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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

A defect stream function, law of the wall/wake method for turbulent boundary layers

The application of the defect stream function to the solution of the two-dimensional, incompressible boundary layer problem is reexamined. A law-of-the-wall/law-of-the-wake formulation for the inner part of the boundary layer is presented which greatly simplifies the computational task near the wall and eliminates the need for an eddy viscosity model in this region. The eddy viscosity model in the outer part of the boundary layer is arbitrary. Formulations for both equilibrium and nonequilibrium boundary layers are presented, and results are compared with previous methods for equilibrium boundary layers. A formulation for primitive variables is presented. The present treatments eliminate the need for resolving the flow in the inner part of the boundary layer computationally, thereby improving computational efficiency.

Barnwell, Richard W.↗
NASA NTRS1988

Three step double layers in the laboratory

A new class of stationary double layer structure, with three or more distinct steps, is demonstrated in the laboratory. A large monotonic potential increase results from a series of smaller double layers. In many respects, these double layer structures resemble those inferred from satellite measurements of auroral double layers. This new class of double layer appears to depend on turbulence for its existence and to be a hybrid structure, intermediate between anomalous resistivity and BGK double layers.

Bailey, Andrew, III↗
NASA NTRS1988

A streamwise vortex embedded in a plane mixing layer

Experimental results are presented on the interaction of a single streamwise vortex with a plane turbulent mixing layer. In the present setup, the vortex is generated by a half-delta wing mounted in the settling chamber of a blower-driven wind tunnel. Initially, the vortex is shown to ride just below the mixing layer, locally distorting the mean velocity and turbulence Reynolds stress distributions in the lower part of the mixing layer. Once the vortex becomes embedded within the mixing layer, the distortions are found to spread through the whole width of the layer, and the induced extra strain rates in the mixing layer result in the generation of additional, relatively large, Reynolds normal and shear stresses, mainly in the regions above the vortex.

Bell, James H.↗
NASA NTRS1989

Three-dimensional structure of a plane mixing layer

An experimental study is reported which shows the spanwise structure of a plane mixing layer originating from laminar boundary layers. Quantitative measurements are obtained for the first time which show the origin and evolution of streamwise vortices within the mixing layer. After the first spanwise vortex rollup occurs, streamwise structures are triggered in clusters within the braid region of the mixing layer. Further downstream, they realign to form pairs of counterrotating vortices. The maximum vorticity diffuses rapidly with increasing downstream distance while the vortices grow, scaling with the mixing layer vorticity thickness. The presence of the streamwise vortices leads to significant spanwise distortions in the mean and turbulence properties of the mixing layer. A consistent peak in the secondary shear stress exists for each streamwise vortex.

Bell, James H.↗
NASA NTRS1989

Diverging boundary layers with zero streamwise pressure gradient

The effects of spanwise divergence on the boundary layer forming between a pair of embedded streamwise vortices with the common flow between them directed toward the wall was studied. Measurements indicate that divergence controls the rate of development of the boundary layer and that large divergence significantly retards boundary layer growth and enhances skin friction. For strongly diverging boundary layers, divergence accounts for nearly all of the local skin friction. Even with divergence, however, the local similarity relationships for two-dimensional boundary layers are satisfactory. Although divergence modifies the mean development of the boundary layer, it does not significantly modify the turbulence structure. In the present experiments with a zero streamwise pressure gradient, it was found that spanwise divergence dit not significantly affect the Reynolds stress and the turbulent triple product distributions.

Pauley, Wayne R.↗
NASA NTRS1989

Nonequilibrium shock layer temperature profiles from arc jet radiation measurements

Shock layer temperature profiles are obtained through analysis of radiation from shock layers produced by a blunt body inserted in arc jet flow. Spectral measurements have been made in a nitrogen flow of 54.4 gm/s at an enthalpy of 8.72 MJ/kg. Vibrational temperatures for N2+ are obtained by matching spectral regions from arc jet spectra with spectra generated using the NEQAIR code. Temperature profiles obtained from the radiation layers show a vibrational temperature higher than the rotational temperature near the front of the shock and both temperatures decrease as the flow approaches the body. The spectral measurements are made and analysis completed for four distances, from the surface of the blunt body. The corresponding shock layer thickness is approximately 3.6 cm. Although the shock layer appears to be in thermal nonequilibrium, the measured rotational temperature approaches the single temperature results of viscous shock layer calculations at this test condition.

