Three-dimensional vortex wake structure of flapping wings in hovering flight

Abstract

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity.

Higher-order theories for the free vibrations of doubly-curved laminated panels with curvilinear reinforcing fibers by means of a local version of the GDQ method

“…Due to their outstanding performance and efficiency shell structures have acquired an important role in many engineering fields, as stated in the review by Liew et al [48] and in the researches carried out by Qatu [49,50]. For example, the importance of these kinds of structures is highlighted in some recent papers about civil, naval [51][52][53][54][55] and aerodynamics [56][57][58][59][60][61][62][63] applications. Furthermore, these characteristics have been emphasized mostly by the use of composite materials, such as laminated composites, in order to obtain the best engineering properties with respect to conventional materials.…”

Hydrodynamics of swimming in stingrays: numerical simulations and the role of the leading-edge vortex

“…LEV dynamics have been widely studied, and the LEV often grows and starts shedding for translating 2D foils (Dickinson & Gotz 1993;Wang 2000;Kim & Gharib 2010;Beem, Rival & Triantafyllou 2012;Pitt Ford & Babinsky 2013) or 2D flapping foils (Anderson et al 1998;Dong et al 2006), but it remains stable/attached for 3D rotating blades (Ellington et al 1996;Van Den Berg & Ellington 1997;Aono, Liang & Liu 2008;Lentink & Dickinson 2009b;Ozen & Rockwell 2012;Harbig, Sheridan & Thompson 2013;Cheng et al 2014). A spanwise flow that causes the LEV spiral towards the tip (Maxworthy 1981;Ellington et al 1996;Van Den Berg & Ellington 1997;Wang 2005;Lentink & Dickinson 2009b) and recently vorticity annihilation (Wojcik & Buchholz 2014) is suggested as a stabilizing mechanism by providing a sink of vorticity to balance the vorticity flux of the leading-edge shear layer.…”

Aerodynamics of dynamic wing flexion in translating wings

“…Furthermore, in 3D flapping wings, because the tip and root vortices may play a critical role in defining the flow structure (Cheng et al 2014;Liu et al 2013), the study of active wing morphing may do well to consider both the 3D and unsteady effects. …”