The influence of mean flow velocity and direction on the acoustic transmission behavior of right-angled duct bends

Many duct systems transporting air flow also transport unwanted noise. The geometrical parameters of duct bends can be tailored to reduce the noise emission by blocking sound transmission through ducts. However, the flow velocity has an influence on sound propagation. So far, it is not clear to what extent this limits the transmission loss of bends and their usage for noise reduction. In this work, the influence of flow on the transmission loss of duct bends is studied numerically on the basis of a right-angled sharp bend with rectangular cross-section. The numerical method consists of two steps: First, a CFD flow simulation is done. These results are used as an input for the subsequent finite element acoustic simulation where the Helmholtz equation is solved. Thereby, the usage of two different flow models allows differing influences purely related to the uniform movement of the fluid from those related to additional flow phenomena like the flow constriction zone downstream of the bend. It is revealed that the impact of mean flow velocities up to 40 m/s is mostly negligible. However, flow direction plays an important role when higher order modes propagate, leading to significant differences between upstream and downstream transmission loss. Inter-modal acoustic scattering as well as mode transformation in the flow constriction zone have been identified as the reasons for these differences. The numerical results are confirmed by measurements.