Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014
Authors agree with Dr Della Valentina’s comment about the possibility to investigate forces fluctuation and not only wind speed up effects on local structures. This is usually done by means of force measurements acting on the local elements or by means of pressure distribution measurements which in this particular project have not been requested.
Regarding the “worst condition” question, the authors do not completely agree that the condition when the generator is just started would necessarily be the worst: although the temperature of the flumes is lower in this case (the temperature can fluctuate from 280 to almost 500 °C), it must be noted that also the flow rate of the exhaust gas is lower, when the generator is still off-line and until the paralleling operation is complete. Since the flow rate is lower, also the pollution concentration is lower.
In response to the comments from Benoît Augier: The wind speed used for the wind tunnel tests are different depending if we are considering the force and speed up measurements or the exhaust dilution investigation.
In the first case wind speed is chosen with reference to the optimal pressure measurement system working range and with reference to the dynamometer working loads. Measures are usually carried at 2 different speed for repeatability check purposes.
For the exhaust dilution investigation tests are then performed using wind speed scaled accordingly to Froude number and usually the full scale wind speed is targeted to low Beaufort wind states which are the worst cases concerning potential discomfort
due to exhausts produced by diesel fuel
combustion as well as potential plume re-ingestion problems or pollutants dilution.
Wind speed of 5m/s is typically considered a reference value at full scale concerning the boat anchored in sheltered waters.
Authors confirm that the aerodynamic center of effort is different from the center of rotation of the hull. Results provided by the wind tunnel tests in terms of aerodynamic forces and moments are actually the input data for the mathematical model of the ship with reference to the various ship design issues.
Figure 11 contains an error in the longitudinal Fx force versus: positive Fx means force directed to the bow, so negative value of CFX between 0-40deg incidence are related to the mainly frontal wind relative incidence. Authors apologise for this and have provided the corrected figure in the final format version of the paper.
In response to the comments from Dr Ignazio Viola: Maxi and mega-yachts application tests are generally performed in the Boundary Layer test section in order to properly take into account atmospheric turbulent characteristics considering scaled simulation of the
flow
natural wind characteristics of the site according with turbulence and boundary layer simulation law.
The wind speed used for the wind tunnel tests are different depending if the exhaust dilution investigation or the force and speed up measurements are considered. Pollutant dilution tests are performed using wind speed scaled accordingly to the Froude similarity law in order to maintain the ratio between the inertia and the weight forces
while force and speed-up measurements are
generally performed considering different wind speeds in order to check the measurement repeatability.
The issue pointed out of a potential Re effect on global forces and local speed up is actually interesting and noticeable of attention: a considerable amount of tests carried out by the authors in the high speed test section of the Politecnico di Milano Wind Tunnel on other yachts and pleasure craft models with similar model scale factor show that non dimensional drag and lift coefficients values highlight a clear asymptotical trend with wind speed leading to the conclusion that wind speed of about 10m/s is appropriate.
Regarding the “comparison with the aerodynamic forces acting on a rectangular prism” question, the authors do not completely agree that this would be an interesting issue because of the variety of superstructure exterior design and superstructure
shapes of a yacht often
resulting in a strong three-dimensionality of the flow leading to very complex flow patterns.
Authors apologize for not providing the key dimensions of the model which are confidential and concerning
finally experimental uncertainty, additional
performance info about instrumentation on force measurements and flow measurements have been included in the paper.
©2014: The Royal Institution of Naval Architects
B-117
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