Abstract:
The modern era of yacht racing involves speeds which present several hydrodynamic
challenges to engineers, one of which is ventilation. Ventilation is a
multiphase phenomenon, similar to but distinct from cavitation, that involves the
ingress of atmospheric air around the body of a hydrofoil. This significantly
alters the foil chord-wise pressure distribution which can halve lift coefficients
and has severe implications for yacht control. Experimental studies have demonstrated
the required flow conditions for ventilation formation, elimination, and
cavity stability and have characterised many of the influencing parameters such
as dynamic effects, the presence of fences, and a foil’s section shape.
The study of ventilation via numerical methods has been shown to be possible
in the last two decades but is far from being a mature science. The ventilation
characteristics of a foil such as its bifurcation and stall ventilation boundaries as
well as its stability cannot be easily determined, which hampers hydrodynamic
design.
This thesis systematically investigates ventilation using the commercial computational
fluid dynamics software ANSYS CFX. Four numerical techniques are
verified and validated including using ventilated initial conditions, moving mesh
simulations and a novel ‘blast’ method. The stability and dynamic response
of four NACA foil sections are compared, and discussed in the context of the
literature.
The ventilation inception mechanism of tail ventilation was found in all cases
tested. It was found that the magnitude of the suction peak did not correlate
with ventilation inception, however, tail ventilation is significantly affected by
the pressure distributions and skin friction coefficients in the rear 50% of the foil.
The NACA foils’ ventilation characteristics are compared, and the performance
of an existing empirical model is assessed. Lastly, the poor performance of
conventional foils in the ventilated flow regime is demonstrated by evaluating
the drag at constant lift, and the lift-to-drag ratios during ventilation.