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The correct density ratio can be achieved in two ways. Heated air can be used where the model release temperature is equal to the full-scale temperature. There are practical disadvantages associated with this method in setting the high temperatures of around 500°C in a wind tunnel. A practical alternative is to release a buoyant gas mixture (e.g. helium-air) at ambient temperature with a density equal to that of the full-scale exhaust plume. The local density decay of the gas mixture is used as a direct analogue of the temperature decay. Any gas mixture can be used provided that there is a convenient way to measure its concentration.
The measurement of wind speeds above the helideck should be carried out using instrumentation capable of resolving velocity and turbulence components. Hot wire anemometry is the most widely used technique although laser anemometry is an alternative.
Computational Fluid Dynamics (CFD)
CFD methods allow engineers to model the behaviour of three dimensional, turbulent, fluid flows by computer. The fundamental aim of CFD is the solution of equations representing the conservation of mass, fluid momentum and energy, throughout a computational domain which contains a geometrical model of the object of interest (e.g. an offshore platform), and is contained within boundaries upon which known values or behaviours of the flow can be defined (boundary conditions).
Solutions are achieved within a defined computational domain using numerical techniques. Among commercially available CFD computer programs, the so-called finite volume method has become the most popular, mainly for reasons of computational speed, versatility and robustness compared to other numerical techniques. As its name suggests, the domain of interest is sub-divided into many smaller volumes or elements to form a three dimensional grid. Volume averaged values of fluid variables are located at points within this grid, and local numerical approximations to the conservation equations used to form a very large system of coupled, simultaneous equations. When known boundary conditions are applied, these equations can be solved to obtain averaged quantities for each variable at every grid point in the flow domain.
The extents of the computational domain should be sufficiently large to avoid any numerical influence of these boundaries on the flow around the platform in accordance with best practice guidelines [Ref: 61]. Typically, this should extend several platform diameters away from the object of interest in all directions with an extended computational domain in the downstream wake region. A marine atmospheric boundary layer profile of velocity and turbulence should be generated at the upstream boundary and maintained throughout the computational domain using suitable roughness properties for the sea.
To obtain good quality CFD solutions, a sufficient number of finite volumes (grid density) must be used, and their ‘quality’ must be such that the numerical approximations used retain their formal mathematical accuracy. The grid density should be sufficient to fit both geometrical features and flow behaviour (such as shear layers and eddies). The overall aim is to achieve, as closely as practicable, so-called ‘grid-independent’ solutions of the numerical formulations of the mass, momentum and energy conservation equations. This becomes more difficult, of course, as the Reynolds Number and the range of geometrical scales is increased.
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离岸直升机起落甲板设计指南 OFFSHORE HELIDECK DESIGN GUIDELINES 2(21)
