Showing papers on "Impulse (physics) published in 2016"

On pressure impulse of a laser-induced underwater shock wave

Abstract: We experimentally examine a laser-induced underwater shock wave paying special attention to the pressure impulse, the time integral of the pressure evolution. Plasma formation, shock-wave expansion and the pressure in water are observed simultaneously using a combined measurement system that obtains high-resolution nanosecond-order image sequences. These detailed measurements reveal a distribution of the pressure peak which is not spherically symmetric. In contrast, remarkably, the pressure impulse is found to be symmetrically distributed for a wide range of experimental parameters, even when the shock waves are emitted from an elongated region. The structure is determined to be a collection of multiple spherical shock waves originating from point-like plasmas in the elongated region.

Effects of a water hammer and cavitation on jet formation in a test tube

Abstract: We investigate the motion of a gas–liquid interface in a test tube induced by a large acceleration via impulsive force. We conduct simple experiments in which the tube partially filled with a liquid falls under gravity and hits a rigid floor. A curved gas–liquid interface inside the tube reverses and eventually forms a so-called focused jet. In our experiments, there arises either vibration of the interface or an increment in the velocity of the liquid jet, accompanied by the onset of cavitation in the liquid column. These phenomena cannot be explained by a considering pressure impulse in a classical potential flow analysis, which does not account for finite speeds of sound or phase changes. Here we model such water-hammer events as a result of the one-dimensional propagation of a pressure wave and its interaction with boundaries through acoustic impedance mismatching. The method of characteristics is applied to describe pressure-wave interactions and the subsequent cavitation. The model proposed is found to be able to capture the time-dependent characteristics of the liquid jet.

Impulse waves generated by snow avalanches: Momentum and energy transfer to a water body

Abstract: When a snow avalanche enters a body of water, it creates an impulse wave whose effects may be catastrophic. Assessing the risk posed by such events requires estimates of the wave’s features. Empirical equations have been developed for this purpose in the context of landslides and rock avalanches. Despite the density difference between snow and rock, these equations are also used in avalanche protection engineering. We developed a theoretical model which describes the momentum transfers between the particle and water phases of such events. Scaling analysis showed that these momentum transfers were controlled by a number of dimensionless parameters. Approximate solutions could be worked out by aggregating the dimensionless numbers into a single dimensionless group, which then made it possible to reduce the system’s degree of freedom. We carried out experiments that mimicked a snow avalanche striking a reservoir. A lightweight granular material was used as a substitute for snow. The setup was devised so as to satisfy the Froude similarity criterion between the real-world and laboratory scenarios. Our experiments in a water channel showed that the numerical solutions underestimated wave amplitude by a factor of 2 on average. We also compared our experimental data with those obtained by Heller and Hager (2010), who used the same relative particle density as in our runs, but at higher slide Froude numbers.

Multi-microphone neural network for sound recognition

Abstract: A neural network is provided for recognition and enhancement of multi-channel sound signals received by multiple microphones, which need not be aligned in a linear array in a given environment. Directions and distances of sound sources may also be detected by the neural network without the need for a beamformer connected to the microphones. The neural network may be trained by knowledge gained from free-field array impulse responses obtained in an anechoic chamber, array impulse responses that model simulated environments of different reverberation times, and array impulse responses obtained in actual environments.

Channel Impulse Responses in Diffusive Molecular Communication with Spherical Transmitters

Abstract: Molecular communication is an emerging paradigm for systems that rely on the release of molecules as information carriers. Communication via molecular diffusion is a popular strategy that is ubiquitous in nature and very fast over distances on the order of a micron or less. Existing closed-form analysis of the diffusion channel impulse response generally assumes that the transmitter is a point source. In this paper, channel impulse responses are derived for spherical transmitters with either a passive or absorbing receiver. The derived channel impulse responses are in closed-form for a one-dimensional environment and can be found via numerical integration for a three-dimensional environment. The point transmitter assumption (PTA) is formally defined so that its accuracy can be measured in comparison to the derived spherical transmitter impulse responses. The spherical transmitter model is much more accurate than the PTA when the distance between a transmitter and its receiver is small relative to the size of the transmitter. The derived results are verified via microscopic particle-based simulations using the molecular communication simulation platform AcCoRD (Actor-based Communication via Reaction-Diffusion). A spherical transmitter variation where molecules are released from the surface of a solid sphere is also considered via simulation.

