Fish attempt to optimize their swimming performance within their environments to minimize energy consumption. In this study, we conducted experiments using the Japanese dace (Tribolodon hakonensis), a simple flat plate, and a flume tank to simulate the swimming behavior in which fish exploit the stagnation area of the flat plate for maintaining their position. Two observed cases were: the fish localized by straightening the tail fin along the flow and by bending the tail fin like a cambered wing. The results of drag and lift forces suggest that fish adaptively flex tail fin based on the flow field, and selectively choose the optimal swimming posture for maintaining position.

Finless porpoises have numerous small tubercles on their dorsal ridges. The fluid dynamics of these tubercles were investigated in terms of sound and impact attenuation upon water entry. The experiments were conducted using streamlined models with small tubercles attached to the body surface. As a result, streamlined models with tubercles uniformly attached to the front part of the body showed a significant sound and impact attenuation effect, indicating that tubercles play an important role in masking the breathing sounds of finless porpoises to avoid predation by natural enemies, such as killer whales.

In this study, we propose an insect-inspired design of unmanned aerial vehicle (UAV), which is developed with a novel thrust vectoring mechanism. Like two-winged insects, our UAV is equipped with two propellers and two sets of thrust vectoring mechanisms, which enable controlling each propeller's position and attitude independently. By designing a control system suitable for the UAV, we successfully demonstrated its controllability of attitude apart from position control in a simulation model, which outperforming the conventional multi-rotor UAVs, enables achieving an arbitral attitude while flying horizontally. Future work may need to explore the optimal flight attitude to create the additional lift while improving the energy efficiency.

Sea cucumber Apostichopus japonicus is strongly affected physically by currents and waves. However, little is known about the flow resistance of this species. In this study, fluid forces acting on a model of sea cucumber were examined in a flume tank test. Parameters for estimating hydrodynamic forces using sea wave data were obtained in tank experiments. Compared to a spheroid body, the sea cucumber model exhibits a large value of drag coefficient, however, a relatively stable value of lift coefficient regardless of the angle of attack.

We performed a numerical simulation of the flow field with wing flapping under conditions simulating mosquito flight. Several simple planar shape wing models with the same aspect ratio as that of an actual mosquito were considered. The wing models were treated as rigid, flat plates. The immersed boundary method was used to account for the wing model behavior. As results, the average aerodynamic forces generated by the real-shaped wing model did not exhibit the greatest value. When comparing the aerodynamic force divided by the wing area, the trend that the longer the wingtip chord length, the greater the force, was observed. Trapezoidal-shaped wing models with long chord lengths at the wing tip showed the highest overall performance.

Aquatic ecosystems have changed significantly in recent years. Consequently, small robotic fish can be utilized to exterminate non-native and farmed fish that have contracted diseases. In this study, we have developed a robotic fish equipped with a spear-shooting mechanism to tag it. Experiments were conducted in which a robotic fish shot a spear underwater into a stationary mackerel. The specific characteristics of the spear required to penetrate a stationary mackerel while the robotic fish was swimming were determined through experiments.

In this study, we introduce a new structure called the “connecting membrane” aiming to improve the performance of hummingbird-mimetic flapping wings. The “connecting membrane” is an extension of the wing membrane at the basal that connects the root shaft and the robot body, it elastically pulls the wing to stop the continuous feathering, thereby curbing feathering in the wing's root chord while promoting twist of the wing. This augmentation in total wing torsion improves lift generation. Tethered flapping experiments conducted at 24 Hz showed that the wing with the connecting membrane integrated achieved 24.2% higher power efficacy and 8.4% higher lift (116 mN) compared to the wing without a connecting membrane that flapped with a rigid limiter.