Acoustics of 90 degree sharp bends. Part II: Low-frequency aeroacoustical response

The aeroacoustical response of 90 degree sharp bends is defined as the response to acoustical perturbations in the presence of a main flow. The acoustical response of bends, in the absence of a main flow, has been considered in part I [1]. Experiments are carried out for bends in pipes with circular cross-sections. These 3D-bends have a sharp inner edge and have either a sharp outer corner or a rounded outer wall. The three-dimensional experimental results are compared with results of numerical simulations, based on the Euler equations for two-dimensional inviscid and compressible flows, and with analytical data obtained by means of two-dimensional quasi-steady flow theories. As observed in the absence of a main flow (part I [1]), the two-dimensional numerical simulations provide a good prediction of the aeroacoustical response of bends with a sharp inner edge and a sharp outer corner. For bends with a rounded outer corner, the prediction is less satisfactory. The two-dimensional quasi-steady flow approximation predicts reasonably well the response of bends up to Strouhal numbers of the order of unity. However, quasi-steady flow theories do not predict the irregularities of the response as a function of the flow velocity. These irregularities are expected to be a Strouhal number effect and are observed both in experiments and numerical simulations.