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


resolved. Whilst LES was once only a tool available to academia it is now seeing increased use in industry.


In terms of airwake calculations, LES removes the assumption of isotropy used in the approach to derive the velocity component fluctuations from the turbulent kinetic energy, as employed in equation (5). This in turn improves the resolution of the turbulent structures in the wake.


However, despite the significant benefits of LES, it is not a panacea.


Whilst the computational requirements are


many orders of magnitude less than for DNS, they are considerable compared to standard steady RANS solutions. For example mesh sizes of 10-50 million cells are typical, the calculation must be run in unsteady mode and the data capture can only commence once the influence of the initial conditions have fully decayed and the flow statistics stabilised. The result is that based on today’s computing power a single LES calculation may take weeks to complete and generally require the use of high performance parallel processing clusters employing typically fifty computing nodes or more.


Therefore, at present LES is not considered a practical route for airwake certification, except for special cases where explicit understanding of the turbulence spectra is required. Instead, a comprehensive programme of steady RANS calculations provides a sufficiently robust and substantially more cost effective approach. However, it is envisaged that as computer power continues to increase, the cost of LES calculations will reduce to the point whereby such techniques will become practical for airwake calculations and LY2 certification.


6. CONCLUSIONS


This paper has discussed the need to understand the flow around a yacht’s superstructure to support certification for rotorcraft operations under commercial charter. The various approaches of experimental and computational analysis of airwake and their associated merits have been discussed.


It is apparent perturbations, that


parameters which are key to the safe operation of rotorcraft,


namely gust velocity presents


a


determination of the airwake and thermal


significant experimental


challenge. The certification agencies recognise these challenges by permitting certification based on detailed CFD assessments.


The results of an example CFD based airwake assessment have been presented for a typical super yacht design. The results show that


velocity and thermal perturbation can be


the variation of gust readily


examined in the assessment volume and that at the selected wind speed the design does not meet the LY2 airwake criteria.


Whilst the analysis considers a single operating condition (i.e. wind and forward speed) it 8. 7.


would be relatively simple to extend the half model to permit assessment under cross wind conditions. Once a model has been constructed the simulation and assessment process may be automated be such that a large number of operating conditions can be considered.


Compared to experimental studies, computational


assessment of airwake is relatively inexpensive. The major costs associated with CFD are the initial model build and mesh construction which for complex geometries may take on the order of 1-2 weeks to complete.


In addition to the outright assessment of airwake, a CFD based approach enables the designer to obtain an understanding of the behaviour of thermal plumes away from the helideck which may influence the comfort of passengers on patios and walkways.


7. ACKNOWLEDGEMENTS


The authors would like to thank Richard Underhill, Simon Rees and Elizabeth Morrison of Frazer-Nash Consultancy Ltd for their advice and guidance during the preparation of this paper.


8. 1.


2. 3. 4.


5. 6.


REFERENCES


Standards’, CAP 437 6th Ed., CAA Safety Regulation Group.


– Guidance


Civil Aviation Authority (2008), ‘Offshore Helicopter Landing Areas


on


Civil Aviation Authority (2008), ‘Validation of the Helicopter Turbulence Criterion to Offshore Platforms’, CAA Paper 2008/02 Study I.


Civil Aviation Authority (2008), ‘A Review of 0.9ms-1 Vertical Wind


Component


Helicopters Operating to Offshore Installations’, CAA Paper 2008/02 Study II.


Martime and Coastguard Agency (2008), ‘The Large Commercial Yacht Code (LY2)’, MSN 1972 (M) Ed. 2.


The Yacht Report (2007), ‘Flying Machines of the Future’, Issue 88, October 2007.


Yeh, Y. and Cummins, H. Z. (1964), ‘Localised Fluid Flow Measurements with a He-Ne Laser Spectrometer’, Applied Physics Letters 4, 176– 178.


Lienhart, H. and Becker, S. (2002), LDA Measurements of the Flow and Turbulence Structures in the Wake of a Simplified Car Model, in ‘Proceedings of the 11th International Symposium


on Applications Swales, C. and Breeze, of Laser


Techniques to Fluid Mechanics’, Lisbon, Portugal.


G. (1997), ‘LDV


Measurements Above the Flight Deck of a Model Frigate’, 35th AIAA Aerospace Sciences Meeting and Exhibit, AIAA-1997-712.


for


©2010: The Royal Institution of Naval Architects


B-31


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60