Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019
range of wind speeds on full scale ships and the reattachment zones for 15 knots and 30 knots of wind speed will occur essentially at the same position.
n) To accurately resolve all the flow features: turbulence, boundary layer, flow separation and interaction of helicopter flow-field with the ship airwake, in a time accurate simulation is a big challenge. The magnitude of the unsteadiness demands long time records which, coupled with large grids associated with such geometries, require enormous computational power. Full-scale or wind tunnel experiments, on the other hand, can easily generate long time records of data, but the associated initial setup cost and time can be substantial. Taking into cognisance the need to provide quick solutions for an improved environment over helodeck for helo operations, and not so much towards capturing the intricate flow details, steady state CFD calculations can be used, with an understanding that the results of the flow improvement techniques with the steady state analysis will not change drastically even when used with unsteady analysis, as far as the steady state CFD model is validated against experimental results.
4.
A ROADMAP FOR FUTURE INVESTIGATION OF SHIP-HELO INTERFACE
The phrase 'Pilot Workload' or 'Pilot Effort Scale' used for grading of the helodeck environment, is a qualitative assessment of a test pilot which involves several environmental factors interacting in a seemingly random manner. These factors acting together, put up a formidable challenge to the helo pilot for ensuring safe operations. As a first step, a graded approach is required to breakdown the complex problem into smaller parts and understand the effect of these components on the pilot workload in isolation. If, during the course of the research, the contribution of various components to the qualitative Pilot workload can be quantified in some manner, a range of problems faced presently with respect to on-site qualification processes adopted for ship-helo interaction, can be solved through experimental and numerical methods. While such a knowledge will help reduce the on- site evaluation efforts being employed presently through SHOL trials, it can also be employed for improving the helodeck environment for safer helo operations. Once a method for quantification is established for the various components, "benchmark minimum acceptable numeric values" can be established based on experimental/ numerical evaluation of a ship helo combination for which SHOL trial results have been established as acceptable. The efficacy of other geometries, with respect to the different components adding up to the Pilot workload, can then be assessed against these benchmarked values for acceptance.
Considering the airwake component, which has been taken up for the present analysis, it is understood that one of the most important gaps, as on date, is the absence of a
criteria set to grade a particular combination of ship and helicopter for ensuring minimum standards of safe operations. Such a criteria-based approach (with numerical limits on flow parameters) has been established by civil aviation authorities, through years of research, for helo operations from offshore structures. Considering that the fields of helo operation on warship and on offshore structures are dissimilar and inherit their peculiar problems from their grossly different geometries and operating environment, informed choices can be made while drawing lessons from such literature, on setting up of a criteria set for helo operations from warships. Thus, while it is understood that the numeric limits used for offshore industry will not be directly applicable for warship helo operations, limited guidelines on selection of flow parameters (to form up the criteria set) can be adopted from them since the flow parameters affecting the Pilot Workload will remain same for both the fields. Accordingly, a criteria set comprising of the following parameters, is proposed for grading a particular design against competing configurations:-
a) As part of the turbulence criteria used for offshore helodeck design, the design guidance document CAP 437 (2010) places an upper limit on the standard deviation of instantaneous vertical velocity. Accordingly, the first criteria points towards a reduced value of Standard Deviation of Instantaneous Vertical Velocity component on rotor plane for reduced Pilot Workload. While working in the numerical environment, the endeavor should be to reduce the average Turbulence Intensity values.
b) Before arriving at the criteria for turbulence, the UK Civil Aviation Authority, for a long time, had stipulated a criteria for maximum limit on the mean of the vertical component of velocity on helodeck. This was termed as the Downdraft Criteria. Though this criteria has been abolished for offshore structures in the wake of the new turbulence criteria, literature brings out that this flow parameter can become very important for warship helo operations. Typically downdraft reduces power and torque margins that are available to the pilot for control of helo, by forcing changes to the lift generation by rotors. For a geometry like that of offshore helodecks (which has a very open architecture), both high turbulence and high downdrafts can be associated with higher winds. High winds mean availability of increased power and torque margins in the hands of pilot which allows the pilot to handle higher degree of control even with the presence of downdraft. Hence abolishing the downdraft criteria for offshore industry, in the wake of turbulence criteria, has been resorted to. However, the downstream of warship superstructure provides an environment on helodeck with low relative wind speeds, a high turbulence and a high downdraft due to the presence of the superstructure airwake. In such a scenario, literature suggests a combination of both turbulence and downdraft to be considered for setting criteria limits. Accordingly, for reducing pilot
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©2019: The Royal Institution of Naval Architects
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