Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019 2. STUDIES ON SHIP AIRWAKE
The problem of ship helo interaction finds considerable resonance with classical problems of 2D Backward Facing Step (BFS) and flow around 3D bluff bodies which have been researched extensively in the period 1980 to 2000. Prior to 1980, several countries having advanced Navies have regularly used wind tunnel experiments (mainly flow visualisation) to understand the complex flow conditions on the super structure of warships. However, coinciding with the advent of high power computing, the application of numerical techniques using CFD to attempt solutions for these problems has been a recent development. Praveen et al (2013 & 2014) have undertaken a review of literature commencing from the 2D BFS, separated flow around bluff bodies, and the experimental and numerical studies conducted on ship airwake. Though not made part of the present review, the research literature referred to in the above publications have been indicated in the references (4 to 28) for the sake of completeness. These publications have been frequently referred to by most of the authors working in the field of warship-helo interaction. In the present publication, it is thus intended to take forward the review process (with some additions of even the older literature not covered in the above references) and arrive at definite conclusions from the literature survey which will help researchers in the field to improve the allowable limits of environmental conditions for safe ship-borne helo operations and arrive at novel solutions towards a good design of helodeck on warships without compromising on stealth.
The early literature specific to ship helo interface originated from the Naval Postgraduate School (NPS), USA, in the late 1980s under the guidance of Prof J Val Healy. The works of NPS, during this time, were purely experimental and mainly related to qualitative analysis of helodeck flow using flow visualisation techniques such as smoke, helium bubble, tufts, oil streaks etc. The effect of cross flow components on the Wind over deck (WOD) angles other than 0 degrees, which occur quite frequently on helodeck at sea have been studied by Johns (1988), Daley (1988) and Rhoades (1990). Johns (1988) has undertaken flow visualisation studies on a DD 963 "Spruance" class destroyer with helium bubbles and smoke and concluded that results obtained give a good insight of the flow. He has shown that at moderate WOD angles, large coherent structures are shed from the side edge of the hangar at regular intervals.
Anderson (1989), as part of continuation of the research at NPS, undertook quantitative measurements using hot wire and hot film anemometry at single points in the flow field over the glide paths of 30 degrees port and starboard and stern approach. He analysed various statistical quantities such as mean, standard deviation, autocorrelation etc and derived velocity components, turbulence intensities and energies present in the turbulence which could aid in mathematically modelling of the flow at a later stage. In order to accurately map the flow field, the work involved
modelling of atmospheric boundary layer in the low speed wind tunnel. It was ascertained during the course of the literature survey that these statistical estimates for turbulence, form the basis of present day practice of fixing criteria for design of marine environment for helo landing, as observed in Offshore Industry. These aspects have been discussed in the following sections.
Though the NPS literature provides several possible solutions for conducting meaningful flow visualisation in wind tunnel and plausible indication towards the quantities which may matter most for evaluating the pilot workload, the results contained are very limited owing probably to the confidential nature of the work and may not be very relevant to the modern ship forms. However, while connecting up these literature survey findings with the future work, especially with Offshore Industry design standards in vogue, it emerges that the path adopted by NPS for research forms a very good base for work on ship helo interface for non aviation ships and is worthy to emulate in certain parts for the present day research.
Most of the researchers, in the recent times, have conducted both experimental and numerical work on the ship-helo interface. Accordingly, the publications of the researchers cover both these aspects in tandem. Considerable amount of work has been undertaken in the field of shipborne helicopter operations in the United Kingdom and especially at University of Liverpool in the past two decades. Kaaria et al (2013) have undertaken experimental investigations using Airdyn on a model scale helicopter mounted on a six axis force balance to measure the unsteady forces and moments on the rotor due to the airwake of ship with an aim to modify the ship airwake on the helodeck using flow modification devices. Investigations have been undertaken on a baseline model and modified ship geometries. They have also reviewed the previous work undertaken in this field by others. Forrest et al (2016) have examined the effectiveness of five hangar edge modifications using CFD and flight simulation modelling and also piloted flight trials in a motion based simulator. Results are presented, in terms of unsteady helicopter loads and pilot workload ratings, for modifications to the windward vertical rear edge of the hangar with an oblique wind. Of the five flow modification devices, chamfer and flap were observed to reduce turbulence over the flight deck while tabs, saw tooth and cylinder on the vertical edge were observed to increase turbulence.
A comprehensive review of the problem of recovery of large helicopters on small naval ships has been undertaken by Lumsden et al (1998). It also reviews the role of CFD and piloted flight simulations towards safer methods of assessment of ship helo operating limits. Newman (2004) has reviewed the changes in the characteristics of forces and moments acting through the main rotor for helicopters while operating on ship helodeck environment. He has also brought out the procedures adopted for the shipborne helo operations.
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©2019: The Royal Institution of Naval Architects
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