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Tam, C. K. W.

Publications and source records attributed to Tam, C. K. W..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Sound generated by instability waves of supersonic flows. I Two-dimensional mixing layers. II - Axisymmetric jets

An investigation is conducted of the phenomenon of sound generation by spatially growing instability waves in high-speed flows. It is pointed out that this process of noise generation is most effective when the flow is supersonic relative to the ambient speed of sound. The inner and outer asymptotic expansions corresponding to an excited instability wave in a two-dimensional mixing layer and its associated acoustic fields are constructed in terms of the inner and outer spatial variables. In matching the solutions, the intermediate matching principle of Van Dyke and Cole is followed. The validity of the theory is tested by applying it to an axisymmetric supersonic jet and comparing the calculated results with experimental measurements. Very favorable agreements are found both in the calculated instability-wave amplitude distribution (the inner solution) and the near pressure field level contours (the outer solution) in each case.

Tam, C. K. W.↗
NASA NTRS1983

On the shock cell structure and noise of supersonic jets

A linear solution modeling the shock cell structure of an axisymmetric supersonic jet operated at off-design conditions is developed by the method of multiple-scales. The model solution takes into account the gradual spatial change of the mean flow in the downstream direction. Turbulence in the mixing layer of the jet has the tendency of smoothing out the sharp velocity and density gradients induced by the shocks. To simulate this effect, eddy viscosity terms are incorporated in the model. It is known that the interaction between the quasi-periodic shock cells and the downstream propagating large turbulence structures in the mixing layer of the jet is responsible for the generation of broadband shock associated noise. Experimentally, the dominant part of this noise has been found to originate from the part of the jet near the end of the potential core. Calculated shock cell spacing at the end of the jet core according to the present model is used to estimate the peak frequencies of the shock associated noise for a range of observation angles. Very favorable agreement with experimental measurements is found.

Tam, C. K. W.↗
NASA NTRS1982

Tone-excited jet: Theory and experiments

A detailed study to understand the phenomenon of broadband jet-noise amplification produced by upstream discrete-tone sound excitation has been carried out. This has been achieved by simultaneous acquisition of the acoustic, mean velocity, turbulence intensities, and instability-wave pressure data. A 5.08 cm diameter jet has been tested for this purpose under static and also flight-simulation conditions. An open-jet wind tunnel has been used to simulate the flight effects. Limited data on heated jets have also been obtained. To improve the physical understanding of the flow modifications brought about by the upstream discrete-tone excitation, ensemble-averaged schlieren photographs of the jets have also been taken. Parallel to the experimental study, a mathematical model of the processes that lead to broadband-noise amplification by upstream tones has been developed. Excitation of large-scale turbulence by upstream tones is first calculated. A model to predict the changes in small-scale turbulence is then developed. By numerically integrating the resultant set of equations, the enhanced small-scale turbulence distribution in a jet under various excitation conditions is obtained. The resulting changes in small-scale turbulence have been attributed to broadband amplification of jet noise. Excellent agreement has been found between the theory and the experiments. It has also shown that the relative velocity effects are the same for the excited and the unexcited jets.

Ahuja, K. K.↗
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 NTRS1980

The radiation of sound by the instability waves of a compressible plane turbulent shear layer

The problem of acoustic radiation generated by instability waves of a compressible plane turbulent shear layer is solved. The solution provided is valid up to the acoustic far-field region. It represents a significant improvement over the solution obtained by classical hydrodynamic-stability theory which is essentially a local solution with the acoustic radiation suppressed. The basic instability-wave solution which is valid in the shear layer and the near-field region is constructed in terms of an asymptotic expansion using the method of multiple scales. This solution accounts for the effects of the slightly divergent mean flow. It is shown that the multiple-scales asymptotic expansion is not uniformly valid far from the shear layer. Continuation of this solution into the entire upper half-plane is described. The extended solution enables the near- and far-field pressure fluctuations associated with the instability wave to be determined. Numerical results show that the directivity pattern of acoustic radiation into the stationary medium peaks at 20 degrees to the axis of the shear layer in the downstream direction for supersonic flows. This agrees qualitatively with the observed noise-directivity patterns of supersonic jets.

Tam, C. K. W.↗
NASA NTRS1979

A statistical model of turbulence in two-dimensional mixing layers

A statistical model of turbulence in fully developed two-dimensional incompressible turbulent mixing layers is proposed. The development of this model is largely motivated by the recent experimental observations of Brown and Roshko (1974). The model is based on the proposition that the turbulence of a fully developed two-dimensional incompressible mixing layer is in a state of quasi-equilibrium. The model is used to predict the second-order turbulence statistics of the flow including single-point turbulent Reynolds stress distribution, intensity of turbulent velocity components, rms turbulent pressure fluctuations, power spectra and two-point space-time correlation functions. It is shown that numerical results compare favorably with available experimental measurements.

Tam, C. K. W.↗
NASA NTRS1978

Excitation of instability waves in a two-dimensional shear layer by sound

The excitation of instability waves in a plane compressible shear layer by sound waves is studied. The problem is formulated mathematically as an inhomogeneous boundary-value problem. A general solution for abitrary incident sound wave is found by first constructing the Green's function of the problem. Numerical values of the coupling constants between incident sound waves and excited instability waves for a range of flow Mach number are calculated. The effect of the angle of incidence in the case of a beam of acoustic waves is analyzed. It is found that for moderate subsonic Mach numbers a narrow beam aiming at an angle between 50 to 80 deg to the flow direction is most effective in exciting instability waves.

