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<jats:p>The reflection of elastic wave from thin bed in porous media is important for oil and gas reservoir seismic exploration. The equations for calculating frequency-dependent reflection amplitude <jats:italic>versus</jats:italic> incident angle (FDAVA) from thin bed in porous media are obtained based on porous media theory. Some conclusions are obtained from numerical analysis, specifically, slow compression wave may be ignored when considering boundary conditions in most situations; the dispersion of reflection from thin bed is much higher than that from thick layer and is periodic in frequency domain, which is affected by the thickness of thin bed, velocity, and incident angle; the reflection amplitude envelope in frequency domain decays exponentially, which is affected by the thickness of thin bed, attenuation, and incident angle; the reflection amplitude increases with thickness of thin bed increasing, and then it decreases when the thickness reaches to a quarter of wavelength.</jats:p>

<jats:p>For conservative linear homogeneous nonholonomic systems, there exists a cotangent bundle with the symplectic structure d<jats:italic>π<jats:sup>μ</jats:sup> </jats:italic> ∧ d<jats:italic>ξ<jats:sub>μ</jats:sub> </jats:italic>, in which the motion equations of the system can be written into the form of the canonical equations by the set of quasi-coordinates <jats:italic>π<jats:sup>μ</jats:sup> </jats:italic> and quasi-momenta <jats:italic>ξ<jats:sub>μ</jats:sub> </jats:italic>. The key to construct this cotangent bundle is to define a set of suitable quasi-coordinates <jats:italic>π<jats:sup>μ</jats:sup> </jats:italic> by a first-order linear mapping, so that the reduced configuration space of the system is a Riemann space with no torsion. The Hamilton–Jacobi method for linear homogeneous nonholonomic systems is studied as an application of the quasi-canonicalization. The Hamilton–Jacobi method can be applied not only to Chaplygin nonholonomic systems, but also to non-Chaplygin nonholonomic systems. Two examples are given to illustrate the effectiveness of the quasi-canonicalization and the Hamilton–Jacobi method.</jats:p>

<jats:p>A free triangular jet (TJ1) and its counterpart initially passing a short circular chamber (TJ2) are numerically modeled using large eddy simulation (LES). This paper compares the near-field characteristics of the two jets in detail. To enable some necessary experimental validations, the LES conditions of TJ1 and TJ2 are taken to be identical to those measured by Xu <jats:italic>et al.</jats:italic> (<jats:italic>Sci. China Phys.</jats:italic> <jats:bold>56</jats:bold> 1176 (2013)) and England <jats:italic>et al.</jats:italic> (<jats:italic>Exp. Fluids.</jats:italic> <jats:bold>48</jats:bold> 69 (2010)), respectively. The LES predictions are found to agree well with those measurements. It is demonstrated that a strong swirl occurs near the chamber inlet plane for the TJ2 flow. At the center of the swirl, there is a cluster of three sink foci, where each focus is aligned midway between the original triangular apexes. In the vortex skeleton constructed from the time-averaged flow field, the vortices arising from the foci are helically twisted around the core of the jet. As the flow passes through the chamber, the foci merge to form a closed-loop “bifurcation line”, which separates the inward swirling flow and the outward oscillating jet. This global oscillation is regarded as a source node near the centerline of the chamber. If the chamber is removed for a “free” jet, i.e., TJ1, a cluster of three pairs of counter-rotating foci is produced and the net swirl circulation is zero, so the overall oscillation of the jet does not occur.</jats:p>

<jats:p>We investigate the discharge and flow characterizations of a double-side siding discharge plasma actuator driven by different polarities of direct current (DC) voltage. The discharge tests show that sliding discharge and extended discharge are filamentary discharge. The irregular current pulse of sliding discharge fluctuates obviously in the first half cycle, ultimately expands the discharge channel. The instantaneous power and average power consumptions of sliding discharge are larger than those of the extended discharge and dielectric barrier discharge (DBD). The flow characteristics measured by a high-frequency particle-image-velocimetry system together with high-speed schlieren technology show that the opposite jet at the bias DC electrode is induced by sliding discharge, which causes a bulge structure in the discharge channel. The bias DC electrode can deflect the direction of the induced jet, then modifying the properties of the boundary layer. Extended discharge can accelerate the velocity of the starting vortex, improving the horizontal velocity profile by 203%. The momentum growth caused by extended discharge has the largest peak value and the fastest growth rate, compared with sliding discharge and DBD. However, the momentum growth of sliding discharge lasts longer in the whole pulsed cycle, indicating that sliding discharge can also inject more momentum.</jats:p>

<jats:p>Plasma control of forebody asymmetric vortices is mostly achieved by means of dielectric barrier discharge (DBD) plasma actuators. However, DBD actuators suffer from some disadvantages such as a weak induced body force, a single-direction induced jet, and an unclear control mechanism. We carry out wind tunnel experiments involving the forebody vortex control of a slender body at high angles of attack using an innovative extended DBD actuator, which has a stronger capacity to induce an electric wind than a DBD actuator. Through synchronous measurements of the pressure distribution and particle image velocimetry (PIV), the spatiotemporal evolution of the dynamic interactions between plasma-actuation-induced vortices and forebody asymmetric vortices is analyzed. The influence of plasma discharge on the boundary layer separation around a slender body and the spatial topological structures of asymmetric vortices are further surveyed, as the optimized actuation parameters. Extended DBD actuators are found to be more capable of controlling asymmetric vortices than DBD actuators, and a linear proportionality of the sectional lateral force versus the duty ratio is achieved. There exists an optimal normalized reduced frequency (<jats:italic>f</jats:italic> <jats:sup>+</jats:sup> = 2<jats:italic>π</jats:italic> <jats:italic>f</jats:italic> <jats:sub>p</jats:sub> <jats:italic>d</jats:italic>/<jats:italic>U</jats:italic> <jats:sub>∞</jats:sub> = 2.39) for asymmetric vortex control under the present experimental conditions. The research results can provide technical guidance for the control and reuse of forebody asymmetric vortices.</jats:p>

