Abstract
To enhance the damage effect of focusing warhead, the investigation about the scattering behavior of focusing warhead is conducted. Based on the momentum method and the detonation wave ...theory, the influence of initiation detonation methods and curvature radius of the charge on fragment velocity is analyzed. The fragments velocity distribution models of the end-initiated, the center-initiated and the two ends-initiated are obtained. Numerical simulation shows that fragments velocity distribution and the propagation of the detonation wave of numerical simulation results have a good fit with the theoretical model. It is shown that fragments velocity is higher in the middle and low on both sides. When it was end-initiated, the location where the fragment velocity reaches the maximum was 18.6cm from the initiation end. When it was center-initiated, the maximum fragment velocity appeared at the center of the projectile axis. When it was two ends-initiated, the maximum fragment velocity appeared symmetric about the center line of the projectile axis. With increasing of the curvature radius of the circular charge, the maximum velocity of the fragments increased.
We consider the coordinated attitude control problem for a group of spacecraft, without velocity measurements. Our approach is based on the introduction of auxiliary dynamical systems (playing the ...role of velocity observers in a certain sense) to generate the individual and relative damping terms in the absence of the actual angular velocities and relative angular velocities. Our main focus, in this technical note, is to address the following two problems: 1) Design a velocity-free attitude tracking and synchronization control scheme, that allows the team members to align their attitudes and track a time-varying reference trajectory (simultaneously). 2) Design a velocity-free synchronization control scheme, in the case where no reference attitude is specified, and all spacecraft are required to reach a consensus by aligning their attitudes with the same final time-varying attitude. In this work, one important and novel feature (besides the non-requirement of the angular velocity measurements), consists in the fact that the control torques are naturally bounded and the designer can arbitrarily assign the desired bounds on the control torques, a priori , through the control gains, regardless of the angular velocities. Throughout this technical note, the communication flow between spacecraft is assumed to be undirected. Simulation results of a scenario of four spacecraft are provided to show the effectiveness of the proposed control schemes.
The acousto-electric (AE) effect associated with the propagation of the Rayleigh wave in ZnO half-space was theoretically investigated by studying the changes in wave velocity and propagation loss ...induced by in-depth inhomogeneous changes in the ZnO electrical conductivity. An exponentially decaying profile for the electrical conductivity was attributed to the ZnO half-space, for some values of the exponential decay constant (from 100 to 500 nm), in order to simulate the photoconductivity effect induced by ultra-violet illumination. The calculated Rayleigh wave velocity and attenuation vs. ZnO conductivity curves have the form of a double-relaxation response as opposed to the single-relaxation response which characterizes the well-known AE effect due to surface conductivity changes onto piezoelectric media. As to the author's knowledge, this is the first time the double-relaxation AE effect has been theoretically predicted.
In this article, a resonant cavity piezoelectric cantilever sensor (RCPCS) is designed and optimized for wind velocity and direction measurement. The RCPCS consists of a resonant cavity with a ring ...groove array and a free-standing piezoelectric cantilever array. The presence of the resonant cavity magnifies the local pressure at the cantilever's boundary and also increases the cantilever's vibration amplitude according to a new resonant cavity mathematical model. A structural optimization model is created using MATLAB and the COMSOL optimization module to increase the sensitivity of RCPCS. The analytical results demonstrate that the optimized cantilever's strain distribution is more uniform than the original results, as well as the physical mechanism of strain and aerodynamic amplification. Several comprehensive experiments are conducted to evaluate the RCPCS prototype's performance utilizing a new identification method of wind velocity and direction. The experimental results agree well with the mathematical solutions for the RCPCS response voltage and sensitivity, and the optimized cantilever's sensitivity is noticeably improved over the unoptimized cantilever. Note that the comparison of experimental results demonstrates a significant reduction in the range of direction error (<inline-formula> <tex-math notation="LaTeX">\boldsymbol {\Delta } \boldsymbol {\theta }_{\mathbf {exp}} </tex-math></inline-formula>) from 0.34°-1.04° to 0.09°-0.29°. Similarly, the range of velocity error (<inline-formula> <tex-math notation="LaTeX">\mathrm { \boldsymbol {\Delta }}\mathbf {v}_{\mathbf {exp}} </tex-math></inline-formula>) is reduced significantly from 1.20%-2.27% to 0.14%-0.80%. Additionally, the measured lower limit can reach 0.2 m/s. As a result of these findings, the superiority of the proposed RCPCS's reaction performance is validated, and a simple, highly sensitive, and high-precision wind sensor is constructed.
