A mixed experimental and theoretical method for noise propagation and radiation prediction of a cowled axial fan

Abstract

A method for predicting acoustic pressure radiated by a duct opening with internal axial flow, based upon mixed experimental and numerical tools, is presented. It is applied to a six-bladed/one stator configuration axial fan cowled by a cylindrical duct 2.3 m long, 0.6 m in diameter and carrying a low-speed axial flow ͑M 0 р0.1͒. Experimental mappings of acoustic pressure, acoustic axial particle velocity, and axial mean flow velocity at blade passing frequencies upstream of the rotor are carried out using a probe made of a microphone and a single hot wire. The results show an acoustic field structure as expected by acoustic propagation laws in ducts. After a modal filtering, experimental radial acoustic potential profiles are input to the database for the prediction process. In the second step, the theoretical radiated far field is computed using a program based on a variational formulation by integral equations accounting for internal flow. The very good agreement between theoretical and experimental results demonstrates the validity of this method.

Experimental setup for measurement of acoustic power dissipation in lined ducts for higher order modes propagation with air mean-flow conditions

“…3͒ consists of two transducers 0.01 m spaced on the radial boom: a 0.003 17-m diameter microphone with its nose cone and a single hot film. This probe, developed and used during previous studies, 11 provides local measurements of acoustic pressure, axial acoustic particle velocity, and axial mean flow velocity. Calibration of the hot film is performed in two steps by an original experimental procedure 12 consisting of aerodynamic calibration by a classical experiment against a pitot tube in a small jet and acoustic calibration.…”

“…As acoustic pressure is not measured at the same radial point as the acoustic axial particle velocity; a correction of the acoustic particle velocity based on a third-order interpolation process is applied. 11 Also, as the microphone cannot move as close as 0.013 m to the duct wall, an extrapolation procedure is applied to deduce measurement near the duct wall. 11 This data collection procedure is performed for the 240 positions ͑16 angularϫ15 radial͒ in both upstream and downstream measurement cross sections.…”

“…It was used to measure, in front of an axial cowled fan, the acoustic potential per mode radiated on blade passing frequencies in presence of a mean flow. 11 Computation of the radiated field was performed with input data supplied by the experiment, a good agreement was found between experimental and predicted pressure radiated fields. In the present paper, a new method of measuring the axial component of the acoustic sound power is introduced.…”

“…Because the two microphones' sound intensity probe can measure sound intensity only when mean flow and sound propagation are colinear, 8 which is invalid for higher order mode propagation conditions and also because intensity measurement is time consuming, acoustic sound power is deduced from a local pressure measurement at a radial position and corrections are applied to take into account the effects of flow and higher order modes pressure distribution. 9,10 In previous works a pressure-velocity probe made of a microphone and a hot film was studied 11 including an acoustic calibration 12 and modal decomposition 13 procedures. It was used to measure, in front of an axial cowled fan, the acoustic potential per mode radiated on blade passing frequencies in presence of a mean flow.…”

Two-Frequency Continuous Wave Ultrasonic System for Temperature Measurement in an Infant Incubator

“…The ultrasonic velocity is also sensitive to the other fluctuations (such as the wind, the humidity or the pressure of the air) [21][22][23][24][25][26][27][28][29][30]. These changes are also measured by the ultrasonic system when the tempera ture is known.…”