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these natural modes of locomotion with engineered systems has been challenging. [11][12][13] In this respect, burgeoning effort has been devoted to developing new simulation tools, physical models, and experimental platforms. Among these engineering tools, soft robotic systems are promising in light of their biologically relevant mechanical compliance, deformability, and modes of locomotion. [4,[14][15][16][17] Untethered soft robots, which can freely move without requiring a physical connection to external hardware and power supplies, are good candidates for studying the locomotion modes of natural invertebrates. [18][19][20][21] Thanks to recent advances in fabrication methods, [22][23][24][25][26] functional materials, [27][28][29] and actuation strategies, [7,[30][31][32][33][34][35] roboticists have proposed several soft robots that are capable of mimicking some features of natural animal locomotion. For example, previous efforts with photo-responsive hydrogels-based robots demonstrated the ability to mimic peristaltic earthworm crawling. [36] Likewise, untethered soft robots powered with shape memory alloy have been shown to exhibit a variety of locomotion modes, including undulation, jumping, crawling through narrow space or walking over rough terrain. [37,38] The application of magnetic soft robots in biomimetic study has received growing attention due to their high controllability. Hu et al. developed a millimeter-scale film robot that achieves multiple locomotion modes,The efficient motility of invertebrates helps them survive under evolutionary pressures. Reconstructing the locomotion of invertebrates and decoupling the influence of individual basic motion are crucial for understanding their underlying mechanisms, which, however, generally remain a challenge due to the complexity of locomotion gaits. Herein, a magnetic soft robot to reproduce midge larva's key natural swimming gaits is developed, and the coupling effect between body curling and rotation on motility is investigated. Through the authors' systematically decoupling studies using programmed magnetic field inputs, the soft robot (named LarvaBot) experiences various coupled gaits, including biomimetic side-to-side flexures, and unveils that the optimal rotation amplitude and the synchronization of curling and rotation greatly enhance its motility. The LarvaBot achieves fast locomotion and upstream capability at the moderate Reynolds number regime. The soft robotics-based platform provides new insight to decouple complex biological locomotion, and design programmed swimming gaits for the fast locomotion of softbodied swimmers.

Data centers have become ubiquitous in the last few years in an attempt to keep pace with the processing and storage needs of the Internet and cloud computing. The steady growth in the heat densities of IT servers leads to a rise in the energy needed to cool them, and constitutes approximately 40% of the power consumed by data centers. However, many data centers feature redundant air conditioning systems that contribute to inefficient air distribution, which significantly increases energy consumption. This remains an insufficiently explored problem. In this paper, a typical, small data center with tiles for an air supply system with a raised floor is used. We use a fluent (Computational Fluid Dynamics, CFD) to simulate thermal distribution and airflow, and investigate the optimal conditions of air distribution to save energy. The effects of the airflow outlet angle along the tile, the cooling temperature and the rate of airflow on the beta index as well as the energy utilization index are discussed, and the optimal conditions are obtained. The reasonable airflow distribution achieved using 3D CFD calculations and the parameter settings provided in this paper can help reduce the energy consumption of data centers by improving the efficiency of the air conditioning.

How muscles are used is a key to understanding the internal driving of fish swimming. However, the underlying mechanisms of some features of the muscle activation patterns and their differential appearance in different species are still obscure. In this study, we explain the muscle activation patterns by using 3D computational fluid dynamics models coupled to the motion of fish with prescribed deformation and examining the torque and power required along the fish body with two primary swimming modes. We find that the torque required by the hydrodynamic forces and body inertia exhibits a wave pattern that travels faster than the curvature wave in both anguilliform and carangiform swimmers, which can explain the traveling wave speeds of the muscle activations. Notably, intermittent negative power (i.e., power delivered by the fluid to the body) on the posterior part, along with a timely transfer of torque and energy by tendons, explains the decrease in the duration of muscle activation towards the tail. The torque contribution from the body elasticity further clarifies the wave speed increase or the reverse of the wave direction of the muscle activation on the posterior part of a carangiform swimmer. For anguilliform swimmers, the absence of the aforementioned changes in the muscle activation on the posterior part is consistent with our torque prediction and the absence of long tendons from experimental observations. These results provide novel insights into the functions of muscles and tendons as an integral part of the internal driving system, especially from an energy perspective, and they highlight the differences in the internal driving systems between the two primary swimming modes.