Non-line-of-sight channel impulse response characterisation in visible light communications

Abstract: One of the important issues in an indoor visible light communication (VLC) system is the multi-path (MP) effect due to reflections. Generally, the MP effect can be characterised by the channel impulse response (CIR). Methods based on ray-tracing/Monte Carlo simulations are primarily used to obtain these impulse responses, but this generally leads to time consuming computer simulations. In this study, an analytical approach is proposed to directly and accurately calculate the non-line-of-sight (NLOS) components of the CIR due to the interior surface reflections in a room. The proposed method could be used in system-level simulations of a networked VLC system (also referred to as a LiFi attocell network), which requires a large number of channels with different transmitter and receiver deployments to be generated in a computationally efficient manner.

Practical stability analysis of fractional-order impulsive control systems.

Abstract: In this paper we obtain sufficient conditions for practical stability of a nonlinear system of differential equations of fractional order subject to impulse effects. Our results provide a design method of impulsive control law which practically stabilizes the impulse free fractional-order system.

Joint Confidence Sets for Structural Impulse Responses

Abstract: Many questions of economic interest in structural VAR analysis involve estimates of multiple impulse response functions. Other questions relate to the shape of a given impulse response function. Answering these questions requires joint inference about sets of structural impulse responses, allowing for dependencies across time as well as across response functions. Such joint inference is complicated by the fact that the joint distribution of the structural impulse response estimators becomes degenerate when the number of structural impulse responses of interest exceeds the number of model parameters, as is often the case in applied work. This degeneracy may be overcome by transforming the estimator appropriately. We show that the joint Wald test is invariant to this transformation and converges to a nonstandard distribution, which can be approximated by the bootstrap, allowing the construction of asymptotically valid joint confidence sets for any subset of structural impulse responses, regardless of whether the joint distribution of the structural impulse responses is degenerate or not. We propose to represent the joint confidence sets in the form of “shotgun plots” rather than joint confidence bands for impulse response functions. Several empirical examples demonstrate that this approach not only conveys the same information as confidence bands about the statistical significance of response functions, but may be used to provide economically relevant additional information about the shape of and comovement across response functions that is lost when reducing the joint confidence set to two-dimensional bands.

Short Time-Scale Sensory Coding in S1 during Discrimination of Whisker Vibrotactile Sequences.

Abstract: Rodent whisker input consists of dense microvibration sequences that are often temporally integrated for perceptual discrimination. Whether primary somatosensory cortex (S1) participates in temporal integration is unknown. We trained rats to discriminate whisker impulse sequences that varied in single-impulse kinematics (5–20-ms time scale) and mean speed (150-ms time scale). Rats appeared to use the integrated feature, mean speed, to guide discrimination in this task, consistent with similar prior studies. Despite this, 52% of S1 units, including 73% of units in L4 and L2/3, encoded sequences at fast time scales (≤20 ms, mostly 5–10 ms), accurately reflecting single impulse kinematics. 17% of units, mostly in L5, showed weaker impulse responses and a slow firing rate increase during sequences. However, these units did not effectively integrate whisker impulses, but instead combined weak impulse responses with a distinct, slow signal correlated to behavioral choice. A neural decoder could identify sequences from fast unit spike trains and behavioral choice from slow units. Thus, S1 encoded fast time scale whisker input without substantial temporal integration across whisker impulses.

Jumping depends on impulse not power

Abstract: To the editor-in-chief,We read with interest the manuscript by Russell, Sparkes, Northeast & Kilduff (2015) and noted their use of power to characterise performance in jumping. Such use is widespre...

Time-Gating-Based Time Reversal Imaging for Impulse Borehole Radar in Layered Media

Abstract: In this paper, the formulation of reverse time migration (RTM) is improved for impulse borehole radar imaging in the subsurface scenarios with layered media. By fully adopting the prior information of surrounding media, the time gating function is designed and applied to the incident wave field and scattering wave field for each imaging point, which strengthens the correlation between the wave fields in the time domain. The clutters partly caused by the multiple reflections between different media layers are suppressed due to the gating function. A normalized zero-offset cross correlation with gated samples is conducted and used to weight the result of RTM. The improved approach is compared with the conventional RTM, the back-projection method, and the Stolt migration algorithm with synthetic data and is then validated by a single-borehole radar experiment in a layered media scenario. The results demonstrate that the developed approach is superior to the conventional methods in locating targets and robust in complex subsurface environments.