Tam, C. K. W.↗
NASA NTRS1978

On the tones and pressure oscillations induced by flow over rectangular cavities

Experimental measurements of the frequencies of discrete tones induced by flow over rectangular cavities were carried out over a range of low subsonic Mach numbers to provide a reliable data base for (aircraft wheel well) cavity noise consideration. A mathematical model of the cavity tones and pressure oscillation phenomenon based on the coupling between shear layer instabilities and acoustic feedback is developed to help in understanding the tone generation mechanism. Good agreement is found between discrete tone frequencies predicted by the model and experimental measurements over a wide range of Mach numbers. Evidence of tones generated by the cavity normal mode resonance mechanism at very low subsonic Mach numbers is also presented.

Tam, C. K. W.↗
NASA NTRS1978

Noise characteristics of upper surface blown configurations: Analytical Studies

Noise and flow results of upper surface blown configurations were analyzed. The dominant noise source mechanisms were identified from experimental data. From far-field noise data for various geometric and operational parameters, an empirical noise prediction program was developed and evaluated by comparing predicted results with experimental data from other tests. USB aircraft compatibility studies were conducted using the described noise prediction and a cruise performance data base. A final design aircraft was selected and theory was developed for the noise from the trailing edge wake assuming it as a highly sheared layer.

Reddy, N. N.↗
NASA NTRS1977

An experimental investigation of the trailing edge noise mechanism

An experimental investigation has been conducted to understand the physical mechanism of noise generation from a turbulent wall jet discharging over a flat plate and interacting with its sharp trailing edge. An aspect ratio 10 rectangular nozzle was used to provide the wall jet. Measurements made consist of farfield noise, surface pressure fluctuations, turbulent velocity fluctuations, and two-point space-time cross-correlations among these quantities. Results are presented which suggest strongly that the generation mechanism is the interaction of the convecting large scale quasi-orderly disturbance in the upper free shear layer of the wall jet with the trailing edge. The interaction also excites large scale strong vortical motion in the trailing edge wake. The dominant part of the sound field is highly coherent and in phase opposition across the trailing edge.

Yu, J. C.↗
NASA NTRS1976

Sound generated in the vicinity of the trailing edge of an upper surface blown flap

Far-field noise data indicated that for practical upper surface blown flap configurations, the noise radiated below the flap is dominated by the noise generated in the vicinity of the trailing edge. Sound field by turbulent mixing in the trailing edge wake is investigated experimentally and theoretically. Hot-wire measurements were made downstream of the trailing edge to determine the gross turbulence mixing characteristics of the flow. This information is used as input to a theoretical analysis of the sound field. Favorable agreement is found between predicted and measured far-field noise directivities at various frequencies and noise power spectra at various directions.

Tam, C. K. W.↗
NASA NTRS1976

A study of sound transmission in curved duct bends by the Galerkin method

A computational procedure based on the Galerkin method is developed to study the sound transmission and reflection characteristics of circularly curved bends of rectangular ducts. This procedure is adaptable to routine computer calculation. An important component of the computer program consists of a QR iterative algorithm which solves the matrix eigenvalue problem generated by the Galerkin method. The present procedure produced very satisfactory convergent results when applied even to cases where the incident wave has high frequency and high wave mode numbers. Some properties of the Galerkin solution are investigated. Its relationship to the classical solution by the method of separation of variables is discussed.

Tam, C. K. W.↗
NASA NTRS1976

Analytical developments for definition and prediction of USB noise

A systematic acoustic data base and associated flow data are used in identifying the noise generating mechanisms of upper surface blown flap configurations of short takeoff and landing aircraft. Theory is developed for the radiated sound field of the highly sheared flow of the trailing edge wake. An empirical method is also developed using extensive experimental data and physical reasonings to predict the noise levels.

Reddy, N. N.↗
NASA NTRS1975

Trailing edge noise

Shadowgraphic observations of the flow structure of a wall jet downstream of the trailing edge of a flat plate were carried out. Shadowgraphs obtained at jet exit Mach number 0.3 to 0.8 consistently showed an orderly large oscillatory flow structure. It is believed that these large scale disturbances are the result of flow instabilities. It is also believed that these orderly disturbances are responsible for generating the dominant part of trailing edge noise either directly or indirectly. The interaction of sound generated by these coherent oscillatory flow disturbances and the plate was investigated theoretically. It is found that the farfield noise directivity is strongly influenced by diffraction of sound at the leading edge of the plate.

Tam, C. K. W.↗
NASA NTRS1972

Effects of interplanetary magnetic field on the propagation of flare-generated interplanetary shock waves.

The effects of an interplanetary magnetic field on the propagation of flare-generated interplanetary shock waves are investigated with an approximate analytical method. It is found that the interplanetary magnetic field is relatively unimportant for strong shocks as far as the shock speed and transit time are concerned. It has more significant effects for weak shocks. However, in all the situations examined, the error committed if the magnetic field is neglected is no more than 10%. It is suggested that a model without a magnetic field gives sufficiently accurate numerical results for the propagation of flare-generated shocks.

Tam, C. K. W.↗