The investigation of aeroacoustic characteristics of flow through a shielded orifice plate
Aeroacoustics is a branch in acoustics science which deals with the noise generation by air flows, and different methods to minimize the noise during the design of aerodynamic systems. The industries which are associated with fluid flow are the major source of aerodynamic noise. The flow of air through the ducts, pipes, and different types of free jets creates a huge amount of aerodynamics noise depending on the associated gradient in pressure. In this project, we examine the aeroacoustics characteristics of flow through the plain orifice plate for different nozzle pressure ratios varying from 1.5 to 6. Next, the effect of tabs and different confinement length are studied and compared with the plain orifice plate. The acoustic pressure fluctuations originating from the orifice plate is measured using free field microphones. Throughout the testing, the data are collected with the help of two free-field microphones placed at a distance of 1 m from the nozzle exit at 60° and 110° respectively. The analog physical signal is converted into digital data and stored in a computer using NI based DAQ system and software. The acquired voltage data is converted to pressure readings with the help of the sensitivity data or calibration coefficients. Here, sound pressure level (SPL) and fast Fourier transform (FFT) obtained from fluctuating acoustic pressure are used for characterizing the jet noise. First, uncertainty analysis is performed by conducting many experiments with a plane orifice plate for a given nozzle pressure ratio (NPR). Next, the noise radiated from the plain orifice plate is examined for different NPR. Finally, the sound pressure level (SPL) is obtained for the same orifice plate with tabs and two different confinement lengths namely 40 mm and 80 mm is obtained. A blockage percentage of about 4 % is used for the tabbed nozzle. The directivity of the emitted noise, dominant frequencies, SPL from these experiments is used for characterizing and comparing noise field and the associated jet flow.
Keywords: aeroacoustics, free jet, noise reduction, fast fourier transform, orifice plate, acoustic measurement