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Title: A CFD Study of the Aerodynamics of a Ship’s Bulky Enclosed Mast

Author(s): leuan Owen, Mark White, Paul Scott, Rebecca Mateer

Abstract: Modern warships, such as the UK’s Daring Class Type 45 destroyer, the German Sachen Class frigate or the Dutch Holland class offshore patrol vessel which carries a Thales integrated mast, have main masts that are solid bulky structures; very different to the lattice or pole masts of earlier ships. While the newer masts are carefully designed to reduce radar cross section, and provide a weather tight enclosure for sensors and systems, they also significantly affect the air flow over the ship. This paper will consider the aerodynamic characteristics of a generic ship’s superstructure with a bulky foremast that is sufficiently far forward that it will not significantly affect the airflow over the landing deck at the rear of the ship; it does however significantly affect the air flow in regions where the ship’s anemometers may be placed, and may entrain exhaust gases from the ship’s engines into the mast’s wake. The mast under consideration, based on a generic ship which has a design typical of modern vessels, has a square cross-section that reduces with height and acts as a bluff body that produces a large unsteady wake. Anemometers mounted on the yard-arm of a bulky mast In the work reported in this paper, CFD has been used to calculate the airflow over the ship for different wind directions with specific attention being paid to the unsteady velocities at various locations where a ship’s anemometer might typically be placed. The flow is seen to be highly disturbed and both the wind strength and angle are significantly different to those of the free stream wind, hence making a true wind over deck measurement difficult. Inaccurate anemometers have serious consequences for the ship’s helicopter, both in terms of determining the original Ship-Helicopter Operating Limits (SHOL) and in establishing the wind over deck conditions during launch and recovery. The results of the CFD calculations are shown to provide very useful guidance for anemometer placement. The generic ship under consideration has a gas turbine exhaust behind the main mast, and in a head wind the hot exhaust gases are seen to be entrained into the unsteady wake from the mast so that the gases are drawn into the lee of the mast with the potential to heat the surface upon which instrumentation may be mounted. The paper will quantify the temperature field due to the exhaust gas, and will also show how vortex shedding from the mast mixes the exhaust gas with the airwake, so quenching the hot gases and reducing temperatures over the helicopter deck.