Catálogo de publicaciones - revistas

<jats:p>Bone fatigue accumulation is a factor leading to bone fracture, which is a progressive process of microdamage deteriorating under long-term and repeated stress. Since the microdamage of the early stage in bone is difficult to be investigated by linear ultrasound, the second harmonic generation method in nonlinear ultrasound technique is employed in this paper, which is proved to be more sensitive to microdamage. To solve the deficiency that the second harmonic component is easily submerged by noise in traditional nonlinear measurement, a weighted chirp coded sinusoidal signal was applied as the ultrasonic excitation, while pulse inversion is implemented at the receiving side. The effectiveness of this combination to improve the signal-to-noise ratio has been demonstrated by <jats:italic>in vitro</jats:italic> experiment. Progressive fatigue loading experiments were conducted on the cortical bone plate <jats:italic>in vitro</jats:italic> for microdamage generation. There was a significant increase in the slope of the acoustic nonlinearity parameter with the propagation distance (increased by 8% and 24% respectively) when the bone specimen was at a progressive level of microdamage. These results indicate that the coded nonlinear ultrasonic method might have the potential in diagnosing bone fatigue.</jats:p>

<jats:p>Abnormal hematocrit (Hct) is associated with an increased risk of pre-hypertension and all-cause death in general population, and people with a high Hct value are susceptible to arterial cardiovascular disease and venous thromboembolism. In this study, we report for the first time on the ability of thermoacoustic imaging (TAI) for <jats:italic>in vivo</jats:italic> evaluating Hct changes in human forearms. <jats:italic>In vitro</jats:italic> blood samples with different Hct values from healthy volunteers (<jats:italic>n</jats:italic> = 3) were prepared after centrifugation. TAI was performed using these samples in comparison with the direct measurements of conductivity. <jats:italic>In vivo</jats:italic> TAI was conducted in the forearm of healthy volunteers (<jats:italic>n</jats:italic> = 7) where Hct changes were produced through a vascular occlusion stimulation over a period of time. The results of <jats:italic>in vitro</jats:italic> blood samples obtained from the 3 healthy subjects show that the thermoacoustic (TA) signals changes due to the variation of blood conductivity are closely related to the changes in Hct. In addition, the <jats:italic>in vivo</jats:italic> TA signals obtained from the 7 healthy subjects consistently increase in the artery/muscle and decrease in the vein during venous or arterial occlusion because of the changed Hct value in their forearms. These findings suggest that TAI has the potential to become a new tool for monitoring Hct changes for a variety of pre-clinical and clinical applications.</jats:p>

<jats:p>We report a numerical study of the Prandtl-number (<jats:italic>Pr</jats:italic>) effects in two-dimensional turbulent Rayleigh–Bénard convection. The simulations were conducted in a square box over the <jats:italic>Pr</jats:italic> range from 0.25 to 100 and over the Rayleigh number (<jats:italic>Ra</jats:italic>) range from 10<jats:sup>7</jats:sup> to 10<jats:sup>10</jats:sup>. We find that both the strength and the stability of the large-scale flow decrease with the increasing of <jats:italic>Pr</jats:italic>, and the flow pattern becomes plume-dominated at high <jats:italic>Pr</jats:italic>. The evolution in flow pattern is quantified by the Reynolds number (<jats:italic>Re</jats:italic>), with the <jats:italic>Ra</jats:italic> and the <jats:italic>Pr</jats:italic> scaling exponents varying from 0.54 to 0.67 and –0.87 to –0.93, respectively. It is further found that the non-dimensional heat flux at small <jats:italic>Ra</jats:italic> diverges strongly for different <jats:italic>Pr</jats:italic>, but their difference becomes marginal as <jats:italic>Ra</jats:italic> increases. For the thermal boundary layer, the spatially averaged thicknesses for all the <jats:italic>Pr</jats:italic> numbers can be described by <jats:italic>δ<jats:sub>θ</jats:sub> </jats:italic> ∼ <jats:italic>Ra</jats:italic> <jats:sup>−0.30</jats:sup> approximately, but the local values vary a lot for different <jats:italic>Pr</jats:italic>, which become more uniform with <jats:italic>Pr</jats:italic> increasing.</jats:p>

<jats:p>A Monte Carlo implicit simulation program, Implicit Stratonovich Stochastic Differential Equations (ISSDE), is developed for solving stochastic differential equations (SDEs) that describe plasmas with Coulomb collision. The basic idea of the program is the stochastic equivalence between the Fokker–Planck equation and the Stratonovich SDEs. The splitting method is used to increase the numerical stability of the algorithm for dynamics of charged particles with Coulomb collision. The cases of Lorentzian plasma, Maxwellian plasma and arbitrary distribution function of background plasma have been considered. The adoption of the implicit midpoint method guarantees exactly the energy conservation for the diffusion term and thus improves the numerical stability compared with conventional Runge–Kutta methods. ISSDE is built with C++ and has standard interfaces and extensible modules. The slowing down processes of electron beams in unmagnetized plasma and relaxation process in magnetized plasma are studied using the ISSDE, which shows its correctness and reliability.</jats:p>

<jats:p>The linear characteristics of the unstable mode of ion-acoustic waves are examined in an electrostatic electron-ion plasma composed of streaming hot electrons, non-streaming cold electrons and dynamical positive ions. The plasma under consideration is modeled by using a non-gyrotropic nonextensive <jats:italic>q</jats:italic>-distribution function in which the free energy source for wave excitation is provided by the relative directed motion of streaming hot electrons with respect to the other plasma species. In the frame work of kinetic model, a linearized set of Vlasov–Poisson’s equations are solved to obtain the analytical expressions for dispersion relation and Landau damping rate. The threshold condition for the unstable ion-acoustic wave is derived to assess the stability of the wave in the presence of nonextensive effects. Growth in the wave spectrum and nontrivial effects of <jats:italic>q</jats:italic>-nonextensive parameter on the ion-acoustic waves can be of interest for the readers in the regions of Saturns’s magnetosphere.</jats:p>

