Detecting Vortex Burst on a Rotating Low-Aspect Ratio Plate
This work experimentally examines the phenomenon of leading-edge vortex (LEV) burst in a low aspect ratio rotating plate. Through stereoscopic particle image velocimetry (SPIV) about chrodwise planes, the evolutionary track of the vortex is detailed from inception to completion of a single revolution at Reynolds number Re=2500. The plate is driven at fixed incidence angle α = 45 ◦ with angularvelocity derived from a simple ramp-up, hold, ramp-down profile. Of particular interest is the SPIV analysis about the midspan and the 60% span positions. As demonstrated by the LEV core dye visualization of Figure 1, the burst process entails a topological shift about the midspan from a well-behaved swirl near the plate root to a more complex formation within the latter half of the plate, showcasing a loss of coherency. The ubiquity of unsteady flow phenomena exploited in biological flight and propulsion has prompted a recent surge in unsteady aerodynamics research. Such phenomena remain integral to the agile capabilities observed in bio-locomotion. However, proposed airborne platform designs aspire to perform beyond the operational limitations of biological counterparts. This includes elevated kinematic and flight speeds. In the pursuit of greater operational speeds, researchers are faced with a fundamental shift in flow characteristics as vortical structures approach saturation where stability may be an issue. Stability here is reserved to the context of flapping flight to indicate prolonged attachment of a given vortex to the airfoil and is not to be confused with the growth or decay of perturbations. Of primary interest has been the production of the leading-edge vortex to which much of the lift generation is attributed, owing to dynamic-stall effects. Under the burst mechanism, the leading-edge vortex experiences an abrupt expansion of cross-sectional area about the midspan. The complexity of the resulting vortex is elevated further with radial position approaching the plate tip. The previously simple homogeneous vorticity composition of the pre-burst vortex is now replaced by homogeneity where opposite-sign vorticity elements are incorporated into the leading-edge vortex. Concurrently, the burst process is cause for stagnation of the axial flow components within the leading-edge vortex core. Subsequently, the core flow undergoes flow reversal as entrained vorticity elements are are deposited within the LEV core. The counter-rotating vorticity composition of the leading-edge vortex presents a unique challenge in identifying a coherent LEV and quantifying its respective attributes. Determination of LEV circulation depends on the selected of contributing regions. The results of three different schemes using a rectangular fixed boundary, a vorticity threshold method and a λ 2-guided convex hull were compared and showed only small variations in magnitude while preserving agreement with established trends. In addition, a proper orthogonal decomposition of the vorticity and out-of-plane velocity field reveal dominant flow patterns characteristic of the pre-burst, bursting and post-burst regimes including of the onset of an inhomogeneous vorticity field hosting an array of entrained opposite-signed elements.
Record created on 2016-04-28, modified on 2016-08-09