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At least 829 records · Page 46

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NASA NTRS1977

Wall cooling effects on hypersonic transitional/turbulent boundary layers at high Reynolds numbers

The characteristics of a thick hypersonic boundary layer turbulent for a length of 175 cm on a 4 deg sharp wedge were measured. The resulting boundary layer was free from transverse curvature effects and only mildly affected by upstream history effects caused by pressure and wall temperature gradients. Heat-transfer distributions were used to locate regions of laminar, transitional, and turbulent flow at an edge unit Reynolds number of 470,000 cm at wall-to-total temperature ratios from about 0.3 to 1. Wall cooling had little effect on the location of the transition region. Pitot and total temperature profiles and skin-friction measurements were obtained at several locations along the model longitudinal centerline. Mixing length and turbulent Prandtl number distributions were derived from the fully turbulent mean profiles.

Watson, R. D.↗

NASA NTRS1977

Effect of compliant wall motion on turbulent boundary layers

A critical analysis of available compliant wall data which indicated drag reduction under turbulent boundary layers is presented. Detailed structural dynamic calculations suggest that the surfaces responded in a resonant, rather than a compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: (1) flexible pipe flows and (2) water-backed membranes in air. Analysis indicates that the wall motion for the remaining data is typified by short wavelengths in agreement with the requirements of a possible compliant wall drag reduction mechanism recently suggested by Langley.

Bushnell, D. M.↗

NASA NTRS1977

An electrostatically driven surface for flexible wall drag reduction studies

This paper describes the theoretical analysis, experimental development and surface motion data from an electrostatically driven flexible wall capable of producing high frequency, large amplitude, short wavelength standing wave motions. The driven wall was developed for use in studying flexible wall-turbulent boundary layer interactions. The theoretical analysis considers the coupled electrochemical system using a continuum model. For the system under consideration the electrical field is uncoupled from the mechanical system leading to a simplified analysis. The structural problem is a non-linear (moderately) large amplitude problem and details of the analysis are presented. Surface motion measurements, made using a modified schlieren type optical system, are discussed and details of the surface motion under different excitations are presented. Fair agreement is obtained between experimental measurements and theoretical calculations.

Weinstein, L. M.↗

NASA NTRS1978

Effect of wall temperature on a supersonic turbulent boundary layer

Measurement of mean flow profiles in a fully developed Mach 3 turbulent boundary layer with negligible pressure gradient is reported. Data were acquired at several streamwise locations for wall-to-total temperature ratios of 0.94, 0.71, and 0.54. The results demonstrate that the velocity defect formulation of the law-of-the-wake, which successfully correlates compressible, adiabatic boundary layers, is also valid for nonadiabatic flows. It is also shown that for adiabatic walls, the linear Crocco relation between total temperature and velocity does not provide a valid test of the nature of the boundary-layer flow for practical cases where the Prandtl number departs from unity. Finally, the turbulent shear stress, mixing length, and eddy viscosity were extracted from the 'time-averaged' conservation equations using the measured mean flow profiles and found to be insensitive to wall temperature. In particular, the latter properties are in good agreement with earlier compressible, adiabatic correlations of turbulent transport properties.

Laderman, A. J.↗

NASA NTRS1978

Duct wall impedance control as an advanced concept for acoustic suppression enhancement

A systems concept procedure is described for the optimization of acoustic duct liner design for both uniform and multisegment types. The concept was implemented by the use of a double reverberant chamber flow duct facility coupled with sophisticated computer control and acoustic analysis systems. The optimization procedure for liner insertion loss was based on the concept of variable liner impedance produced by bias air flow through a multilayer, resonant cavity liner. A multiple microphone technique for in situ wall impedance measurements was used and successfully adapted to produce automated measurements for all liner configurations tested. The complete validation of the systems concept was prevented by the inability to optimize the insertion loss using bias flow induced wall impedance changes. This inability appeared to be a direct function of the presence of a higher order energy carrying modes which were not influenced significantly by the wall impedance changes.

