Aerodynamic Analysis of the Bi-Radial MKII Sail for the Olympic Sailing Class Dinghy ILCA 7
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Naval architecture and ocean engineering (MPNAV), MSc
Publicerad
2022
Författare
Magnander, Gustaf
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
The most widely used model for sail force coefficients is probably Hazen’s model. The
model, which is presented in [18], offers great flexibility and can be used for a wide range
of different sail plans. However, even if it has been extensively used since Hazen presented
the model in 1980 and is still used today 4 decades later, it has shown not to be fully
satisfying for downwind analysis of sails. To properly analyze the Olympic sailing class
dinghy ILCA 7 sailing downwind, the aerodynamic sail coefficients of the new bi-radial
MKII sail first has to be retrieved from experimental wind tunnel tests.
The sail coefficients can then be used in a Velocity Prediction Program (VPP), where they
are balanced against the corresponding hydrodynamic coefficients, obtained from experimental
towing-tank tests of a full-scale ILCA dinghy in 2014 at SSPA, a Swedish Maritime
Consulting Company, situated at the main campus of Chalmers University of Technology
in Gothenburg Sweden. With the output from the VPP, in the form of a polar plot, the
optimal way to sail the ILCA 7 dinghy can be established, which is of great value for the
Swedish elite ILCA 7 dinghy sailor, who will have the honor of representing Sweden at the
Summer Olympics 2024.
A 1:7 scale model of the new bi-radial MKII sail, released in 2016, was manufactured by
North Sails and tested for downwind sailing in flat water conditions, in the low turbulence
subsonic wind tunnel, at Chalmers University of Technology. The sail model was tested
for two different apparent wind speeds that represented light and strong wind conditions.
The Reynolds numbers for these two conditions were 1.8×105 and 3.0×105, respectively.
Even though the flow at these Reynolds numbers presumably is turbulent, a grid, that
induced turbulence with an intensity of 2% was used and the sail model was therefore with
certainty tested in turbulent flow. For the light wind condition, the sail model was tested
for five different headings, three different heel angles (upright condition included), seven
different sheet angles and for each configuration of these, three different settings with the
kicker were tested. For the strong wind condition, only the heading corresponding to when
the ILCA 7 dinghy is sailed dead downwind in a planing mode was of interest; for that
heading, the sail model was tested for the upright sailing condition, i.e. with a heel angle
of 0°, for four different sheet angles and three different settings with the kicker. For each
configuration of these variables, for both wind conditions, the other two trim controls,
the outhaul and the cunningham were continuously trimmed. Altogether 327 different
configurations of the above mentioned variables were tested.
This Master’s Thesis describes how the aerodynamic sail coefficients were retrieved from
experimental tests in Chalmers low-turbulence subsonic wind tunnel and how the measurements
were corrected for blockage effects, after performing a numerical analysis with
Reynolds Averaged Navier-Stokes (RANS) simulations, using the commercial CFD-code
STAR-CCM+. The final results is a full set of sail coefficients needed for optimizing
downwind sailing of the ILCA 7 dinghy, using a VPP.
Beskrivning
Ämne/nyckelord
ILCA 7 , MKII Sail , Scaling , Chalmers Subsonic Wind Tunnel , Blockage Effects , Sail Coefficients , STAR-CCM+ , RANS , DES , Verification & Validation