<jats:p>A one-dimensional self-consistent calculation model of capacitively coupled plasma (CCP) discharge and electromagnetic wave propagation is developed to solve the plasma characteristics and electromagnetic wave transmission attenuation. Numerical simulation results show that the peak electron number density of argon is about 12 times higher than that of helium, and that the electron number density increases with the augment of pressure, radio frequency (RF) power, and RF frequency. However, the electron number density first increases and then decreases as the discharge gap increases. The transmission attenuation of electromagnetic wave in argon discharge plasma is 8.5-dB higher than that of helium. At the same time, the transmission attenuation increases with the augment of the RF power and RF frequency, but it does not increase or decrease monotonically with the increase of gas pressure and discharge gap. The electromagnetic wave absorption frequency band of the argon discharge plasma under the optimal parameters in this paper can reach the Ku band. It is concluded that the argon CCP discharge under the optimal discharge parameters has great potential applications in plasma stealth.</jats:p>

<jats:p>The discharge characteristics of capacitively coupled argon plasmas driven by very high frequency discharge are studied. The mean electron temperature and electron density are calculated by using the Ar spectral lines at different values of power (20 W–70 W) and four different frequencies (13.56 MHz, 40.68 MHz, 94.92 MHz, and 100 MHz). The mean electron temperature decreases with the increase of power at a fixed frequency. The mean electron temperature varies non-linearly with frequency increasing at constant power. At 40.68 MHz, the mean electron temperature is the largest. The electron density increases with the increase of power at a fixed frequency. In the cases of driving frequencies of 94.92 MHz and 100 MHz, the obtained electron temperatures are almost the same, so are the electron densities. Particle-in-cell/Monte-Carlo collision (PIC/MCC) method developed within the Vsim 8.0 simulation package is used to simulate the electron density, the potential distribution, and the electron energy probability function (EEPF) under the experimental condition. The sheath width increases with the power increasing. The EEPF of 13.56 MHz and 40.68 MHz are both bi-Maxwellian with a large population of low-energy electrons. The EEPF of 94.92 MHz and 100 MHz are almost the same and both are nearly Maxwellian.</jats:p>

<jats:p>A three-dimensional fluid model is developed to investigate the radio-frequency inductively coupled H<jats:sub>2</jats:sub> plasma in a reactor with a rectangular expansion chamber and a cylindrical driver chamber, for neutral beam injection system in CFETR. In this model, the electron effective collision frequency and the ion mobility at high <jats:italic>E</jats:italic>-fields are employed, for accurate simulation of discharges at low pressures (0.3 Pa–2 Pa) and high powers (40 kW–100 kW). The results indicate that when the high <jats:italic>E</jats:italic>-field ion mobility is taken into account, the electron density is about four times higher than the value in the low <jats:italic>E</jats:italic>-field case. In addition, the influences of the magnetic field, pressure and power on the electron density and electron temperature are demonstrated. It is found that the electron density and electron temperature in the <jats:italic>xz</jats:italic>-plane along permanent magnet side become much more asymmetric when magnetic field enhances. However, the plasma parameters in the <jats:italic>yz</jats:italic>-plane without permanent magnet side are symmetric no matter the magnetic field is applied or not. Besides, the maximum of the electron density first increases and then decreases with magnetic field, while the electron temperature at the bottom of the expansion region first decreases and then almost keeps constant. As the pressure increases from 0.3 Pa to 2 Pa, the electron density becomes higher, with the maximum moving upwards to the driver region, and the symmetry of the electron temperature in the <jats:italic>xz</jats:italic>-plane becomes much better. As power increases, the electron density rises, whereas the spatial distribution is similar. It can be summarized that the magnetic field and gas pressure have great influence on the symmetry of the plasma parameters, while the power only has little effect.</jats:p>

<jats:p>A detailed understanding of anode heat transfer is important for the optimization of arc processing technology. In this paper, a two-temperature chemical non-equilibrium model considering the collisionless space charge sheath is developed to investigate the anode heat transfer of nitrogen free-burning arc. The temperature, total heat flux and different heat flux components are analyzed in detail under different arc currents and anode materials. It is found that the arc current can affect the parameter distributions of anode region by changing plasma characteristics in arc column. As the arc current increases from 100 A to 200 A, the total anode heat flux increases, however, the maximum electron condensation heat flux decreases due to the arc expansion. The anode materials have a significant effect on the temperature and heat flux distributions in the anode region. The total heat flux on thoriated tungsten anode is lower than that on copper anode, while the maximum temperature is higher. The power transferred to thoriated tungsten anode, ranked in descending order, is heat flux from heavy-species, electron condensation heat, heat flux from electrons and ion recombination heat. However, the electron condensation heat makes the largest contribution for power transferred to copper anode.</jats:p>

<jats:p>Extreme ultraviolet (EUV) source produced by laser-induced discharge plasma (LDP) is a potential technical means in inspection and metrology. A pulsed Nd:YAG laser is focused on a tin plate to produce an initial plasma thereby triggering a discharge between high-voltage electrodes in a vacuum system. The process of micro-pinch formation during the current rising is recorded by a time-resolved intensified charge couple device camera. The evolution of electron temperature and density of LDP are obtained by optical emission spectrometry. An extreme ultraviolet spectrometer is built up to investigate the EUV spectrum of Sn LDP at 13.5 nm. The laser and discharge parameters such as laser energy, voltage, gap distance, and anode shape can influence the EUV emission.</jats:p>