Wake generator control of flow distortion induced tonal noise in fans

Kota, Viswanath (2005) Wake generator control of flow distortion induced tonal noise in fans. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

One of the means of generation of tonal noise in aircraft engines is the interaction of fan blades with flow disturbances. A novel way to control this noise is to generate further disturbances which interact with the fan to produce anti-noise. Flow disturbances can be created using an array of cylindrical rods protruding into the duct. The variation of the lengths and orientation of these rods results in the production of wakes of varying extent which then interact with the fan blades to produce unsteady blade forces and these radiate noise.

Adaptive control of this form of noise is possible by formulating the acoustic response of the rod obtained through the theoretical relationship between the rod length and the magnitudes of the unsteady blade forced and in turn the magnitudes of the acoustic modes in the duct. Since the control of a single mode could cause the amplitude of the other modes to increase, optimal control of a set of modes is possible by formulating the problem of minimisation of the cost function which is the in-duct sound power level comprising the squares of the modal amplitudes. Convergence of the optimal rod lengths is obtained through the use of the steepest descent algorithm.

Experimental investigations confirm that it is possible to implement the adaptive control of fan tonal noise using wake generators. The concept has been implemented on a test fan rig and an 8-rod controller array using the pressure signals acquired from duct microphones.

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