Active control of axial-flow fan noise

Abstract

Discrete-frequency axial-flow fan noise reduction using active noise control is described. The unique aspect of the current research is the use of the fan itself as the antinoise source in the active noise control scheme. This is achieved by driving the entire fan unit axially with an electrodynamic shaker which mechanically couples the solid surfaces of the fan to the acoustic medium. The fan unit is thus transformed into a crude loudspeaker. A near-field microphone serves as an error sensor, where transfer function measurements between the electrical input to the shaker and the electrical output of the microphone are found to be reasonably free of phase distortions and linear. A feedforward algorithm utilizing the output of a tachometer as a reference signal is used. The experimental apparatus is composed of a baffled fan unit in a free field. A small cylindrical flow obstruction is placed on the inlet side of the fan to enhance noise emissions at the blade-pass frequency and harmonics. The experiment successfully demonstrates the concept of active control of tonal fan noise using a shaken fan as the cancellation source. For the fan operating in a planar baffle, the fundamental blade-passage frequency sound-pressure level at the location of the error sensor is reduced by 20 dB, while the second and third harmonic levels are reduced by 15 and 8 dB, respectively. Placing a cabinet enclosure over the baffled fan did not affect these results significantly, and free-field sound power measurements indicate similar level reductions with the active control in operation.

Noise control by a rotating rod in a rod-airfoil configuration

“…The grid is discretized by 250 points around the cylinder and 398 points around the airfoil. The first cell of the mesh is located at a distance of 4 10 c  and 5 10 c  normal to the rod and airfoil walls, respectively. The achieved wall y  is lower than 1, verifying that the grid is adequately fine for the computations.…”

“…Several examples may be encountered in the literature, such as introducing boundary layer excitation 1 , taking advantage of cavity resonance effects 2 and distorting the flow [3][4][5][6] in order to optimize the noise characteristics of aircraft engines, high-lift devices and wings. A potential noise reduction technique which, to our knowledge, has not been explored in the literature is the rotating cylinder as part of an airframe configuration.…”

Reactive control of subsonic axial fan noise in a duct

“…It is used along the propagating path by virtue of the array of actuators to produce the cancelling sound field. These array actuators are composed of an array of microphones (Koopmann et al, 1988;Gerhold, 1997), resonant type drivers (Walker, 1999), or conventional electromagnetic drivers (Lauchle et al, 1997). However, the issues of cost, sophistication of the design, and reliability have so far prevented this technology from finding widespread use in engineering.…”

Active control of drag noise from a small axial flow fan

“…Active fan noise control is further divided into active minimization of the source strength by interfering with the aerodynamics ͑e.g., Neuhaus et al, 2003;Rao et al, 2001;Simonich et al, 1993͒, and the cancellation of the radiated sound by secondary sources ͑e.g., Gerhold, 1997;Thomas et al, 1993Thomas et al, , 1994Quinlan, 1992;Lauchle et al, 1997;Gee and Sommerfeldt, 2004͒. One common feature in most reported works is that the rotational plane of the fan is placed in a baffle in order to simplify the acoustic field before the global control is contemplated.…”