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Title: Introduction to Ship Airwake

Author(s): Susan Polsky

Abstract: This work summarizes efforts of NATO Research Task Group AVT-217 related to the impact of ship geometry on the airwake flow field, airwake modeling and simulation (M&S), and analysis of airwake as part of the ship design process. The flow of air over a ship’s superstructure creates an unsteady and complex airwake, the nature of which is highly dependent on the geometry of the ship. This complexity means that good M&S of a ship’s airwake is challenging. Techniques and guidelines for good airwake modelling have been under development for decades. Modelling the airwake without ship motion is the most basic form of simulation and can be achieved directly using either CFD or experimental methods. This type of modelling will be the focus of this presentation. Simulations including the coupled effects of the helicopter rotor on the airwake are more robust in that they include the effects of the helicopter, rotor, pilot, control system, or some combination of these factors. Some approaches adopt one-way coupling, in which the airwake influences the rotor but the rotor has no impact on the flowfield. This is a simplifying assumption since, in reality, the rotor impacts the flowfield. This approach is often used in real-time computational analysis, where a predetermined airwake model is used to compute equivalent fuselage and rotor forces assuming that the presence of the helicopter does not in turn affect the flowfield. This approach has been used with success for helicopter positions that are well back of the hangar face. Several studies have shown the importance of full or “two-way” coupling, especially in considering the presence of the downwash of a helicopter rotor on the fuselage. Computationally, this coupling can be estimated using numerical techniques, such as wake models, which can be superimposed on one-way coupled simulations to improve the fidelity. A summary US Navy and other techniques in this area will also be presented.