Free surface interaction of a ‘T-foil’ hydrofoil

This thesis presents an investigation of the flow field around the tip vortex of a t-foil hydrofoil. The objective of the investigation was to gain understanding of the mechanisms surrounding the inception of tip vortex ventilation of t-foil hydrofoils which will inform the evolution of the next generation of t-foil hydrofoil design for both commercial passenger and high performance sail craft. Qualitatively – Observations from images taken during high speed towing tank testing at velocities between 2 to 12 ms\(^{-1}\) were compared and estimations made of the wake age, and the submergence, at which tip vortex filament cavitation and tip vortex ventilation may occur in a controlled environment. Quantitatively – Particle Image Velocimetry (PIV) velocity fields were obtained experimentally for a vertically mounted flat plate at angle of incidence and a NACA 0012 t-foil hydrofoil at an angle of attack (\(AoA\)) of 8°, with a submergence to chord ratio (\(h/c\)) of between 0.11 and 1.70 and at streamwise distance to chord length ratio or wake age (\(x/c\)) of between 0 and 5. The flow fields were compared using the metrics of circulation, peak tangential velocity, vorticity and velocity profiles of the components of the planar cross velocity. A methodology was developed for tracking the location of the wandering vortex core and the experimental and numerical results compared. At high angle of attack the numerical results identified that vortex shedding from the leading edge separation of the test geometry is a possible contributory factor to the vortex wandering phenomena. The vortex centre and the point of extreme core velocity were found not to be co-located. The point of extreme streamwise velocity within the vortex core was located within half the vortex radius of the vortex centre. During investigation of the t-foil hydrofoil, the inception of both vortex filament cavitation and ventilation were observed within the tip vortex. The trajectory of the tip vortex was affected by the velocity and submergence of the t-foil with progressive descent of the tip vortices observed. With increase in wake age up to an \(x/c\) of 5, and freestream velocity of 2 ms\(^{-1}\), the tip vortex developed an asymmetry in the span-wise component of the flow, resulting in a region of accelerated flow within the vortex, located between the free surface and the vortex centre. The intensity of this region varied inversely with submergence and was found to converge upon the maximum intensity of span-wise velocity at the greatest submergence. Prior to ventilation inception at low submergence, the tip vortex was observed to interact with vortices parallel to the wave wake surface. From these results, a indication of high risk regions may be anticipated for a range of vessel velocities, whereby a rudder hydrofoil or downstream vessel may be affected by the presence of turbulent eddies and vortex structures with high cavity content that may cause control issues.