Self-sustained oscillations in a closed side branch system

Self-sustained oscillations of the flow in a closed side branch system due to a coupling of vortex shedding with acoustical resonances are considered. The configuration consists of two closed side branches of same length placed opposite to each other along a main pipe. This is called a cross-junction. Numerical simulations, based on the Euler equations for two-dimensional inviscid and compressible flows, are performed. As the radiation into the main pipe is negligible at the resonance frequency, this acoustically closed system is a good test-case of such Euler numerical calculations. The numerical results are compared to acoustical measurements and flow visualization obtained in a previous study. Depending on the flow conditions, the predicted pulsation amplitudes are about 30–40% higher than the measured amplitudes. This is partially due to the absence of visco-thermal dissipation in the numerical model but also to the effect of wall vibrations in experiments. A simple analytical model is proposed for the prediction of the pulsation amplitudes. This model is based on Nelson's representation of the shear layer as a row of discrete vortices convected at constant velocity from the upstream edge towards the downstream edge. When the downstream edge is sharp, this results in a spurious interaction between the singularity of the vortices and of the edge flow. This artefact is partially compensated by suppressing the singularity of the acoustical flow at the edge, or when a junction with rounded edges, as found in engineering practice, is considered. In spite of its crudeness, the analytical model provides a fair prediction (within 30%) which makes it useful for engineering applications

The aero-acoustic resonance behavior of partially covered slender cavities

The present investigation focuses on the aero-acoustic resonance of cavities with a width much larger than their length or depth and partially covered, as often encountered in automotive door gaps. The cavities are under influence of a low Mach number flow with a relatively thick boundary layer. Under certain conditions, these cavities can acoustically resonate with the flow. The upstream and downstream edge of the opening as well as the cover lip overhang location and boundary layer thickness are parametrically varied in an experimental campaign, and the effect of the parameters on the resonance amplitude is investigated. Slender rectangular cavity geometries with an opening length of 8 mm and spanwise width of 500 mm are used. The cavity flow-induced acoustic response is measured with pressure transducers at different spanwise locations inside the cavity. Hot-wire measurements are performed to quantify the boundary layer characteristics. Furthermore, high-speed time-resolved particle image velocimetry is used to capture the instantaneous velocity field around the opening geometries. When the boundary layer thickness is increased, the cavity resonance amplitude diminishes. The cover lip overhang location has a large influence on the resonance response, which can be attributed to changes in the cavity driven flow properties. Rounding of the upstream edge promotes resonance, whereas rounding of the downstream edge can diminish it. A possible explanation of the phenomenon is given on the basis of the PIV observations.Aerodynamics and Wind EnergyAerospace Engineerin

Acoustics of 90 degree sharp bends, Part I: Low-frequency acoustical response

The acoustical response of 90 degree sharp bends to acoustical perturbations in the absence of a main flow is considered. The aeroacoustical response of these bends is presented in part II [1]. The bends considered have a sharp 90 degree inner edge and have either a sharp or a rounded outer corner. They are placed in pipes with either a square cross-section (2D-bends) or a circular cross-section (3D-bends). The acoustical performance of a numerical method based on the non-linear Euler equations for two-dimensional inviscid and compressible flows is checked and its ability to predict the response of 3D-bends is investigated. The comparison between 2-D and 3-D data is made for equal dimensionless frequencies f/fc where f is the frequency of the acoustical perturbations and fc is the cut-off frequency of the bends. In the case of a bend with a sharp inner edge and a sharp outer corner, the 2-D numerical predictions agree with 2-D analytical data obtained from a mode expansion technique and with 2-D experimental data from literature and our own 3-D experimental results. In the case of a bend with a sharp inner edge and a rounded outer corner, the 2-D numerical simulations predict accurately the 2-D experimental data from literature. However, the 2-D numerical predictions do not agree with our 3-D experimental data. The acoustical response of 3D-bends appears to be independent of the shape of the outer corner. This behavior is quite unexpected