Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


derived from the local rearranging equation (4),


σ = 2k 3


turbulent kinetic energy by (5)


This result can be compared against the LY2 criterion of a maximum standard deviation of vertical velocity of 1.75ms-1 in order to perform the airwake assessment. This approach has previously been used by Frazer-Nash to perform airwake assessments for other luxury yachts and various naval vessels [14].


Figure 8 show contours of gust velocity plotted on the centreline plane of the model.


such that regions above the 1.75ms-1 gust criterion are coloured red.


The contours range is marked on Figure 8 for clarity.


The region of airwake assessment is From Figure 8 it is


evident that the gust criterion is exceeded in this case and the LY2 airwake requirements are not met.


Further


interrogation of the CFD solution indicates that 12.4% of the assessment volume exceeds the 1.75ms-1 perturbation criterion.


It is notable that the region of high gust


velocity is concentrated in the lower parts of the assessment volume in the wake of the bridge and mast. The upper regions of the volume have comparatively low predicted gust velocities.


There are two possible means of mitigating this result which are dependent on the timing assessment.


If the assessment is conducted during the


design or bid phasing it may be possible to redesign of the superstructure to try to reduce gust velocities. This may include the addition of smooth fairings to eliminate sharp surface discontinuities and the repositioning of masts. If the assessment is conducted late in the design or in the final phases of the build this may be impractical and it may be preferable to impose an operational limit for commercial rotorcraft operations


maximum wind velocity in order to meet certification requirements.


of the CFD


Figure 9: An isosurface of absolute helicity showing flow disturbances induced in the region of the superstructure.


4.5 (c) Local Temperature Perturbations


Temperature perturbations are defined in terms of the increase in local temperature above ambient conditions (30°C). The LY2 standard requires the temperature perturbations


averaging period.


times longer than the mean residence time of passing flow through the assessment volume and therefore relates to a long-term disturbance of the thermal


field. based on a


Therefore, in this case it is appropriate to make the assessment


based on the temperature indicated by the steady flow CFD results.


Figure 10 shows an isosurface of a temperature of 32°C (2°C above ambient) which marks the extent of the maximum allowable thermal perturbation stipulated by LY2. Further interrogation of the results indicates that 1.6% of the assessment volume exceeds the


2°C


perturbation criterion. This figure shows the propagation of the plumes of warm air issuing from the ventilation outlets and passing through the lower section of the assessment volume.


Figure 8: Contours of gust velocity (ms-1) on a plane through the centre of the model. The airwake assessment volume is shown using transparency.


Figure 9 shows an isosurface of absolute helicity with a value of 10ms-2. Helicity is a scalar quantity which


©2010: The Royal Institution of Naval Architects


Comparing Figure 10 to Figure 8 it is postulated that the entrainment of the hot ventilation plume into the flow around the superstructure is also likely to contribute to the development of unsteadiness above the helideck.


In theory thermal perturbations above the helideck could be reduced by either an increase in ventilation flow through the yacht to reduce the temperature rise for a given heat load and/or


B-29 disturbance


quantifies the extent to which the fluid undergoes a spiral motion, and is the dot product of the local vorticity and flow velocity. Helicity is a particularly effective metric with which to visualise flow disturbance. From Figure 9 it is seen that that high helicity occurs at the leading and trailing edges of each deck, at the bow and aft of the bridge and mast. The high helicity in the assessment volume is a combination of flow disturbances generated following separation from the bridge and mast, and from the leading edge of the upper deck.


to be assessed with reference to a 3s This period is approximately three


repositioning the ventilation


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