Dean, P. D.↗

NASA NTRS1978

Pressure and velocity measurements in a three dimensional wall jet

A flat plate and a curved wall surface, intended to model a wing-flap combination in a high V/STOL configuration, were used to measure pressure and flow velocity in a freely expanding coflowing jet and a three dimensional wall jet. The effects of increasing the velocity ratio of the jet exit plane velocity to the free stream velocity were investigated. Velocity measurements were made using a two color laser Doppler velocimeter with a phase-locked loop processor. Fluctuating pressures were monitored with condenser-type microphones. Quantities measured include the width of the mixing region, the mean velocity field, turbulent intensities, and time scales. Wall and static pressure-velocity correlations and coherences are presented.

Catalano, G. D.↗

NASA NTRS1979

Prediction of compliant wall drag reduction, part 2

A numerical model of turbulent boundary layer flows over compliant walls was investigated. The model is based on Burton's observation that outer flow structures in turbulent boundary layers produce large scale pressure fluctuations near the wall. The results of calculations indicate that certain small wavelength wall motions can have a significant effect upon the stability of turbulent boundary layers.

Orszag, S. A.↗

NASA NTRS1979

Design and calibration of slotted walls for transonic airfoil wind tunnels

The traditional procedure for estimating the performance of slotted walls for airfoil wind tunnels is reviewed, and a modification which improves the accuracy of this procedure is described. Unlike the traditional procedure, the modified procedure indicates that the design of airfoil wind-tunnel walls which induce minimal blockage and streamline-curvature effects is feasible. The design and testing of such a slotted wall is described. It is shown experimentally that the presence of a model can affect the plenum pressure and thus make the use of the plenum pressure as a calibration reference questionable. Finally, an ONERA experiment which shows the effect of the sidewall boundary layer on the measured model normal force is discussed.

Barnwell, R. W.↗

NASA NTRS1979

Pressure and velocity measurements in a three dimensional wall jet

In three recent papers, some results of an experimental investigation of a freely expanding coflowing jet as well as a three dimensional wall jet have been presented. A flat plate as well as a curved wall surface intended to model a wing-flap combination in a high lift V/STOL configuration have been investigated. In these papers, the ratio of the jet exit plane velocity to the free stream velocity, was 5.1. This paper explores the effects of increasing the velocity ratio. The quantities measured include the width of the mixing region, the mean velocity field, turbulent intensities and time scales. In addition, wall and static pressure-velocity correlations and coherences are presented. The velocity measurements are made using a laser Doppler velocimeter with a phase-locked loop processor. The fluctuating pressures are monitored using condenser-type microphones.

Catalano, G. D.↗

NASA NTRS1980

TWINTAN: A program for transonic wall interference assessment in two-dimensional wind tunnels

A method for assessing the wall interference in transonic two dimensional wind tunnel test was developed and implemented in a computer program. The method involves three successive solutions of the transonic small disturbance potential equation to define the wind tunnel flow, the perturbation attriburable to the model, and the equivalent free air flow around the model. Input includes pressure distributions on the model and along the top and bottom tunnel walls which are used as boundary conditions for the wind tunnel flow. The wall induced perturbation fields is determined as the difference between the perturbation in the tunnel flow solution and the perturbation attributable to the model. The methodology used in the program is described and detailed descriptions of the computer program input and output are presented. Input and output for a sample case are given.

Kemp, W. B., Jr.↗

NASA NTRS1980

The development of a self-streamlining flexible walled transonic test section

This design eliminates the uncertainties in data from conventional transonic test sections. Sidewalls are rigid, and the flexible floor and ceiling are positioned by motorized jacks controlled by on-line computer to minimize run times. The tunnel-computer combination is self-streamlining without reference to the model. Data is taken from the model only when the walls are good streamlines, and is corrected for the small, known but inevitable residual interferences. Two-dimensional validation testing in the Mach range up to about 0.85 where the walls are just supercritical shows good agreement with reference data using a height:chord ratio of 1.5. Techniques are under development to extend Mach number above 1. This work has demonstrated the feasibility of almost eliminating wall interferences, improving flow quality, and reducing power requirements or increasing Reynolds number. Extensions to three-dimensional testing are outlined.

Goodyer, M. J.↗

NASA NTRS1980

Investigation of blown boundary layers with an improved wall jet system

Measurements were made in a two dimensional incompressible wall jet submerged under a thick upstream boundary layer with a zero pressure gradient and an adverse pressure gradient. The measurements included mean velocity and Reynolds stresses profiles, skin friction, and turbulence spectra. The measurements were confined to practical ratios (less than 2) of the jet velocity to the free stream velocity. The wall jet used in the experiments had an asymmetric velocity profile with a relatively higher concentration of momentum away from the wall. An asymmetric jet velocity profile has distinct advantages over a uniform jet velocity profile, especially in the control of separation. Predictions were made using Irwin's (1974) method for blown boundary layers. The predictions clearly show the difference in flow development between an asymmetric jet velocity profile and a uniform jet velocity profile.