<jats:p>We consider a self-assembled hybrid system, composed of a bilayer vesicle to which a number of colloids are adhered. Based on known results of membrane curvature elasticity, we predict that, for sufficiently deflated prolate vesicles, the colloids can self-assemble into a ring at a finite distance away from the vesicle equator, thus breaking the up–down symmetry in the system. Because the relative variation of the position of the colloidal ring along the vesicle endows the system with an effective elasticity, periodic cycles of inflation and deflation can lead to non-reciprocal shape changes of the vesicle–colloid hybrid, allowing it to swim in a low Reynolds number environment under reciprocal actuation. We design several actuation protocols that allow control over the swimming direction.</jats:p>

<jats:p>We investigate the collective motion of rotlets that are placed in a single plane. Due to the hydrodynamic interactions, the particles move through the two-dimensional (2D) plane and we analyze these diffusive motions. By analyzing the scaling of the values, we predict that the diffusion coefficient scales with <jats:italic>ϕ</jats:italic> <jats:sup>0.5</jats:sup>, the average velocity with <jats:italic>ϕ</jats:italic>, and relaxation time of the velocity autocorrelation function with <jats:italic>ϕ</jats:italic> <jats:sup>–1.5</jats:sup>, where <jats:italic>ϕ</jats:italic> is the area fraction of the particles. In this paper, we find that the predicted scaling could be seen only when the initial particle position is homogeneous. The particle collective motions are different by starting the simulation from random initial positions, and the diffusion coefficient is the largest at a minimum volume fraction of our parameter range, <jats:italic>ϕ</jats:italic> = 0.05. The deviations based on two initial positions can be explained by the frequency of the collision events. The particles collide during their movements and the inter-particle distances gradually increase. When the area fraction is large, the particles will result in relatively homogeneous configurations regardless of the initial positions because of many collision events. When the area fraction is small (<jats:italic>ϕ</jats:italic> &lt; 0.25), on the other hand, two initial positions would fall into different local solutions because the rare collision events would not modify the inter-particle distances drastically. By starting from the homogeneous initial positions, the particles show the maximum diffusion coefficient at <jats:italic>ϕ</jats:italic> ≈ 0.20. The diffusion coefficient starts to decrease from this area fraction because the particles start to collide and hinder each other from a critical fraction ∼ 23 %. We believe our current work contributes to a basic understanding of the collective motion of rotating units.</jats:p>

<jats:p>To study helium (He) supersonic molecular beam injection (SMBI) into H-mode tokamak plasma, a simplified multicomponent-plasma model under the assumption of quasi-neutral condition is developed and implemented in the frame of BOUT ++. The simulation results show that He species propagate inwards after He SMBI, and are deposited at the bottom of the pedestal due to intensive ionization and weak spreading speed. It is found that almost all injected helium particles strip off all the bounded electrons. He species interact intensively with background plasma along the injection path during He SMBI, making deuterium ion density profile drop at the He-deposited location and resulting in a large electron temperature decreasing, but deuterium ion temperature decreasing a little at the top of the pedestal.</jats:p>

<jats:p>Effects of oblique collisions of the dust acoustic (DA) waves in dusty plasma are studied by considering unmagnetized fully ionized plasma. The plasma consists of inertial warm negatively charged massive dusts, positively charged dusts, superthermal kappa distributed electrons, and isothermal ions. The extended Poincaré–Lighthill–Kuo (ePLK) method is employed for the drivation of two-sided Korteweg–de Vries (KdV) equations (KdVEs). The KdV soliton solutions are derived by using the hyperbolic secant method. The effects of superthermality index of electrons, temperature ratio of isothermal ion to electron, and the density ratio of isothermal ions to negatively charged massive dusts on nonlinear coefficients are investigated. The effects of oblique collision on amplitude, phase shift, and potential profile of right traveling solitons of DA waves are also studied. The study reveals that the new nonlinear wave structures are produced in the colliding region due to head-on collision of the two counter propagating DA waves. The nonlinearity is found to decrease with the increasing density ratio of ion to negative dust in the critical region. The phase shifts decrease (increase) with increasing the temperature ratio of ion to electron (<jats:italic>κ</jats:italic> <jats:sub>e</jats:sub>). The hump (compressive, <jats:italic>κ</jats:italic> <jats:sub>e</jats:sub> &lt; <jats:italic>κ</jats:italic> <jats:sub>ec</jats:sub>) and dipshaped (rarefactive, <jats:italic>κ</jats:italic> <jats:sub>e</jats:sub> &gt; <jats:italic>κ</jats:italic> <jats:sub>ec</jats:sub>) solitons are produced depending on the angle (<jats:italic>θ</jats:italic>) of oblique collision between the two waves.</jats:p>

<jats:p>Laser-induced breakdown spectroscopy (LIBS) is an important technique which is widely used to analyze element composition. In order to improve the sensitivity of LIBS, much effort has been made to enhance the spectral intensity of LIBS by proposing a number of methods. In addition, we find that laser polarization has great influence on the emission intensity of femtosecond LIBS. By comparing the emission intensity of femtosecond LIBS in the circular polarization with that in the linear polarization, the spectral intensity in the case of circular polarization is stronger than that in the case of linear polarization. Moreover, this phenomenon is more obvious as laser energy increases. The polarization plays an important role in LIBS signal intensity. Based on the observation, the enhanced mechanism of the laser polarization for the spectral intensity is discussed in this paper, which will be helpful in spectral analysis and component analysis.</jats:p>