Showing papers on "Impulse (physics) published in 2015"

Wireless positioning systems

Abstract: A time-reversal positioning system includes a storage storing first data representing channel impulse responses derived from probe signals sent from a plurality of positions and second data representing coordinates of the positions. A data processor determines a position of a terminal device based on the stored channel impulse responses and a time-reversed signal determined based on a time-reversed version of a channel impulse response that is estimated based on a channel probing signal sent from the terminal device.

Variable-speed quadrupedal bounding using impulse planning: Untethered high-speed 3D Running of MIT Cheetah 2

Abstract: This paper introduces a bounding gait control algorithm that allows a variable-speed running in the MIT Cheetah 2. A simple impulse planning algorithm is proposed to design vertical and horizontal force profiles which make net impulse on the system during one cycle zero. This design of force profiles leads to the conservation of linear momentum over a complete step, providing periodicity in horizontal and vertical velocity. When designed profiles are applied to the system, periodic orbits with an ability to change running speed are obtained. A virtual compliance control in the horizontal and vertical direction has been added onto the designed force profiles to stabilize the periodic orbits. The experimental results show that the algorithm successfully achieved untethered 3D running of the MIT Cheetah 2, with speeds ranging from 0 m/sec to 4.5 m/sec on treadmills as well as on grassy fields.

Stability of highly cooled hypervelocity boundary layers

Abstract: The influence of high levels of wall cooling on the stability of hypervelocity boundary layers is investigated. Such conditions are relevant to experiments in high-enthalpy impulse facilities, where the wall temperature is much smaller than the free-stream temperature, as well as to some real flight scenarios. Some effects of wall cooling are well known, for instance, the stabilization of the first mode and destabilization of the second mode. In this paper, several new instability phenomena are investigated that arise only for high Mach numbers and high levels of wall cooling. In particular, certain unstable modes can travel supersonically with respect to the free stream, which changes the nature of the dispersion curve and leads to instability over a much wider band of frequencies. The cause of this phenomenon, the range of parameters for which it occurs and its implications for boundary layer stability are examined. Additionally, growth rates are systematically reported for a wide range of conditions relevant to high-enthalpy impulse facilities, and the stability trends in terms of Mach number and wall temperature are mapped out. Thermal non-equilibrium is included in the analysis and its influence on the stability characteristics of flows in impulse facilities is assessed.

Cavitation and bubble dynamics: the Kelvin impulse and its applications.

Abstract: Cavitation and bubble dynamics have a wide range of practical applications in a range of disciplines, including hydraulic, mechanical and naval engineering, oil exploration, clinical medicine and sonochemistry. However, this paper focuses on how a fundamental concept, the Kelvin impulse, can provide practical insights into engineering and industrial design problems. The pathway is provided through physical insight, idealized experiments and enhancing the accuracy and interpretation of the computation. In 1966, Benjamin and Ellis made a number of important statements relating to the use of the Kelvin impulse in cavitation and bubble dynamics, one of these being ‘One should always reason in terms of the Kelvin impulse, not in terms of the fluid momentum…’. We revisit part of this paper, developing the Kelvin impulse from first principles, using it, not only as a check on advanced computations (for which it was first used!), but also to provide greater physical insights into cavitation bubble dynamics near boundaries (rigid, potential free surface, two-fluid interface, flexible surface and axisymmetric stagnation point flow) and to provide predictions on different types of bubble collapse behaviour, later compared against experiments. The paper concludes with two recent studies involving (i) the direction of the jet formation in a cavitation bubble close to a rigid boundary in the presence of high-intensity ultrasound propagated parallel to the surface and (ii) the study of a ‘paradigm bubble model’ for the collapse of a translating spherical bubble, sometimes leading to a constant velocity high-speed jet, known as the Longuet-Higgins jet.

Impulse-induced vibration suppression of an axially moving beam with parallel nonlinear energy sinks

Abstract: Excessive vibration of the beam with varying axial speed could be suppressed by nonlinear targeted energy transfer Parallel nonlinear energy sink (NES) devices were attached to the beam for absorbing the vibration energy Galerkin method was applied to discretize the equation of the integrated translating beam–NES system derived from Newton’s second law The numerical method was used to display the effect of vibration suppression Results showed that the parallel NES could effectively suppress the vibration of the axially moving beam By contrast with the single NES under the same condition except the attached mass, not only the one was less and the suppressed effect was better

Critical Earthquake Response of Elastic–Plastic Structures Under Near-Fault Ground Motions (Part 1: Fling-Step Input)

Abstract: The double impulse input is introduced as a substitute of the fling-step near-fault ground motion and a closed-form solution of the elastic-plastic response of a structure by the ‘critical double impulse input’ is derived. Since only the free-vibration appears under such double impulse input, the energy approach plays an important role in the derivation of the closed-form solution of a complicated elastic-plastic response. It is shown that the maximum inelastic deformation can occur either after the first impulse or after the second impulse depending on the input level. The validity and accuracy of the proposed theory are investigated through the comparison with the response analysis to the corresponding one-cycle sinusoidal input as a representative of the fling-step near-fault ground motion.