Globally exponential stability of delayed impulsive functional differential systems with impulse time windows

Abstract: In the practical application of impulsive differential systems, impulse does not always occur at the fixed-time point; it may occur in a little range of time. Namely, impulse occurs in a time window, which is more general and more nearing to reality than those fixed-time impulses. Therefore, it is necessary to investigate the dynamical behaviors of impulsive differential systems with impulse time windows. In this paper, the exponential stability of these systems is researched. By means of Lyapunov functions, Razumikhin technique and other analysis methods, several novel exponential stability criteria for delayed impulsive functional differential equations with impulse time windows are obtained, which are different from the previously published results for fixed-time impulses. What is more, based on the analysis of this paper, it is worth noting that choosing an efficient impulse time window may be easier and more effective than choosing fixed-time impulsive sequences. Finally, three examples and their simulations are provided to illustrate the effectiveness of our results.

Dynamic Impulse Systems Theory And Applications

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Impulse and Sampled-Data Optimal Control of Heat Equations, and Error Estimates

Abstract: We consider the optimal control problem of minimizing some quadratic functional over all possible solutions of an internally controlled multidimensional heat equation with a periodic terminal state constraint. This problem has a unique optimal solution, which can be characterized by an optimality system derived from the Pontryagin maximum principle. We define two approximations of this optimal control problem. The first one is an impulse approximation and consists of considering a system of linear heat equations with impulse control. The second one is obtained by the sample-and-hold procedure applied to the control, resulting in a sampled-data approximation of the controlled heat equation. We prove that both problems have a unique optimal solution, and we establish precise error estimates for the optimal controls and optimal states of the initial problem with respect to its impulse and sampled-data approximations.

Preliminary stage synchronization algorithm of auto-compensation quantum key distribution system with an unauthorized access security

Abstract: A two-pass auto-compensation fiber-optic quantum key distribution system (QKDS) with phase-encoded photon states in synchronization mode has been investigated. Preliminary stage sync initialization algorithm of QKDS that requires impulse-repetition period division into time windows as well as signal time window detection has been proposed. The peculiarity of the algorithm being investigated is that it is implemented in the single-photon mode which improves the synchronization mode security. The second peculiarity is that in case the number of accumulated impulses in two adjacent time windows is equal, it is decided to receive a photon impulse by any of these windows, if the number of accumulated impulses exceeds the number of registered impulses in the rest of the windows. Algorithm modeling has discovered that when the number of time windows is changed from 2 to 100000 probabilities limit values difference does not exceed 0.5 %. Requirements of time window duration ratio to the photon impulse have been formulated. It has been demonstrated that when the number of time windows is increased from 2 to 1024 the probability reduces by less than 1.5 % in case of time uncertainty decrease in respect to the photon impulse occurrence in 512 times. An analytical expression for computation of a signal time window correct detection probability in the proposed algorithm of the preliminary stage QKDS synchronization.

Spatial impulse waves: wave height decay experiments at laboratory scale

Abstract: Impulse waves generated by rapid subaerial landslides into water bodies may pose a threat to riparian settlements and infrastructure. Empirically derived prediction equations based on experiments at laboratory scale provide information on key wave characteristics for preliminary hazard assessment. This research discusses existing prediction methods for spatial wave propagation features and compares their results with own impulse wave height decay experiments. While some prediction methods are based on simplified approaches for wave generation such as rigid body slides, others take only limited sets of slide parameters into account, narrowing their range of applicability considerably. The prediction methods are intentionally applied outside their ranges of applicability with the aim to assess their characteristics on an extended parameter range. It is found that a combination of separate terms for wave generation and wave propagation from two different existing prediction methods provides the best representation of the experimental data.

Finite element simulation of impulse wave generated by landslides using a three-phase model and the conservative level set method

Abstract: A numerical model has been developed using the finite element method for the simulation of impulse waves generated by landslides. The fluid-like landslide is modeled as a generalized non-Newtonian visco-plastic fluid. A three-phase flow model based on the incompressible viscous Navier–Stokes equations is solved using the finite element method to describe the motion of the three types of fluid in landslide. The conservative level set method is expanded to n-phase flow cases and employed to capture the interface of the three phases: air, water, and the landslide. The overall performance of the approach is checked by a number of validation cases: a Rayleigh–Taylor instability problem to illustrate the capability of the proposed method to deal with interface capturing, a benchmark test of a subaerial landslide generated by an impulse wave is carried out and compared with the published experimental data and numerical results, and finally, the 1958 Lituya Bay landslide generated impulse wave, and its results are compared against a scaled-down experiment and other published numerical results. It can be noted that the current model has an excellent ability to capture the complex phenomena that occurs during the whole process of the landslide-generated impulse wave, and considering the simplified treatment of the landslide and the numerical model, fairly good agreement between computed and experimental results has been observed for all simulation cases.