Blackwell, Harvel E.↗
NASA NTRS1989

Compressibility and shock wave interaction effects on free shear layers

Two compressible free shear layers with convective Mach numbers of .51 and .86 were studied as baseline configurations to investigate the effects of compressibility on the turbulence characteristics. These shear layers were then disturbed by the placement of an obstruction in the shear layer in an attempt to enhance the shear layer growth rate. These models produced a curved shock in the supersonic side of the shear layer. The results indicate a significant reduction in turbulence levels with increased compressibility. However, there are not any significant changes due to the bow shock interaction with the shear layer.

Samimy, M.↗
NASA NTRS1990

The dynamics and control of fluctuating pressure loads in the reattachment region of a supersonic free shear layer

The primary aim is to investigate the mechanisms which cause the unsteady wall-pressure fluctuations in shock wave turbulent shear layer interactions. The secondary aim is to find means to reduce the magnitude of the fluctuating pressure loads by controlling the unsteady shock motion. The particular flow proposed for study is the unsteady shock wave interaction formed in the reattachment zone of a separated supersonic flow. Similar flows are encountered in many practical situations, and they are associated with high levels of fluctuating wall pressure. Wall pressure fluctuations were measured in the reattachment region of the supersonic free shear layer. The free shear layer was formed by the separation of a Mach 2.9 turbulent boundary layer from a backward facing step. Reattachment occurred on a 20 deg ramp. By adjusting the position of the ramp, the base pressure was set equal to the freestream pressure, and the free shear layer formed in the absence of a separation shock. An array of flush-mounted, miniature, high-frequency pressure transducers was used to make multichannel measurements of the fluctuating wall pressure in the vicinity of the reattachment region. Contrary to previous observations of this flow, the reattachment region was found to be highly unsteady, and the pressure fluctuations were found to be significant. The overall behavior of the wall pressure loading is similar in scale and magnitude to the unsteadiness of the wall pressure field in compression ramp flows at the same Mach number. Rayleigh scattering was used to visualize the instantaneous shock structure in the streamwise and spanwise direction. Spanwise wrinkles on the order of half the boundary layer thickness were observed.

Smits, A. J.↗
NASA NTRS1989

Experimental investigation of convective stability in a superposed fluid and porous layer when heated from below

Experiments have been carried out in a horizontal superposed fluid and porous layer contained in a test box 24 cm x 12 cm x 4 cm high. The porous layer consisted of 3 mm diameter glass beads, and the fluids used were water, 60 and 90 percent glycerin-water solutions, and 100 percent glycerin. The depth ratio d, which is the ratio of the thickness of the fluid layer to that of the porous layer, varied from 0 to 1.0. Fluids of increasingly higher viscosity were used for cases with larger d in order to keep the temperature difference across the tank within reasonable limits. The size of the convection cells was inferred from temperature measurements made with embedded thermocouples and from temperature distributions at the top of the layer by use of liquid crystal film. The experimental results showed: (1) a precipitous decrease in the critical Rayleigh number as the depth of the fluid layer was increased from zero, and (2) an eightfold decrease in the critical wavelength between d = 0.1 and 0.2. Both of these results were predicted by the linear stability theory reported earlier (Chen and Chen, 1988).

Chen, Falin↗
NASA NTRS1990

Mechanisms leading to net drag reduction in manipulated turbulent boundary layers

Turbulent boundary layers have been manipulated successfully using passive devices called BLADES (boundary-layer alteration devices) leading to net drag reduction. Measurements of various turbulence quantities including intermittency have led to the identification of some of the mechanisms involved in skin-friction reduction. Velocity and vorticity fluctuations associated with oncoming large scales are inhibited significantly by the manipulator blades and the vorticity shed in their wake. The manipulated boundary layers exhibit a reduced intermittency in the outer part of the layer. The effects of manipulation relax with downstream distance and the boundary layer returns toward normal conditions after 100 or 150 boundary-layer thicknesses.

