Several previous experimental and theoretical studies have shown that a leading edge vortex (LEV) on an airfoil or wing can provide lift enhancement. In this paper, unsteady two-dimensional (2D) potential flow theory is employed to model the flow field of a pitching flat plate wing. A multi-vortices model is developed to model both the leading edge and trailing edge vortices (TEVs), which offers improved accuracy compared with using only single vortex at each separation location. The lift is obtained by integrating the unsteady Blasius equation. It is found that the motion of vortices contributes significantly to the overall aerodynamic force on the flat plate. A Kutta-like condition is used to determine the vortex intensity and location at the leading edge for large angle of attack cases; however, it is proposed to relax this condition for small angle of attack cases and apply a 2D shear layer model to calculate the circulation of the new added vortex. The results of the simulation are then compared with classical numerical, theoretical, and experimental data for canonical unsteady flat plat problems. Good agreement with these data is observed. Moreover, these results suggested that the leading edge vortex shedding for small angles of attack should be modeled differently than that for large angles of attack. Finally, the results of vortex motion vs. lift indicate that the slow convection of the LEV creates less negative lift while the rapid shedding of the TEV creates more positive lift. The difference between these two contributions of lift results in a total positive lift that lasts for about two chord-length travel of the plate. It is therefore concluded that the lift enhancement during the LEV “stabilization” above the wing is a combined effect of both the LEV and TEV motion. This also provides the insights for future active flow control of micro aerial vehicles (MAVs) that the formation and shedding process of LEVs and TEVs can be manipulated to provide lift enhancement.

Unsteady inviscid flow models of wings and airfoils have been developed to study the aerodynamics of natural and man-made flyers. Vortex methods have been extensively applied to reduce the dimensionality of these aerodynamic models, based on the proper estimation of the strength and distribution of the vortices in the wake. In such modelling approaches, one of the most fundamental questions is how the vortex sheets are generated and released from sharp edges. To determine the formation of the trailing-edge vortex sheet, the classical steady Kutta condition can be extended to unsteady situations by realizing that a flow cannot turn abruptly around a sharp edge. This condition can be readily applied to a flat plate or an airfoil with cusped trailing edge since the direction of the forming vortex sheet is known to be tangential to the trailing edge. However, for a finite-angle trailing edge, or in the case of flow separation away from a sharp corner, the direction of the forming vortex sheet is ambiguous. To remove anyad hocimplementation, the unsteady Kutta condition, the conservation of circulation as well as the conservation laws of mass and momentum are coupled to analytically solve for the angle, strength and relative velocity of the trailing-edge vortex sheet. The two-dimensional aerodynamic model together with the proposed vortex-sheet formation condition is verified by comparing flow structures and force calculations with experimental results for several airfoil motions in steady and unsteady background flows.

High-incidence lift generation via flow reattachment is studied. Different reattachment mechanisms are distinguished, with dynamic manoeuvres and tip vortex downwash being separate mechanisms. We focus on the latter mechanism, which is strictly available to finite wings, and isolate it by considering steadily translating wings. The tip vortex downwash provides a smoother merging of the flow at the trailing edge, thus assisting in establishing a Kutta condition there. This decreases the strength/amount of vorticity shed from the trailing edge, and in turn maintains an effective bound circulation resulting in continued lift generation at high angles of attack. Just below the static lift-stall angle of attack, strong vorticity is shed at the trailing edge indicating an increasingly intermittent reattachment/detachment of the instantaneous flow at mid-span. Above this incidence, the trailing-edge shear layer increases in strength/size representing a negative contribution to the lift and leads to stall. Lastly, we show that the mean-flow topology is equivalent to a vortex pair regardless of the particular physical flow configuration.

This article describes a soft suction cup end effector with squid-inspired suction generation and an octopus-inspired cup design that uses a dielectric elastomer actuator (DEA) to generate suction for adhesion. The fabrication process for the end effector is described in detail, and a mechanical model for generated pressure differential as a function of voltage is presented. When actuated, the DEA exerts an electrostatic stress on the walls of the end effector, resulting in pressure reduction in its water-filled cavity. The actuator is soft, flexible, and creates suction without a reliance on typical DEA elements such as rigid supporting structures and elastomer prestrain. It does not require net fluid flux out of the sucker, allowing faster attachment and easier release. It can be actuated underwater and has been validated with pull-off tests. The sucker generates a pressure differential of 3.63 ± 0.07 kPa (±SD) when driven at 10.75 kV in water and should reach a 4.90 kPa pressure differential when energized at its theoretical failure point of 12.4 kV. Data normalized by the input voltage show that 90% of the maximum pressure differential can be achieved within 50 ms of voltage application. Weighing less than 30 g in air, this elastomer end effector is capable of pulling with a force of 8.34 ± 0.10 N (±SD) and reversibly lifting 26.7 times its own mass underwater when actuated at 10.75 kV.

Studies have shown that many behavioral decisions of fish are facilitated by the lateral line system which provides hydrodynamic information about the surrounding fluid. Inspired by the functionality of the system, a distributed pressure sensory system is developed. The system is intended for use on autonomous underwater vehicles to aid station keeping and accurate maneuvering by allowing the vehicle to react to the changes in the fluid environment before they result in body perturbation. The system can also be used for mission level decision making such as obstacle detection. This paper presents a prototype sensory system using differential, as opposed to absolute or gauge, pressure sensors as the sensing elements, which allows for higher measurement precision. Experimental tests are designed to characterize the system's ability to estimate the hydrodynamic force and to detect the presence of a wall. The hydrodynamic force estimated by the system is validated with an independent force measuring apparatus. The impending wall could also be detected by analyzing the pressure distribution obtained from the sensory system. Particularly, it is found that the wall distance and angle may be inferred by comparing the amplitude and phase of the Fourier components in the pressure distribution against those without the obstacle.