Warning signals protect unpalatable prey from predation because predators who learn the association between the warning signal and prey unprofitability decrease attacks on the prey. Most of the research have focused on visual aposematic signals that are constantly presented and visible to the predators. But a variety of chemically defended insects are rather cryptic when resting, and only in response to predator attacks (post-attack) they perform displays of conspicuous abdomens or hindwings normally hidden under forewings. The function of those displays in unpalatable insects is not well understood. We examined two adaptive hypotheses on this facultative aposematic display using wild-caught oriental tits (Parus minor) as predators. First, we tested whether the display increases the rejection of the prey by predators upon seeing the display (i.e., at the moment of attack) through learning trials (aposematic signaling hypothesis). Second, we tested whether the display facilitates the memory formation between cryptic visible form of the prey and prey defense so that it prevents the predators initiate an attack upon seeing the cryptic form (facilitation hypothesis). We found that predators learned to avoid attacking the prey which supports the facilitation hypothesis. However, the support for the aposematic signaling hypothesis was equivocal. Our results open new directions of research by highlighting the possibility that similar facilitation effects may contribute to the evolution of various forms of post-attack visual displays in chemically, or otherwise, defended animals.
In this article, a design and analysis on 200-m-class hybrid underwater glider actuated by a buoyancy engine and a thruster was studied. The hull shape of the hybrid underwater glider was designed to reduce underwater resistance using Myring hull profile equations. A computational fluid dynamics analysis was conducted, and a resistance coefficient similar to that of existing underwater vehicle was calculated such that simulation reliability was verified. The relationship between the control value of the ballast discharged by the buoyancy engine and the velocity of a glider according to the path angle was analyzed. Through the analysis, the relationship between the optimal glide angle and the designed control value of the ballast water was drawn, and using this relationship, the optimal glider velocity was estimated. Also, the velocity of the hybrid underwater glider using the thruster only was measured through an experiment. Using both the thruster and ballast water control, it was shown that the maximum velocity of the developed hybrid underwater glider is over 2.4 knots.
Net buoyancy, as the main power source for the motion of an underwater glider, is affected by the pump or bladder that the glider adopts to change its buoyancy force in water. In this study, a new underwater glider that can dive to a depth of 400 m at a cruising speed of 2 knots, which is faster than conventional underwater gliders and is less affected by sea currents, is investigated. The UG resisting 400 m pressure on the buoyancy engine and achieving 2 knots’ speed was designed and constructed. For this UG, its steady-state attitude was studied according to the variance of the buoyancy center and the center of gravity with the buoyancy engine influenced by the displacement of the movable mass block. In motion simulation of the UG, the attitude of the UG under different displacement conditions was simulated in Simulink according to the displacements of the piston and the movable mass block. To validate the simulation performance, a UG was constructed and experiments were conducted. The simulation and experimental results were compared to show the reliability of the simulation results under limited conditions.
Flow analysis was carried out for a double-suction centrifugal pump. An impeller-only model and a full pump model were used to simulate the velocity and the pressure field of the pump. Pump head and efficiency were calculated with flow rate in order to obtain general performance of the pump. The calculation results were compared to the experimental data, and satisfactory results were obtained. Also, the velocity and the pressure field of this pump were analyzed for the rated point and off-design points. Changes of the velocity and the pressure field with flow rate were investigated at impeller eye and impeller exit.
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