Guezennec, Y. G.↗
NASA NTRS1990

Color and albedo of the south polar layered deposits on Mars

Five color/albedo units, including polar frost, have been recognized and mapped in the southern layered deposits on Mars. Atmospheric dust scattering was measured in shadows and modeled in order to remove the component of brightness in Mars images due to the atmosphere and quantify the albedo and color of the surface. The layered deposits appear to be mantled by red dust, except where eolian stripping has exposed the underlying bedrock. Dark material has been deposited in topographic depressions in much of the south polar region, including the layered deposits. The available observational data suggest that the layered deposits are composed of bright dust, ice, and a small amount of dark material. If the dark material is sand, a periodic change in polar winds seems required in order to transport the sand poleward into the layered terrain. In any case, the observations are not consistent with the layered deposits being composed only of bright dust and ice.

Herkenhoff, Ken E.↗
NASA NTRS1990

Effects of shock on the stability of hypersonic boundary layers

A set of linearized shock boundary conditions is derived, which is then imposed at the shock to account for the interaction of the shock wave with the boundary/shock layer instability wave; these boundary conditions are used to study the effect of shock on hypersonic boundary layer stability under the assumption of quasi-parallel flow. The result show that the shock has little effect on the boundary layer instability (subsonic first and second mode disturbances) when the shock is located outside the boundary layer edge. When the shock is located near the boundary layer edge, it exerts a stabilizing influence on the first and second modes. The shock also induces unstable supersonic modes with oscillatory structure in the shock layer, but these modes grow slower than the subsonic modes.

Chang, Chau-Lyan↗
NASA NTRS1990

A perspective on coherent structures and conceptual models for turbulent boundary layer physics

Direct numerical simulations of turbulent boundary layers have been analyzed to develop a unified conceptual model for the kinematics of coherent motions in low Reynolds number canonical turbulent boundary layers. All classes of coherent motions are considered in the model, including low-speed streaks, ejections and sweeps, vortical structures, near-wall and outer-region shear layers, sublayer pockets, and large-scale outer-region eddies. The model reflects the conclusions from the study of the simulated boundary layer that vortical structures are directly associated with the production of turbulent shear stresses, entrainment, dissipation of turbulence kinetic energy, and the fluctuating pressure field. These results, when viewed from the perspective of the large body of published work on the subject of coherent motions, confirm that vortical structures may be considered the central dynamic element in the maintenance of turbulence in the canonical boundary layer. Vortical structures serve as a framework on which to construct a unified picture of boundary layer structure, providing a means to relate the many known structural elements in a consistent way.

Robinson, Stephen K.↗
NASA NTRS1990

Transition from moving to stationary double layers in a single-ended Q machine

Large-amplitude (less than about 100 percent) relaxation oscillations in the plasma potential are known to be generated when the cold endplate of a single-ended Q machine is biased positively. These oscillations are associated with double layers that form near the hot plate (plasma source) and travel toward the endplate at about the ion-acoustic velocity. At the endplate they dissolve and then form again near the hot plate, the entire process repeating itself in a regular manner. By admitting a sufficient amount of neutral gas into the system, the moving double layers were slowed down and eventually stopped. The production of stationary double layers requires an ion source on the high-potential side of the double layers. These ions are provided by ionization of the neutral gas by electrons that are accelerated through the double layer. The dependence of the critical neutral gas pressure required for stationary double-layer formation on endplate voltage, magnetic field strength, and neutral atom mass has been examined. These results are discussed in terms of a simple model of ion production and loss, including ion losses across the magnetic field.

