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Tanna, H. K.

Publications and source records attributed to Tanna, H. K..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Tone excited jets. I - Introduction

The major objective of the present study is an understanding of the mechanism of broadband jet noise augmentation due to upstream excitation, in terms of the relationship between excitation characteristics, changes in large scale and/or small scale turbulence structure of the jet, and sound radiated to the far field under both static and simulated forward velocity conditions. A systematic set of acoustic measurements was accordingly made for a range of flow conditions in the acoustic research facilities of a large transport aircraft manufacturer. After analyzing acoustic results, turbulence measurements were made with a laser velocimeter for those jet conditions at which jet noise amplification was important. A theory was developed to account for static and in-flight simulation results.

Tanna, H. K.↗
NASA NTRS1985

Shock associated noise of inverted-profile coannular jets. I - Experiments

The reduction of shock-associated noise in inverted-velocity-profile coannular jets is quantified and explained by conducting extensive optical and acoustic measurements for a suitable range of outer-stream and inner-stream presure-ratio combinations, and by interpreting the measured noise results with the aid of new theoretical models. It is shown that the shock noise from the outer stream is virtually eliminated when the inner stream is operated at a Mach number just above unity, regardless of all the other jet operating parameters. Furthermore, this reduction can be achieved at inverted-velocity-profile as well as normal-velocity-profile conditions.

Tanna, H. K.↗
NASA NTRS1985

Shock associated noise of inverted-profile coannular jets. II - Condition for minimum noise. III - Shock structure and noise characteristic

The generation of noise by the shock-turbulence interaction with shock cells in an inverted-profile coannular jet with nozzle exit velocity aligned with the jet axis is investigated analytically, interpreting the optical measurements of Tanna et al. (1985). The noise-intensity minimum at slightly supersonic primary-flow velocities is related to the weakness of the primary-stream shock-cell structure and the lack of such a pattern in the outer fan stream. The discrepancies between this finding and those of Dosanjh et al. (1977 and 1978) are attributed to nozzle design, the definition of minimum noise, and different interpretative approaches. A first-order shock-cell model is then developed to derive formulas for the peak frequencies and the scaling of noise intensity. The results of computations using these formulas are presented in graphs and found to be in good agreement with the experimental data.

Tam, C. K. W.↗
NASA NTRS1983

Reduced Aircraft-Engine Noise

Shock structure modified to eliminate associated noise. Shock wave repeats in supersonic flow adjoining subsonic flow layer. When supersonic layer is added, shock wave is partially transmitted at interface. Result is almost total elimination of shock pattern after one or two reflections. Technique applicable to inverted as well as to normal-velocity-profile coannular jets and used to eliminate or reduce shock noise of turbojet engines.

Tanna, H. K.↗
NASA NTRS1983

Refinement and application of acoustic impulse technique to study nozzle transmission characteristics

An improved acoustic impulse technique was developed and was used to study the transmission characteristics of duct/nozzle systems. To accomplish the above objective, various problems associated with the existing spark-discharge impulse technique were first studied. These included (1) the nonlinear behavior of high intensity pulses, (2) the contamination of the signal with flow noise, (3) low signal-to-noise ratio at high exhaust velocities, and (4) the inability to control or shape the signal generated by the source, specially when multiple spark points were used as the source. The first step to resolve these problems was the replacement of the spark-discharge source with electroacoustic driver(s). These included (1) synthesizing on acoustic impulse with acoustic driver(s) to control and shape the output signal, (2) time domain signal averaging to remove flow noise from the contaminated signal, (3) signal editing to remove unwanted portions of the time history, (4) spectral averaging, and (5) numerical smoothing. The acoustic power measurement technique was improved by taking multiple induct measurements and by a modal decomposition process to account for the contribution of higher order modes in the power computation. The improved acoustic impulse technique was then validated by comparing the results derived by an impedance tube method. The mechanism of acoustic power loss, that occurs when sound is transmitted through nozzle terminations, was investigated. Finally, the refined impulse technique was applied to obtain more accurate results for the acoustic transmission characteristics of a conical nozzle and a multi-lobe multi-tube supressor nozzle.

