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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Turbulent shear stresses in compressible boundary layers

Hot-wire anemometer measurements of turbulent shear stresses in a Mach 3 compressible boundary layer were performed in order to investigate the effects of heat transfer on turbulence. Measurements were obtained by an x-probe in a flat plate, zero pressure gradient, two dimensional boundary layer in a wind tunnel with wall to freestream temperature ratios of 0.94 and 0.71. The measured shear stress distributions are found to be in good agreement with previous results, supporting the contention that the shear stress distribution is essentially independent of Mach number and heat transfer for Mach numbers from incompressible to hypersonic and wall to freestream temperature ratios of 0.4 to 1.0. It is also found that corrections for frequency response limitations of the electronic equipment are necessary to determine the correct shear stress distribution, particularly at the walls.

Laderman, A. J.↗
NASA NTRS1978

Investigation of turbulent diffusion in the extreme lower atmosphere

Turbulent diffusion in the extreme lower layer of the atmosphere (up to 5 m) has been investigated. Turbulent flow was simulated under dry, stable and calm conditions by means of a 3.04 m diameter fan installed at a field site situated on flat grassland. The ambient wind was continuously monitored by means of a cup anemometer placed outside the wake, and the temperature distribution was measured by four thermometers placed on an 18 m tower, also outside the wake. Balloons and red smoke were used to visualize the wake flow and investigate the predominant sizes of turbulent eddies and their streamwise behavior. The mean and turbulent velocities along the turbulence line were measured using an array of hot-wire anomometers. Results provide substantial verification of a recently proposed model (Koper and Sadeh, 1975; Koper et al., 1978) relating the Lagrangian to the Eulerian turbulent velocity autocorrelation. In this model the Lagrangian autocorrelation is given by a domain integral over a set of ordinary Eulerian autocorrelations acquired simultaneously at all points within the flow field in question, which is viewed as a turbulence 'box'.

Koper, C. A., Jr.↗
NASA NTRS1980

"Grinding" cavities in polyurethane foam

Grinding tool installed on conventional milling machine cuts precise cavities in foam blocks. Method is well suited for prototype or midsize production runs and can be adapted to computer control for mass production. Method saves time and materials compared to bonding or hot wire techniques.

Brower, J. R.↗
NASA NTRS1979

Characteristics of lightly loaded fan rotor blade wakes

Low subsonic and incompressible wake flow downstream of lightly loaded rotor was studied. Measurements of mean velocity, turbulence intensity, Reynolds stress, and static variations across the rotor wake at various axial and radial locations were investigated. Wakes were measured at various rotor blade incidences to discern the effect of blade loading on the rotor wake. Mean velocity and turbulence measurements were carried out with a triaxial hot wire probe both rotating with the rotor and stationary behind the rotor. Results indicate that increased loading slows the decay rates of axial and tangential mean velocity defects and radial velocities in the wake. The presence of large radial velocities in the rotor wake indicate the extent of the interactions between one radius and another. Appreciable static pressure variations across the rotor wake were found in the near wake region. Similarity in the profile shape was found for the axial and tangential components of the mean velocity and in the outer layer for axial, tangential, and radial turbulence intensities.

Reynolds, B.↗
NASA NTRS1979

Amplification of Reynolds number dependent processes by wave distortion

The amplification of a Reynolds number dependent process by wave distortion and the possibility of applying the results to other similar Reynolds number dependent processes were investigated. The process investigated was that associated with the operation of a constant-temperature hot-wire anemometer. The application of vaporization limited combustion, the type of combustion typically associated with liquid propellant rocket engines, was studied. A series of experiments were carried out to determine the effect of wave distortion on a Reynolds number dependent process and to establish the analogy between the anemometer process and the combustion process. Parametric trends, behavior common to different chamber geometries, and stability boundaries were identified. The results indicate a high degree of similarity between the two processes and the possibility of using the anemometer system to investigate combustion instability. The nonlinear aspects of a Reynolds number dependent process appear to be the dominant mechanisms controlling instability.

Ventrice, M.↗
NASA NTRS1976

An experimental study of the structure and acoustic field of a jet in a cross stream

The plane of symmetry of a high speed circular jet was surveyed to measure the mean and turbulent velocity fields by using constant temperature hot wire anemometry. The intensity of the noise radiated from the jet was determined in the tunnel test section by utilizing the cross-correlation at a particular time delay between the signals of two microphones suitably located along a given direction. Experimental results indicate that the turbulent intensity inside the crossflow jet increases by a factor of (1 + 1/2) as compared to the turbulent intensity of the same jet under free conditions, with r indicating the ratio of the jet velocity by the cross stream velocity. The peak observed in the turbulence spectra obtained inside the potential core of the jet has a frequency that increases by the same factor with respect to the corresponding frequency measured in the case of the free jet. The noise radiated by the jet becomes more intense as the crossflow velocity increases. The measured acoustic intensity of the crossflow jet is higher than the value which would be expected from the increase of the turbulent intensity only.

