Aeroacoustics of merging flows at duct junctions

Abstract

This thesis involves a numerical and experimental investigation of aeroacoustics of merging flow at duct junctions, which are composed of a main duct and a side branch with the same duct width. Since the aeroacoustics of internal flow is complicated, the flow dynamic and acoustic disturbances generated are always mixed. It is very difficult to differentiate their evolutions experimentally, so a numerical tool is developed to investigate the duct junction aeroacoustics. This tool is based on the Conservation Element and Solution Element method, which solves the unsteady compressible Navier-Stokes equations and the ideal gas law, to perform direct aeroacoustic simulation. To account for the effects of flow turbulence, implicit LES strategy is adopted by combining the MILES approach and wall modeling derived from the classical logarithm wall law. The numerical code is verified fully with both external and internal benchmark aeroacoustic problems. The numerical investigations are performed in two dimensions (2D) with Reynolds number (Re) based on duct width equal to 10⁵. The cases under investigation are defined by different combinations of the ratio of side-branch to main duct flow velocities, VR (= 0.5, 0.67, 1.0, 2.0) and merging angle, θ (= 30°, 45°, 60°, 90°). The numerical investigation continues with a three dimensional (₃D) calculation (VR = 1 and θ = 90°) due to limited computational resources available. The general aeroacoustics of 2D merging flow and its variations with VR and θ are discussed. The acoustic power generated is found to increase with VR and θ, leading to the noisiest case at VR = 2.0 with θ = 90°. The numerical results of both 2D and ₃D studies are compared and discussed. A test rig is developed for investigating different combinations of VR (0.5, 0.67, 1.0, 2.0) and θ (45°, 90°) in experiments. Due to the limited capability of facilities available, a smaller maximum Re is attained (10⁴). The merging flow was driven by using a two-fan system. The velocity of the flow and pressure fluctuations were measured by a cobra probe and a probe microphone respectively. The experimental results are discussed and compared with the numerical results, which provide us the insights in the aeroacoustics generated by the merging flow at duct junction.