High-Speed Video Observations and Acoustic-Impact Measurements on a NACA 0015 Foil

A 6-cm long 2D NACA hydrofoil was tested in a cavitation tunnel at flow speeds of around 17 m/s. The objective of the
tests was to investigate the collapsing behaviour of sheet cavitation and possible erosion mechanisms by observation
using high-speed cameras - top and side view - and to register the structure-borne and fluid-borne noise emitted by collapsing cavitation with three resonant acoustic - emission sensors - one fixated to the hydrofoil mounting and two on the tunnel walls.
The loudest impacts were traced to small cavitating vortices generated during the shedding mechanism collapsing and rebounding near the foil's surface. Soft-paint tests showed damage to the paint in the area of the vortex collapse. The
correlation was good although a portion of impacts are believed to either occur off-camera or optically blocked by the cavitation itself and could not be visibly traced to collapsing structures.
Digital image filtering was used to find the second temporal derivative of the image intensity - hypothesized to
capture a rebounding structure - and did result in high gradients at the time of a collapse. However, the filter could not
differentiate between high- and low-impact collapses and was too sensitive to lighting direction and other experimental conditions not related to cavitation.