Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


The bare-hull model was stiff enough to avoid any deformations while the rigging allowed a similar mast bend to that one achieved in full-scale. The sailcloth used for the sails was the same as used in full-scale but with a properly scaled thickness. This allowed the model-scale sails to stretch and fly as in full-scale.


3.2 PRESSURE MEASUREMENT SYSTEM AND PRESSURE TAP LOCATIONS


A pressure system capable of acquiring up to 512 channels


modules, was used for the investigation. Only 2 of the 8 modules were used. The differential


simultaneously, made up of 8 64-channel transducers are


temperature-compensated Honeywell XSCL04D and were used for every channel. The 2 modules were placed in the model cockpit. Miniature lightweight


plastic pressure


taps 20mm long, 10mm wide and 4mm height were made and attached with double-sided tape to the sail on the opposite side to that under investigation. A 1 mm diameter hole was made in the sail to allow pressure transmission to the tap, resulting in no modification of the local pressure coefficient due to the pressure tap itself. Pressure taps were placed on 5 horizontal sections at heights of ⅛, ¼, ½, ¾ and ⅞ of the mitre (line made up of the points on the sail surface equally far from the leech and the luff). Eleven pressure taps were located on each section: at 1/12, 1/6, ¼, ⅜, ½, ⅝, ¾, 5/6 and 11/12 of the curve length, plus one as close as possible to the luff and one as close as possible to the leech. To minimise the number of tubes on the model that could affect


the


aerodynamics, the pressure distributions were measured on one sail surface at a time, hence 5x11=55 pressure measurements were recorded simultaneously.


Fifty-five tubes connected the pressure taps to the modules (Figure 1). To minimise the effect of the tubes on the flow field, the tubes connected to the top two sections of the spinnaker were fed through the head of the mast and then along the windward side shrouds to the modules on the cockpit; the tubes connected to the forward half of the bottom sections were horizontally suspended to the mast and then collected with the upper tubes on the windward shrouds; the tubes connected to the aft half of the bottom sections came to the cockpit along the spinnaker sheet. The pressure tubes of the 2 top sections were 3.2m in length and the other tubes were 2.4m long. The internal


diameter of the tubes was


1.5mm. The length of the tubes did not allow high frequency pressure fluctuations to be measured. Hence a relatively low sampling rate of 100Hz was used to minimise the post-processing computational


time.


Pressures were recorded and averaged over a 70s acquisition period.


All the transducers were pneumatically connected to a reference static pressure measured with a Pitot-static probe located


topmast height. Five other Pitot-static probes at different locations were used to correct the


approximately 10m upstream at the reference static


pressure although the pressure differences were small enough to be neglected. The total pressure from the reference Pitot-static probe was connected to an additional transducer, which measured the


dynamic pressure q (roughly 7.5Pa). The pressure differences from each channel were divided by q to provide the pressure coefficients Cp.


More than one transducer measured the dynamic reference pressure and the average value was used.


reference


Figure 1: the AC33 1/15th scale model set up for tests.


3.3 FORCES AND MOMENTS MEASUREMENT SYSTEM


The model was fixed to a 6-component balance via 3 brackets and the hull was partially immersed in a pool of water to act as an airtight seal between the hull and the wind tunnel floor. The forces and moments measured by the balance were transformed into


non-dimensional


coefficients by dividing the forces by the product of q and SA, and by dividing the moments by the product of q, SA and the height of the model h (2.3m). Forces and moments were acquired for 70s at 200Hz.


3.4 INFLOW CONDITIONS


The YRU’s Twisted Flow Wind Tunnel has a special vane device to twist the flow upstream of the model test section (Figure 1) [17], which has been recently enhanced after the investigation performed in [18] and which can also be removed from the test section to


©2010: The Royal Institution of Naval Architects


B-43


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