Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014


narrow passages. In this case aerodynamic forces are very important and, in the first case mentioned above, can be predominant. The experimental assessment of the ship aerodynamic behaviour through wind tunnel tests on a scale model can be very significant for the performance of the installed DPS system.


The lack of data concerning the peculiar mega-yacht forms and the required accuracy lead to the need to set up scale model wind tunnel tests. They are asked for more and more frequently by designers and surveyors and considered a fundamental reference for any further CFD simulation.


Comfort on board a luxury yacht is obviously a key feature in all its aspects, like ship motions, noise and vibrations, windage and exhausts gases issues. The on board comfort assessment on the various living areas of the yacht decks can be performed by two point of view. The first one is related to the flux acceleration and can be considered by means of local wind speed measurements, i.e. speed up comfort. The second one is connected to the air pollutant concentrations of gases released from the stacks of the yacht: it can be taken into account by means of pollutant concentration measurements.


Since the speed-up comfort results are heavily dependent on the yacht exterior profile, these measurements are to be carried out at an early stage: excessive speed-up values in external areas like sunbathing or dining may be overcome by modifying the exterior shape,


in way of the helideck, in order to make sure that the helicopter can develop enough lift.


This paper describes experimental methodologies developed at Politecnico di Milano Wind Tunnel in order to assess mega-yacht aerodynamics in terms of aerodynamic loads, comfort analysis, pollutants dilution and re-ingestion analysis using wind tunnel tests on a scale model.


Aerodynamic load assessment of global aerodynamic forces are investigated at


different wind exposures using a six


component dynamometer, while comfort evaluation on the various yacht decks and living areas are performed by means of speed-up measurements in different positions on each yacht terrace and roof.


Finally in order to detect potential discomfort due to exhausts produced by diesel fuel combustion as well as potential plume re-ingestion


problems, or


measurements. Some


pollutants experiments are carried out dilution, lab-scale using tracer gas dilution results obtained with reference to aerodynamic


investigations carried out, using a scale model of a 270’ LOA (~90 m) mega-yacht in smooth and turbulent flow at different wind exposures, are presented and discussed. This data represent a reference for


a better consideration of or by


inserting glass or plexiglass windscreens, which can be fixed or removable. These elements usually have an impact on the aesthetics of the yacht, so they must be carefully designed together with the exterior stylist.


The positioning of HVAC external grids and generator exhausts can also be dependent on the aerodynamics of the flow around the yacht. Grids are usually placed according to the exterior profile design, so that they have the smallest possible aesthetic impact, and by using common practices in order to avoid pollution between exhaust and intake air (for example, usually the intake air grids are put on the opposite side from the exhausts)[15]. However, exhausts gas flow around the yacht superstructure may not be intuitive, therefore an experimental evaluation is very useful in order to detect possible re-ingestion problems. Most of the yachts now have diesel generator exhausts running up to the stack, as high as possible, but limitations come from the maximum allowable backpressure and from the fact that they often have to be incorporated into the mast or superstructure design, so that they are not directly visible. Although exhausts are usually provided with particle soot filters and/or catalytic filters, great care is to be taken in order to ensure that gas is diluted or is flowing far enough from the external guest living areas.


If there were a certified helideck on-board, wind tunnel tests would be required by the rules stated in [16] to estimate the temperature rise, due to the exhausts gases,


aerodynamic aspects in maxi and mega-yacht design. 2.


EXPERIMENTAL SET UP 2.1 POLITECNICO DI MILANO WIND TUNNEL


The Politecnico di Milano Wind Tunnel is located in the Campus Bovisa Sud of the Politecnico di Milano University.


The wind tunnel is a closed circuit facility with a vertical arrangement having two test sections, a 4x4 [m] high speed low turbulence and a 14x4 [m] low speed boundary layer test Section (Figure 1); the overall wind tunnel characteristics are summarized in Table 1.


Figure 1: MilanoWind Tunnel facility.


A peculiarity of the facility is the presence of two test sections of very different characteristics, offering a very wide spectrum of flow conditions.


B-72


© 2014: The Royal Institution of Naval Architects


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