The Influence of Rotor Detuning on Rotor-Stator Interaction Generated Acoustic Response

Abstract

In a turbomachine, the stator row is excited by the upstream rotor and responds at multiples of rotor blade pass frequency, resulting in a discrete blade pass frequency tone. Rotor detuning, i.e., unequal circumferential rotor blade spacing, decreases this fundamental tone by breaking the fundamental periodicity of blade passing. However, the stator row responds at additional multiples of rotor shaft pass frequency. This paper is directed at investigating the influence of rotor detuning on discrete frequency acoustic response. Complete data were acquired including acoustic mode and stator vane unsteady surface pressure data. Microphones in a circumferential array upstream of the rotor and stator are sampled simultaneously over one rotor revolution, and an ensemble averaged data set is acquired. A dual transform is then utilized to determine the acoustic pressure amplitude as a function of frequency and spatial mode. The stator vane acoustic response is also calculated from unsteady stator vane surface pressure measurements, and the spectrum of the acoustic response is compared with data from the circumferential microphone array. Rotor detuning is shown to have a significant effect on the discrete frequency noise. Although the stator when forced by the detuned rotor responded at more frequencies, there is an overall reduction in the total intensity level for a range of rotor speeds. Nomenclature Symbol Description ax freestream speed of sound c stator chord I acoustic intensity EL intensity level k reduced frequency k = coc/W k axial wave number kg tangential wave number Ic ... criticalNyquistmode 1 ^ an eigenvalue M axial mean flow Mach number n rotor harmonic N number of microphones number of rotor blades number of stator vanes p acoustic pressure P spatial transform of acoustic pressure s stator pitch spacing SPL sound pressure level r radial coordinate Uoo mean axial velocity Ujj axial acoustic velocity v stator index, v = 0,1,..., NVmes W Freestream velocity in two dimensions x axial direction z blade chord coordinate a mean stator angle-of-attack ctyp Two dimensional axial wave number P Two dimensional tangential wave number s level of detuning Tw unsteady bound vorticity complex magnitude 6 circumferential coordinate r^jj tuned rotor tangential spacing freestream density freestream density frequency co = nNBIadesQ rotor circular frequency pressure pattern phase speed axial coordinate A6,