Unsteady measurements of a three-dimensional high-lift system

An experimental study of the unsteady fluid dynamics around a three-element high-lift system was performed. Four different measurement techniques were used to quantify the flowfield: hot-wire anemometry, unsteady pressure measurements, acoustic measurements, and particle image velocimetry. From these measurements, it was discovered that the unsteady flowfield around a leadingedge slat is extremely complex. In addition to unsteadiness from separating shear layers, the fluid dynamics are affected by a coupling between the acoustic and hydrodynamic disturbances. The slat trailing edge and slat gap play a primary role in contributing to unsteadiness in the slat cove. An acoustic resonance in the slat gap interacts with the vortex shedding of the slat trailing edge, and also stimulates shedding from the cusped edge on the lower surface of the slat. This acoustic excitation amplifies certain frequencies of vortex shedding and also induces an effect known as “lockin”. The magnitude of the shedding is largely a function of the state of the boundary layer at the slat trailing edge. Configurations with laminar boundary layers have larger amplitude disturbances in velocity and pressure. The magnitude of the slat disturbances may also be affected by the relative thickness of the slat trailing edge. A slight nonlinear behavior of the vortex shedding frequency may be attributed to an acoustic feedback mechanism. The total sound pressure level of the slat noise scales with velocity to the fifth power. Serrating the trailing edge of the slat significantly reduces the radiated noise, as well as fluctuations of velocity and pressure in the slat cove. The discovered noise mechanisms may scale to full-size aircraft. The flowfield around the flap tip is dominated by a primary vortex rolling out from the lower surface of the flap. A secondary vortex also exists which originates from the upper surface, although, this secondary vortex is quickly assimilated with the primary vortex and is only of importance in the near field.