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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Synthetic Jets in Cross-flow: Round Jet - Part 1

Results of an experimental investigation on synthetic jets from round orifices with and without cross-flow are presented. Jet Reynolds number up to 46,000 with a fully turbulent approach boundary layer, and Stokes number up to 400. are covered. The threshold of stroke length for synthetic jet formation. in the absence of the cross-flow, is found to be Lo /D approximately 0.5. Above Lo /D is approximately 10, the profiles of normalized centerline mean velocity appear to become invariant. It is reasoned that the latter threshold may be related to the phenomenon of saturation of impulsively generated vortices. In the presence of the cross-flow, the penetration height of a synthetic jet is found to depend on the momentum- flux ratio . When this ratio is defined in terms of the maximum jet velocity and the cross-flow velocity. not only all data collapse but also the jet trajectory is predicted well by correlation equation available for steady jets-in-cross-flow. Distributions of mean velocity, streamwise vorticity as well as turbulence intensity for a synthetic jet in cross-flow are found to be similar to those of a steady jet-in-cross-flow. A pair of counter-rotating streamwise vortices, corresponding to the bound vortex pair of the steady case, is clearly observed. Mean velocity distribution exhibits a dome of low momentum fluid pulled up from the boundary layer, and the entire domain is characterized by high turbulence.

Zaman, K. B. M. Q.↗
NASA NTRS1986

Computation of turbulent boundary layer flows with an algebraic stress turbulence model

An algebraic stress turbulence model is presented, characterized by the following: (1) the eddy viscosity expression is derived from the Reynolds stress turbulence model; (2) the turbulent kinetic energy dissipation rate equation is improved by including a production range time scale; and (3) the diffusion coefficients for turbulence equations are adjusted so that the kinetic energy profile extends further into the free stream region found in most experimental data. The turbulent flow equations were solved using a finite element method. Examples include: fully developed channel flow, fully developed pipe flow, flat plate boundary layer flow, plane jet exhausting into a moving stream, circular jet exhausting into a moving stream, and wall jet flow. Computational results compare favorably with experimental data for most of the examples considered. Significantly improved results were obtained for the plane jet flow, the circular jet flow, and the wall jet flow; whereas the remainder are comparable to those obtained by finite difference methods using the standard kappa-epsilon turbulence model. The latter seems to be promising with further improvement of the expression for the eddy viscosity coefficient.

Kim, Sang-Wook↗
NASA NTRS1999

Development of an Empirical Methods for Predicting Jet Mixing Noise of Cold Flow Rectangular Jets

This report presents an empirical method for predicting the jet mixing noise levels of cold flow rectangular jets. The report presents a detailed analysis of the methodology used in development of the prediction method. The empirical correlations used are based on narrow band acoustic data for cold flow rectangular model nozzle tests conducted in the NASA Langley Jet Noise Laboratory. There were 20 separate nozzle test operating conditions. For each operating condition 60 Hz bandwidth microphone measurements were made over a frequency range from 0 to 60,000 Hz. Measurements were performed at 16 polar directivity angles ranging from 45 degrees to 157.5 degrees. At each polar directivity angle, measurements were made at 9 azimuth directivity angles. The report shows the methods employed to remove screech tones and shock noise from the data in order to obtain the jet mixing noise component. The jet mixing noise was defined in terms of one third octave band spectral content, polar and azimuth directivity, and overall power level. Empirical correlations were performed over the range of test conditions to define each of these jet mixing noise parameters as a function of aspect ratio, jet velocity, and polar and azimuth directivity angles. The report presents the method for predicting the overall power level, the average polar directivity, the azimuth directivity and the location and shape of the spectra for jet mixing noise of cold flow rectangular jets.

Russell, James W.↗
NASA NTRS1982

Correlation of flight effects on centerline velocity decay for cold-flow acoustically excited jets

Acoustic excitation can influence the large scale shear layer structure of jet flow, thereby causing the jet centerline velocity to decay more rapidly. this phenomena has numerous practical applications, such as the reduction of jet/flap impingement velocity in order to reduce flap structural loads for under-the-wing STOL aircraft concepts. Cold flow centerline velocity decay data obtained with acoustic excitation of the jet exhaust flow are correlated with and without flight effects. Initially, static data are correlated in terms of an acoustic parameter involving the threshold sound level needed to initiate acoustic excitation of the jet plume. It is shown that for a given flight speed, the same acoustic excitation parameter as that developed for the static condition is valid. Finally, an aerodynamic flight effects parameter is included to correlate the static and flight centerline velocity decay data with and without acoustic excitation.

Vonglahn, U. H.↗
NASA NTRS1981

Flow field investigation of atmospheric braking for high drag vehicles with forward facing jets

Flow field phenomena associated with a supersonic jet issuing upstream into a hypervelocity flow field were investigated experimentally in support of a new space vehicle aerobraking concept developed by Boeing for Orbital Transfer Vehicles (OTV's). Tests were made on OTV models in the NASA Langley 22 in., Mach 20 helium tunnel with jet exit Mach numbers from 1.0 to 6.18 and ballute half angles of 45 and 60 deg. Force data were taken at zero angle of attack to determine the effect of ballute angle, jet Mach number and jet flow rate on vehicle drag. Bow shock structures were examined in terms of flow steadiness to define acceptable jet flow rate regimes for use in drag modulation. Limited tests were made to obtain pressure and temperature distributions around the ballute and to determine the ballute center of pressure. Test results are presented and discussed relative to OTV application and similar previous experimental investigations.

