Laminar Separation Bubble Development on a Finite Wing

Abstract

This experimental study considers the development of a laminar separation bubble formed over a finite wing and contrasts it with the flow evolution over a two-dimensional airfoil. The experiments were performed on a reference NACA 0018 airfoil model and a finite wing with an aspect ratio of 2.5 at a Reynolds number of 125,000 involving surface pressure and particle image velocimetry measurements. For equivalent effective angles of attack, the results reveal significant differences between the two- and three-dimensional configurations. Specifically, increasing the effective angle of attack causes the separation bubble to shift upstream on the two-dimensional airfoil, whereas its mean position and streamwise extent remain invariant to spanwise changes in the effective angle on the finite wing. Similarly, the dominant shear layer frequency and characteristics of shear layer rollers do not vary appreciably over the wingspan. This suggests that the sectional analogy between the local effective angle on the wing and that on the airfoil is not universally applicable to the wing sections subjected to the formation of laminar separation bubbles. Instead, the spanwise characteristics of the bubble on the wing are well approximated by those obtained on the airfoil at the angle of attack matching the effective angle of the wing root. It should be noted, however, that these findings are exclusive of the regions in the immediate vicinity of the wing tip and/or root, which were not considered in the present investigations.