Salikuddin, M.↗
NASA NTRS1981

Shock-associated noise reduction from inverted-velocity-profile coannular jets

The reduction of shock-associated noise in inverted velocity profile coannular jets is investigated and quantified by making optical and acoustic measurements as a function of outer and inner stream pressure ratio combinations and interpreting noise results with the aid of new theoretical models. Schlieren photographs are obtained in three series of test conditions with a varying primary stream pressure ratio, and acoustic experiments are conducted on a polar arc radius of 3.05 m over a frequency range of 200 Hz to 80 kHz. A constant-thrust comparison is made, and data are plotted and discussed with emphasis on the variation of jet noise levels with primary stream Mach number and equivalent single jets on an absolute basis. It is shown that shock noise from the outer stream is almost eliminated when the inner stream is operated at a Mach number above unity at inverted and normal velocity profile conditions, and optical measurements indicate that for a fixed supercritical fan stream pressure ratio, the repetitive shock structure in the outer stream is minimized when the primary stream ratio becomes supercritical.

Tanna, H. K.↗
NASA NTRS1981

Shock associated noise reduction from inverted-velocity-profile coannular jets

Acoustic measurements show that the shock noise from the outer stream is virtually eliminated when the inner stream is operated at a Mach number just above unity, regardless of all the other jet operating conditions. At this optimum condition, the coannular jet provides the maximum noise reduction relative to the equivalent single jet. The shock noise reduction can be achieved at inverted-as well as normal-velocity-profile conditions, provided the coannular jet is operated with the inner stream just slightly supersonic. Analytical models for the shock structure and shock noise are developed indicate that a drastic change in the outer stream shock cell structure occurs when the inner stream increases its velocity from subsonic to supersonic. At this point, the almost periodic shock cell structure of the outer stream nearly completely disappears the noise radiated is minimum. Theoretically derive formulae for the peak frequencies and intensity scaling of shock associated noise are compared with the measured results, and good agreement is found for both subsonic and supersonic inner jet flows.

Tanna, H. K.↗
NASA NTRS1981

An experimental study of transmission, reflection and scattering of sound in a free jet flight simulation facility and comparison with theory

When a free jet (or open jet) is used as a wind tunnel to simulate the effects of flight on model noise sources, it is necessary to calibrate out the effects of the free jet shear layer on the transmitted sound, since the shear layer is absent in the real flight case. In this paper, a theoretical calibration procedure for this purpose is first summarized; following this, the results of an experimental program, designed to test the validity of the various components of the calibration procedure, are described. The experiments are conducted by using a point sound source located at various axial positions within the free jet potential core. By using broadband excitation and cross-correlation methods, the angle changes associated with ray paths across the shear layer are first established. Measurements are then made simultaneously inside and outside the free jet along the proper ray paths to determine the amplitude changes across the shear layer. It is shown that both the angle and amplitude changes can be predicted accurately by theory. It is also found that internal reflection at the shear layer is significant only for large ray angles in the forward quadrant where total internal reflection occurs. Finally, the effects of sound absorption and scattering by the shear layer turbulence are also examined experimentally.

Ahuja, K. K.↗
NASA NTRS1980

Coannular jets - Are they really quiet and why

The noise reductions from both normal- and inverted-velocity-profile (IVP) shock-free coannular jets are critically assessed on the basis of existing data. The fully mixed equivalent (FMEQ) single jet, defined as having a uniform exit profile and the same thrust, mass flow rate and exit area as the coannular jet, is used throughout as the reference for noise comparisons. It is shown that uninverted-velocity-profile coannular jets are consistently noisier than FMEQ jets, whereas IVP jets are slightly quieter. The practical implications are discussed in detail. The 'inverted velocity' effects are isolated experimentally from the 'inverted temperature' effects and the resulting noise reductions or increases are explained by using the measurements of mean flow, turbulence structure, and source location in coannular jets.

Tanna, H. K.↗
NASA NTRS1979

Effects of simulated forward flight on jet noise, shock noise and internal noise

Inflight simulation experiments are conducted in an anechoic free-jet facility to examine the flight effects on various combinations of jet noise, shock noise and internal noise. The jet mixing noise component reduces with forward velocity at all angles and frequencies. When jet mixing noise is contaminated with internal noise, forward motion provides a noise reduction in the rear arc and a noise increase in the forward arc, with little change at 90 deg. The results are similar for shock-containing jets. It is found that the existing anomalies between full-scale flight data and model-scale flight simulation data could well be due to the contamination of the flight data by internal noise.

