Investigation of the near-field tip vortex behind an oscillating wing

Abstract

The near-field tip-vortex flow structure behind an oscillating NACA 0015 wing was investigated at ${\hbox {{\it Re}}}\,{=}\,1.86 \times 10^{5}$. For attached-flow and light-stall oscillations, a small hysteretic property existed between the pitch-up and pitch-down motion, and many of the vortex flow features were found to be qualitatively similar to those of a static wing. For deep-stall oscillations, the wing oscillations imposed a strong discrepancy in contour shapes and magnitudes between the pitch-up and pitch-down phases of the oscillation cycle. The vortex was less organized during pitch-down (as a result of leading-edge-vortex-induced massive flow separation) than during pitch-up. The tangential velocity, circulation and lift-induced drag increased progressively with the airfoil incidence, and had higher magnitudes during pitch-up than during pitch-down, while varying slightly with the downstream distance. The vortex size, however, was larger during pitch-down than during pitch-up. The axial flow was always wake-like during the deep-stall oscillation cycle. The normalized circulation within the inner region of the tip vortex also exhibited a self-similar structure, similar to that of a static wing, and was insensitive to the reduced frequency.

The near and far wake of Pallas' long tongued bat (Glossophaga soricina)

“…The merger of two like-signed vortices (e.g. Melander et al, 1988;Meunier and Lewecke, 2005) is accompanied by an increased circulation of the new merged object, compared with each of its components (the total circulation is conserved), and the increase in circulation of trailing vortices as they entrain vorticity from their surroundings has also been shown in the near wake of an oscillating wing (Birch and Lee, 2005).…”

Tracer particle momentum effects in vortex flows

“…Velocity and pressure field measurements in the wake of an aircraft model are a valuable resource in predicting the performance characteristics of the aircraft (Spalart 1998b;Rossow 1999), as well as a means to investigate the more fundamental nature of vortex flows (see, for example, Bandyopadhyay et al (1991) and Phillips & Graham (1984)). However, single-point scans of vortex flows (such as the hot-wire and multi-hole pressure probe data of Beninati & Marshall (2005), Birch et al (2004), Birch & Lee (2005), Chow et al (1997) and Dacles-Mariani et al (1995), to list but a few) will tend to underpredict vortex strength owing to the smoothing effect of vortex 'meandering', or the random, low-frequency modulation in the trajectory of the vortex centre. While a number of schemes have been proposed to correct single-point scans of vortex wakes for the effect of meandering (such as those of Devenport et al (1996), Bailey & Tavoularis (2008) and Iungo et al (2009)), it has also been shown that, with meandering amplitudes which are large but within the range of those observed experimentally, the reconstruction of the vortex velocity field from point measurements becomes impossible (Birch 2012).…”

Calibration and Use of n-Hole Velocity Probes

“…Note that C j = 0 identically for the hole registering the largest pressure. These definitions are based upon the same reasoning used to obtain (1) and (3): that the error in the approximations of local stagnation and static pressure will become velocity-independent when normalized against the approximation of local dynamic pressure. The pressure coefficients defined above have the advantages of being continuous throughout the range of calibration, and of being independent of the hole arrangement and the probe tip geometry.…”

“…Furthermore, because they can provide local measurements of the three components of fluid velocity as well as of the local static and total pressure, they are of particular use in wake surveys (see [1,2,3,4] and references cited therein), for which optical methods may present difficulties owing to the potential flow interference arising from particle injection [5] and particle momentum effects [6].…”