Lightning Response of Grounding Grids: Simulated and Experimental Results

Abstract: The response of grounding grids subjected to currents with typical waveforms of first and subsequent lightning return strokes is analyzed, based on experimental and simulated results. Parameters, which characterize this response, such as the grounding potential rise, impulse impedance, impulse coefficient, and effective grid area are determined for grids buried in soils of low, moderate, and high resistivity, varying the grid dimension and considering the frequency dependence of soil parameters. Based on such results, expressions to obtain the impulse impedance from the low-frequency resistance are derived.

Statistical Channel Model with Multi-Frequency and Arbitrary Antenna Beamwidth for Millimeter-Wave Outdoor Communications

Abstract: This paper presents a 3-dimensional millimeter- wave statistical channel impulse response model from 28 GHz and 73 GHz ultrawideband propagation measurements. An accurate 3GPP-like channel model that supports arbitrary carrier frequency, RF bandwidth, and antenna beamwidth (for both omnidirectional and arbitrary directional antennas), is provided. Time cluster and spatial lobe model parameters are extracted from empirical distributions from field measurements. A step-by- step modeling procedure for generating channel coefficients is shown to agree with statistics from the field measurements, thus confirming that the statistical channel model faithfully recreates spatial and temporal channel impulse responses for use in millimeter-wave 5G air interface designs.

On the Analysis of Impulse Test Results on Grounding Systems

Abstract: In this paper, an analysis of experimental results on the behavior under impulse currents of various grounding electrodes: rod and horizontal electrodes, a ground grid, and tower footings, is presented. The parameters used for analyzing transient performance are reviewed, and the differences are highlighted based on test data. The analysis is extended to the following: 1) to quantify the effect of impulse shape; 2) to quantify the effect of current magnitude; 3) to compare low-frequency and impulse performances; 4) to compare impulse and high-frequency performances; and 5) to examine the effects of the test setup on measured results, e.g., in the case of field tests, the effect of current return leads, and proximity and extent of return electrodes. A generalized impulse index is introduced to help elucidate the differences between different parameters used for the analysis of transient test results on ground electrodes. It is found that the analysis of test data based on different parameters may lead to different assessments of impulse performance. The results also show that the impulse parameters used for the analysis of test data can be influenced by several factors such as electrode length, impulse current rise time, and experimental setup. In addition, variable-frequency test results are analyzed by introducing a “harmonic coefficient” which quantifies the deviations of the harmonic impedance from the low-frequency resistance over different frequency ranges covering the entire lightning frequency spectrum. Significant variations of the harmonic coefficient with frequency were observed, highlighting the importance of taking the frequency dependence of soil properties when modeling the impulse and high-frequency behavior of grounding systems.

Wave propagation of a functionally graded beam in thermal environments

Abstract: In this paper, the effect of material-temperature dependent on the wave propagation of a cantilever beam composed of functionally graded material (FGM) under the effect of an impact force is investigated. The beam is excited by a transverse triangular force impulse modulated by a harmonic motion. Material properties of the beam are temperature-dependent and change in the thickness direction. The Kelvin–Voigt model for the material of the beam is used. The considered problem is investigated within the Euler-Bernoulli beam theory by using energy based finite element method. The system of equations of motion is derived by using Lagrange\'s equations. The obtained system of linear differential equations is reduced to a linear algebraic equation system and solved in the time domain and frequency domain by using Newmark average acceleration method. In order to establish the accuracy of the present formulation and results, the comparison study is performed with the published results available in the literature. Good agreement is observed. In the study, the effects of material distributions and temperature rising on the wave propagation of the FGM beam are investigated in detail.

Progress in Lightning Impulse Characteristics of Grounding Electrodes With Soil Ionization

Abstract: Grounding is the fundamental measure of lightning protection for power systems, industries, and buildings, and the lightning impulse behavior of grounding electrodes plays a very important role. How to correctly consider the lightning impulse characteristics of grounding electrodes is the fundament in lightning protection design. This paper summarized the progress in lightning impulse characteristics of grounding electrodes with soil ionization, including the images of soil ionization, the residual resistivity of ionized soil, the critical breakdown electrical field, the impulse breakdown delay characteristics of soil, and the impulse breakdown mechanism of soil. Finally, the numerical simulation methods for lightning impulse characteristics of grounding electrodes are classified, and some key performances of grounding electrodes under lightning impulse were presented.