Ahuja, K. K.↗
NASA NTRS1979

The noise and flow characteristics of inverted-profile coannular jets

A basic understanding of the noise reduction mechanisms in shock-free inverted-velocity-profile coannular jets was studied. Acoustic measurements are first conducted in an anechoic facility to isolate the effects of inverted velocity and inverted temperature for coannular jets having constant total thrust, mass flow rate and exit area. To obtain physical explanations of the measured noise changes, several types of experiments are conducted. These include (1) source location experiments using the polar correlation technique, (2) mean flow surveys using a combination pressure/temperature probe, and (3) detailed mean flow and turbulence measurements using a two-point four-channel laser velocimeter. The results from these experiments are presented and discussed in detail. Finally, the measured variations of coannular jet mixing noise with fan-to-primary velocity ratio and static temperature ratio are interpreted by utilizing the results from the various experimental phases in conjunction with the existing Lockheed single jet noise prediction model.

Tanna, H. K.↗
NASA NTRS1978

The free jet as a simulator of forward velocity effects on jet noise

A thorough theoretical and experimental study of the effects of the free-jet shear layer on the transmission of sound from a model jet placed within the free jet to the far-field receiver located outside the free-jet flow was conducted. The validity and accuracy of the free-jet flight simulation technique for forward velocity effects on jet noise was evaluated. Transformation charts and a systematic computational procedure for converting measurements from a free-jet simulation to the corresponding results from a wind-tunnel simulation, and, finally, to the flight case were provided. The effects of simulated forward flight on jet mixing noise, internal noise and shock-associated noise from model-scale unheated and heated jets were established experimentally in a free-jet facility. It was illustrated that the existing anomalies between full-scale flight data and model-scale flight simulation data projected to the flight case, could well be due to the contamination of flight data by engine internal noise.

Ahuja, K. K.↗
NASA NTRS1978

Calculation of far-field jet noise spectra from near-field measurements using true source location

Jet mixing noise data at different measurement distances are compared with values calculated from the Lockheed prediction method. Although the method does not include any acoustic near-field effects, the measured and predicted results agree well where the measured data deviates from the inverse square law. It is therefore suggested that departures from the inverse square law are primarily the result of (1) the non-negligible distance between the nozzle exit plane and the true axial source location and (2) the jet mixing noise directionality, as modeled in the prediction method. Allowing for these effects, jet noise data at 8 and 96 diameters over a wide range of frequencies, angles and jet conditions are shown to collapse with reasonable accuracy.

Ahuja, K. K.↗
NASA NTRS1977

An experimental study of transmission, reflection and scattering of sound in a free-jet flight simulation facility and comparison with theory

Acoustic time delays across a free-jet shear layer are measured and compared with predictions based on (1) ray paths refracted abruptly across a cylindrical vortex sheet and (2) ray paths traced through a more realistic diverging flow model. The close agreement between measurement and theory confirms that Snell's law provides an accurate prediction of wavefront refraction or angle changes across a diverging shear layer. Microphones are placed on calculated ray paths to determine the coherent transmission and internal reflection characteristics of the shear layer and also the scattering of sound by the shear-layer turbulence. The transmission data essentially verify the proposed, theoretical calibration factor which forms part of a computational procedure that is being developed to convert model jet data from a free-jet facility to inflight conditions.

Ahuja, K. K.↗
NASA NTRS1976

Inflight simulation experiments on turbulent jet mixing noise

The effects of aircraft forward motion on pure turbulent mixing noise from unheated jets are examined experimentally in the inflight simulation mode. Both acoustic and flow characteristics are determined by testing model-scale nozzles in an anechoic free-jet facility and a wind tunnel, respectively. Scaling laws are derived from each set of experiments and are found to be complementary. The implications are discussed in detail. In particular, it is shown that the measured reduction in noise at 90 deg to the jet axis is a pure source alteration effect.

Tanna, H. K.↗