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978-3-8439-2423-8, Reihe Ingenieurwissenschaften
Aeroacoustic Investigation of Controlled Subsonic Single Jets
257 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2014), Softcover, A5
The free jet examined in this work is a model scale example of an experimental cold single subsonic jet. The major fluid dynamical and acoustical characteristics of free jets are compiled first to understand the sensitivities of the jet concerning the evolution of its flow field and the resulting noise emissions. Based on these insights one active and one passive method is proposed for impacting the noise emission of the jet by influencing its flow field. These controlled jets are assessed in a new test facility, which has been validated prior to the actual jet experiments in terms of distribution and magnitude of mean and turbulent flow field quantities as well as properties of far and near field noise radiation.
Starting with an acoustic evaluation of each controlled jet, the most effective configuration with the largest noise reduction of both methods is selected to be further investigated in a detailed flow field assessment to account for the lower sound levels. The actively controlled jet is equipped with 8 moving nozzle flaps that periodically intersect with the nozzle flow, when the flaps bend inwards. In this position the flaps produce a bypass flow, which shields the emission of pronounced tonal noise components at frequencies of maximum sound levels. They are indeed inherent to the nozzle attachment. For the passively controlled jet small magnets are installed in the nozzle efflux boundary layer to induce streamwise vorticity for accelerating the spectral evolution of the velocity fluctuations and the development of the shear layer.
The so-called mini tabs are varied in shape, height, size, and number. This approach creates a broadband noise reduction for all mini tab configurations with the most effective noise suppression for a configuration of 30 vortex generators exhibiting not only lower noise emissions from 10 to 100kHz, but also down to 1kHz including maximum sound levels. The installation of mini tabs trips the initially laminar efflux boundary layer, the shear layer becomes more stable, and its spectrum is smoother. The initiation of streamwise vorticity at the nozzle exit causes a widespread increase of turbulent quantities across and along the jet, which enhances the turbulent mixing and thus the spreading of the jet. It further speeds up the downsizing of length scales in the cascade process, which indicates an earlier decay of the sound generating turbulent entities, which leads to the observed noise reduction.