Anthony Gérard

Tonal Noise Control of Centrifugal Fan Using Flow Obstructions - Experimental and Numerical Approaches

Placing a flow obstruction at the inlet of a fan is a cheap, yet efficient way to reduce the tonal noise. However, the optimization of this method is not straightforward since the aeroacoustic mechanisms responsible for the noise reduction have not yet been fully understood. As an attempt to highlight these mechanisms, an experimental and numerical study of the interaction between an obstruction and a centrifugal fan is presented. Four different types of obstructions are tested experimentally and their effect on the aerodynamic performance and the acoustic signature is characterized. A direct aeroacoustic simulation is then performed with a sinusoidal obstruction. The tonal noise reduction at the blade passing frequency is well captured, but the optimal angular position show some discrepancies with the experimental results.

Optimization problem for the automatic positioning of flow obstructions to control tonal fan noise

Tonal noise from subsonic axial fans can be controlled by adding obstructions in the downstream or upstream flow field of the rotor. These obstructions must be located with care to create secondary circumferential modes of the blade unsteady lift of equal magnitude but opposite in phase with the primary unsteady lift modes responsible for the tonal noise. The general optimization problem of controlling N circumferential modes using N obstructions with 2 degrees of freedom or 2N obstructions with 1 degree of freedom is first posed. This optimization problem can be greatly simplified using nB‐perdiodic obstructions (where B is the number of blade) designed to control the most radiating mode of the blade unsteady lift at frequencies n×BPF. In this case, the control of each frequency is uncoupled and a single error microphone in the axis of the fan can be used to globally control the selected frequencies.

An Inverse Aeroacoustic Model for Fans

An inverse aeroacoustic model for subsonic axial flow fans is proposed. The model aims at determining the circumferential blade loading variations (responsible for tonal sound radiation) from far-field acoustical measurements. The direct model is based on the Helmholtz integral equation, as formulated by Morse & Ingard. The inversion of the direct problem is mathematically ill-posed, and this problem is overcome by the Thikonov regularisation technique to improve the accuracy of the source strength reconstruction. A sensitivity analysis is first presented to show the influence of the regularisation parameter choice, signal to noise ratio and frequency. In this paper, simulations show that the loading harmonics can be partially reconstructed. Experimental results also demonstrate the ability of the inverse model to locate hot spots of rotor/stator interaction. Nomenclature a1 = Inner rotor radius (m) a2 = Outer rotor radius (m) B = Number of blades c = Speed of sound (m.s -1 ) e = Error vector (N.m -2 ) f = Axial force component acting on the rotor per surface unit (N.m -2 )

Industrial fan noise control using flow obstructions

Fans are used in a lot of industrial processes and are sometimes a source of important noise for workers. We have focused on fans having a high tonal content, for which the Simple Silence technology can be applied, i.e. tonal fan noise control using obstructions in the flow. Evaporator fan noise control in a cold storage room, based on previous work on fan noise control in free field is first presented. Then, in-duct gold mine underground air-extractor fan noise control is presented, an original analytical model is derived for noise generation and propagation inside the duct, and preliminary experimental results are provided.

Active control of automotive fan noise

Active control for globally reducing the noise radiated by automotive axial engine cooling fans is investigated. First, an aeroacoutic model of the fan is combined with acoustic directivity measurements to derive a distribution of equivalent dipole sources on the fan surface. The results reveal that the fan behaves like a distributed dipole at blade passage tones when the upstream flow through the fan is spatially nonuniform. Numerical simulations of active noise control in the free field have been carried out using the previous aeroacoustic model of the fan and a dipole secondary source in front of the fan. The numerical results show that a single dipole control source is effective in globally controlling the sound radiation of the fan at the blade passage frequency and its first harmonic. Last, an experimental investigation of active control is presented. It consists of a SISO feedforward configuration with either a LMS algorithm (for FIR filters) or a back‐retropopagation algorithm (for neural networks...

A compact active control system for tonal noise reduction in axial fans

The objective of this paper is to show the feasibility of active control of tonal noise from automotive engine cooling fans using a single unbaffled loudspeaker in front of the fan. A simplified model is described for preliminary design using this configuration. A single input single output (SISO) filtered X-LMS has been implemented to control the first tones in real time. At the error microphone, the sound pressure attenuation at blade passage frequency (BPF) and its first harmonics are reduced down to the broadband noise level. At low frequency, for large sound wavelength compared to the fan radius, the sound radiation is reduced in all directions, since the dipolar directivity of the secondary noise emitted by the unbaffled loudspeaker is similar to the directivity of the primary noise emitted by the fan.

Unsteady lift and flow velocity evaluation for subsonic fans using inverse aeroacoustic models

This paper deals with the inversion of the Blake model for tonal noise from subsonic fans. The unsteady lift formulation is first used to reconstruct the circumferential blade loading variations from the tonal noise radiation in free field. Then the unsteady lift is related to the inflow velocity distortions by a compressible Sears-like functions. Discretizing the lift and velocity direct models leads to ill-conditionned aeroacoustic transfer matrices. The Tikhonov regularization technique is used to stabilize the inversion. The L-curve and its curvature are plotted to choose the regularization parameter such that the source strength vectors are optimally reconstructed. One experimental case is considered to demonstrate the capability of the inverse model to reconstruct the variation of loading and inflow velocity during the rotation of an an automotive engine cooling fan.