Synchronized Swept-Sine: Theory, Application, and Implementation

Abstract: Exponential, or sometimes called logarithmic, swept-sine signal is very often used to analyze nonlinear audio systems. In this paper, the theory of exponential swept-sine measurements of nonlinear systems is reexamined. The synchronization procedure necessary for a proper analysis of higher harmonics is detailed leading to an improvement of the formula for the exponential swept-sine signal generation. Moreover, an analytical expression of spectra of the swept-sine signal is derived and used in the deconvolution of the impulse response. A Matlab code for generation of the synchronized swept-sine, deconvolution, and separation of the impulse responses is given with discussion of some application issues and an illustrative example of harmonic analysis of current distortion of a woofer is provided.

Intensity impulse response of SDM links.

Abstract: We study the response of space-division multiplexed fiber links to an excitation by a short impulse of the optical intensity. We show that, in the presence of full mixing, the intensity impulse response is Gaussian, confirming recently reported experimental observations, and relate its variance to the mean square of the mode dispersion vector of the link τ. The good agreement between our theory and the previously published experiments provides solid foundations to the random coupling model of SDM fiber links, and provides a tool for efficient design of MIMO-DSP receivers.

A Robust Calibration Technique for Acoustic Emission Systems Based on Momentum Transfer from a Ball Drop

Abstract: We describe a technique to estimate the seismic moment of acoustic emissions and other extremely small seismic events. Unlike previous calibration tech- niques, it does not require modeling of the wave propagation, sensor response, or signal conditioning. Rather, this technique calibrates the recording system as a whole and uses a ball impact as a reference source or empirical Green's function. To correctly apply this technique, we develop mathematical expressions that link the seismic mo- ment M0 of internal seismic sources (i.e., earthquakes and acoustic emissions) to the impulse, or change in momentum Δp, of externally applied seismic sources (i.e., me- teor impacts or, in this case, ball impact). We find that, at low frequencies, moment and impulse are linked by a constant, which we call the force-moment-rate scale factor C F _ MM0=Δp. This constant is equal to twice the speed of sound in the material from which the seismic sources were generated. Next, we demonstrate the calibration technique on two different experimental rock mechanics facilities. The first example is a saw-cut cylindrical granite sample that is loaded in a triaxial apparatus at 40 MPa confining pressure. The second example is a 2 m long fault cut in a granite sample and deformed in a large biaxial apparatus at lower stress levels. Using the empirical cali- bration technique, we are able to determine absolute source parameters including the seismic moment, corner frequency, stress drop, and radiated energy of these magnitude −2:5 to −7 seismic events.

Quadrupedal galloping control for a wide range of speed via vertical impulse scaling.

Abstract: This paper presents a bio-inspired quadruped controller that allows variable-speed galloping. The controller design is inspired by observations from biological runners. Quadrupedal animals increase the vertical impulse that is generated by ground reaction forces at each stride as running speed increases and the duration of each stance phase reduces, whereas the swing phase stays relatively constant. Inspired by this observation, the presented controller estimates the required vertical impulse at each stride by applying the linear momentum conservation principle in the vertical direction and prescribes the ground reaction forces at each stride. The design process begins with deriving a planar model from the MIT Cheetah 2 robot. A baseline periodic limit cycle is obtained by optimizing ground reaction force profiles and the temporal gait pattern (timing and duration of gait phases). To stabilize the optimized limit cycle, the obtained limit cycle is converted to a state feedback controller by representing the obtained ground reaction force profiles as functions of the state variable, which is monotonically increasing throughout the gait, adding impedance control around the height and pitch trajectories of the obtained limit cycle and introducing a finite state machine and a pattern stabilizer to enforce the optimized gait pattern. The controller that achieves a stable 3 m s(-1) gallop successfully adapts the speed change by scaling the vertical ground reaction force to match the momentum lost by gravity and adding a simple speed controller that controls horizontal speed. Without requiring additional gait optimization processes, the controller achieves galloping at speeds ranging from 3 m s(-1) to 14.9 m s(-1) while respecting the torque limit of the motor used in the MIT Cheetah 2 robot. The robustness of the controller is verified by demonstrating stable running during various disturbances, including 1.49 m step down and 0.18 m step up, as well as random ground height and model parameter variations.

