Effect of unsteady and sinusoidally varying free stream on turbulent boundary layer separation

The problem of turbulent boundary layer separation due to adverse pressure gradient is an important factor in the design of many devices such as jet engines, airfoils, rocket nozzles, and helicopter blades and the design of many fluidic logic systems. As a result, wind tunnel experiments were conducted to investigate the separation developing on a flat-plate in steady and unsteady flow. The nature of upstream flow subjected to the effect of unsteady free stream velocity which changes sinusoidally with time due to the rotation of an elliptical cylinder placed downstream of the flat-plate has been studied in the wind tunnel test section. Separation points (or line) were measured in different angles of incidence and at various stations. Pitot-static-tube and hot-wire anemometry were used to measure the steady and instantaneous velocities and turbulence. The results indicated that the more the increase in the angle of incidence of the flat-plate in steady and unsteady flow, the more is the increase in the thickness of separated region which extends further towards the flat-plate trailing edge. Reduced frequency also caused the separation points to move up away from the flat-plate surface but this movement was restricted to reduced frequencies less than 2 after which the changes were negligible.