Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014
The facilities of Politecnico di Milano have all the characteristics to ensure very accurate measurements. Scale effects are well analysed and described in the paper. The use of large models in facilities with large sections leads
to accurate measurements with low
blockage effects. Although the Paper shows the widest variety currently possible in aerodynamic investigations, a possible area of further
developments is in the
measurements of forces fluctuation and not only wind speed up effects on local structures. This information can be used for better design local structures and predict the possible sources of vibrations.
Although the “enhanced scaling relationships” make sure that realistic conditions are represented at model scale, a point of attention remains in the selection of the gas temperatures
and densities to be represented: the
conservative experimental approach used in the paper leads to a negative buoyancy of the plumes assuming that an higher temperature of the fumes at full scale would facilitate the dispersion. This approach seems reasonable although, to my opinion, the worst condition to be investigated at full scale is when the flumes are still “cold”: despite being transitory, this condition represents the situation with the highest risk of re-ingestions of undesirable gasses in the ventilation system.
The clear structure of the Paper, the back ground research with the use of the most relevant references in literature, make this work almost immune to criticism.
Benoît Augier, UCSD, La Jolla, CA, USA
Dr Ignazio Viola, Institute for Energy Systems School of Engineering, University of Edinburgh
The paper presents one of the few examples of wind tunnel tests of super yachts and focuses on the capabilities of the Politecnico di Milano Wind Tunnel to perform these tests. It is very positive that aerodynamic measurements were performed in a high-quality facility for a prestigious yacht firm and I hope to see more of these papers in the coming years. These tests are not easy to be performed and few issues are discussed hereby.
Despite the large test section of the facility – 14 m wide and 4 m high – the tested Reynolds number is 20-30 times lower than the full scale one. In fact, as often happens in yacht applications, the maximum wind tunnel flow speed is similar to the full-scale wind speed and thus the ratio between the model and full scale Re is the same as the ratio between the model and full-scale geometrical dimensions. Unfortunately this limitation can be overcome only in very few very large facilities with very high flow speed. It would be interesting if the authors could comment on the potential Re effect on global forces, local speed up and pollutant dilution.
What are the wind speed targeted at Full scale? Regarding the good results of the exhausts dilution, what would have been the scale speed without considering the density ratio?
The center of forces of the windage is certainly different from the center of rotation of the hull. How could you take into account the resultant attitude angles (mainly heel) in the projected area and the windage forces and moment?
Figure 11. Considering that 0deg is the boat head to wind and 180deg is the boat running, the negative value of Cfx is surprising between 0- 40deg. Anyways, what is the reason for such a perfect superposition for
curves of the 2
investigated speeds? The curves Cmz and Cfy are quite symmetrical when Cfx is not. What could be the reason and the consequences?
4.3 exhausts dilution. regarding the repartition of the user position, AWA=90deg seems not to be the worst case. What are the results for AWA=0 and 180deg?
The force coefficients presented in Figures 9 and 10 are potentially very interesting and could be compared with those measured on other yachts and ships or on simple 3D geometries. For instance, an interesting comparison could be with the aerodynamic forces acting on a rectangular prism. In order for this new valuable data to be used by other authors, the key dimensions of the model and which of those dimensions were used to define the coefficients should be stated. Also, it would be useful to know which is the experimental uncertainty for the coefficients in Figures 9 and 10.
AUTHORS’ RESPONSE
The authors are very grateful for the several interesting comments and questions received from their colleagues; please find in the following our attempt to better address each of the raised points.
The issue pointed out, by Dr Ian Dand, of a negligible surge force for a wide range of wind incidence is very interesting and noticeable of attention.
Authors strongly agree that aerodynamic loading for mega-yachts is not an easy issue because of the complex geometry often resulting in a strong three-dimensionality of the flow.
A strong three-dimensionality of the flow is for example highlighted in
Figure 16 which shows streamlines
starting at main deck level: in particular changes in streamlines produced by wind incidence moving from 30° to 55° angle of attack are shown.
B-114
©2014: The Royal Institution of Naval Architects
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