Ramani Mani

A moving source problem relevant to jet noise

Abstract The total power and power spectrum due to a point source convecting at a uniform subsonic velocity along the axis of a round jet are calculated. The source is a discrete frequency source in its own frame of reference and the jet is assumed to have a slug flow velocity profile with velocity equal to that of the source. The model problem is intended to provide some guidance concerning the issue of “convective amplification” in jet noise and the solution confirms that the convective amplification is very much frequency dependent. Some implications with regard to Strouhal scaling are pointed out.

The radiation of sound from a propeller at angle of attack

The mechanism by which the noise generated at the blade passing frequency by a propeller is altered when the propeller axis is at an angle of attack to the freestream is examined herein. The measured noise field is distinctly non-axially symmetric under such conditions with far field sound pressure levels both diminished and increased relative to the axially symmetric values produced with the propeller at zero angle of attack. Attempts have been made to explain this non-axially symmetric sound field based on the unsteady (‘once per revolution’) loading experienced by the propeller blades when the propeller axis is at non-zero angle of attack. A calculation based on this notion appears to greatly underestimate the measured azimuthal asymmetry of noise for high-tip-speed, highly loaded propellers. A new mechanism is proposed herein; namely, that at angle of attack, there is a non-axially symmetric modulation of the radiative efficiency of the steady loading and thickness noise, which is the primary cause of the non-axially symmetric sound field at angle of attack for high-tip-speed, heavily loaded propellers with a large number of blades. A calculation of this effect to first order in the cross-flow Mach number (component of freestream Mach number normal to the propeller axis) is carried out and shows much better agreement with measured noise data on the angle of attack effect.

Further studies on moving source solutions relevant to jet noise

Abstract This study expands further on an earlier study reported in this journal wherein the power spectrum and total power of a moving point source in a round, slug flow jet were calculated. In the present study three further aspects are reported on. Firstly the effect of non-axial lines of source convection is explored with the aid of a plane jet, line source model problem. Secondly, for centerline source convection in a round jet, the effect of a small (non-zero) shear layer thickness is studied. This procedure is first illustrated by an application to the classical problem of reflection of plane sound waves from a velocity discontinuity. Finally inferences regarding the peak angle in the radiation pattern are drawn from the study and shown to be in rough agreement with experimental data.

Investigation of Fan-Wake/Outlet-Guide-Vane Interaction Broadband Noise

Fan-wake/outlet-guide-vane interaction broadband noise in turbofan jet engines is studied. The mechanism and some issues are first discussed using a two-dimensional gust-prediction model. An oblique gust-prediction model is then developed. Quasi-three-dimensional unsteady lift is calculated using a two-dimensional equivalence method. It is coupled with annular duct modes to obtain the sound power spectrum density. Spanwise turbulence integral length scales and their impact on power spectrum density predictions are investigated. A spanwise integration limit suitable for the complete frequency range is proposed. The model is validated using the NASA Source Diagnostic Test data. Sound power scaling with vane count B is examined. If solidity is maintained, the cascade response does not converge on the single-airfoil response, even for low vane counts. The sound power varies inversely with B at low frequency; it scales with B at very high frequency. The power spectrum density trend with the fan tip Mach number...

Investigation of a Parabolic Iterative Solver for Three-Dimensional Configurations

Aparabolic iterative solution procedure is investigated that seeks to extend the parabolic approximation used within the internal propagation module of the duct noise propagation and radiation code CDUCTLaRC. The governing convected Helmholtz equation is split into a set of coupled equations governing propagation in the ‘positive’ and ‘negative’ directions. The proposed method utilizes an iterative procedure to solve the coupled equations in an attempt to account for possible reflections from internal bifurcations, impedance discontinuities, and duct terminations. A geometry consistent with the NASA Langley Curved Duct Test Rig is considered and the effects of acoustic treatment and non-anechoic termination are included. Two numerical implementations are studied and preliminary results indicate that improved accuracy in predicted amplitude and phase can be obtained for modes at a cut-off ratio of 1.7. Further predictions for modes at a cut-off ratio of 1.1 show improvement in predicted phase at the expense of increased amplitude error. Possible methods of improvement are suggested based on analytic and numerical analysis. It is hoped that coupling the parabolic iterative approach with less efficient, high fidelity finite element approaches will ultimately provide the capability to perform efficient, higher fidelity acoustic calculations within complex 3-D geometries for impedance eduction and noise propagation and radiation predictions.

Fine-Scale Turbulent Noise Predictions from Non-Axisymmetric Jets

In this work a model for fine-scale turbulent noise predictions from non-axisymmetric, heated and unheated jets is developed. This model, called GENO, improves upon the previous noise prediction methods JeNo and MGBK by including the effects of mean flow refraction from non-circular geometry, as well as a more complete model for the self, shear, and enthalpy noise sources. The Greens function for non-axisymmetric flows is determined by solving the Lilley-Goldstein equation in terms of coupled azimuthal modes. Additionally, the noise source model evaluates the fourth order velocity correlation tensor I ijkl for the general case and the relationship between non-zero components is found. This is combined with an empirical noise source model for heated jets. Acoustic results using GENO are shown for aspect ratio 2:1 and 4:1 rectangular jets, under unheated (SP7) and heated (SP46) conditions, and the comparisons agree favorably with experimental data.