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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Laser Doppler velocimeter measurements of boundary layer velocity and turbulent intensities in Mach 2.5 flow

In recent years, the interest in developing a high-speed civil transport has increased. This has led to an increase in research activity on compressible supersonic flows, in particular the boundary layer. The structure of subsonic boundary layers has been extensively documented using conditional sampling techniques which exploit the knowledge of both u and v velocities. Researchers using these techniques have been able to explore some of the complex three-dimensional motions which are responsible for Reynolds stress production and transport in the boundary layer. As interest in turbulent structure has grown to include supersonic flows, a need for simultaneous multicomponent velocity measurements in these flows has developed. The success of conditional analysis in determining the characteristics of coherent motions and structures in the boundary layer relies on accurate, simultaneous measurement of two instantaneous velocity components.

Sewell, Jesse↗
NASA NTRS1977

Experimental and theoretical investigation of three-dimensional turbulent boundary layers and turbulence characteristics inside an axial flow inducer passage

Analytical and experimental investigations of the characteristics of three dimensional turbulent boundary layers in a rotating helical passage of an inducer rotor are reported. Expressions are developed for the velocity profiles in the inner layer, where the viscous effects dominate, in the outer layer, where the viscous effects are small, and in the interference layer, where the end walls influence the flow. The prediction of boundary layer growth is based on the momentum integral technique. The equations derived are general enough to be valid for all turbomachinery rotors with arbitrary pressure gradients. The experimental investigations are carried out in a flat plate inducer 3 feet in diameter. The mean velocity profiles, turbulence intensities and shear stresses, wall shear stress, and limiting streamline angles are measured at various radial and chordwise locations by using rotating probes. The measurements are in general agreement with the predictions. The radial flows are well represented by an expression which includes the effect of stagger angle and radial pressure gradient. The radial flows in the rotor channel are higher than those on a single blade. The collateral region exists only very near the blade surface. The radial component of turbulence intensity is higher than the streamwise component because of the effect of rotation.

Anand, A. K.↗
NASA NTRS1986

Boundary-layer transition effects on airplane stability and control

Surface contamination of laminar-flow airfoils can significantly modify the location of transition from laminar-to-turbulent boundary-layer flow. The contamination can be the result of insect debris, environmental effects such as ice crystals and moisture due to mist or rain, surface damage, or other contamination adhering to the surface. Location and mode of transition have a dominant effect on the lift-and-drag characteristics of a lifting surface. The influences of laminar boundary-layer flow behavior on airplane stability and control are examined through theoretical results and experimental (wind-tunnel and free-flight) data. For certain airfoils with a relatively steep pressure recovery it is shown that loss of laminar flow near the leading edge can result in premature separation of the turbulent boundary layer and, consequently, in loss of lift and control effectiveness. Aerodynamic modifications which minimize boundary-layer transition effects on airplane stability and control are also discussed.

Van Dam, C. P.↗
NASA NTRS1984

Developing flow in S-shaped ducts. 2: Circular cross-section duct

Laser-Doppler velocimetry measured the laminar and turbulent streamwise flow in a S-duct. The wall pressure distribution and one component of cross-stream velocity were also obtained for the turbulent flow case. Boundary layers near the duct inlet were about 25 percent of the hydraulic diameter in the laminar flow and varied around the periphery of the pipe between 10 percent and 20 percent in turbulent flow. Pressure-driven secondary flows develop in the first half of the S-duct and are attenuated and reversed in the second half. For both Reynolds numbers there is a region near the outer wall of the second half of the duct where the sign of the radial vorticity results in an enforcement of the secondary flow which was established in the first half of the S-duct. The core flow migrates, for both Reynolds numbers, to the outside wall of the first half and lies towards the inside wall of the second half of the S-duct at the outlet. The thinner inlet boundary layers in the turbulent flow give rise to weaker secondary motion.

Taylor, A. M. K. P.↗
NASA NTRS1985

Backward boundary layers in downward flame spread

The gas-phase combustion of a vertical fuel slab pyrolyzing at a finite rate under downward flame spread conditions is investigated theoretically. The backward boundary layer character of the flow is exploited, and a transformation that shifts the singularity associated with boundary-layer equations to a location, upstream of flame lift-off, where the surface temperature is nearly ambient, is employed. The boundary-layer flow is matched asymptotically for large Grashof number to a potential flow that allows the flow induced by the flame and along the upstream surface of the fuel bed to be calculated. Gas-phase temperature and velocity predictions are compared to experiment, and although analysis of the gas phase is emphasized, an approximate spread rate, based on a solid-phase energy balance, is calculated.

Vedha-Nayagam, M.↗
NASA NTRS1981

The boundary layer on compressor cascade blades

The flow field about an airfoil in cascade at a Reynolds number of 5 x 10 to the 5th power is described. Hot wire and laser anemometry are combined with flow visualization techniques in order to obtain detailed flow data (e.g., boundary layer profiles, points of separation, and the transition zone) on a cascade of relatively highly loaded blades. Benchmark data is provided for the evaluation of current and future predictive models, in this way aiding in the compressor design process.

