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At least 1,117 records · Page 62

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NASA NTRS1989

The measurement of boundary layers on a compressor blade in cascade. Volume 2: Data tables

Measurements were made of the boundary layers and wakes about a highly loaded, double-circular-arc compressor blade in cascade. These laser Doppler velocimetry measurements have yielded a very detailed and precise data base with which to test the application of viscous computational codes to turbomachinery. In order to test the computational codes at off-design conditions, the data have been acquired at a chord Reynolds number of 500,000 and at three incidence angles. Average values and 95 percent confidence bands were tabularized for the velocity, local turbulence intensity, skewness, kurtosis, and percent backflow. Tables also exist for the blade static-pressure distributions and boundary layer velocity profiles reconstructed to account for the normal pressure gradient.

Zierke, William C.↗

NASA NTRS1987

Goertler instability on an airfoil

An effective computational scheme was developed to study the growth/damping of Goertler vortices along walls of variable curvature. Computational experiments indicate that when the amplification rates for the u-, v-, and w-perturbations are the same, the finite difference approach to solve the initial value problem and the normal mode approach give identical results for the Blasius boundary layer on constant curvature concave walls. The growth of Goertler vortices was rapid in the concave regions and was followed by sharp damping in the convex region. However, multiple sets of counter-rotating vortices were formed and remained far downstream in the convex region. The current computational scheme can be easily extended to more realistic problems including variable pressure gradients and suction effects.

Kalburgi, Vijay↗

NASA NTRS1989

A review of quasi-coherent structures in a numerically simulated turbulent boundary layer

Preliminary results of a comprehensive study of the structural aspects of a numerically simulated number turbulent boundary layer are presented. A direct Navier-Stokes simulation of a flat-plate, zero pressure gradient boundary layer at Re0 = 670 was used. Most of the known nonrandom, coherent features of turbulent boundary layers are confirmed in the simulation, and several new aspects of their spatial character are reported. The spatial relationships between many of the various structures are described, forming the basis for a more complete kinematical picture of boundary layer physics than has been previously known. In particular, the importance of vortex structures of various forms to the generation of Reynolds shear stress is investigated.

Robinson, S. K.↗

NASA NTRS1989

Shear-induced inflation of coronal magnetic fields

Using numerical models of force-free magnetic fields, the shearing of footprints in arcade geometries leading to an inflation of the coronal magnetic field was examined. For each of the shear profiles considered, all of the field lines become elevated compared with the potential field. This includes cases where the shear is concentrated well away from the arcade axis, such that B(sub z), the component of field parallel to the axis, increases outward to produce an inward B(sub z)squared/8 pi magnetic pressure gradient force. These results contrast with an earlier claim, shown to be incorrect, that field lines can sometimes become depressed as a result of shear. It is conjectured that an inflation of the entire field will always result from the shearing of simple arcade configurations. These results have implications for prominence formation, the interplanetary magnetic flux, and possibly also coronal holes.

Klimchuk, James A.↗

NASA NTRS1989

Group-kinetic theory and modeling of atmospheric turbulence

A group kinetic method is developed for analyzing eddy transport properties and relaxation to equilibrium. The purpose is to derive the spectral structure of turbulence in incompressible and compressible media. Of particular interest are: direct and inverse cascade, boundary layer turbulence, Rossby wave turbulence, two phase turbulence; compressible turbulence, and soliton turbulence. Soliton turbulence can be found in large scale turbulence, turbulence connected with surface gravity waves and nonlinear propagation of acoustical and optical waves. By letting the pressure gradient represent the elementary interaction among fluid elements and by raising the Navier-Stokes equation to higher dimensionality, the master equation was obtained for the description of the microdynamical state of turbulence.

Tchen, C. M.↗

NASA NTRS1989

Thermal histories of the samples of two KOSI comet nucleus simulation experiments

Temperatures recorded during two KOSI comet nucleus simulation experiments strongly suggest that heat transport by vapor flow into the interior of the sample is very important. Two comet nucleus simulation experiments have been done by the KOSI team in a big space simulator. The thermal evolution of the sample during insolation and the results of simplified thermal evolution calculations are discussed. The observed thermal histories cannot be explained by a simple model with heat transferred by heat conduction at a constant conductivity, so a coupled heat and mass transfer problem was considered. The porous ice matrix was assumed to have a constant thermal conductivity and to be in thermal equilibrium with vapor in the pores, the internal pressure being the vapor pressure. The vapor was modelled as an ideal gas because, at the temperatures relevant to the problem, the mean free path length of the vapor molecules is large in comparison with the pore dimensions. The heat capacity at constant volume per unit mass of the two phase mixture was also assumed constant. The vapor was allowed to flow and transfer heat in response to an internal pressure gradient.

Spohn, T.↗

NASA NTRS1990

Navier-Stokes analysis of turbine blade heat transfer

Comparisons with experimental heat transfer and surface pressures were made for seven turbine vane and blade geometries using a quasi-three-dimensional thin-layer Navier-Stokes analysis. Comparisons are made for cases with both separated and unseparated flow over a range of Reynolds numbers and freestream turbulence intensities. The analysis used a modified Baldwin-Lomax turbulent eddy viscosity mode. Modifications were made to account for the effects of: (1) freestream turbulence on both transition and leading edge heat transfer; (2) strong favorable pressure gradients on relaminarization; and (3) variable turbulent Prandtl number heat transfer. In addition, the effect of heat transfer on the near wall model of Deissler is compared with the Van Driest model.

