Global mode visualisation in cavitating flows

A technique for visualising global frequency modes and phasing information is presented using fast Fourier transforms applied to individual pixel intensities of high-speed photography image frames. A singular value decomposition is applied to the resulting complex data set to allow the application of windowing to the transforms. A high-speed image set of cloud cavitation about a sphere is used to demonstrate the technique and identify the dominant cavitation shedding modes. Three frequencies were found and the modal analysis show these to correspond to large-scale near symmetric shedding, intermediate-scale oblique shedding, and smallscale near symmetric shedding, respectively. Results obtained by the presented image analysis technique show close agreement to shedding frequencies obtained from pressure data.