Song, Bin↗
NASA NTRS1990

An experimental study of the turbulent boundary layer on a transport wing in subsonic and transonic flow

The upper surface boundary layer on a transport wing model was extensively surveyed with miniature yaw probes at a subsonic and a transonic cruise condition. Additional data were obtained at a second transonic test condition, for which a separated region was present at mid-semispan, aft of mid-chord. Significant variation in flow direction with distance from the surface was observed near the trailing edge except at the wing root and tip. The data collected at the transonic cruise condition show boundary layer growth associated with shock wave/boundary layer interaction, followed by recovery of the boundary layer downstream of the shock. Measurements of fluctuating surface pressure and wingtip acceleration were also obtained. The influence of flow field unsteadiness on the boundary layer data is discussed. Comparisons among the data and predictions from a variety of computational methods are presented. The computed predictions are in reasonable agreement with the experimental data in the outboard regions where 3-D effects are moderate and adverse pressure gradients are mild. In the more highly loaded mid-span region near the trailing edge, displacement thickness growth was significantly underpredicted, except when unrealistically severe adverse pressure gradients associated with inviscid calculations were used to perform boundary layer calculations.

Spaid, Frank W.↗
NASA NTRS1990

Ion composition in and near the frontside boundary layer

A unique set of magnetopause crossings by the AMPTE/CCE spacecraft from near its 8.8 R(E) apogee is used to identify 13 crossings within the first two years after launch time that contain significant data intervals in the magnetosheath, boundary layer, and nearby outer magnetosphere. It is concluded that shocked solar wind or magnetosheath ions as well as ions from the outer magnetosphere overlap the boundary layer. Low-energy He(2+) as well as high charge state CNO, Si, and Fe group ions clearly overlap the boundary layer from their solar wind source. Similarly, O(+), N(+), O2(+) and NO(+) + O2(+) overlap the boundary layer from their magnetospheric source region. It is found that ion densities in the boundary layer are roughly one half those observed in the respective source regions, and that ion transport across the boundary layer is a one-way process with dawn-dusk asymmetry.

Eastman, T. E.↗
NASA NTRS1991

Silicon dioxide with a silicon interfacial layer as an insulating gate for highly stable indium phosphide metal-insulator-semiconductor field effect transistors

A novel gate insulator consisting of silicon dioxide (SiO2) with a thin silicon (Si) interfacial layer has been investigated for high-power microwave indium phosphide (InP) metal-insulator-semiconductor field effect transistors (MISFETs). The role of the silicon interfacial layer on the chemical nature of the SiO2/Si/InP interface was studied by high-resolution X-ray photoelectron spectroscopy. The results indicated that the silicon interfacial layer reacted with the native oxide at the InP surface, thus producing silicon dioxide, while reducing the native oxide which has been shown to be responsible for the instabilities in InP MISFETs. While a 1.2-V hysteresis was present in the capacitance-voltage (C-V) curve of the MIS capacitors with silicon dioxide, less than 0.1 V hysteresis was observed in the C-V curve of the capacitors with the silicon interfacial layer incorporated in the insulator. InP MISFETs fabricated with the silicon dioxide in combination with the silicon interfacial layer exhibited excellent stability with drain current drift of less than 3 percent in 10,000 sec, as compared to 15-18 percent drift in 10,000 sec for devices without the silicon interfacial layer. High-power microwave InP MISFETs with Si/SiO2 gate insulators resulted in an output power density of 1.75 W/mm gate width at 9.7 GHz, with an associated power gain of 2.5 dB and 24 percent power added efficiency.

Kapoor, V. J.↗
NASA NTRS1991

Instabilities and large structures in reattaching boundary layers

The underlying premise in this work is that large structures in turbulent shear flows are not universal and similar large structures in different flows owe their origins to basically similar instabilities. The turbulent boundary-layer flow over a backward-facing step and also the reattaching transitional boundary layer behind a large spanwise rod have been studied experimentally in parallel primarily to understand the origin of the curious large structures observed in the far downstream region (after 39 step heights) of the former flowfield. Simultaneous flow visualization and hot-wire anemometry and also plain-flow visualization have been carried out. In the backward-facing step, the three-dimensional mixing layer developing after detachment persists even after reattachment. The flow does not recover fully back to a regular turbulent boundary layer even far downstream of reattachment. The turbulence-producing instability of a turbulent boundary layer continues to compete with the wall-bounded mixing layer instability, giving rise to the curious schismatic arrays of large structures.

Bandyopadhyay, Promode R.↗