Saripalli, K. R.↗

NASA NTRS1981

Existence of a natural instability not predicted by theory and connected to a wall deformation in a laminar boundary layer

Natural instability related to negative wall deformation was studied. It was shown that natural instabilities which propagate in a laminar boundary layer of a flat plate are in agreement with stability theory. It is found that if a wall has a deformation, a second frequency does exist, which is not predicted and is twice the first frequency. This second instable frequency only appears if there is a negative velocity gradient. The phenomenon is located very closely to the wall and drops off rapidly when moved away from it.

Gougat, P.↗

NASA NTRS1980

Composite wall concept for high-temperature turbine shrouds - Heat transfer analysis

The variables affecting the design of a composite turbine shroud, consisting of a metal base, an interlayer of porous metal, and an outer layer of yttria-stabilized zirconia, are analyzed. Results show that significant reductions in the cooling-air to gas-flow ratio are indicated for the composite shrouds compared to an all-metal shroud that was only impingement air cooled. The good insulating properties of the ceramic reduced the temperatures of the porous metal and support wall significantly. For a given porous metal density and coolant- to gas-flow ratio, decreasing the thickness of the porous metal and increasing ceramic thickness resulted in lower support wall temperatures. To maintain given allowable inter-layer temperatures and coolantto gas-flow ratios, porous-metal density or thermal conductivity must increase as the ratio of the thickness of the ceramic-to-porous metal decreases. It is concluded that a 1.78 mm thickness of porous material with a density of 0.2 and a 1.78 mm thickness of ceramic appears to be a good composite wall configuration for the assumed conditions

Ludwig, L. P.↗

NASA NTRS1981

A survey of the acoustical field of a rectangular wall jet

The acoustic field of a rectangular wall jet was surveyed for Reynolds numbers between 2000 and 9200. The study includes measurements of amplitude directivity, acoustic power spectra, and flow visualization. The effects of jet speed, wall length and trailing edge angle were examined. The flow visualization survey suggests an important role of discrete vortex convection in wall tone generation.

Horne, W. C.↗

NASA NTRS1982

Three-dimensional flow studies on a slotted transonic wind tunnel wall

Three-dimensional flow field measurements were made near a transonic slotted wall. Field velocity vectors and static pressure distributions have been obtained. The boundary layer displacement thickness was found to vary in the transverse plane with its maximum at the slot center line but decreased with increasing suction rate through the slot. The boundary layer characteristics were sensitive to the mass transfer through the slot. The projection on the flow field velocity vectors on the transverse plane reveals a vortex-like flow formation. The center of this secondary flow was located nearly at the edge of the wall shear layer and decreased in strength with applied suction. The secondary vortex motion may be attributed to the mean flow skewing, inhomogeneous transverse plane boundary layer and the wall turbulence anisotropy.

Wu, J. M.↗

NASA NTRS1982

Transonic wind tunnel wall interference corrections for three-dimensional models

A procedure for the evaluation of wall interference corrections for three-dimensional models is presented. The Mach number and angle-of-attack corrections require the numerical solution of the potential equation about a simplified representation of the experimental model. Pressure measurements are required near the wind tunnel walls. The correction procedure also requires knowledge of the free-stream Mach number, the model angle of attack, and the lift force experienced by the model. The procedure provides an estimate of the accuracy of the correction. For slender configurations at Mach numbers close to one, the Equivalence Rule formulation is adopted to calculate the wall interference effects. Preliminary results are presented for both general and slender-body configurations.

Rizk, M. H.↗

NASA NTRS1984

Neutron Probe of Building-Wall Composition

Walls of historic buildings charted by neutron radiography. Neutron source and Gamma-Ray Detector alined with each other yield map of composition of wall. Points spaced for minimal overlap based on mean free path of gamma rays emitted from wall materials. Map indicates nature and extent of changes in building materials so proper treatment is applied.

Trombka, J. I.↗