Modal Decomposition of Large- And Small-Scale Cloud Cavitation

Abstract

Cavitation clouds cover a frequency ω and wavenumber k spectrum limited by an upper bound due to the span of a blade or the diameter of a nozzle. The lower bound is given by the size of the eddies of a turbulent flow. Interpreting cavitation clouds as ring vortices and the formation as a challenge between re-entrant jet and asymptotic sheet growth, this becomes clear. To investigate the temporal and spatial contain of cavitation clouds, experiments were conducted on an unmodified and a modified hydrofoil with an artificial roughness i.e. an obstacle. From the high-speed measurements, we observe two cavitation regimes, (i) periodic large-scale cloud cavitation for the modified and (ii) small-scale cloud cavitation for the unmodified hydrofoil. For the latter, a coherent pattern is not visible to the naked eye. Sparsity-Promoting Dynamic Mode Decomposition is applied to the high-speed measurements to capture the dominant coherent structures from the complex flow field. This allows us to identify the underlying physical mechanism leading to cloud detachments. Within the paper we give the temporal frequencies of cloud detachments in both observed cavitation regimes.