| Near-surface flow patterns along a basic delta wing of moderate sweep angle, representative of key features of Unmanned Combat Air Vehicles (UCAVs) and Micro Air Vehicles (MAVs), are visualized by a technique of high-image-density digital particle image velocimetry (DPIV), which provides quantitative representations of the whole-field flow patterns. Due to the highly three-dimensional nature of the flow patterns, they are also visualized by stereoscopic particle image velocimetry (SPIV). Qualitative dye visualization is employed to complement the DPIV technique.;The flow structure is represented by patterns of dye, velocity vectors, streamwise, transverse and out-of-plane velocity components, streamline topology and vorticity. The surface topology, i.e., surface streamlines, and patterns of surface velocity and vorticity oriented normal to the surface of the wing, are investigated by making use of topological rules and critical point theory.;For the case of DPIV measurements, the focus is on the time evolution of the surface topology during relaxation of the flow after termination of a pitching maneuver, for a wide range of pitch rates. It is demonstrated that there exists a critical universal state, which marks an abrupt transformation between two distinctly different states of the near-surface pattern of critical points. Moreover, an approach that predicts the occurrence of three-dimensional separation from the surface of the wing, for a wide range of pitch rate, is introduced.;For the case of SPIV measurements, the relationship between the three-dimensional flow structure above the surface of the wing and the near-surface topology along the wing has been established, at successive instants following termination of the maneuver. Features of the leading-edge vortex and its breakdown location were quantitatively determined at the termination of the pitching maneuver. For the relaxed state of the flow structure, there is a reference elevation above the wing surface, at which a marked change in the sectional patterns of streamline topology occurs. Prior to the present investigation, this structure of the large-scale vortical motion associated with three-dimensional separation and stall had not yet been identified.;Finally, the effect of a passive control technique in the form of a sinusoidally-shaped leading-edge, on near-surface flow patterns was determined for a wide range of amplitude and wavelength. The near-surface topology was found to be very sensitive to the slight changes in the leading-edge geometry, and is a strong function of its amplitude and wavelength. The dimensionless ratio lambda/ϕ of wavelength to amplitude was found to be a predominant parameter. It turns out that the near-surface flow structure is substantially altered, for relatively small values of lambda/ϕ, and the largest changes were obtained by keeping the wavelength lambda/C small and the amplitude ϕ/C large. These alterations involve either a decrease in the extent of three-dimensional separation or its elimination altogether. In fact, the patterns on a wing with a sinusoidally-shaped leading-edge at high angle-of-attack resemble those on a wing with a straight leading-edge at a substantially lower angle-of-attack. |
