Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
by combining wind-tunnel experiments with 3D RANS simulations.
2. EXPERIMENTAL SET-UP
The wind-tunnel experiments took place in the low speed section of the 7 by 5 wind-tunnel of the University of Southampton. The slow speed working section has dimensions of 4.6m x 3.7m, this wind-tunnel is fitted with a six component dynamometer mounted on a turntable, dedicated to sailing yacht testing.
Flow
circulation underneath the hull is prevented by a water tank filled to the waterline level of the model. Wind speed was kept at 4.1m s-1during all tests.
2.1 THE SAIL RIGS
The sails tested were 1:15 scale models of the Ocean Racing Multihull Association (ORMA) 60’ trimarans, mounted on a One Metre class monohull with a circular section mast of diameter 0.01m. The hull was set at 25° to the wind and the sails trimmed to maximize driving force. The process of determining the maximum drive force from the settings of the sails was via the live data, being displayed and stored on the data acquisition system, directly measured from the dynamometer. The tell tales that were fitted to the main and jib sails were also used to aid this
process. During testing it is apparent be at
maximum drive force that the top part of the sail was fluctuating in a minor manner. This indicated
the
presence of a vortex separation zone from the head of teh sail. This was of course to
expected. The
particularities of the One Metre’s rig led to a gap of 0.06m between the deck and the foot of the sails. The tests were performed with no heel, since the ORMA 60’ are allowed to cant the mast laterally to maintain a vertical mast.
One jib and two mainsails were tested, as represented in Figure 1. The two mainsails, which will be denoted as Small Main and Large Main for ease of understanding, differed only with the chord length of the square head. The chord length, at model, at the square head: 0.16m for the Small Main and 0.29m for the Large Main. With a mean chord for the whole rig of 0.70m and a wind speed of 4.15m.s-1, the Reynolds number is in the region of 180 000. Lateral planform areas of
the various
components are given in table 1. 2.2 GEOMETRY AQUISITION
A comparison between wind-tunnel measurements and numerical predictions can only be valid if the geometries used for CFD are identical to those in the wind-tunnel. It has thus been necessary to acquire the flying shape of the sails for the various wind-tunnel tests. To do so, two views were taken from a digital camera: one from the top and one from the rear, as seen in Figures 2 and 3. The freeware Accumeasure was then used to scan the geometries from the pictures, using a method similar to
the one used by Couser and Deane [7], where Bezier curves are fitted to the various camber stripes. In Accumeasure, these curves are fitted manually and their parameters used in the software ANSYS ICEM 10.0 to generate the sails’ surfaces and generate the mesh.
Figure 1: Side view of the jib and mainsails tested Lateral planform areas
Large Main Small Main
Hull 0.3026m2 Jib 0.4116m2 0.7355m2 0.6599m2
Mast 0.0578m2 Table 1: Lateral planform areas of the hull, mast and sails
Figure 2: Top view of the picture and digitalized shape
B-2
©2010: The Royal Institution of Naval Architects
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