Measurement and analysis of the flowfields induced by suction perforations

The flow physics of laminar flow control suction surfaces is revealed by performing a detailed fundamental experimental investigation of an isolated suction perforation. A unique series of nonintrusive, high-resolution measurements are obtained using a three-component laser Doppler velocimetry system, and experiments are conducted in a low-speed, low-turbulence wind tunnel. The suction perforation flowfields are mapped for a range of sub- and supercritical suction rates and are found to be highly three dimensional. A rich variety of flowfield features is observed, including a pair of counter-rotating longitudinal vortices, multiple corotating longitudinal vortices, span wise variations of the mean flow, and inherently unstable boundary-layer profiles. Critical suction limits, over a range of freestream speeds, are determined, and a new design criterion for critical suction is established. It is also shown that for sufficiently small perforations, irrespective of suction flow, boundary-layer transition does not occur. Further analyses of the measurements explore the possibility of interaction between the crossflow vortices and the suction-induced longitudinal vortices. The suction-induced transition process is discovered to commence with an instability of the longitudinal vortices. Engineering design criteria, i.e., suction limits and perforation spacings, are established.