Deutsch, S.↗
NASA NTRS1984

The boundary layer on compressor cascade blades

The flow field about an airfoil in a cascade at chord Reynolds number (R sub C) near 50,000. The program is experimental and combines laser Doppler anemometry (LDA) with flow visualization techniques in order to obtain detailed flow data (e.g., boundary layer profiles, points of separation and the transition zone) on a cascade of highly-loaded compressor blades. The information provided is to serve as benchmark data for the evaluation of current and future compressor cascade predictive models, in this way aiding in the compressor design process. The completed pressure surface mean velocity profiles, as well as two detailed near wake velocity profiles, all at a single incidence angle are provided.

Deutsch, S.↗
NASA NTRS1996

Delay of Turbulent Boundary Layer Detachment by Mechanical Excitation: Application to Rearward-facing Ramp

A vane oscillating about a fixed point at the inlet to a two-dimensional 20 deg rearward-facing ramp proved effective in delaying the detachment of a turbulent boundary layer. Flow-field, surface static pressure, and smoke-wire flow visualization measurements were made. Surface pressure coefficient distributions revealed that two different effects occurred with axial distance along the ramp surface. The surface pressure coefficient varied as a complex function of the vane oscillation frequency and its trailing edge displacement amplitude; that is, it varied as a function of the vane oscillation frequency throughout the entire range of frequencies covered during the test, but it varied over only a limited range of the trailing edge displacement amplitudes covered.The complexity of these findings prompted a detailed investigation, the results of which revealed a combination of phenomena that explain qualitatively how the mechanically generated, periodic, sinusoidal perturbing signal produced by the oscillating vane reacts with the fluid flow to delay the detachment of a turbulent boundary layer experiencing transitory detachment.

McKinzie, Daniel J., Jr.↗
NASA NTRS1978

Theoretical investigation of maintaining the boundary layer of revolution laminar using suction slits in incompressible flow

The transition of the laminar boundary layer into the turbulent state, which results in an increased drag, can be avoided by sucking of the boundary layer particles near the wall. The technically-interesting case of sucking the particles using individual slits is investigated for bodies of revolution in incompressible flow. The results of the variational calculations show that there is an optimum suction height, where the slot separations are maximum. Combined with favorable shaping of the body, it is possible to keep the boundary layer over bodies of revolution laminar at high Reynolds numbers using relatively few suction slits and small amounts of suction flow.

Thiede, P.↗
NASA NTRS1985

An experimental study of three-dimensional shock wave/turbulent boundary layer interactions in a supersonic flow

Wall static pressure distributions, surface flow patterns, pitot pressures, and yaw angle profiles were measured in a skewed three-dimensional shock wave/turbulent boundary layer interaction region. The test section was axisymmetric with a constant diameter. The nominal freestream Mach number was 4. Upstream of the interaction, the boundary layer thickness was 0.31 in. (0.787 cm). The three-dimensional flow was produced by azimuthal pressure gradients which were generated by an 8-degree cone aligned with the primary flow direction, but with the cone axis displaced 0.3 in. (0.76 cm) from the channel centerline. The yaw angle was found to be a function of both the azimuthal angle and the distance from the beginning of the interaction. It was observed that yaw angle increased substantially near the wall. The maximum yaw angle for the whole flow field was obtained in the 90 degree azimuthal plane.

Chou, J. H.↗
NASA NTRS2003

Blended Wing Body Systems Studies: Boundary Layer Ingestion Inlets With Active Flow Control

A CFD analysis was performed on a Blended Wing Body (BWB) aircraft with advanced, turbofan engines analyzing various inlet configurations atop the aft end of the aircraft. The results are presented showing that the optimal design for best aircraft fuel efficiency would be a configuration with a partially buried engine, short offset diffuser using active flow control, and a D-shaped inlet duct that partially ingests the boundary layer air in flight. The CFD models showed that if active flow control technology can be satisfactorily developed, it might be able to control the inlet flow distortion to the engine fan face and reduce the powerplant performance losses to an acceptable level. The weight and surface area drag benefits of a partially submerged engine shows that it might offset the penalties of ingesting the low energy boundary layer air. The combined airplane performance of such a design might deliver approximately 5.5% better aircraft fuel efficiency over a conventionally designed, pod-mounted engine.

Geiselhart, Karl A.↗
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.↗
NASA NTRS1974

Experimental and theoretical investigation of a slotted noise shield model for wind tunnel walls

Tests at Mach 6 and 8 have been conducted on a model of a wind tunnel sound radiation shield which consists of interchangeable circular and wedge-shaped longitudinal rods with gaps between the rods for boundary layer removal. In view of the large effective sweep angle of the circular rods with respect to the local flow, the boundary layer behavior was surprisingly well represented by swept cylinder theory. Laminar flow was maintained up to a maximum local unit Reynolds number of 8 million per foot which would presumably be independent of model length for a given rod diameter by analogy with results on swept cylinders. Reduction in wind-tunnel sound levels within the shielded region of the circular rod model were about 90% of the maximum possible reduction for an ideal shield.

Harvey, W. D.↗
NASA NTRS1976

Outer flow and turbulence in boundary layers

Results presented in report indicate that perturbations travel through boundary layer and give rise to turbulence production process which occurs near viscous sublayer. Part of turbulence produced near wall, in turn, moves outward and eventually produces convoluting outer edge of boundary layer to reproduce and sustain itself.

Cliff, W. C.↗