The propagation of pressure disturbances is investigated in an annular duct with mean potential axial and swirling flow. The swirling mean motion causes refraction of the acoustic waves as they propagate along the duct. A normal mode analysis leads to a non-linear eigenvalue problem. The numerical solutions are validated with an asymptotic analysis in the limit of large radius. The results show that while for a uniform flow the evanescent modes propagate at the same speed only upstream, for a swirling flow such modes propagate at different speeds both upstream and downstream. Moreover, most non-axisymmetric acoustic modes propagate opposite to the mean flow swirl.

An experimental and analytical study of the tonal trailing-edge noise of a symmetric NACA-0012 aerofoil and of a cambered SD7003 aerofoil has been achieved. It provides a complete experimental database for both aerofoils and improves the understanding of the underlying mechanisms. The analysis stresses the high sensitivity of the tonal noise phenomenon to the flow velocity and the angle of attack. Several regimes of the noise emission are observed depending on the aforementioned parameters. The contributions of the pressure and the suction sides are found to vary with the flow parameters too. A special attention has been paid to the role of the separation bubble in the tonal noise generation. Hot-wire measurements and flow visualization prove that the separation bubble is a necessary condition for the tonal noise production. Moreover, the bubble must be located close enough to the trailing edge. Several tests with small-scale upstream turbulence confirm the existence of the feedback loop. Analytical predictions with a classical trailing-edge noise model show a good agreement with the experimental data; they confirm the cause-to-effect relationship between the wall-pressure fluctuations and the radiated sound. Finally, previously reported works on fans and propellers are shortly re-addressed to show that the tonal noise associated with laminar-boundary-layer instabilities can take place in rotating blade technology.

High-fidelity simulations of the unsteady flow and radiated noise of an unmanned aerial vehicle’s propeller are presented herein. The aim of this study is to capture the broad-band noise associated...

The article discusses an airfoil unsteady aerodynamic response to upstream unsteady flow disturbances in flow regimes characteristic of Micro Air Vehicle (MAV) applications. Because of their small size and speed, the flight control of these vehicles in gusty urban environments poses tremendous challenges addressed in the current work. Following a thorough review of experimental and numerical studies characterizing various scales and configurations of urban flow disturbances, three canonical gust models are proposed corresponding to the time-harmonic gust, the sharp-edge gust, and the Taylor vortex. A high-accuracy numerical analysis is applied to examine MAV airfoil response to such perturbations and conclude on relative importance of contributing viscous and inviscid forces.