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In recent years, underwater video technologies allow us to explore the ocean in scientific and noninvasive ways, such as environmental monitoring, marine ecology studies, and fisheries management. However the low-light and high-noise scenarios pose great challenges for the underwater image and video analysis. We here propose a CNN knowledge transfer framework for underwater object recognition and tackle the problem of extracting discriminative features from relatively low contrast images. Even with the insufficient training set, the transfer framework can well learn a recognition model for the special underwater object recognition task together with the help of data augmentation. For better identifying objects from an underwater video, a weighted probabilities decision mechanism is introduced to identify the object from a series of frames. The proposed framework can be implemented for real-time underwater object recognition on autonomous underwater vehicles and video monitoring systems. To verify the effectiveness of our method, experiments on a public dataset are carried out. The results show that the proposed method achieves promising results for underwater object recognition on both test image datasets and underwater videos.

Abstract. microRNAs (miRNAs) are small non-coding RNA molecules of 21-24 nt that regulate the expression of other genes by transcriptional inhibition or translational repression. Multiple lines of evidence suggest that miRNAs play important roles in tumor development and progression.

The amount of the electric charge injected by the streamer corona bursts during the stage of leader inception determines the energy deposited to thermalize the corona stem into a leader segment. This paper is aimed at investigating the critical charge required for positive leader inception in air by using a thermo-hydrodynamic model with a detailed kinetic scheme. In order to simplify the analysis and to speed up the simulation, a reduced kinetic scheme for air is proposed. Numerical comparisons show that the reduced scheme can obtain almost the same results as the previous comprehensive kinetic scheme but with only half of the number of species and reactions. The thermo-hydrodynamic model with the reduced kinetics is then used to solve the radial dynamics of a single stem heated by current pulses typical of streamer corona bursts. The critical charge necessary for the direct transition of a first streamer corona into a leader under electrodes with large curvature radius is estimated between 0.08 and 0.5 µC per stem. Furthermore, the simulation shows that the gas heating of corona stem formed from electrodes with small curvature radius is mainly determined by the total accumulated charge injected by previous streamer corona bursts and the length of the dark periods in between the current pulses. The shape and the number of the corona current pulses in the discharge also play a role and their effects are discussed. It is suggested that the transition into a leader is triggered when a secondary streamer burst is initiated after the gas temperature is increased by the heating of previous streamers to about 1200 K. In addition, it is found that the heating produced by the charge injected by previous streamer corona bursts can be neglected if the dark period to the next burst is larger than few hundreds of µs for a corona stem with moderate initial stem radius. This indicates that the critical charge criterion obtained from laboratory experiments does not hold to evaluate the inception of positive leaders under conditions when long dark periods are present.

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Leader discharge plays a key role in the breakdown of long air gap and the evolution of natural lightning discharges. Prior to the formation of continuous leaders, streamer corona bursts with dark periods in between generally appear. To attain a better physical understanding of the dynamics of discharge during dark periods, experimental studies with a specially designed electrode based on an improved quantitative schlieren system were performed. The experiments measured gas density and temperature changes of the leader discharge channel within the dark period for the first time. Temperature changes in a single discharge channel and corresponding current were obtained simultaneously. Measurements show that at the axis of the streamer stem and the unstable leader within the dark period the gas temperature is below 2000 K and decreases monotonically with time. These measurements confirmed the prediction of a previous theoretical study. To analyse the properties of discharge during the dark period, the measurements were compared with simulation results based on a detailed 1D thermo-hydrodynamic model. The measured temperature drop was less significant than that computed from simulation. This discrepancy is believed to be caused by heating from the ionic current, which was neglected in the previous studies. The ionic current helps to maintain the gas temperature during the long dark period but cannot reverse its decline. From a comparison and analysis, several suggestions are proposed to improve both the measurement system and the numerical model for further quantitative investigations of the dynamics of leader initiation.