The aerodynamics of a modern warship

Although modern warships routinely operate with on-board helicopters, the effect of the ship superstructure aerodynamics on the flying environment around the ship is generally not considered in the design process. The modern warship is now well designed for stealth in warfare, but the now bulkier, smooth-sided and sharp-edged superstructures create a significant disturbance to the oncoming wind, resulting in turbulent flow features in the lee of the ship, most often across the stern where the ship's flight deck is located. Such conditions, coupled with hot exhaust gases from the ship's gas turbine engines, everchanging weather resulting in various sea-states, sea-spray, rain, fog, etc. create a potentially very hazardous operational environment. In addition, as the launch of unmanned aircraft (UA) vehicles from the flight deck is becoming commonplace, their successful launch and recovery is also directly impacted by the scale of the turbulent flow features in the vicinity of the flight deck. Through the use of time-accurate Computational Fluid Dynamics (CFD), this thesis addresses the impact of modern warship aerodynamics on helicopter and UAV operations. Full-scale, time-accurate, unsteady airwakes have been produced for an early design of the UK Royal Navy's new Global Combat Ship (GCS), the Type 26 City-Class, and its predecessor, the Type 23 Duke-Class. The larger, stealthier design of the GCS has been shown to produce a more aggressive airwake compared with its predecessor, resulting in higher turbulence levels across the flight deck and greater aerodynamic loading on a helicopter. The larger main mast of the GCS also has a clear effect on the accuracy of the ship's anemometer readings when compared with Type 23 thereby making it difficult to determine if the wind conditions are safe for helicopter launch and recovery. Simulation of the Type 23 and GCS exhaust gas efflux has shown that the more turbulent airwake of the GCS aids the cooling and dispersion of the ship's exhaust gases with air temperatures above the flight deck being comparatively lower for the GCS in headwind and Green 30 wind-over-deck (WOD) conditions. As such, the dispersing engine exhaust plume of Type 23 presents a greater threat to the helicopter. Detailed analysis of the air flow around the GCS main mast has shown how CFD can be used to predict flow distortion at the ship's anemometers. Issues with current practice for positioning anemometers on bulky main mast structures, detailed in the defence standard, DEFSTAN 00-133 Part 2, have been highlighted. Findings show that the defence standard should be modified to take account of the new ship geometries that are emerging; at the least, anemometers should be placed as high up the mast structure as possible on yard arms of suitable length. Additionally, an appropriately placed aft anemometer is recommended to yield more accurate wind measurements from astern. Analysis of the exhaust gas dispersion from the GCS has shown that significant cooling of the plume is achieved through the hot exhaust gases mixing with the ship's turbulent airwake, but elevated, unsteady temperatures are present in the vicinity of the flight deck, particularly in the port side hover position in a Green 30 WOD, that can adversely affect an operating helicopter. The addition of an eductor to the exhaust outlet does reduce temperatures above the flight deck, but the temperatures remain elevated and unsteady. A new method for simulating the launch of lightweight unmanned aircraft into the turbulent CFD-generated ship airwake has been also developed. The simulation environment has shown potential for developing Unmanned Aircraft (UA) for maritime applications, for developing appropriate flight controllers, and for developing appropriate launch and recovery operational procedures.