Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014 DISCUSSION
EXPERIMENTAL ASSESSMENT OF MEGA-YACHT AERODYNAMIC PERFORMANCE AND CHARACTERISTICS
F Fossati, F Robustelli and M Belloli, Politecnico di Milano – Department of Mechanics, Italy, C Bertorello, University of Naples Federico II, Italy, S Dellepiane Azimut - Benetti, Italy (Vol 156, Part B2, 2014)
COMMENT Dr Ian Dand, FRINA, BMT Isis Ltd
The authors have provided an interesting paper on the subject of mega-yacht aerodynamics. The results they give in Figure 9 for the aerodynamic loads on such a vessel will be of interest to naval architects as such information is relatively scarce in the open literature. Aerodynamic loading for merchant and naval vessels is relatively easy to come by, but that for mega-yachts is not.
1. It is of interest to note in Figure 9 that the surge force is effectively zero for relative wind incidence angles between 50o to about 110o off the bow. Can the authors provide any explanation for this in relation to the design of the superstructure?
2. The importance of minimising the effects of accelerated airflow past passage or doorway openings in the superstructure is emphasised in the paper and the authors show some interesting measurements in Figures 12 and 13. The accelerations near what appears to be a break in the superstructure led to flow speeds about 70% of wind speed there. What are acceptable percentage values for accelerated flow speeds, and, if a 70% value is unacceptable, how was it reduced to an acceptable level?
3. The authors rightly emphasize the necessity to avoid entrainment of the exhaust discharge into the flow local to the living and leisure spaces on deck. The example shown in Figure 14 appears to be for a beam wind with a relative incidence of -90o. This does not appear to the writer as a very onerous condition compared to a head or following wind. Were similar results obtained in such wind directions?
4. Finally, it appears from Figures 2, 4 and 15 that the model was mounted on the floor of the wind tunnel, presumably on the dynamometer turntable. Was this the case and, if so, were any attempts made to remove the unwanted effects of the tunnel boundary layer in the vicinity of the model?
©2014: The Royal Institution of Naval Architects Professor Chris Baker, University of Birmingham
This is an interesting paper and the experimental work that
it contains has been carefully carried out and
reported. The experimental and analytical methodology is sound and the authors are to be congratulated on the work that is presented. However I would be interested in knowing a little more about what the implications of the results are and how they will be used. Specifically:
How are the force coefficients used in the vehicle control algorithm – what are good and bad force coefficients in this regard?
Are the wind speed up measurements put into any form of probability analysis that allows a calculation of what proportion of the time the wind conditions are suitable for different activities – this would be the norm in urban pedestrian level wind measurements?
What are acceptable levels in this context?
Dr Della Valentina, MARIN
This paper represents, to my opinion, an excellent and complete example of aerodynamic investigation for motor yachts. It can be considered a sort of guideline for the selection and interpretation of the most suitable tests and approaches to answer the majority of the aerodynamic challenges on yacht design.
The inventory made in the Paper on the possible
investigations is certainly very complete. I would like to extend the range of applicability of the wind tunnel data to other fields of expertise. Given the increase demand on comfort, safety and smart systems (such as DP) on board, the integration and cooperation among different disciplines became the key factor for the success of the final design: the use of the aerodynamic coefficients, for example, can be used for more accurate speed power predictions and for more realistic analysis of the sustained speeds in wind and waves. A correct dimensioning in terms of power and geometry of the bow and stern thrusters has large consequences not only on the DP capabilities of the yacht but also on her resistance. The impact of local elements such as extended canopies on the sun decks, cranes, tenders and toys on the total resistance (hydrodynamic + aerodynamic) is regularly underestimated. Too often data, coming from other ship types, is used as reference for aerodynamic assessments on yachts. Credits should therefore be given to Azimut – Benetti because they agreed on publishing their results. The entire industry can profit from sharing this type of information. Furthermore engineers and researchers can use this data as validation material for CFD calculations.
B-113 pollutant concentration
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