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The relaxation process of the discharge channel near the anode in a long air gap was observed using a Schlieren system with a temporal resolution of 5 µs and a spatial resolution of 70 µm. The dynamic characteristics of the decay process in the vicinity of the anode are obtained. The discharge channel evolves just as a growing mushroom in nature during the relaxation phase. Two physical quantities, angle θ and velocity v, are defined to describe the process in this paper. The average value of the angle and velocity under lightning impulses are 71.7° and 3.3 m s −1 respectively, while 7.7 m s −1 under switching impulses. A simplified model was established to simulate the formation of mushroom-shaped channel. The simulation and experimental results show that the formation and development of the mushroom-shaped channel are due to two factors. One is the convection of the high temperature and high pressure air near the anode produced by the first corona discharge; the other is the ionic migration. These two factors result in the phenomena that the cooling process in the vicinity of the anode is much more efficient than further into the gap, whereas the thermal conductivity of the anode may have little contribution to that.

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Cooperatives differ in their intensity of horizontal and vertical communications, their innovation policies, and their centralization of decision-making power. We aim to establish relationships between these communication, innovation, and decision-making aspects of cooperatives, and to identify the circumstances when a particular configuration adds most value. Horizontal and vertical communications are analyzed in a decentralized and centralized cooperative. Horizontal communication (HC) is characterized as the exchange of information between farmers in the society of members. It is associated with process innovation. Vertical communication (VC) is the exchange of information between a member and the CEO of the cooperative enterprise. It is associated with product innovation. The CEO decides regarding the deliveries of the member and the level of vertical communication in the centralized cooperative, while these decisions are taken by the members in the decentralized cooperative. We establish that the decentralized cooperative is efficient at an intermediate level of the VC cost coefficient and when the HC cost coefficient is above a certain level, while the centralized cooperative is efficient in the other cases.

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Conductive gels are developing vigorously as superior wearable sensing materials due to their intrinsic conductivity, softness, stretchability, and biocompatibility, showing a great potential in many aspects of lives. However, compared to their wide application on land, it is significant yet rather challenging for traditional conductive gels to realize sensing application under water. The swelling of gels and the loss of conductive components in the aqueous environment, resulted from the diffusion across the interface, lead to structural instability and sensing performance decline. Fortunately, great efforts are devoted to improving the water resistance of conductive gels and employing them in the field of underwater wearable sensing in recent years, and some exciting achievements are obtained, which are of great significance for promoting the safety and efficiency of underwater activities. However, there is no review to thoroughly summarize the underwater sensing application of conductive gels. This review presents a brief overview of the representative design strategies for developing water‐resistant conductive gels and their diversified applications in the underwater sensing field as wearable sensors. Finally, the ongoing challenges for further developing water‐resistant conductive gels for underwater wearable sensing are also discussed along with recommendations for the future.

Block transmissions are commonly used in underwater acoustic communications due to the large delay spread of the underlying channels. In this paper, we propose a novel jamming protocol based on the half-duplex nature of the underwater transceivers and the block-based transmission structure, taking advantage of the large propagation delays of underwater acoustic channels to create interference at the eavesdropper without affecting the reception of the intended user. This half-duplex jamming protocol acts like a self-protection scheme, which does not need any helpers but instead relies on the legitimate receiver itself to transmit jamming signals for the improvement of secrecy rate. The simulation results evaluate the secrecy rate improvement as a function of the eavesdropper locations.Index Terms-Underwater acoustic communications, halfduplex, jamming, secrecy rate, physical layer security.which the relay was equipped with multiple antennas and proposed a system design for determining the antenna weights and transmit power of the source and relay, so that the system secrecy rate was maximized subject to a total transmit power constraint or the transmit power was minimized subject to a secrecy rate constraint. Further, the work in [5] addressed secure communications considering three cooperative schemes: decode-and-forward (DF), amplify-and-forward (AF), and cooperative jamming (CJ), and aimed at the determination of relay weights and the allocation of transmit power.Recently, using a full-duplex receiver as the jamming source without help from relays has received some attention. In [6], the approach that the full-duplex legitimate receiver could generate artificial noise to jam the eavesdropper was proposed, and the outage secrecy region was used to analyze the secrecy performance from the perspective of geometry. The work in [7] proposed to use both the transmitter and receiver for full duplex beamforming, to improve the throughput and secrecy under the constraint of the guaranteed secrecy and QoS for legitimate users. The joint time and power allocation in fullduplex (FD) wireless-powered communication network was studied in [8], where the average and peak transmit power were constrained to maximize the weighted sum-rate.However, the characteristics of underwater acoustic channels, such as large delay spread, limited acoustic link capacity, and multipath effect have differed the underwater acoustic communication significantly from the ground wireless communication [9], [10]. The approaches aforementioned are neither applicable nor effective to protect the underwater communication information. Until now, most work about underwater communication security is done on the network layer and little attention has been paid on the use of jamming signals to improve the physical layer security.

The diameter of a stem, a common origin point for streamers where the transition from the streamer to the leader takes place, has a significant effect on the simulation of streamer-to-leader transition. To measure the diameter of the stem, a schlieren system was established with the temporal and spatial resolutions of 5 μs and 70 μm, respectively. Experiments were conducted under positive lightning and switching impulses; the applied voltage, discharge current, and schlieren images were recorded synchronously. A method was proposed to measure the diameter of the stem based on the schlieren images, with which the stem diameter as a function of time was obtained. Moreover, the initial diameters of the stem under different conditions were obtained through the linear fitting. At last, a relation between the initial cross-sectional area of the stem and the discharge current was given within a certain range.