T. Lee

Investigation of flow over an oscillating airfoil

The characteristics of the unsteady boundary layer and stall events occurring on an oscillating NACA 0012 airfoil were investigated by using closely spaced multiple hot-film sensor arrays at

Structure and Induced Drag of a Tip Vortex

Re\,{=}\,1.35\,{\times}\,10^{5}

Flowfield of a rotating-wing micro air vehicle

. Aerodynamic forces and pitching moments, integrated from surface pressure measurements, and smoke-flow visualizations were also obtained to supplement the hot-film measurements. Special attention was focused on the behaviour of the spatial-temporal progression of the locations of the boundary-layer transition and separation, and reattachment and relaminarization points, compared to the static values, for a range of oscillation frequency and amplitude both prior to, during and after the stall. The initiation, growth and rearward convection of a leading-edge vortex, and the role of the laminar separation bubble leading to the dynamic stall, as well as the mechanisms responsible for the stall events observed at different test conditions were also characterized. The hot-film measurements were also correlated with the aerodynamic load and pitching moment results to quantify the values of lift increment and stall angle delay as a result of the observed boundary layer and stall events. The results reported here provide an insight into the detailed nature of the unsteady boundary-layer events as well as the stalling mechanisms at work at different stages in the dynamic-stall process.

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

The three-dimensional flow structure of a tip vortex in the near wake of both a rectangular, square-tipped NACA 0015 airfoil and a high-lift cambered airfoil was investigated by using a seven-hole pressure probe at Re = 2.01 x 10 5 . Lift-induced drag was computed based on vorticity and was compared with force-balance data. For both the airfoils tested, the vortex strength reached a maximum immediately behind the trailing edge and remained nearly constant up to two chord lengths downstream. As the airfoil incidence increased, the increase in the lift force resulted in a basically linear increase in the vortex strength and the peak values of the tangential velocity and vorticity, whereas the vortex radius did not appear to have a clear dependence on the vortex strength. Depending on the airfoil incidence, the core axial velocity could be wake-like or jet-like. The normalized circulation within the inner region of the nearly axisymmetric tip vortex exhibited a universal, or self-similar, structure. The NACA 0015 airfoil, however, possessed smaller tangential velocities but similar vortex core diameters compared to those of a cambered airfoil

Two improved methods for low-speed hot-wire calibration

Experiments were conducted to measure the performance of a rotor, typical of that used on a rotating-wing micro air vehicle, which was shown to develop relatively low hovering efficiency. This inefficiency can be traced to high profile drag losses on the blades and also to the relatively large turbulent and rotational aerodynamic losses that are associated with the structure of the rotor wake. High-resolution flow visualization images have divulged several interesting flow features that appear unique to rotors operating at low Reynolds numbers. The wake sheets trailing from the rotor blades were found to be much thicker and also more turbulent than their higher chord Reynolds number counterparts. Similarly, the viscous core sizes of the tip vortices were relatively large as a fraction of blade chord. However, the flows in the tip vortices themselves were found to be similar, with laminar flow near their core axis and an outer turbulent region. Particle image velocimetry measurements were made to quantify the structure and strength of the wake flow, including the tip vortices. An analysis of the vortex aging process was conducted. A new nondimensional equivalent time scaling parameter is proposed to normalize the rate of growth of the vortex cores that are generated at substantially different vortex Reynolds numbers.

Oscillating Wing Loadings with Trailing-Edge Strips

The near-field tip-vortex flow structure behind an oscillating NACA 0015 wing was investigated at

Effect of Winglet Dihedral on a Tip Vortex

{\hbox {{\it Re}}}\,{=}\,1.86 \times 10^{5}

Active Trailing-Edge Flap Control of Oscillating-Wing Tip Vortex

. 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.

Aerodynamic Characteristics of Airfoil with Perforated Gurney-Type Flaps

Two improved methods for hot-wire anemometer calibration in the low-speed range (15-95 cm/s) are described: a laminar pipe-flow method, and a shedding-frequency method. For the laminar pipe-flow method the calibration is performed in the exit plane of a fully-developed laminar pipe flow. For the shedding-frequency method the shedding frequency of a parallel mode cylinder wake is measured; the velocity is then obtained from a continuous Strouhal-Reynolds number relationship. Calibration results from the present methods were compared with each other and with results from a commercially available calibration jet. The present methods were demonstrated as a simple, but accurate means for low-speed hot-wire calibration.

Unsteady Airfoil with Dynamic Leading- and Trailing-Edge Flaps

The dynamic-load loops of an oscillating NACA 0012 airfoil, obtained from surface-pressure measurements, fitted with small full-span trailing-edge strips, positioned on the lower (i.e., the Gurney flaps) and/or upper (the inverted strips) surface at the wing trailing edge, of height of 1.6 and 3.2% of the airfoil chord were studied at Re = 1.07 x 105. The results show that, similar to a static wing, the Gurney flap concept was also fairly generally applicable, in terms of lift performance enhancement, to an oscillating airfoil, despite the large and small increases in the peak negative pitching moment and maximum drag force, respectively, as well as a promoted dynamic stall. The increase (decrease) in the nose-down pitching moment (dynamic-stall angle) can be alleviated by the use of inverted strips at the price of a reduced maximum lift coefficient. The asymmetric strips were found to provide a compromise in the dynamic aerodynamic performance between the regular and inverted Gurney flaps, including a 49-deg flap, of an oscillating airfoil. The present passive oscillating-wing C 1 -C d -C m control can be valuable, because it can be used as an experimental guideline for the active control of the dynamic stall and/or nose-down pitching moment via a spanwise trailing-edge flap.