Study of an Adaptive Mechanical Turbulator for Control of Laminar Separation Bubbles

Abstract

Low Reynolds number experiments have been conducted in the North Carolina State University Subsonic Wind Tunnel Facility to determine the feasibility of using an adaptive mechanical turbulator to control in a predetermined manner a laminar separation bubble. The experimental set-up consists of a flat plate with an inverted airfoil mounted above its surface. This set-up permits laminar separation bubbles to be examined over a range of Reynolds numbers and pressure distributions. An SMA-actuated mechanical turbulator is developed in the present work. A multi-element hot-film array is used to detect the laminar separation, transition to turbulence, and turbulent reattachment in the laminar separation bubble. The effects of turbulator height and Reynolds number on the mean, the standard deviation, the skewness, and the kurtosis of the hot-film signals are examined. The experimental results agree well with the theoretical predictions of laminar separation, transition, and turbulent reattachment obtained using a viscous/inviscid analysis code.For the range of trip heights examined, that is up to 30 percent of boundary layer thickness, the adaptive boundary layer trip is not suited to provide proportional control of a laminar separation bubble, as no monotonic variation of location in separation, transition, or reattachment is observed with varying trip height. However, the hot-film measurements show promise for providing in real time the identification of the primary features of the laminar separation bubble.