The interaction between an incident shock wave and a Mach-6 undisturbed hypersonic laminar boundary layer over a cold wall is addressed using direct numerical simulations (DNS) and wall-modelled ...large-eddy simulations (WMLES) at different angles of incidence. At sufficiently high shock-incidence angles, the boundary layer transitions to turbulence via breakdown of near-wall streaks shortly downstream of the shock impingement, without the need of any inflow free-stream disturbances. The transition causes a localized significant increase in the Stanton number and skin-friction coefficient, with high incidence angles augmenting the peak thermomechanical loads in an approximately linear way. Statistical analyses of the boundary layer downstream of the interaction for each case are provided that quantify streamwise spatial variations of the Reynolds analogy factors and indicate a breakdown of the Morkovin's hypothesis near the wall, where velocity and temperature become correlated. A modified strong Reynolds analogy with a fixed turbulent Prandtl number is observed to perform best. Conventional transformations fail at collapsing the mean velocity profiles on the incompressible log law. The WMLES prompts transition and peak heating, delays separation and advances reattachment, thereby shortening the separation bubble. When the shock leads to transition, WMLES provides predictions of DNS peak thermomechanical loads within $\pm 10\,\%$ at a computational cost lower than DNS by two orders of magnitude. Downstream of the interaction, in the turbulent boundary layer, the WMLES agrees well with DNS results for the Reynolds analogy factor, the mean profiles of velocity and temperature, including the temperature peak, and the temperature/velocity correlation.
The ion tails of bright comets have long been considered as a natural tracers of the solar wind (SW) near these objects. Studies of comets and their ion tails allow inexpensive monitoring of key SW ...structures in the inner heliosphere, much of which is otherwise only accessible by in situ SW spacecraft measurements. Here, we present a novel technique to mine the rich archive of amateur, professional and spacecraft observations of cometary ion tails. To demonstrate this, we focus on Near‐Sun comet C/2011 L4 (Pan‐STARRS) during Carrington Rotations (CR) 2134 and 2135 and comet C/2013 R1 (Lovejoy) during CR 2118. We outline the technique’s shortcomings, including its geometric limitations, and present a catalog of radial SW velocities derived in the near‐comet environment and information on the heliospheric conditions inferred from the measured SW. Complementary measurements, derived from folding ion rays and a velocity profile map built from consecutive images, are provided as an alternative means of quantifying the SW ‐cometary ionosphere interaction. We find that comets are generally good indicators of SW structure, but the quality of the results is strongly dependent on the observing geometry.
Plain Language Summary
Comets, as they move through the inner solar system, can be considered as natural laboratories of the solar wind (SW). The SW is a continuous stream of fast charged particles that carries with it a remnant of the solar magnetic field into the solar system. Sourcing images from the internet and astrophotographers, we developed a new technique and the software to measure the SW speed in the comet’s orbital plane by using the ion tails of comets. The ion tail behaves similarly to a transparent windsock and indicates the direction of the SW. This allowed us to create a snapshot map of the SW variations along a comet’s orbit when it is close to the Sun. We also outline the reliability and the limitations of the technique and a catalog of SW velocities from comets C/2011 L4 (Pan‐STARRS) and C/2013 R1 (Lovejoy). We find that with the right geometry, comets are good and efficient probes of the SW. This catalog of speeds will help us to better understand the three‐dimensional structure and variability of the SW.