Direction of arrival estimation of reflections from room impulse responses using a spherical microphone array

Abstract: This paper studies the direction of arrival estimation of reflections in short time windows of room impulse responses measured with a spherical microphone array. Spectral-based methods, such as multiple signal classification (MUSIC) and beamforming, are commonly used in the analysis of spatial room impulse responses. However, the room acoustic reflections are highly correlated or even coherent in a single analysis window and this imposes limitations on the use of spectral-based methods. Here, we apply maximum likelihood (ML) methods, which are suitable for direction of arrival estimation of coherent reflections. These methods have been earlier developed in the linear space domain and here we present the ML methods in the context of spherical microphone array processing and room impulse responses. Experiments are conducted with simulated and real data using the em32 Eigenmike. The results show that direction estimation with ML methods is more robust against noise and less biased than MUSIC or beamforming.

Particle visualization in high-power impulse magnetron sputtering. II. Absolute density dynamics

Abstract: Time-resolved characterization of an Ar-Ti high-power impulse magnetron sputtering discharge has been performed. The present, second, paper of the study is related to the discharge characterization in terms of the absolute density of species using resonant absorption spectroscopy. The results on the time-resolved density evolution of the neutral and singly-ionized Ti ground state atoms as well as the metastable Ti and Ar atoms during the discharge on- and off-time are presented. Among the others, the questions related to the inversion of population of the Ti energy sublevels, as well as to re-normalization of the two-dimensional density maps in terms of the absolute density of species, are stressed.

Critical earthquake response of elastic–plastic structures under near-fault ground motions (Part 2: Forward-directivity input)

Abstract: The triple impulse input is used as a simplified version of the forward-directivity near-fault ground motion and a closed-form solution of the elastic-plastic response of a structure by this triple input is obtained. It is noteworthy that only the free-vibration appears under such triple impulse input. An almost critical excitation is defined and its response is derived. The energy approach plays an important role in the derivation of the closed-form solution of a complicated elastic-plastic response. It is shown that the maximum inelastic deformation can occur after the second impulse or the third impulse depending on the input level. The validity and accuracy of the proposed theory are discussed through the comparison with the response analysis result to the corresponding three wavelets of sinusoidal waves as a representative of the forward-directivity near-fault ground motion.

A Comprehensive Approach for Transient Performance of Grounding System in the Time Domain

Abstract: It is important to identify the impulse performance of the grounding system for lightning protection. However, the performance is nonlinear and dynamic under impulse condition. The electrical parameters of the grounding system are not only frequency dependent, but are also time dependent. Moreover, the mutual couplings between the grounding conductors cannot be ignored. Based on the hybrid electromagnetic method and vector fitting, a time-domain approach for the transient performance of the grounding system is developed. All the aforementioned phenomena can be covered in this method. Meanwhile, the sudden change of the parameters resulting from the soil ionization is also successfully dealt with in this paper. Compared with previous study results and field tests, the method proves to be reliable and effective.

Critical Input and Response of Elastic–Plastic Structures Under Long-Duration Earthquake Ground Motions

Abstract: The multiple impulse input is introduced as a substitute of the long-duration earthquake ground motion, mostly expressed in terms of harmonic waves, and a closed-form solution is derived of the elastic-plastic response of a single-degree-of-freedom structure under the ‘critical multiple impulse input’. Since only the free-vibration appears under such multiple impulse input, the energy approach plays an important role in the derivation of the closed-form solution of a complicated elastic-plastic response. It is shown that the critical inelastic deformation and the corresponding critical input frequency can be captured depending on the input level by the substituted multiple impulse input in the form of original and modified input sequence. The validity and accuracy of the proposed theory are investigated through the comparison with the response analysis to the corresponding sinusoidal input as a representative of the long-duration earthquake ground motion.

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: III. Shock Wave Characteristics With Three Discharge Loads

Abstract: As an important plasma-assisted technology to generate shock waves (SWs), underwater pulsed discharge has drawn much attention in recent years for its complex physical process. Based on three discharge loads, the water gap (WG) load, the electrical wire (EW) load, and the energetic material (EM) load, the discharge processes are briefly introduced and the characteristics of the associated SWs are analyzed. First, the experimental setups were built and typical structures of the three loads were presented. Second, the inherent characteristics of SWs under the three loads, such as their peak pressure, impulse, and time duration of positive pressure and power spectral density (PSD), were studied and compared. Finally, a cracking effect experiment is carried out to study the SW fracturing characteristics. The results show that SWs generated with the WG load have the lowest peak pressure, impulse, and power density, SWs generated with the EW load have a better energy conversion efficiency and the largest peak pressure, and SWs generated with the EM load have the maximum impulse and power density. Furthermore, SW fracturing characteristics are mainly affected by its inherent characteristics. The peak pressure and impulse determine the shock number of fracturing, and the fracture pattern is significantly affected by the PSD.

