Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


use of the turbulence criteria measured in terms of the standard deviation of the vertical velocity was also indicated in research at NPS, USA by Prof Val Healy earlier for the ship helo interaction problem. An important lesson that can be drawn from the above is that lower the standard deviation of vertical velocity on the planes of rotation of helicopter rotor, lower is the turbulence and hence the pilot workload.


Figure 5 depicts the methodology adopted for the research work which led to the determination of the Turbulence Criteria for CAP 437. The left side column shows the reality of flight operations on offshore helodeck. The right side columns represent the simulation and modelling efforts. This included estimation of pilot effort scales in the given ship environment through piloted flight simulations involving test pilots and desktop simulations with a FLIGHTLAB model of helicopter. As part of validation discussions, the research brings out that both the piloted flight simulation and the desktop simulation depend on the wind tunnel adequately modelling the turbulent flow around the offshore platform. They both also depend on the FLIGHTLAB simulation software being an adequate representation of the aerodynamic properties and dynamic response of the helicopter.


With regards to correctness of the turbulence measurements in the wind tunnel, the document brings out the following:- "So far as is known there have been no direct field measurements of turbulence in the wake of offshore platforms that have been compared with equivalent wind tunnel measurements of turbulence to provide a direct validation of the turbulence measured in the wind tunnel model. It would be quite difficult and expensive to perform such a validation. The key potential scale effect that could cause the model to differ from reality is that due to viscosity, and represented by Reynolds Number. The Reynolds Number is much lower in the wind tunnel than on the full-scale platform. However, classical fluid mechanics tells us that the flow around bluff sharp-edged bodies is not much influenced by Reynolds number, and all the circumstantial evidence (e.g. the independence of drag force on Reynolds number for such shapes) indicates that, provided the natural wind is well represented in the wind tunnel, we can expect the wake flows around the platform to be well represented also." This conclusion has a direct bearing on wind tunnel experiments being conducted on warship-helodeck configurations and indicates that such an exercise is worthwhile in representing the real separated flow behind the hangar on the helodeck.


Figure 5. Modelling and Simulation work linking turbulence with pilot workload and flight safety (Courtesy- CAA 2004/03 (CAA, 2004))


A-406


©2019: The Royal Institution of Naval Architects


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