Sound generation and propagation in circular ducts for frequencies beyond the cut-on frequencies of several higher order acoustic modes is investigated. To achieve this, experimental and numerical set-ups are designed and used to research aeroacoustic interactions between in-duct components and to conceive noise mitigation strategies. Describing in-duct sound for frequencies with a moderate number of propagating modes is important, for example, for improving the noise emission from mid-size ventilation systems. Challenges that are largely unacknowledged in the literature involve efficient test rig design, quantification of limits in the methods, numerical modelling, and development of effective noise mitigation strategies for higher order modes. In this thesis, in-duct sound is mapped on a set of propagating pressure eigenmodes to describe aeroacoustic components as multi-ports with sound scattering (passive properties) and a source strength (active properties). The presented analysis includes genetic algorithms and Monte Carlo Methods for test rig enhancement and evaluation, multi-port network predictions to identify model limitations, and scale resolving (IDDES) and Linearized Navier Stokes computations for numerical multi-port eduction and the silencer design. It is first shown that test rig optimization improves the quality of multi-port data significantly. Subsequently, measurements on orifice plates are used to test the network prediction model. The model works with high accuracy for two components that are sufficiently separated. For small separations, strong coupling effects are observed for the source strength but not for the scattering of sound. The measurements are used for numerical validation, which gives reliable results for coupled and uncoupled systems. The total acoustic power of tandem orifices is predicted with less than 2 dB deviation and the passive properties for most frequencies with less than 5 % difference from the measurement. The numerical (FEM) models are also used to design a completely integrated silencer for spinning modes that is based on micro-perforated plates and gives broadband attenuation of 3-6 dB per duct diameter silencer length. The multi-port method is a powerful tool when describing aerodynamically decoupled in-duct components in the low- to mid-frequency range. Due to a robust passive network prediction, multi-port methods are particular interesting for the design of silencer stages. Furthermore, the demonstrated applicability to numerical data opens novel application areas. / <p>QC 20170522</p> / IdealVent

<p dir="ltr">Gas turbines are widely used to provide propulsion, electrical-power, and mechanical power. Though tremendous advances have been made since Frank Whittle’s patent of a turbojet in 1930 and Hans von Ohain’s patent of the first operational turbojet in 1936, industry still has aggressive goals on improvements in efficiency and service life. One area where further advances are needed is better control of the flow across the gap between the blade tip and the shroud, referred to as tip-leakage flow (TLF). This is because TLF accounts for up to one-third of the aerodynamic losses in a turbine stage.</p><p dir="ltr">In this study, hybrid LES-RANS based on IDDES and steady RANS based on the SST turbulence model were used to study the compressible flow in a 1.5-stage turbine with geometry and operating conditions that are relevant to power-generation gas turbines. The focus is on the flow in the tip-gap region that account for the flow features created by the upstream stator vanes, stator-rotor interactions, and downstream stator vanes. Results obtained reveal the flow structures about the tip-gap region and the flow mechanisms that create them. Results obtained also show where steady RANS with mixing plane could predict correctly when compared with results from IDDES that resolve the unsteadiness of the turbulence and the motion of the rotor blades passing the stator vanes. Turbulent statistics from the IDDES were generated to guide the development of better RANS models. Results were also obtained by using RANS to examine the effects of blade loading, where mass flow rate through the 1.5 stage turbine was varied with the rotor’s rotational speed fixed at 3,600 RPM – the speed at which power-generation gas turbines operate in the U.S.</p><p dir="ltr">Key findings are as follows: In the first-stage stator, horseshoe, passage, and corner vortices were found to be confined within 10 to 15% span from the hub and shroud, and both steady RANS and IDDES generated similar results. Steady RANS and IDDES, however, differed considerably in how they predicted the wake downstream of the vane’s trailing edge. This coupled with the use of mixing plane, steady RANS was unable to account for effects of stator-rotor interactions and their effects on the tip-leakage flow. In the rotor, steady RANS predicted passage vortices that extended up to 50% span from the hub and 25% span from the shroud. The flow through the tip gap was found to induce a separation bubble on the blade tip and one large and two small vortical structures on the suction side of the blade and a vortical structure next to the shroud. These structures were found to grow along the axial chord of the blade. Steady RANS also predicted the large tip leakage vortex that contained the fluid from the tip-leakage flow to breakdown. IDDES did not predict the vortex breakdown because all of the coherent vortical structures identified including the separated region on the blade tip were unsteady and constantly shedding. As a result, IDDES predicted much smaller mean passage vortices – albeit the instantaneous structures were nearly as large as those predicted by steady RANS.</p>