Grenich, A. F.↗
NASA NTRS1973

Computer program for predicting symmetric jet mixing of compressible flow in jets

Finite-difference computer program has been developed for treating mixing of two parallel and compressible air streams; one of them may be supersonic. This development is restricted to symmetric jet mixing in which high-speed jet is located on axis of channel and no provision is made for blowing or suction along channel walls.

Gilbert, G. B.↗
NASA NTRS1981

Flow and acoustic properties of low Reynolds number supersonic underexpanded jets

Flow and acoustic measurements are made of cold model jets exhausting from a choked nozzle at pressure conditions corresponding to those of Mach 1.4 and 2.1 jets to investigate noise production properties of underexpanded supersonic jets. Mean flow measurements are made using pitot and static pressure probes, with flow fluctuation measurements made with a hot-wire probe and acoustic measurements made with a transversing microphone. Two convergent nozzles with exit diameters of 7.0 and 7.9 mm are used with an exciter consisting of a 0.8 mm tungsten electrode positioned 2 mm from the exit. Shock structure is observed as having a significant effect on the development of the flow field, while large-scale instabilities have higher growth rates in the shock containing underexpanded jets. The role of the asymmetric n = + or - 1 sinusoidal instability is clarified, and results suggest that the broadband shock associated noise of conventional high Reynolds number jets is not related to large-scale jet instability.

Hu, T. F.↗
NASA NTRS1976

Fluctuating pressures in flow fields of jets

The powered lift configurations under present development for STOL aircraft are the externally blown flap (EBF), involving direct jet impingement on the aircraft flaps, and the upper surface blown (USB), where the jet flow is attached on the upper surface of the wing and directed downwards. Towards the goal of developing scaling laws to predict unsteady loads imposed on the structural components of these STOL aircraft from small model tests, the near field fluctuating pressure behavior for the simplified cases of a round free cold jet and the same jet impinging on a flat plate was investigated. Examples are given of coherences, phase lags (giving convection velocities), and overall fluctuating pressure levels measured. The fluctuating pressure levels measured on the flat plate are compared to surface fluctuating pressure levels measured on full-scale powered-lift configuration models.

Schroeder, J. C.↗
NASA NTRS1975

Minimization of jet and core noise by rotation of flow

Jet and core noise can be reduced and flame lengths may be significantly decreased when exhaust gases are caused to rotate or swirl about longitudinal axis of exhaust. Combustion in rotating flows is steady and quiet, and is not accompanied by pulsations or violent fluctuations.

Schwartz, I. R.↗
NASA NTRS1989

Three dimensional flow field measurements of a 4:1 aspect ratio subsonic jet

Flow field measurements for a subsonic rectangular cold air jet with an aspect ratio of 4:1 (12.7 x 50.8 mm) at a Mach number of 0.09 and Re of 100,000 have been carried out using a three-dimensional laser Doppler anemometer system. Mean velocity measurements show that the jet width spreads more rapidly along the minor axis than along the major axis. The outward velocities, however, are not significantly different for the two axes, indicating the presence of enhanced mixing along the minor axis. The jet slowly changes from a rectangular jet to a circular jet as the flow progresses downstream.

Morrison, G. L.↗
NASA NTRS1982

Experiments on the flow and acoustic properties of a moderate-Reynolds-number supersonic jet

Flow and acoustic properties of a jet at Reynolds number of 70,000 were studied at Mach 2.1. Measurements in a free jet test facility were made with pitot tubes and hot-wire anemometry. Center-line Mach number distributions for natural and excited jets were obtained. A slow initial growth rate was in the potential core region of the jet, indicating a transition from laminar to turbulent flow in moderate Reynolds number jets. The transition occurred within the first 2-3 diameters. Spectral components were calculated for the fluctuating flowfield, and sound pressure levels were measured for the overall near-field noise. The centroid of noise was located about 8 nozzle diameters downstream. The growth rates of instabilities were determined to be in agreement with linear stability theory predictions over a broad frequency range.

Troutt, T. R.↗
NASA NTRS1991

On the anomalies in single-jet hover suckdown data

The data from nine different investigations of the suckdown induced in ground effect by a single jet issuing from plates of various sizes and shapes have been examined and compared. The results show that the generally accepted method for estimating suckdown significantly underestimated the suckdown for most of the configurations. The study identified several factors that could contribute to the differences. These include ground board size, plate edge effects, jet flow quality, jet impingement angle, the size of the chamber in which the tests were run, and obstructions in the region above the model. Most of these factors have not been investigated and in many cases items such as the size of the test chamber, jet flow quality, ground board size, etc., have not even been shown in the documents reporting the investigation. A program to investigate the effects of these factors is recommended.

Kuhn, Richard E.↗
NASA NTRS1959

Wind-Tunnel Investigation of a Small-Scale Sweptback-Wing Jet-Transport Model Equipped with an External-Flow Jet-Augmented Double Slotted Flap

A wind-tunnel investigation at low speeds has been made to study the aerodynamic characteristics of a small-scale sweptback-wing Jet-transport model equipped with an external-flow jet-augmented double slotted flap. Included in the investigation were tests of the wing alone to study the effects of varying the spanwise extent of blowing on the full-span flap. The results indicated that the double-slotted-flap arrangement of the present investigation was more efficient in terms of lift and drag than were the external-flow single-slotted-flap arrangements previously tested and gave a substantial reduction In the thrust-weight ratio required for a given lift coefficient under trimmed drag conditions. An increase in the spanwise extent of blowing on the full-span flap was also found to increase the efficiency of the model in terms of the lift and drag but, as would be expected on a sweptback-wing configuration, was accompanied by significant increases in negative pitching moment.

Johnson, Joseph L., Jr.↗