Aeroacoustics at low mach number merging flows at duct junction

Complicated ductworks are common in modern air-conditioned buildings, in which duct junctions are often found. Air flows merge at these junctions and initiate shear layers at junction edge due to the different velocities and directions of flows upstream of a junction. These shear layers eventually roll up to form vortical structures, whose interactions produce additional noise in the ductwork. Previous investigations mainly focused on the propagation of noise at duct junctions. However, the aeroacoustical generation of noise, and its propagation, of merging flow at duct junction are seldom studied. Thus, this paper aims to report a numerical investigation of the aeroacoustics of merging flow at a T-junction and this is done by solving Navier-Stokes equations with the conservation element and solution element (CE/SE) scheme. This method has already been demonstrated as a successful tool in investigating various aeroacoustics problems such as jet screeching noise, noise generated at cavities. The scheme is built on a comparatively simple, yet noble, framework which saves computational resources while it yields accurate results. The first part of this paper presents a study of wave propagation at T-junctions, which illustrates the capability of CE/SE method in capturing the acoustics accurately. The second part shows an investigation of 90° impinging merging flows at T-junctions. Since the flows in ducts are usually turbulent in practical situation, MILES approach with wall modelling is adopted in the present study to account for the effects of turbulence. From the instantaneous vorticity at T-junction, the vortices are found shedding at the edge downstream of T-junction. Furthermore, identifying the regions where the acoustic or aerodynamic effect dominates is always a task in one-step aeroacoustics simulation. A simple method using two neighbouring time signals is also introduced to identity these regions and its effectiveness is reported in this paper.