Boyle, R. J.↗

NASA NTRS1990

Extra compressibility terms for Favre-averaged two-equation models of inhomogeneous turbulent flows

Forms of extra-compressibility terms that result from use of Favre averaging of the turbulence transport equations for kinetic energy and dissipation are derived. These forms introduce three new modeling constants, a polytropic coefficient that defines the interrelationships of the pressure, density, and enthalpy fluctuations and two constants in the dissipation equation that account for the non-zero pressure-dilitation and mean pressure gradients.

Rubesin, Morris W.↗

NASA NTRS1989

Flow unsteadiness effects on boundary layers

The development of boundary layers at high subsonic speeds in the presence of either mass flux fluctuations or acoustic disturbances (the two most important parameters in the unsteadiness environment affecting the aerodynamics of a flight vehicle) was investigated. A high quality database for generating detailed information concerning free-stream flow unsteadiness effects on boundary layer growth and transition in high subsonic and transonic speeds is described. The database will be generated with a two-pronged approach: (1) from a detailed review of existing literature on research and wind tunnel calibration database, and (2) from detailed tests in the Boundary Layer Apparatus for Subsonic and Transonic flow Affected by Noise Environment (BLASTANE). Special instrumentation, including hot wire anemometry, the buried wire gage technique, and laser velocimetry were used to obtain skin friction and turbulent shear stress data along the entire boundary layer for various free stream noise levels, turbulence content, and pressure gradients. This database will be useful for improving the correction methodology of applying wind tunnel test data to flight predictions and will be helpful for making improvements in turbulence modeling laws.

Murthy, Sreedhara V.↗

NASA NTRS1990

Effect of Suction on Controlling the Secondary Instability of Boundary Layers

The effect of suction on controlling the 3-D secondary instability is investigated for a boundary layer with pressure gradient in the presence of small but finite amplitude Tollmien-Schlichting wave. The focus is on principal parametric resonance responsible for strong growth of subharmonics in low disturbance environment. Calculations are presented for the effect of suction on the onset and amplification of the secondary instability in Blasius and Falkner-Skan flows, as well as its effect on controlling the production of the vortical structure.

El-Hady, Nabil M.↗

NASA NTRS1990

Control of submersible vortex flows

Vortex flows produced by submersibles typically unfavorably influence key figures of merit such as acoustic and nonacoustic stealth, control effectiveness/maneuverability, and propulsor efficiency/body drag. Sources of such organized, primarily longitudinal, vorticity include the basic body (nose and sides) and appendages (both base/intersection and tip regions) such as the fairwater, dive planes, rear control surfaces, and propulsor stators/tips. Two fundamentally different vortex control approaches are available: (1) deintensification of the amplitude and/or organization of the vortex during its initiation process; and (2) downstream vortex disablement. Vortex control techniques applicable to the initiation region (deintensification approach) include transverse pressure gradient minimization via altered body cross section, appendage dillets, fillets, and sweep, and various appendage tip and spanload treatment along with the use of active controls to minimize control surface size and motions. Vortex disablement can be accomplished either via use of control vortices (which can also be used to steer the vortices off-board), direct unwinding, inducement of vortex bursting, or segmentation/tailoring for enhanced dissipation. Submersible-applicable vortex control technology is also included derived from various aeronautical applications such as mitigation of the wing wake vortex hazard and flight aircraft maneuverability at high angle of attack as well as the status of vortex effects upon, and mitigation of, nonlinear control forces on submersibles. Specific suggestions for submersible-applicable vortex control techniques are presented.

Bushnell, D. M.↗

NASA NTRS1989

On the structure of pressure fluctuations in simulated turbulent channel flow

Pressure fluctuations in a turbulent channel flow are investigated by analyzing a database obtained from a direct numerical simulation. Detailed statistics associated with the pressure fluctuations are presented. Characteristics associated with the rapid (linear) and slow (nonlinear) pressure are discussed. It is found that the slow pressure fluctuations are larger than the rapid pressure fluctuations throughout the channel except very near the wall, where they are about the same magnitude. This is contrary to the common belief that the nonlinear source terms are negligible compared to the linear source terms. Probability density distributions, power spectra, and two-point correlations are examined to reveal the characteristics of the pressure fluctuations. The global dependence of the pressure fluctuations and pressure-strain correlations are also examined by evaluating the integral associated with Green's function representations of them. In the wall region where the pressure-strain terms are large, most contributions to the pressure-strain terms are from the wall region (i.e., local), whereas away from the wall where the pressure-strain terms are small, contributions are global. Structures of instantaneous pressure and pressure gradients at the wall and the corresponding vorticity field are examined.

