Duct mode evaluation for traction motor outlets

Abstract: This study investigates duct mode propagation at outlets of traction motors, featuring an open fan air-cooled architecture. Both acoustical measurements and simulations are employed. Measurements are performed for two duct outlet geometries, a radial and a non-radial outlet, both combined with an equidistant and a non-equidistant blade fan. Specifically, six microphones are flush-mounted circumferentially at the entrance of each duct outlet (downstream of fan blade). Additionally, five microphones are placed circumferentially above the exit of each duct outlet. Measurements from both positions are utilized for circumferential mode reconstruction (CMR) of the acoustic near field. The radiated acoustic power of detected modes, from CMR analysis, is used for performance evaluation of different fan-duct outlet combinations. CMR analysis from measurements at the entrance of duct outlets, demonstrate that the non-radial duct outlet combined with an equidistant blade fan outperforms the rest combinations. The CMR analysis from measurements above the exit of duct outlets, reflects the effect of fan rotational speed. At low fan rotational speeds, the combination of the radial duct with equidistant blade fan is favorable. At high fan speeds, both outlets perform similarly with either duct outlet. The CMR analysis performed for all fan-duct outlet combinations employed, does not indicate significant acoustic coupling phenomena. Furthermore, acoustic simulation is implemented with the finite element software for acoustics, ACTRAN. The employed methodology is developed by MSC Software Corporation. Circumferential modes of different order, are injected at the entrance of the duct outlet geometry. The far field acoustic radiation, for each injected mode is calculated. A set of fifty variants of the baseline duct outlet is evaluated, with respect to major geometrical characteristics. The results accrued through finite element analysis confirm the validity of the employed methodology, on a qualitative basis. Overall, this work proposes experimental and computational methods, along with recommended practices for the evaluation of acoustic coupling in fan-duct outlet configurations.