NATO STO

Both underwater vehicles (UV) and aircraft are increasingly being called upon to perform non-traditional maneuvers, which involve low speeds and precision maneuvering. During these maneuvers, the angles of incidence often exceed 10-15o where the flow separates, often times forming large-scale vortex structures. These flow separations are the cause of extreme lift, drag, and pitching moments. Many classes of UVs are based on bodies of revolution, which feature continuous, doubly-curved convex surfaces. The hydrodynamics over these types of geometries are difficult to predict because there are no geometric discontinuities to trigger separation at exact, and therefore, easily predictable, locations. Correspondingly aircraft, ranging from low-aspect ratio fixed-wing planforms to curved helicopter fuselages, suffer from similar flow separation processes, particularly when operating in transient (gusty) environments. The flow separations on smoothly curved bodies are dependent on the flow history, which may include laminar-turbulent transition, streamline curvature, pressure gradients and hull roughness as well as extrinsic variables such as the steady and unsteady body motions and rapid changes in angles of incidence via for instance external currents. Intrinsic flow forcing may occur in steady mean flows due to large-scale shedding phenomena. Furthermore, UV hull surfaces are never hydrodynamically smooth as manufacturing imperfections (e.g., weld seams) and biofouling are endemic. Roughness increases the boundary layer thickness and may ameliorate the pressure gradients meaning that it may play an important role in the physics of separation of smoothly-curved bodies.