Measurement of plasma momentum exerted on target by a small helicon plasma thruster and comparison with direct thrust measurement

Abstract: Momentum, i.e., force, exerted from a small helicon plasma thruster to a target plate is measured simultaneously with a direct thrust measurement using a thrust balance. The calibration coefficient relating a target displacement to a steady-state force is obtained by supplying a dc to a calibration coil mounted on the target, where a force acting to a small permanent magnet located near the coil is directly measured by using a load cell. As the force exerted by the plasma flow to the target plate is in good agreement with the directly measured thrust, the validity of the target technique is demonstrated under the present operating conditions, where the thruster is operated in steady-state. Furthermore, a calibration coefficient relating a swing amplitude of the target to an impulse bit is also obtained by pulsing the calibration coil current. The force exerted by the pulsed plasma, which is estimated from the measured impulse bit and the pulse width, is also in good agreement with that obtained for the steady-state operation; hence, the thrust assessment of the helicon plasma thruster by the target is validated for both the steady-state and pulsed operations.

Novel Technique to Determine SparkJet Efficiency

Abstract: E NERGY deposition has recently received significant interest as a powerful technique in a variety of high-speed flow-control applications [1–5]. To achieve energy deposition, an electromagnetic local flow/flight control (ELFC) device generates pulsed electromagnetic fields. A wide variety of ELFC devices have been developed, including, for example, laser and/or microwave discharge, electron beam, and surface dc/ac discharge [2,3,6] and the nanosecond pulse mode of dielectric barrier discharge operation [7]. The Johns Hopkins University Applied Physics Laboratory has developed a unique ELFC device denoted the SparkJet for flow and flight control [8–14]. Figure 1 illustrates schematically the three stages of energy deposition by a SparkJet device. A spark is discharged within a typical volume of several cubic centimeters (stage 1). The high-pressure gas discharges through a converging nozzle, thereby generating an impulse (stage 2). Provided there is a mechanism for recharging the gas in the cavity (stage 3), the sequence can be repeated. Research has been conducted on the SparkJet by different groups. Narayanaswamy et al. [15] have performed experiments to investigate the effect of a SparkJet discharged at different frequencies and locations upstream of a 30 deg wedge on the separation induced by a shock/boundary-layer interaction. Caruana et al. [16] have carried out numerical and experimental research on a plasma synthetic jet generated by a similar device. They have shown that this method can be applicable for separation control and noise reduction. Anderson and Knight [17] have performed analytical and numerical studies to predict the impulse from the jet and the temporal pressure and temperature inside the cavity upon discharge. They have shown that an array of SparkJets can effectively replace a flap and hence be practical in flight control. The objective of this note is the determination of the electromechanical efficiency of a particular design of a SparkJet. The electromechanical efficiency is the fraction of the electrical energy that results in the generation of the SparkJet mechanical impulse based upon a perfect gas model. The spark discharge generates a plasma with excited electronic, rotational, and vibrational states. A portion of the energy dissipated across the spark gap is channeled into heating of the gas (i.e., increasing the translational–rotational temperature), which leads to a high pressure and subsequent jet exiting through the converging nozzle, thus creating the mechanical impulse. Various definitions of the SparkJet efficiency have been published. Haack et al. [13] have experimentally found an efficiency of 35% based on the measured peak pressure inside the SparkJet. In a later study, Haack et al. [14] have experimentally shown an average efficiency of 20–30% in different operating conditions based on the measured pressure and voltage. In this paper, a novel method for determining the electromechanical efficiency is proposed based upon comparison of predictions of a theoretical model and experimental measurements.

Sparseness-Controlled Proportionate Affine Projection Sign Algorithms for Acoustic Echo Cancellation

Abstract: In order to reduce the steady-state misalignment of real-coefficient proportionate affine projection sign algorithm (RP-APSA) for sparse impulse responses, a memory RP-APSA is proposed in this paper by exploiting the historical values of proportionate factors, called MP-APSA. Further, to improve the robustness of MP-APSA and the recently proposed memory-improved RP-APSA (MIP-APSA) for impulse responses with mutative sparseness, two sparseness-controlled algorithms (SC-MP-APSA and SC-MIP-APSA) are developed by estimating the sparseness of the impulse response at each iteration. Simulation results in the context of acoustic echo cancellation show that the proposed algorithms retain good robustness against impulsive interferences. More importantly, the proposed sparseness-controlled algorithms can not only obtain low steady-state misalignment in both the sparse and dispersive situations, but also adapt to the variations in the sparseness of the impulse response.