To analyze the genetic background of ‘white’ type Northern snakehead ( Channa argus ), and provide atheoretical basis for breeding of C. argus , the investigation of genetic diversity and population structure were investigated based on the complete sequences of mitochondrial DNA D-loop region for three cultured ‘white’ type C. argus populations, and four ‘bicolor’ type C. argus populations were used to compare with them; 28 mutation loci and 30 haplotypes were found in the D-loop sequence of all individuals with a total length of 907 bp. The highest haplotype diversity ( H d ) and nucleotide diversity ( P i ) in the ‘white’ type C. argus populations were 0.505 and 0.00057, respectively, which lower than those in the ‘bicolor’ type C. argus populations ( H d = 0.911, P i = 0.00326). Population differentiation values ( F ST ) show that the four ‘bicolor’ type C. argus populations had obvious genetic differentiation ( Fst : 0.21902–0.49428. p < 0.01), but not in the three ‘white’ type C. argus populations ( Fst : −0.00571 to 0.07261. p > 0.05). The phylogenetic tree and Median Joining (MJ) network showed that the genetic distance among ‘white’ type C. argus populations is very close. Therefore, much attention should be paid to protecting population genetic diversity and avoiding inbreeding in the breeding of ‘white’ type C. argus .
Micro-vibrations in satellites mainly originate from the control moment gyroscope because of its mass imbalance, which seriously affect the image accuracy of high-resolution optical payload, so it must be isolated. Bellows-type fluid viscous damper (FVD) can be used to isolate micro-vibrations. In this paper, a simplified model of the damping element in bellowstype FVD under medium-and high-frequency excitation is proposed according to the concept of bellows effective area. Based on this theoretical model, nonlinear stiffness and damping at different design parameters are extracted by nonlinear fitting method. Then, the main cause of nonlinear stiffness and damping is analyzed by the velocity distribution in the cross section of damping orifice. The factors that affect the intensity of nonlinear stiffness and damping are discussed by using the flow resistance. The results show that the velocity term in the corrected hydraulic resistance is the cause of nonlinear damping and stiffness. Damping orifice diameter, length and silicone oil viscosity will affect the intensity of nonlinear characteristics of this damper. At low viscosity, the nonlinear damping is more obvious. The larger the diameter, the more obvious the nonlinear damping. In the frequency domain between 1 and 200 Hz, the velocity index is not a constant, but changes with frequency. Hydraulic stiffness can be divided by linear stiffness and nonlinear stiffness; both of them can affect the elastic force.
Three-parameter isolation system can be used to isolate microvibration for control moment gyroscopes. Normalized analytical model for three-parameter system in the time domain and frequency domain is proposed by using analytical method. Dynamic behavior of three-parameter system in the time domain and frequency domain is studied. Response in the time domain under different types of excitations is analyzed. In this paper, a regulatory factor is defined in order to analyze dynamic behavior in the frequency domain. For harmonic excitation, a comparison study is made on isolation performance between the case when the system has optimal damping and the case when regulatory factor is 1. Besides, phase margin of three-parameter system is obtained. Results show that dynamic behavior in the time domain and frequency domain changes with regulatory factor. Phase margin has the largest value when the value of regulatory factor is 1. System under impulse excitation and step excitation has the shortest settling time for the response in the time domain when the value of regulatory factor is 1. When stiffness ratio is small, isolation performances of two cases are nearly the same; when system has a large stiffness ratio, isolation performance of the first case is better.
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