Key Points
Multi‐point multi‐latitudinal solar wind (SW) velocities can be derived from cometary ion tails
Images acquired from observatories, STEREO B provide comparable results to amateur astronomers
Results validated against 3D MHD models offer snapshots of the SW structure
Surface wave data from the 25 April 2015 Nepal earthquake of
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7.8 and its aftershocks recorded at 11 stations in India were used to estimate group velocities of both Love and Rayleigh waves. All ...stations are at regional distances from the earthquake sources. It is observed that the group velocities for both Love and Rayleigh waves obtained from mainshock data are lower than those obtained from aftershock data for stations located west and southwest of the earthquake source region. The differences are generally lower towards the southwest as compared to those observed for stations located towards the west. The differences decrease progressively towards the east, and when the stations are located close to the direction of rupture propagation, the group velocity obtained from the mainshock is either equal to or higher than that obtained from aftershocks for both Love and Rayleigh waves. Such variation in group velocity obtained from mainshock and aftershock data for different stations may be due to source directivity for the mainshock affecting the source group time, which in turn affects the travel time of surface waves at different periods. It was previously believed that source group time affects group velocity estimations significantly only at periods longer than 75 s and only when the focal depth is less than 25 km. In this work, we show that when the source rupture is unidirectional, group velocity estimates are also affected for shorter periods.
•The restitution coefficient related to velocity model was first investigated in the random vibration system.•The restitution coefficient is segmented according to the level of the yield ...velocity.•The stochastic averaging procedure is used to study the response of vibro-impact system.•The obtained stationary PDF in a closed-form provides an effective way for optimal design.•The reasonable of the theoretical results is tested by numerical results.
Hertz contact model has not included the impact of inelastic, while the modified Hertz contact models are too complicated to handle. The classical impact model only uses a material-related constant to describe the relationship between the velocity before and after a collision, without considering the energy dissipation mechanism in the impact process. On the other hand, many recent experiments have shown that the restitution factor was a function of impact velocity. Such impact velocity dependent restitution factor offers a comprehensive and reasonable impact model. In the present paper, the random vibration of single-degree-of-freedom (SDOF) vibro-impact oscillators with the restitution factor related to velocity under wide-band noise excitations is investigated. The given stochastic vibro-impact oscillator is firstly transformed into a nonlinear system without impact by a non-smooth transformation. Then, a Markov approximation is applied to obtain the Fokker–Plank-Kolmogorov (FPK) equation governing the evolution of the probability density function (PDF) of the energy envelope. The stationary PDFs of the energy envelope and amplitude envelope are solvable in a closed-form. Finally, two examples are given to verify the effectiveness of the proposed method. The results showed that the classical impact model can simplify the calculations to some extent. However, the error caused by this method cannot be ignored. The revised model presented in this paper is more in line with the actual working conditions. For complex dynamical system, especially for multi-point impact vibration systems, the work of this paper is helpful to the optimization of the system.
Abstract
Passive surface-wave methods using dense seismic arrays have gained growing attention in near-surface high-resolution imaging in urban environments. Deep learning (DL) can release a ...tremendous workload brought by dense seismic arrays. We presented a case study of shear-wave velocity (Vs) structure imaging in the Hangzhou urban area (eastern China) using DL inversion. Noise data were recorded by dense linear arrays with approximately 5 m spacing deployed along two crossing roads for investigating the top 80 m of the subsurface. Phase-velocity dispersion curves are extracted from virtual shot gathers using multichannel analysis of surface waves. We divided the area where the low-velocity layer (LVL) may exist into three layers with a thickness of 5 m. We gave the four layers weak constraints to generate training dataset and adopted a convolutional neural network to directly invert fundamental-mode Rayleigh-wave phase velocity for 1D Vs models. To improve the accuracy, we further applied the sensitivities to weight the loss function in DL inversion. The obtained pseudo-2D Vs profiles correspond to the velocities estimated from logging data and previous survey. The well-trained neural network successfully identified that the LVL is located at 50-60 m deep. And this network was also achieved accurately the inversion of a dense seismic network nearby. The results of this survey demonstrate the accuracy and efficiency of delineating near-surface structures from traffic-induced noise using the DL technique, which has great potential for monitoring subsurface changes in urban areas.
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