Acoustic instabilities may develop in ducts with a mean flow due to vortex shedding. A reduced-order model for vortex-structure interaction is derived in this paper. A duct with two baffles is considered as a general configuration for developing this model. Formation of vortices at the upstream baffle is affected by acoustic oscillations. A new quasi-steady model is applied for determining a moment of vortex separation in an unsteady flow. A vortex impingement on the downstream baffle is assumed to occur instantaneously in order to utilize a 'kicked' oscillator model. If vortex collisions with the baffle happen in the appropriate phase with an acoustic pressure perturbation, then acoustic eigen modes of the duct can be excited. Peculiarities of a numerical implementation of the model are discussed.
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