Noise induced in a cylindrical towed model from separated/reattached turbulent flows

Results from an underwater experiment on turbulent flow-induced noise in the interior of a cylindrical towed model are presented. The thin axisymmetric model consist of an elastic hose filled with a fluid and the towing experiments were performed under sea conditions at three different towing velocity U = 2.5, 3.3, and 4.2 m s-1 (flow velocities U' = 2.0, 2.7, and 3.6 m s-1) at a towing depth of about 10 m. During a tow the elastic hose is excited from an axisymmetric turbulent boundary layer flow that has reattached after being triggered upstream at axisymmetric obstacle due to flow separation. Turbulent flow-induced noise in the (quiescent) interior of the model is measured with a hydrophone array located on the axis of symmetry and analyzed by wavenumber-frequency analysis. By scaling with outer variables (model diameter d, fluid density, flow velocity U') the power spectral densities of flow-induced noise collapse onto each and decay according to an f-3 law (at low U' between f-2 and f-3) below a corner frequency of about 150 Hz. In this regime (interior) noise is governed by properties of the (outer) turbulent boundary layer. Above the corner frequency a substantial drop in spectral level is observed going along with an incomplete collapse of the scaled power spectral densities.