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


outlets away from the helideck. In practice, increasing the ventilation flow will produce high ventilation exit velocities and therefore may contribute to further increases in gust velocity above the helideck or indeed unacceptable noise.


Therefore, it may be preferred to


consider a redesign of the internal ventilation system to relocate the outlet to a lower position. This is discussed further in Section 4.5(d).


location in order to resolve the momentum and dilution of warm air, and allow thermal perturbations in the patio region to be quantified. However, for the purposes of exploring various new ventilation concepts, the tracking of flow pathlines on an existing solution is informative.


Figure 10: Isosurface of a temperature perturbation of 2°C above ambient (red) showing intersection with the airwake assessment volume (light blue)


4.5 (d) Comfort


Whilst repositioning the ventilation outlets away from the helideck may enable the certification of rotorcraft operations


via LY2, it may significantly alter the


perception of the yacht in terms of owner’s comfort. Namely the possibility for warm air being undesirably directed across external walkways and patio areas for some wind and forward speed combinations.


Figure 11 examines the possibility of repositioning the ventilation outlets on the hull sides and shows the likely propagation of the warm air. Two ventilation outlets are considered: an upper location coloured yellow and lower location coloured green with pathlines which are coloured corresponding to the outlets from which they are released.


The pathlines from the upper outlet (yellow) indicate that there is a tendency for warm air to be directed along the lower walkways and across the patio. The lower outlet configuration (green) eliminates flow into the walkways and reduces the area of the patio over which the warm ventilation air could flow.


Although the lower outlet


appears to be preferable in this case, there are additional factors which will influence the final design decision, namely down flooding and stability.


It should be noted that this depiction uses the existing CFD solution. In reality, it would be necessary to perform


a supplementary CFD solution with the ventilation flows applied in the revised and correct


Figure 11: Pathlines released from two candidate ventilation outlet locations (shown in yellow and green), indicating potential for interaction with owner’s patio area.


5. FUTURE ADVANCES IN MODELLING As noted in Section 4.3, one of the fundamental


requirements of Computational Fluid Dynamics is the ability to model reliably the influence of turbulence. As computing power has advanced another possibility has emerged which is


the Simulation as opposed to Modelling of turbulence.


It has long been known that in theory the influence of turbulence can be resolved


modelling of very low Reynolds number flows (so called Direction Numerical Simulation, DNS).


without the need for The challenge


in DNS is to conduct the calculation with a mesh and time step fine enough to resolve the smallest eddies which influence the largest eddies. It is possible to show from first principles that the number of calculations which would be required to conduct such a solution is proportional to Re3. For practical engineering flows, for example airwakes, Re >> 106 and the computational requirement is vast and could not be achieved even with modern high performance computers.


One route around the computational requirement of DNS is to directly simulate the large eddy structures (i.e. those comparable in size to a rotorcraft or plume), whilst using a simplified numerical model to represent the dissipation of energy at the small energy scales.


Large Eddy Simulation (LES). The benefit of the LES approach is that the dependency on modelling is reduced such that only the smaller and more universal simpler) eddies need be approximated.


The larger and more complex three-dimensional eddies are directly


This is termed (i.e.


B-30


©2010: The Royal Institution of Naval Architects


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