The energy consumption of fast-growing data centers is drawing attentions from not only energy organizations and institutions all over the world, but also charity groups, such as Greenpeace, and research shows that the power consumption of air conditioning makes up a large proportion of the electricity cost in data centers. Therefore, more detailed investigations of air conditioning power consumption are warranted. Three types of airflow distributions with different aisle layouts (the open aisle, the closed cold aisle, and the closed hot aisle) were investigated with Computational Fluid Dynamics (CFD) methods in a typical data center of four rows of racks in this study. To evaluate the results of thermal and bypass phenomenon, the temperature increase index (β) and the energy utilization index (ηr) were used. The simulations show that there is a better trend of the β index and ηr index both closed cold aisle and closed hot aisle compared with free open aisle. Especially with high air flow rate, the β index decreases and the ηr index increases considerably. Moreover, the results prove the closed aisles (both closed cold aisle and closed hot aisle) can not only significantly improve the airflow distribution, but also reduce the mixture of cold and heat flow, and therefore improve energy efficiency. In addition, it proves the design of the closed aisles can meet the increasing density of installations and our simulation method could evaluate the cooling capacity easily.

Author summaryFor undulatory swimming, fish form posteriorly traveling waves of body bending by 1 activating their muscles sequentially along the body. However, experimental 2 observations have showed that the muscle activation wave does not simply match the 3 bending wave. Researchers have previously computed the torque required for muscles 4 along the body based on classic hydrodynamic theories and explained the higher wave 5 speed of the muscle activation compared to the curvature wave. However, the origins 6 of other features of the muscle activation pattern and their variation among different 7 February 18, 2019 1/16 species are still obscure after decades of research. In this study, we use 3D 8 computational fluid dynamics models to compute the spatiotemporal distributions of 9 both the torque and power required for eel-like and mackerel-like swimming. By 10 examining both the torque and power patterns and considering the energy transfer, 11 storage, and release by tendons and body viscoelasticity, we can explain not only the 12 features and variations in the muscle activation patterns as observed from fish 13 experiments but also how tendons and body elasticity save energy. We provide a 14 mechanical picture in which the body shape, body movement, muscles, tendons, and 15 body elasticity of a mackerel (or similar) orchestrate to make swimming efficient. 16 45 negative torques both occupy half of the period all along the body, the decrease of the 46 EMG duration in carangiform swimmers remains an obscure phenomenon. 47 157 external power along the body is within the numerical error (< 5%). 158 Results and Discussion 159 Body movement and fluid flow 160 The free swimming speeds (U ) are 0.29 and 0.25 in nondimensionalized units for the 161 eel and the mackerel, respectively. The corresponding Strouhal numbers are 0.63 and 162 0.68. These values are consistent with previous numerical studies at similar Reynolds 163numbers (Re ≈4000) (e.g., [14]). For both fishes, double row vortices are shed behind 164 the tail, similar to previous numerical results (see Fig 2). The velocity field behind the 165

At the millimeter scale and in the intermediate Reynolds number ( Re) regime, the midge and mosquito larvae can reach swimming speeds of more than one body length per cycle performing a “figure eight” gait, in which their elongated bodies periodically bend nearly into circles and then fully unfold. To elucidate the propulsion mechanism of this cycle of motion, we conducted a three-dimensional (3D) numerical study, which investigates the hydrodynamics of undergoing the prescribed kinematics. We found novel propulsion mechanisms, such as modulating the body deformation rate to dynamically increase the maximum net propulsion force, using asymmetric kinematics to generate torque and the appropriate rotation, and controlling the radius of the curled body to manipulate the moment of inertia. The figure eight gait is found to achieve propulsion at a wide range of Re but is most effective at intermediate Re. The results were further validated experimentally, via the development of a soft millimeter-sized robot that can reach comparable speeds using the figure eight gait.

The fast start of fish is a rapid event that involves fast actuation in musculature and highly unsteady hydrodynamics. Fast-start capability is of great significance for fish to either hunt prey or escape from predators. In this study, we used a three-dimensional CFD model to study the hydrodynamics of a crucian carp during a C-type fast start. This study confirms the previous observations from both experiments and simulations that the jets are induced by the fast start for force generation, and the vortex rings generated in both the preparation and propulsion stages connect to each other. In addition, an obvious vortex ring generated by the head during the propulsion stage was observed, which potentially benefits the rotational motion during the fast start. According to the hydrodynamic information from CFD modeling, we established a model to analyze the internal torque, which represents the muscular actuation. The backward traveling speed of internal torque is 1.56 times the curvature speed, which confirms the existence of neuromechanical phase lag during the fast start of fish. This study potentially benefits the design of robot fish in terms of kinematics and driving mode.