LaminarFlowSeparation andTransition on a Low-Reynolds-Number Airfoil

A NUMBER of military and civilian applications require efficient operation of airfoils in low chord Reynolds numbers. The applications include propellers, sailplanes, ultralight mancarrying/man-powered aircraft, high-altitude vehicles, wind turbines, unmanned aerial vehicles, and micro air vehicles. Careful management of boundary layers on low-Reynolds-number airfoils is required to alleviate the deterioration in airfoil performance. Although extensive investigations have been conducted to study laminar flow separation and transition on low-Reynolds-number airfoils, most previous experimental studies were carried out by using single-point-based flowdiagnostic techniques such as hot-wire anemometry [1,2] or laser Doppler velocimetry [3,4] to conduct flow measurements. A common shortcoming of single-point-based flow measurements is the incapability of providing spatial correlation of unsteady flow structures to effectively reveal the transient behavior of laminar flow separation. Temporally synchronized and spatially resolved flowfield measurements are highly desirable to elucidate underlying physics to improve our understanding about laminar flow separation and transition on low-Reynolds-number airfoils. Although several studies have been conducted recently by using the particle image velocimetry (PIV) technique to provide temporally synchronized and spatially resolved flowfield measurements to reveal transient behavior of laminar flow separation on lowReynolds-number airfoils [5–7], very little in the literature can be found to correlate detailed flowfield measurements with airfoil surface-pressure measurements to gain further insight into the fundamental physics associatedwith laminarflow separation on lowReynolds-number airfoils. In the present study, an experimental investigation was conducted to elucidate the underlying physics associated with separation, transition, and reattachment of a laminar boundary layer on a low-Reynolds-number airfoil. In addition to mapping surface-pressure distribution around the airfoil with pressure sensors, a high-resolution PIV system was used to make detailed flowmeasurements to quantify the occurrence and behavior of laminar boundary-layer separation, transition, and reattachment on the low-Reynolds-number airfoil. The detailed flowfield measurements were quantitatively correlated with the surfacepressure measurements. To the best knowledge of the authors, this is the first effort of its nature. The objective of the present study is to gain further insight into the fundamental physics of laminar flow separation and transition and the evolution of laminar separation bubbles formed on low-Reynolds-number airfoils.