Phased array microphones and beamforming technology are applied herein in order to better understand the noise sources associated with the installation effects of a counter-rotating open rotor propulsion system. The study compares an installed (equipped with a pylon) and an uninstalled (standalone) engine for takeoff nominal flight conditions, using beamforming results to investigate the far-field acoustic character and to localize noise sources, sorting them into various components accordingly. The study demonstrates how the various noise sources falling in the blade passing frequency bins can be separated into those components which are truly associated with the blade passing frequency and those which are associated with blade-wake interactions. This is done by taking into consideration the nature of the noise sources, and distinguishing between rotating coherent and stationary coherent noise sources. The paper also demonstrates that the impact of the pylon wake on the rotating coherent noise sources (such as blade passing frequency tones and interaction tones) and on the rotating incoherent noise sources (such as rotating broadband noise sources and shaft order noise sources) is small. Though the paper presents the results of a specific comparison, the methodology for separating apart the various components noise presented herein is generally applicable in other investigations.
Counter-Rotating Open Rotors (CROR) are known to have advantageous propulsive efficiency properties, but at the same time many noise emission issues related to this technology still need to be investigated. The noise they generate consists of tonal and broadband components, of which the tonal components often appear in narrow frequency bands with large amplitudes. During beamforming investigations this often makes sorting them out from among other noise sources in the same frequency bin rather straight forward.On the other hand, the broadband noise sources can be characterized as having small amplitudes, and in general they are not the dominant noise sources in many frequency bins, but overall a significant noise component that needs to be investigated. The literature has provided a single microphone signal pre-processing method for removing the tonal components and hence isolating the broadband noise of a CROR, the signal of which has been shown to be appropriate for the investigation of broadband spectra. In this article, this preprocessing method is further developed into a beamforming method which can be used in order to localize the broadband noise sources of CROR. The resulting beamforming maps have been compared to the results of earlier broadband noise CROR studies, showing the
Fans are widely used for industrial as well as household applications. As their numbers increase, so does their effect on our comfort and health via noise pollution. In most instances, these fans are built into a ducted environment, and therefore acoustic investigations should also be carried out in a ducted environment in order to guarantee proper flow conditions. On the other hand, localizing noise sources in a ducted environment is a difficult task. A useful method for localizing noise sources is the beamforming of phased array microphone data, but building an array of microphones into a duct runs with many complications. This has provided motivation for the design of an acoustically transparent duct, which provides a means for using a microphone array located outside of the duct system for the investigation of fan noise sources within the duct. The acoustically transparent duct section provides an acoustic window for the microphone array while guaranteeing that the duct is hydrodynamically impenetrable. This investigation presents the design process as well as a case study of a low pressure fan in order to demonstrate the effectiveness of the design.
Pylons are commonly used for the mounting of engines in the aircraft industry. On the other hand, the installation of a pylon influences the noise generation mechanisms and therefore alters the broadband noise characteristics of a given turbomachinery setup. In this investigation, a counter-rotating open rotor with and without a pylon is investigated in order to determine its effects on broadband noise sources. The various broadband noise sources and their typical frequency ranges have been determined using beamforming maps and spectral analysis. In order to attain a clear impression regarding the broadband noise sources, the Double Filtering beamforming method has been utilized in the investigation. This method removes the tonal components from the recorded signal of a microphone array, resulting in a purely broadband signal. Using beamforming maps, the dominant broadband noise source amplitudes and locations can therefore be investigated in great detail. Compared to other methods, the investigation of measurement data and beamforming maps helps determine the amplitude, the frequency range, and the significance of the various types of broadband noise sources that are truly present in the emitted noise. It has been found for lower frequencies, that the broadband noise sources at the blade root of the aft rotor are dominant, while for higher frequencies, the significant broadband noise sources are localized to the trailing edge region of the forward rotor and the leading edge of the aft rotor. The installation of a pylon has resulted in an additional broadband noise source appearing at the blade tip of the aft rotor.
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