Showing papers by "Christopher K. W. Tam published in 1992"

Instability of rectangular jets

Abstract: The instability of rectangular jets is investigated using a vortex sheet model. It is shown that such jets support four linearly independent families of instability waves. Within each family there are infinitely many modes. A way to classify these modes according to the characteristics of their mode shapes or eigenfunctions is proposed. A parametric study of the instability wave characteristics has been carried out. A sample of the numerical results is reported here. It is found that the first and third modes of each instability wave family are corner modes. The pressure fluctuations associated with these instability waves are localized near the corners of the jet. The second mode, however, is a center mode with maximum fluctuations concentrated in the central portion of the jet flow. The center mode has the largest spatial growth rate. It is anticipated that as the instability waves propagate downstream the center mode would emerge as the dominant instability of the jet.

Impingement tones of large aspect ratio supersonic rectangular jets

Abstract: The frequency structure of large aspect ratio jet impingement tones is analyzed Feedback loops consisting of downstream propagating instability waves of the jet flow and upstream propagating acoustic waves from the wall to the nozzle lip produce as many as 20 or more tones when a supersonic rectangular jet of large aspect ratio is directed normal on a wall It is seen that there are only two basic tones The basic tone of the lower frequency is associated with a symmetric instability feedback mode of the jet, while that of the higher frequency is associated with an antisymmetric instability feedback mode The remaining tones are combination tones of the two basic frequencies and their harmonics It is determined that the measured basic impingement tone frequencies fall in the theoretical permissible frequency bands over a wide range of jet Mach number

Broadband shock associated noise from supersonic jets measured by a ground observer

Abstract: The theory of broadband shock associated noise from supersonic jets in flight previously developed in the nozzle-fixed coordinates is extended to the coordinate system of a stationary ground observer.

Broadband shock associated noise from supersonic jets measured by a ground observer

Abstract: The theory of broadband shock associated noise from supersonic jets in flight previously developed in the nozzle-fixed coordinates is extended to the coordinate system of a stationary ground observer. The extended theory is relevant to community noise and aircraft certification noise prediction. Since the noise source is unaffected by which coordinate system of reference the radiated noise is measured, the same noise source model is retained in the present formulation. Only kinematic arguments are needed in the analysis. A formula for the noise power spectrum as measured by a ground observer is derived. The formula is applicable to hot as well as cold jets. This formula exhibits a form of Doppler shift in the noise spectrum. However, there is no high power convective amplification factor, as some other investigators have proposed. I. Introduction B ROADBAND shock associated noise is an important component of supersonic jet noise. During the last several years a theory of this noise component has been developed by the present author.1'2 In these works the jets were assumed to be issued into a static environment. The theory is based on the large turbulence structures/instability waves shock cell interaction noise generation mechanism. The mean flow of a jet is highly unstable. It sustains downstream propagation large turbulence structures/instability waves. As these large turbulence structures/instability waves propagate through the quasiperiodic shock cell structure of an imperfectly expanded jet, coherent interactions take place. These interactions create unsteady disturbances. A part of these disturbances has supersonic phase velocity relative to the ambient speed of sound. This part is the source of broadband shock associated noise. Extensive comparisons between the calculated noise spectra, directivities, and near-field sound pressure level contours and the measurements of Norum and Seiner3 and Yu4 were carried out. Favorable overall agreements were found. In a more recent paper the broadband shock associated noise theory was extended to include the effect of forward flight.5 The extended theory was developed in the nozzle-fixed reference frame. Changes in the noise sources due to the external flow as well as the convection effect of the external flow on the propagation of the radiated noise are accounted for in the theory. In the presence of a uniform mean flow outside the jet, the spacing of the shock cells is lengthened. The speed of propagation of the instability waves is increased, whereas the spatial growth and decay rates of the instability waves are reduced. These changes were incorporated into the extended theory. Since flight data were not available, the open wind-tunnel simulation noise measurements of Norum and Shearin6'7 were used for comparison and validation. In the simulation experiments the radiated sound has to traverse through the outer shear layer of the open wind tunnel before reaching the measurement microphones. In transmitting through the shear layer the noise spectrum and directivity are inevitably modified. In Ref. 5 the effects of the outer shear layers were approximated by those of a vortex sheet model. In this way the theory is self-contained without the necessity of applying shear layer corrections developed by various investi

Acoustic interactions between an altitude test facility and jet engine plumes: Theory and experiments

Abstract: The overall objective of the described effort was to develop an understanding of the physical mechanisms involved in the flow/acoustic interactions experienced in full-scale altitude engine test facilities. This is done by conducting subscale experiments and through development of a theoretical model. Model cold jet experiments with an axisymmetric convergent nozzle are performed in a test setup that stimulates a supersonic jet exhausting into a cylindrical diffuser. The measured data consist of detailed flow visualization data and acoustic spectra for a free and a ducted plume. It is shown that duct resonance is most likely responsible by theoretical calculations. Theoretical calculations also indicate that the higher discrete tones observed in the measurements are related to the screech phenomena. Limited experiments on the sensitivity of a free 2-D, C-D nozzle to externally imposed sound are also presented. It is shown that a 2-D, C-D nozzle with a cutback is less excitable than a 2-D C-D nozzle with no cutback. At a pressure ratio of 1.5 unsteady separation from the diverging walls of the nozzle is noticed. This separation switches from one wall to the opposite wall thus providing an unsteady deflection of the plume. It is shown that this phenomenon is related to the venting provided by the cutback section.

Discretization errors inherent in finite difference solution of propeller noise problems

Abstract: Global errors arising from the use of a finite difference approximation to solve propeller noise problems are investigated. It is found that the calculated waveforms of the finite difference solutions are subjected to severe distortion due to dispersive effects. This is so even when the Fourier amplitudes of the different blade-passage harmonics of the acoustic disturbance of the propeller are adequately predicted. In addition, it is found that finite difference solutions can produce spurious acoustic radiation. The spurious radiation is a form of aliasing error. The relative intensity of the spurious acoustic radiation can become significant at high subsonic flight Mach number and at high subsonic blade-tip rotational Mach number. Also, the relative intensity is higher for the higher order blade-passage harmonics.

Theoretical aspects of supersonic jet noise

Abstract: The topics covered include the following: the three components of supersonic jet noise; shock cell structure of imperfectly expanded jets; large turbulence structures/instability waves; supersonic jet noise theory; generation of turbulent mixing noise; comparisons between predicted peak noise frequency and direction of radiation with measurements; Strouhal number of maximum SPL of hot supersonic jets; near field sound pressure level contours; generation of broadband shock associated noise; calculated and measured far field shock noise spectra; generation of screech tones; and calculated and measured Strouhal number of screech tones.