Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec
Figure 20 gives the driving and side forces resulting from the simulation, calculated the same way as above. The same peculiar jumps in driving and side force area as in the wind tunnel measurements can be observed.
8.3 COMPARISON OF RESULTS
To validate the FSI simulation, flying shapes as well as forces have to be compared. Figure
profile shapes. Table 1 gives 21 gives a
superposition of the measured and calculated flying shapes at AWAs 90° and 150°. As can be seen, the flying shapes agree well with respect to luff and leech positions and
the maximum
deviations of luff, leech and clew positions as percentage of leech length. Obviously the largest deviations appear at the leech as this has the most freedom to move. The largest deviations per trim occur at AWAs 120° and 180°. As stated above, the trim at AWA 120° was quite peculiar, while at 180° the sail was a little unstable in the wind tunnel as well as during simulations.
the mast in the simulations to improve meshing facility. According to Paton et al. [12], this can lead to a significant increase of lift and reduction of drag of the attached sail.
AWA Luff Leech 90°
105° 120° 135° 150° 165° 180° Avg.
0.90% 1.39% 1.57% 2.17% 1.15% 1.11% 1.95% 1.46%
Clew
1.63% 2.13% 2.41% 1.66% 1.86% 2.34% 4.52% 2.36%
0.49% 0.58% 2.40% 1.58% 0.83% 1.53% 2.20% 1.37%
Table 1: Maximum deviation between measured and calculated leech and clew positions in percent of leech length
]
m ²
y [
A,]
A x [
m ²
-0.5 0.0 0.5 1.0 1.5 2.0
90 Ax TFWT
120 Ay TFWT
150 Ax FlexSail
AWA [deg] Ay FlexSail
180
Figure 22: Driving and side forces from wind tunnel measurements and FlexSail simulations
9. CONCLUSIONS
The development of a program for the simulation of the flow around and the structural behaviour of downwind sails has been described. A physically correct wrinkling model has been detailed, a short overview of a solver, stable even at large geometrical as well as structural non- linearities, has been given.
Figure 21: Comparison of flying shapes from wind tunnel (dashed) and FSI simulation (continuous) at AWA 90° (top) and 150° (bottom)
Figure 22 gives the resulting force areas from wind
tunnel tests and FlexSail simulations in comparison. As can be seen the simulations mostly replicate the trends found by the wind tunnel measurements, yet at an offset. At AWA = 120° the simulation results indicate an earlier change from partially attached to fully separated flow. It is as yet unclear whether the differences are due shortcomings in the wind tunnel measurements or flow simulations. A possible reason might be the omission of
©2011: The Royal Institution of Naval Architects
The necessity for a wrinkling model has been shown, detailing the significance of the errors introduced by the locking effect of the basic CST-elements under compressive deformation.
The capabilities of the entire FSI method have been validated by comparison to a bespoke test case, based on measurements at the YRU-Kiel twisted flow wind tunnel. Whilst the absolute forces are a bit off, the trends are accurately replicated. The flying shape is calculated quite accurately over the whole tested range of AWAs.
Generally it can be said that the method presented is able to provide the sail designer with the information necessary to optimise the sail’s design and construction.
B-79
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