A study of the vortex flows of downwind sails
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Date
28/11/2019Author
Arredondo Galeana, Abel
Metadata
Abstract
A leading-edge vortex (LEV) can be a robust lift generation mechanism on both the
wings of natural fliers and delta wings. A spinnaker-type of sail is a thin wing that
promotes the formation of LEVs due to a sharp leading edge. Recent numerical simulations
(Viola et al., 2014) have demonstrated that this type of sail can prevent LEV
shedding and instead, keeps it trapped near the leading edge. In such cases, the LEV
could enhance lift generation (Saffman and Sheffield, 1977; Huang and Chow, 1982),
and so there is a need to investigate the existence of the LEV and its role for sails.
To study the LEV in the context of sails, a rigid model scale spinnaker was tested in
water at low Reynolds numbers and uniform flow. It was found that the flow separates
at the leading edge, followed by turbulent reattachment, forming an LEV. For finite
periods the LEV breaks down into weaker LEVs that are shed downstream; otherwise,
the LEV remains coherent at the leading edge. On the lower half of the sail, the LEV
has negligible diameter, and trailing edge separation occurs after the first quarter of the
chord.
To understand whether there is a benefit from having the LEV trapped near the leading
edge, as opposed to being shed downstream into smaller LEVs, the local circulation
was measured and its value utilised in a complex potential model. The model maps
a circular arc into a rotating cylinder and assumes the Kutta condition, to provide a
bound circulation value that is a function of the position and circulation of each LEV
(Pitt Ford and Babinsky, 2013; Nabawy and Crowther, 2017). It is found that when the
LEV is trapped near the leading edge, the LEV provides a marginally higher lift than
when it breaks down and sheds. Surprisingly, with the conservative assumption of the
Kutta condition, the LEV contributes between 10% to 20% to the sail’s sectional lift.
In actual sailing conditions, the spinnaker experiences a twisted onset flow, that could
not be replicated in the water flume, such that the angle of attack varies along the span
of the sail. To explore this effect three spinnaker models were made, where the original
sail was twisted from top to bottom by different angles. PIV and force measurements
were compared. It was observed that a low twist sail allows the LEVs to remain close to
the body of the sail, whereas a high twist sail causes them to drift away and generates
counter vorticity on the surface of the sail. This viscous effect results in a marginal
reduction in lift, but significant reduction of induced drag.
The results presented in this PhD thesis aim to provide an improved understanding of
the aerodynamics of downwind sails, where vortex flow is a dominant feature. The
existence of trapped and shedding LEVs is demonstrated and an attempt is made to
model LEVs through a complex potential model in order to assess their contribution to
the sectional lift of the sail. Finally, the effect of twist is evaluated with regard to the
aerodynamics of sails.