Camelier, I.↗
NASA NTRS1980

Turbulence measurements in the boundary layer of a low-speed wind tunnel using laser velocimetry

Laser velocimeter measurements in an incompressible, turbulent boundary layer along the wall of a low-speed wind tunnel are presented. The laser data are compared with existing hot-wire anemometer measurements of a flat plate, incompressible, turbulent, boundary layer with zero pressure gradient. An argument is presented to explain why previous laser velocimeter measurements in zero pressure gradient, turbulent boundary layers have shown an unexpected decrease in turbulent shear stresses near the wall.

Schairer, E. T.↗
NASA NTRS1979

First results of a study on turbulent boundary layers in oscillating flow with a mean adverse pressure gradient

The development of a turbulent unsteady boundary layer with a mean pressure gradient strong enough to induce separation, in order to complete the extend results obtained for the flat plate configuration is presented. The longitudinal component of the velocity is measured using constant temperature hot wire anemometer. The region where negative velocities exist is investigated with a laser Doppler velocimeter system with BRAGG cells. The boundary layer responds by forced pulsation to the perturbation of potential flow. The unsteady effects observed are very important. The average location of the zero skin friction point moves periodically at the perturbation frequency. Average velocity profiles from different instants in the cycle are compared. The existence of a logarithmic region enables a simple calculation of the maximum phase shift of the velocity in the boundary layer. An attempt of calculation by an integral method of boundary layer development is presented, up to the point where reverse flow starts appearing.

Houdeville, R.↗
NASA NTRS1980

Three dimensional mean flow and turbulence characteristics of the near wake of a compressor rotor blade

The investigation was carried out using the rotating hot wire technique. Measurements were taken inside the end wall boundary layer to discern the effect of annulus and hub wall boundary layer, secondary flow, and tip leakage on the wake structure. Static pressure gradients across the wake were measured using a static stagnation pressure probe insensitive to flow direction changes. The axial and the tangential velocity defects, the radial component of velocity, and turbulence intensities were found to be very large as compared to the near and far wake regions. The radial velocities in the trailing edge region exhibited characteristics prevalent in a trailing vortex system. Flow near the blade tips found to be highly complex due to interaction of the end wall boundary layers, secondary flows, and tip leakage flow with the wake. The streamwise curvature was found to be appreciable near the blade trailing edge. Flow properties in the trailing edge region are quite different compared to that in the near and far wake regions with respect to their decay characteristics, similarity, etc. Fourier decomposition of the rotor wake revealed that for a normalized wake only the first three coefficients are dominant.

Ravindranath, A.↗
NASA NTRS1980

Flow measurements in a model swirl combustor

Flow measurements in a model combustor composed of two confined coaxial swirling jets under noncombusting conditions are presented. Mean flow results are obtained for five different flow conditions to determine the effect of outer swirl on the recirculation zone (which is used for flame stabilization under combustion conditions). As the outer swirl magnitude is first decreased from maximum counter-swirl to zero and then increased to give co-swirl conditions, the size and the reverse flow velocity in the recirculation zone diminish. Detailed time mean and fluctuating flow measurements are obtained for a co-swirl and a counter-swirl condition with a directional pitot probe and hot-wire anenometry. For these two cases, recirculation zone occurs only with counter-swirl, near the exit of the inner jet. The recirculation zone is in the form of a one celled toroidal vortex having very low swirl velocity. Axial development of fluctuation levels, energy dissipation rates and turbulence length scales are described for the two flow conditions. Spectral analysis reveals periodic oscillations in both flows. The oscillations originate from the inner jet and up to 4 harmonics are observed. The fundamental frequencies are comparable to the rotational frequencies of the inner jets at the exit which are approximately under solid body rotation. The significance of the present results for the combustion process is discussed.

Vu, B. T.↗
NASA NTRS1980

Blade loading and spanwise effects on the near and far wake characteristics of a compressor rotor blade

An experimental investigation of the blade loading and spanwise effects on the rotor wake is presented. The investigation was limited to a study of a low subsonic and incompressible wake flow found downstream of a lightly loaded rotor. Measurements were made with a tri-axial hot wire probe mounted in the stationary frame of reference at six radial and nine axial positions. At each measurement location, the rotor was run at different operating conditions to discern the effects of blade loading on the wake. Near and far wake measurements are given, including mean velocity and turbulence intensity characteristics. The loading and spanwise effects on rotor wake characteristics were found to be substantial.

Reynolds, B.↗
NASA NTRS1980

Additional flow quality measurements in the Langley Research Center 8-Foot Transonic Pressure Tunnel

Additional tests were conducted to further define the disturbance characteristics of the Langley 8-Foot Transonic Pressure Tunnel. Measurements were made in the settling chamber with hot wire probes and in the test section with pressure transducers when various methods were used to choke the flow. In addition to presenting rms values measured at various locations and tunnel condition, autocorrelations and cross correlation data are also presented.

