Broadband and discrete frequency radiation from subsonic rotors

https://acoustique.ec-lyon.fr/publi/posson_jsv11.pdf

Broadband noise prediction of fan outlet guide vane using a cascade response function

This paper describes the prediction of broadband self noise from ducted fans. The source mechanism is assumed to be the interaction of the turbulent boundary layer with the trailing edges of the fan blades. The source levels are obtained from the measurements of self noise from isolated blades by Brooks, Pope and Marcolini and these are corrected to give the in duct sound power from a high solidity fan. It has been found that the blade surface pressures are not uncorrelated on each fan blade and corrections must be included for the scattering from multiple trailing edges. A method is introduced for coupling the modes in a circular duct to the modes of a linear cascade so that the sound power is not singular at the mode cut on frequencies. Results show that the in duct sound power scales with the fifth power of the fan speed at low Mach numbers, but this changes to the sixth power or greater at high Mach numbers. The angle of attack of the blade increases the self noise as 2.4 dB per degree and there are significant increases in low frequency noise when blade stall occurs.

Broadband Self-Noise from a Ducted Fan

The prediction of broadband noise from wind turbines

It is known experimentally that an aerofoil immersed in a uniform stream at a moderate Reynolds number emits tones. However, there have been major differences in the experimental observations in the past. Some experiments reported the observation of multiple tones, with strong evidence that these tones are most probably generated by a feedback loop. There is also an experiment reporting the observation of a single tone with no tonal jump or other features associated with feedback. In spite of the obvious differences in the experimental observations published in the literature, it is noted that all the dominant tone frequencies measured in all the investigations are in agreement with an empirically derived Paterson formula. The objective of the present study is to perform a direct numerical simulation (DNS) of the flow and acoustic phenomenon to investigate the tone generation mechanism. When comparing with experimental studies, numerical simulations appear to have two important advantages. The first is that there is no background wind tunnel noise in numerical simulation. This avoids the signal-to-noise ratio problem inherent in wind tunnel experiments. In other words, it is possible to study tones emitted by a truly isolated aerofoil computationally. The second advantage is that DNS produces a full set of space–time data, which can be very useful in determining the tone generation processes. The present effort concentrates on the tones emitted by three NACA0012 aerofoils with a slightly rounded trailing edge but with different trailing edge thickness at zero degree angle of attack. At zero degree angle of attack, in the Reynolds number range of to , the boundary layer flow is attached nearly all the way to the trailing edge of the aerofoil. Unlike an aerofoil at an angle of attack, there is no separation bubble, no open flow separation. All the flow separation features tend to increase the complexity of the tone generation processes. The present goal is limited to finding the basic tone generation mechanism in the simplest flow configuration. Our DNS results show that, for the flow configuration under study, the aerofoil emits only a single tone. This is true for all three aerofoils over the entire Reynolds number range of the present study. In the literature, it is known that Kelvin–Helmholtz instabilities of free shear layers generally have a much higher spatial growth rate than that of the Tollmien–Schlichting boundary layer instabilities. A near-wake non-parallel flow instability analysis is performed. It is found that the tone frequencies are the same as the most amplified Kelvin–Helmholtz instability at the location where the wake has a minimum half-width. This suggests that near-wake instability is the energy source of aerofoil tones. However, flow instabilities at low subsonic Mach numbers generally do not cause strong tones. An investigation of how near-wake instability generates tones is carried out using the space–time data provided by numerical simulations. Our observations indicate that the dominant tone generation process is the interaction of the oscillatory motion of the near wake, driven by flow instability, with the trailing edge of the aerofoil. Secondary mechanisms involving unsteady near-wake motion and the formation of discrete vortices in regions further downstream are also observed.

Aerofoil tones at moderate Reynolds number

Experiments and theory relating to fan noise sources are reviewed with emphasis on axial flow machines At supersonic rotor speeds, the steady shock pattern attached to a rotor is an efficient radiator of sound In most practical cases of subsonic rotor operation, however, direct radiation from the rotor‐locked pressure field is negligible compared with the indirect radiation, or scattering, caused by circumferential distortions in the steady flow field surrounding the rotor Random timewise modulation of the distortion changes the scattered spectrum from discrete to continuous, with a gradual progression from narrow‐band tones to broad‐band noise as the modulation bandwidth is increased Similar scattering occurs when a non‐uniform unsteady flow impinges on stator vanes, but here the radiated frequency is that of the impinging flow Finally, for blades operating in flows free from circumferential distortions, self‐generated turbulence becomes an important source of noise The paper describes the physical mechanisms involved in each of these processes, including the generating of unsteady lift by turbulence Order‐of‐magnitude sound power estimates are compared, where possible, with experiment

C. L. Morfey

Papers

Sources of Noise in Axial Flow Fans