conducted. The effects of isolated roughness elements and acoustic forcing, both alone and in combination, were considered. The flow showed no receptivity to intense monochromatic acoustic forcing except in the presence of sizable roughness. A case was investigated where the growth of the primary instability, seeded by an isolated roughness element, was positively altered in the presence of sound, leading to a downstream movement of transition. In agreement with the work of several investigators, the results of this study suggest that receptivity to long wavelength phenomena, such as traveling sound waves, is contingent upon the presence of strong localized pressure gradients (such as those introduced by roughness) to reduce the length scales of the disturbance to those which are consistent with unstable wavelengths in the flow field.

Waitz, Ian A.↗
NASA NTRS1988

On the nonlinear interaction of Goertler vortices and Tollmien-Schlichting waves in curved channel flows at finite Reynolds numbers

The flow in a two-dimensional curved channel driven by an azimuthal pressure gradient can become linearly unstable due to axisymmetric perturbations and/or nonaxisymmetric perturbations depending on the curvature of the channel and the Reynolds number. For a particular small value of curvature, the critical neighborhood of this curvature value and critical Reynolds number, nonlinear interactions occur between these perturbations. The Stuart-Watson approach is used to derive two coupled Landau equations for the amplitudes of these perturbations. The stability of the various possible states of these perturbations is shown through bifurcation diagrams. Emphasis is given to those cases which have relevance to external flows.

Daudpota, Q. Isa↗
NASA NTRS1989

Dynamic, High-Temperature, Flexible Seal

New seal consists of multiple plies of braided ceramic sleeves filled with small ceramic balls. Innermost braided sleeve supported by high-temperature-wire-mesh sleeve that provides both springback and preload capabilities. Ceramic balls reduce effect of relatively high porosity of braided ceramic sleeves by acting as labyrinth flow path for gases and thereby greatly increasing pressure gradient seal can sustain. Dynamic, high-temperature, flexible seal employed in hypersonic engines, two-dimensional convergent/divergent and vectorized-thrust exhaust nozzles, reentry vehicle airframes, rocket-motor casings, high-temperature furnaces, and any application requiring non-asbestos high-temperature gaskets.

Steinetz, Bruce M.↗
NASA NTRS1988

Summary of an experimental investigation on the ground vortex

The results of an experimental investigation into the position and characteristics of the ground vortex are summarized. A 48-inch wind tunnel was modified to create a testing environment suitable for the ground vortex study. Flow visualization was used to document the jet-crossflow interaction and a two-component Laser Doppler Velocimeter (LDV) was used to survey the flowfield in detail. Measurements of the ground vortex characteristics and location as a function of freestream-to-jet velocity ratio, jet height, pressure gradient and upstream boundary layer thickness were obtained.

Billet, Michael L.↗
NASA NTRS1988

Simulation of 2-dimensional viscous flow through cascades using a semi-elliptic analysis and hybrid C-H grids

A semi-elliptic formulation, termed the interacting parabolized Navier-Stokes (IPNS) formulation, is developed for the analysis of a class of subsonic viscous flows for which streamwise diffusion is neglible but which are significantly influenced by upstream interactions. The IPNS equations are obtained from the Navier-Stokes equations by dropping the streamwise viscous-diffusion terms but retaining upstream influence via the streamwise pressure-gradient. A two-step alternating-direction-explicit numerical scheme is developed to solve these equations. The quasi-linearization and discretization of the equations are carefully examined so that no artificial viscosity is added externally to the scheme. Also, solutions to compressible as well as nearly compressible flows are obtained without any modification either in the analysis or in the solution process. The procedure is applied to constricted channels and cascade passages formed by airfoils of various shapes. These geometries are represented using numerically generated curilinear boundary-oriented coordinates forming an H-grid. A hybrid C-H grid, more appropriate for cascade of airfoils with rounded leading edges, was also developed. Satisfactory results are obtained for flows through cascades of Joukowski airfoils.

Ramamurti, R.↗
NASA NTRS1988

Design of high-Reynolds-number flat-plate experiments in the NTF

The design of an experiment to measure skin friction and turbulent boundary layer characteristics at Reynolds numbers exceeding 1 x 10 to the 9th is described. The experiment will be conducted in a zero-pressure-gradient flow on a flat plate in the National Transonic Facility (NTF). The development of computational codes to analyze the aerodynamic loads and the blockage is documented. Novel instrumentation techniques and models, designed to operate in cryogenic environments, are presented. Special problems associated with aerodynamic loads, surface finish, and hot-wire anemometers are discussed.

Saric, William S.↗
NASA NTRS1988

Modeling of transitional flows

An effort directed at developing improved transitional models was initiated. The focus of this work was concentrated on the critical assessment of a popular existing transitional model developed by McDonald and Fish in 1972. The objective of this effort was to identify the shortcomings of the McDonald-Fish model and to use the insights gained to suggest modifications or alterations of the basic model. In order to evaluate the transitional model, a compressible boundary layer code was required. Accordingly, a two-dimensional compressible boundary layer code was developed. The program was based on a three-point fully implicit finite difference algorithm where the equations were solved in an uncoupled manner with second order extrapolation used to evaluate the non-linear coefficients. Iteration was offered as an option if the extrapolation error could not be tolerated. The differencing scheme was arranged to be second order in both spatial directions on an arbitrarily stretched mesh. A variety of boundary condition options were implemented including specification of an external pressure gradient, specification of a wall temperature distribution, and specification of an external temperature distribution. Overall the results of the initial phase of this work indicate that the McDonald-Fish model does a poor job at predicting the details of the turbulent flow structure during the transition region.

Lund, Thomas S.↗
NASA NTRS1987

A low-Reynolds-number two-equation turbulence model for predicting heat transfer on turbine blades

A modified form of the Lam-Bremhorst low-Reynolds number kappa-epsilon turbulence model was developed for predicting transitional boundary layer flows under conditions characteristic of gas turbine blades. The application of the model to flows with pressure gradients is described. Tests against a number of turbine blade cascade data sets are included. Some additional refinements of the model that were made in recent months are explained.

Patankar, Suhas V.↗
NASA NTRS1988

Hot wire anemometer measurements in the unheated air flow tests of the SRB nozzle-to-case joint

Hot-Wire Anemometer measurements made in the Solid Rocket Booster (SRB) nozzle-to-case joint are discussed. The study was undertaken to glean additional information on the circumferential flow induced in the SRB nozzle joint and the effect of this flow on the insulation bonding flaws. The tests were conducted on a full-scale, 2-D representation of a 65-in long segment of the SRB nozzle joint, with unheated air as the working fluid. Both the flight Mach number and Reynolds number were matched simultaneously and different pressure gradients imposed along the joint face were investigated. Hot-wire anemometers were used to obtain velocity data for different joint gaps and debond configurations. The procedure adopted for hot-wire calibration and use is outlined and the results from the tests summarized.

Ramachandran, N.↗