Kim, John↗

NASA NTRS1989

Near wall flow parameters in the blade end-wall corner region

The effects of secondary end-wall corner flows on near wall flow parameters in turbomachinary are studied. Important near wall flow parameters such as the wall shear stress vector, the mean wall pressure, the wall pressure fluctuations, and the correlation of the wall pressure fluctuation with the velocity fluctuation in three-dimensional turbulent flows are first experimentally investigated. The blade end-wall corner region is simulated by mounting airfoil section of symmetric blades on both sides of the flat plate with semicircular leading edge. Observed changes in the maximum values of the wall shear stress and its location from the corner line could be associated with the streching and attenuation of the horseshoe vortex. The values of wall pressure fluctuation intensity in the blade end-wall corner region are found to be influenced by the changes of the strength of the horseshoe vortex. The correlation of the wall pressure fluctuation with the velocity fluctuation indicated higher values of correlation coefficient in the inner region as compared to the outer region of the shear layer. The values of wall pressure-velocity correlation coefficient in the blade end-wall corner region also decrease in the streamwise direction while increasing in the presence of favorable and adverse pressure gradients.

R K Bhargava↗

NASA NTRS1989

Goertler vortices in supersonic and hypersonic boundary layers

The problem of Goertler vortices in compressible boundary layers over concave walls is considered. At O(1) wavelengths, the instability is governed by parabolic partial differential equations that are solved numerically to determine the effect of various initial conditions on the development of Goertler vortex instability in compressible boundary layers. The results show that both the velocity and temperature fluctuations may lead to a Goertler vortex. The vortex growth rates determined from the present method are found to differ somewhat from those given by a normal mode solution. At both the supersonic and hypersonic Mach numbers, cooling has a small destabilizing effect. In addition, the most unstable disturbances shift toward lower wavelengths because of thinning of the boundary layer. The results also show that compressibility has a stabilizing effect on the Goertler instability, while the effect of an adverse pressure gradient is found to be destabilizing. The behavior of the Goertler vortex structure with Mach number is also examined. At hypersonic Mach numbers, vortices are located near the edge of the boundary layer for adiabatic wall conditions. However, the entire boundary layer is affected when the wall is cooled.

Spall, R. E.↗

NASA NTRS1989

Collisionless reconnection in a quasi-neutral sheet near marginal stability

Particle simulations are used to investigate the process of collisionless reconnection in a magnetotail configuration which includes a pressure gradient along the tail axis and tail flaring. In the absence of electron stabilization effects, the tearing mode is stabilized when the ion gyrofrequency in the normal field exceeds the growth rate in the corresponding one-dimensional current sheet. The presence of a low-frequency electromagnetic perturbation in the lobes can serve to destabilize a marginally stable current sheet by producing an extended neutral-sheet region which can then undergo reconnection. These results help to explain how X-type neutral lines, such as those associated with the onset of magnetospheric substorms, can be formed in the near-earth plasma sheet.

Pritchett, P. L.↗

NASA NTRS1989

Three-dimensional aerodynamics of an annular airfoil cascade including loading effects

A series of experiments are described which investigate and quantify the effect of loading on the three-dimensional flow through a subsonic annular cascade of cambered airfoils. At two levels of loading, detailed data quantify the cascade inlet velocity, the intrapassage flow field, the airfoil surface pressure distributions, the exit flow field, and the total pressure loss distributions. Aerodynamic loading is shown to strengthen the radial pressure gradient, the passage vortex structure, the vortex-endwall boundary layer interactions, and the losses.

Fleeter, S.↗

NASA NTRS1989

A magnetohydrodynamical model of the inner coma of Comet Halley

The magnetometer onboard the Giotto spacecraft observed a diamagnetic cavity surrounding the nucleus of Comet Halley. The location of the boundary of this diamagnetic cavity is determined by a balance between an inward magnetic pressure gradient force and an outward ion-neutral drag force, associated with collisions between the outwardly flowing neutrals and the stagnated ions. A one-dimensional time-dependent MHD model has been developed for the inner coma of Comet Halley, and includes ion-neutral collisions, photochemical production and loss of plasma, and finite conductivity. This model is used to investigate the plasma dynamics in the vicinity of the diamagnetic cavity boundary surface. A narrow transition layer with enhanced plasma density is shown to exist just inside the boundary, although a full understanding of this layer will require a two- or three-dimensional MHD model. The flux of cometary ions into this shocklike layer is removed by electron-ion recombination. The thickness of this layer is determined by the Mach number of the incident flow.

Cravens, T. E.↗

NASA NTRS1990

Amplitude effects on dynamic stall of an oscillating airfoil

Amplitude effects on dynamic stall of an oscillating airfoil in locally transonic flow were studied using stroboscopic schlieren flow visualization. The dynamic stall vortex was photographed for various conditions and its properties were documented. Results show a pronounced effect of the amplitude of oscillation. The airfoil flow can sustain the larger amplitudes with the vortex retained on the surface and thus, produce dynamic lift to higher angles of attack at higher amplitudes. A possible explanation for this is offered in terms of the vorticity generation due to surface acceleration and local streamwise pressure gradient. Also, for the first time a shock has been photographed on the airfoil upper surface and some of its characteristics are discussed.

Chandrasekhara, M. S.↗