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Many engineering flows, including platform aerodynamics, are dominated by the effects of turbulence. There is no single turbulence model that applies universally to all flows. However there are a number of approaches for engineering applications that have known ranges of validity and can be used with good judgement. It is, nevertheless, best practice to validate CFD results by comparison with physical measurements, or to follow procedures that have been established as valid in this way [Ref: 61].
Direct and Large Eddy Simulation techniques have shown potential to predict turbulence with reasonable accuracy but are not practical for helideck design due to the excessive computing power and simulation time required. The most common approach is to use a RANS turbulence model in which time averaged (or occasionally ensemble averaged, for transient flows) values of the flow quantities are solved. The role of the turbulence model is twofold. Firstly, it modifies the mean flow field velocities, pressures and temperatures, and secondly it provides a measure of the turbulence within the flow. Most commonly, this takes the form of the turbulent kinetic energy and the dominant length or time scale of the energy containing eddies. Both can be directly related to simple statistical properties of the turbulence.
Strengths and Weaknesses of the Modelling Techniques
Both CFD and wind tunnel testing can provide key information for the design of offshore helidecks. The main strengths and weaknesses of each can be summarised as (assuming best practice in each case):
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On balance, wind tunnel tests can provide reliable flow data for the safe design of particular helidecks, whereas CFD is a tool best employed to provide guidance on the effect of design variations and local flow features
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Wind tunnel testing will give, directly, measured data for turbulent fluctuations, such as peak values, necessary for comparison with helideck design guidance
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Extracting quality estimates for turbulence data from CFD requires specialist expertise in application and interpretation
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Wind tunnel tests for helideck wind flows are normally not affected by modelling at small model scale (Reynolds Number effects), but care should be taken to ensure that this is the case and to suitably condition the experiments if necessary
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CFD can provide results at full-scale flow conditions and hence model consistently buoyancy (Froude Number) and turbulence (Reynolds Number) effects
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Although some comparisons with full-scale measurements have been made, neither technique can be said to have been fully validated at full scale
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CFD results are available for the entire flow field. Wind tunnel data is available at the instrumented measurement locations, although a large number of measurements can be obtained in a relatively short period of time
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Used without sufficient training and experience of the problem in hand, poor quality spurious results are easy to achieve with CFD, and the accessibility of this tool makes this, perhaps more likely, than with wind tunnel testing.
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本文链接地址：离岸直升机起落甲板设计指南 OFFSHORE HELIDECK DESIGN GUIDELINES 2(22)