Brooks, J. D.↗
NASA NTRS1979

Investigations of coherent structures in free turbulent shear flows

The dynamics of large-scale coherent structures in general, and studies based on flow-visualization and phase-locked hot-wire measurements of the structures in the near fields of circular jets are considered. A study involving the nature of coherent structures under conditions of stable vortex pairing induced by controlled axisymmetric acoustic excitation reveals that azimuthal coherence initially enhanced by the excitation is lost before the end of the potential core through evolution of azimuthal lobe structures and turbulent breakdown. The evolution of a spark-induced spot in an axisymmetric turbulent mixing layer is also investigated, and the coherent Reynolds stress associated with the spot is found to be much higher than the background turbulence Reynolds stress. In addition, the state of organization in a high Reynolds number unperturbed axisymmetric mixing layer is examined, and it is shown that the shear layer is infrequently organized and the motions are intensely three-dimensional, while tearing and fractional pairing occur as frequently as complete pairing.

Hussain, A. K. M. F.↗
NASA NTRS1980

An axial flow research compressor facility designed for flow measurement in rotor passages

An axial flow research compressor facility, which is designed for relative flow measurement, is described in this paper. The facility has a rotating probe traverse mechanism which is capable of traversing hot wire, pitot and other probes at 0.09 deg intervals across the rotor blade passage. The data transmission system includes rotating transducers, pressure transfer device, ten-channel mercury slip ring unit, scanivalve, etc. The instrumentation includes on-line data processing capability. A brief description of probes used as well as some typical data on the rotor blade static pressure, rotor endwall flow and rotor wake characteristics are given in the paper.

Lakshminarayana, B.↗
NASA NTRS1980

Turbulence suppression in free shear flows by controlled excitation

The mechanism of turbulence suppression in the near field of a free shear flow under controlled acoustic excitation is investigated. The near fields of four circular air jets, a plane air jet and a large single-stream plane free air mixing layer with laminar efflux boundary layers were monitored with hot-wire anemometers. The suppression effect is found to occur over the entire frequency range in all the flows studied, with maximal suppression (as much as 80%) at about 400 initial shear layer momentum thicknesses downstream from the exit and for excitation at a Strouhal number of 0.017. Suppression is shown to be a consequence of excitation-induced modification of the shear layer instability mode and the resulting structure, and to occur at the excitation frequency corresponding to the maximally unstable disturbance frequency of the initial free shear layer. Flow visualization reveals the presence of strong, large-scale vortical structures in the unexcited flow, which are weakened and diminished by excitation. It is concluded that excitation produces rapid roll-up and early breakdown of the shear layer, thus inhibiting the formation of energetic large-scale vortices and reducing the large fluctuation intensities caused by the passage and interaction of these vortices.

Zaman, K. B. M. Q.↗
NASA NTRS1980

Development and structure of a rectangular jet in a multiple jet configuration

Results of hot-wire measurements in an incompressible jet issuing from an array of rectangular lobes, equally spaced with their small dimensions in a line, are presented. The quantities measured include mean velocity and the Reynolds stress in the two central planes of the jet at stations covering up to 115 widths (small dimension of the lobe) downstream of the nozzle exit. Measurements are carried out on a single rectangular free jet and on the same jet in a multiple jet configuration. In the case of a multiple jet, the flowfield for downstream distance X greater than 60D (D = width of the lobe) resembles that of a jet exiting from a two-dimensional nozzle with its short dimension being the long dimension of the lobe. The field of turbulence is found to be nearly isotropic in the plane containing the small dimension of the lobes for X greater than 60D.

Krothapalli, A.↗
NASA NTRS1979

The mechanics of vortex pairing in a circular jet under controlled excitation

The vortex pairing phenomenon in the near field circular jet under controlled excitation is monitored by the method of phase-averaging. Hot-wire measurements were made in a 7.62 cm diameter air at Re(D) = 32,000, excited at St(D) = 0.85 with an excitation amplitude of 3%. The trajectories of the vortex centers and their longitudinal and transverse convection velocities during the pairing program are measured. Spatial distributions of the average velocity, vorticity, coherent Reynolds numbers, streamlines and pseudo-streamfunction are presented at a particular phase of the pairing. These represent field measurements at one particular time and do not involve the use of Taylor hypothesis.

Zaman, K. B. M. Q.↗
NASA NTRS1980

Flow field and acoustic properties of a Mach number 0.9 jet at a low Reynolds number

An experimental study of the flow field and acoustic properties of a low Reynolds number M = 0.9 jet has been performed in a low pressure anechoic test chamber. The mean flow field was surveyed with a conventional pitot pressure probe and flow fluctuations were detected with a normal hot wire probe. Also, condenser microphone measurements were made in the acoustic field. The flow fluctuations within the jet were found to be dominated initially by a relatively discrete, large-scale, wave-like instability centered around a Strouhal number of 0.44. The axial wavelength of this instability was determined to be 1.45 jet diameters and its azimuthal character includes the n = 0 and n = plus or minus 1 modes. The growth of this instability coupled with its nonlinear breakdown are major contributors to the termination of the potential core region of the jet. The acoustic field of the jet, in contrast to the flow field, has a broad frequency spectrum with a peak amplitude near a Strouhal number of St = 0.2. The results indicate that a nonlinear mechanism involving the large-scale flow instability is responsible for a dominant portion of the noise generated from this jet.

Stromberg, J. L.↗