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


to examine the dispersion of the plumes around the yacht.


For some limited cases it may be possible to combine the measurement of the velocity, thermal field and concentration


using a single complex experimental


arrangement or a multiple probe holder. However, it is envisaged that usually such measurements would be conducted independently and therefore the timescale of the overall


experimental


considerable. 3.2


MODELLING APPROACHES Figure 2: Schematic showing the optical bench arrangement of a multi-component DGV system [12].


In summary, some laser methods provide considerable benefits over probe methods in terms of efficient data capture within planar regions. These benefits afford reduced timescales to map the airwake field and better understanding of the


flow structure, such that the


additional costs of hiring equipment may be readily offset. However, the notable disadvantage with such methods is the required level of expertise and familiarity with the techniques, not withstanding the additional safety implications of


using high-powered Class IV


lasers. 3.1 (c) Experimental Programme


Whilst the discussion of techniques presented in Sections 3.1(a) and (b) has concentrated on measurement of the velocity field within the airwake, it is also important for certification to characterise the behaviour of thermal plumes.


Practically it is difficult to ensure correct


scaling of the flow advection and buoyancy induced flows within the wind tunnel. A key feature of LY2 is the requirement to calculate the 3s averaged temperature rise within the landing region. For a scale model, where the incoming flow speed is set by matching Re, the averaging time will reduce by the scale squared (i.e. 0.03s for a 10th scale model). This will in turn require the use of a probe with a very fine thermocouple junction such that the thermal fluctuations can be resolved on this timescale and for some model scales this may not be achievable.


Although not directly required by LY2, poor dispersion of exhaust plumes can adversely affect comfort and air quality aboard the yacht and the effectiveness


machinery space ventilation systems. Whilst it is in theory feasible to release gases of suitable composition to explicitly measure


dispersion by sampling the air


composition locally, a more pragmatic approach is to conduct qualitative flow visualisation by releasing smoke


of 3.2 (b) Flow Resolution


Although the most advanced laser based experimental techniques are termed wholefield they are essentially planar in nature. Therefore the entire flow field must be constructed by combining results


3.2 (a) Model Build


The major cost associated with CFD analysis is the initial build of the computer model and the construction of the mesh which is used to spatially discretise the flow domain. Modern CFD modelling tools can import geometry direct from CAD packages used for naval architecture.


As noted in Section 2.2, both CAP437 and LY2 recognise the recent advances in computational modelling techniques and permit assessment of airwake for helicopter operations by either experimental testing or CFD analysis.


CFD techniques divide the flow domain into a large number of small volumes in which the flow properties (i.e. velocities, temperature and pressure) are to be determined. The local flow conditions are calculated by iteratively solving the


fundamental Navier-Stokes


equations which describe the conservation of mass, momentum and energy within the fluid. CFD techniques have a number of notable advantages, and disadvantages, over experimental testing and these are discussed below.


programme would be


Most modern mesh generators routinely


handle in excess of 5 million cells and are remarkably resilient to poor CAD connectively such that the CFD model can, to a large part, be built directly from the supplied geometry.


Once the model is built and the boundary conditions set, the only real expense is computer processing time with little human input required. computational


power, namely ever smaller design features can be


Given recent advances in the development


of


multiple core processors and the introduction of parallel computing, simulations which would have once taken weeks to complete are now achieved within days. Additionally,


included in the model thereby improving the resolution of the solution.


from a number of


©2010: The Royal Institution of Naval Architects


B-25


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