Showing papers on "Impulse (physics) published in 2006"

Position Based Dynamics.

Abstract: The most popular approaches for the simulation of dynamic systems in computer graphics are force based Internal and external forces are accumulated from which accelerations are computed based on Newton's second law of motion A time integration method is then used to update the velocities and finally the positions of the object A few simulation methods (most rigid body simulators) use impulse based dynamics and directly manipulate velocities In this paper we present an approach which omits the velocity layer as well and immediately works on the positions The main advantage of a position based approach is its controllability Overshooting problems of explicit integration schemes in force based systems can be avoided In addition, collision constraints can be handled easily and penetrations can be resolved completely by projecting points to valid locations We have used the approach to build a real time cloth simulator which is part of a physics software library for games This application demonstrates the strengths and benefits of the method

Wire network mapping method and apparatus using impulse responses

Abstract: A method and apparatus for mapping a wire network is disclosed. The method includes obtaining a reflectometry test signal of the wire network. An estimated network impulse response is estimated from the reflectometry response. A wire network model is then initialized, and iteratively improved by simulating an impulse response of the wire network model and adjusting the wire network model to reduce differences between the simulated impulse response and estimated network impulse response.

The impulsive response of sandwich beams: Analytical and numerical investigation of regimes of behaviour

Abstract: An analytical model is developed to classify the impulsive response of sandwich beams based on the relative time-scales of core compression and the bending/stretching response of the sandwich beam. It is shown that an overlap in time scales leads to a coupled response and to the possibility of an enhanced shock resistance. Four regimes of behaviour are defined: decoupled responses with the sandwich core densifying partially or completely, and coupled responses with partial or full core densification. These regimes are marked on maps with axes chosen from the sandwich beam transverse core strength, the sandwich beam aspect ratio and the level of blast impulse. In addition to predicting the time-scales involved in the response of the sandwich beam, the analytical model is used to estimate the back face deflection, the degree of core compression and the magnitude of the support reactions. The predictions of the analytical model are compared with finite element (FE) simulations of impulsively loaded sandwich beams comprising an anisotropic foam core and elastic, ideally plastic face-sheets. The analytical and numerical predictions are in good agreement up to the end of core compression. However, the analytical model under-predicts the peak back face deflection and over-predicts the support reactions, especially for sandwich beams with high strength cores. The FE calculations are employed to construct design charts to select the optimum transverse core strength that either minimises the back face deflections or support reactions for a given sandwich beam aspect ratio or blast impulse. Typically, the value of the transverse core strength that minimises the back face deflection also minimises the support reactions. However, the optimal core strength depends on the level of blast impulse, with higher strength cores required for greater blasts.

Analysis of contrast in images generated with transient acoustic radiation force.

Abstract: Several mechanical imaging methods are under investigation that use focused ultrasound (US) as a source of mechanical excitation. Images are then generated of the tissue response to this localized excitation. One such method, acoustic radiation force impulse (ARFI) imaging, utilizes a single US transducer on a commercial US system to transmit brief, high-energy, focused acoustic pulses to generate radiation force in tissue and correlation-based US methods to detect the resulting tissue displacements. Local displacements reflect relative mechanical properties of tissue. The resolution of these images is comparable with that of conventional B-mode imaging. The response of tissue to focused radiation force excitation is complex and depends upon tissue geometry, forcing function geometry (i.e., region of excitation, or ROE) and tissue mechanical and acoustic properties. Finite element method (FEM) simulations using an experimentally validated model and phantom experiments have been performed using varying systems, system configurations and tissue-mimicking phantoms to determine their impact on image quality. Image quality is assessed by lesion contrast. Due to the dynamic nature of ARFI excitation, lesion contrast is temporally-dependent. Contrast of spherical inclusions is highest immediately after force cessation, decreases with time postforce and then reverses, due to shear wave interaction with internal boundaries, differences in shear modulus between lesions and background and inertial effects. In images generated immediately after force cessation, contrast does not vary with applied force, increases with lesion stiffness and increases as the ROE size decreases relative to the size of the structure being imaged. These studies indicate that improved contrast in radiation force-generated images will be achieved as ROE size decreases; however, frame rate and thermal considerations present trade-offs with small ROE size.

High-specific impulse Hall thrusters, part 1: Influence of current density and magnetic field

Abstract: A laboratory-model Hall thruster with a magnetic circuit designed for high-specific impulse (2000‐3000 s) was evaluated to determine how current density and magnetic field affect thruster operation. Results have shown for the first time that a minimum current density and optimum magnetic field shape exist at which efficiency will monotonically increase with specific impulse. At the nominal mass flow rate of 10 mg/s and between discharge voltages of 300 and 1000 V, total specific impulse and total efficiency ranged from 1600 to 3400 s and 51 to 61%, respectively. Comparison with a similar thruster showed how efficiency can be optimized for specific impulse by varying the shape of the magnetic field. Plume divergence decreased from a maximum of 48 deg at 400 V to a minimum of 35 deg at 1000 V, but increased between 300 and 400 V as the likely result of a large increase in discharge current oscillations. The breathing-mode frequency continuously increased with voltage, from 14.5 kHz at 300 V to 22 kHz at 1000 V, in contrast to other Hall thrusters where a sharp decrease of the breathing-mode frequency was found to coincide with increasing electron current and decreasing efficiency. These findings suggest that efficient, high-specific impulse operation was enabled through the regulation of the electron current with the applied magnetic field.

PDE-based image restoration: a hybrid model and color image denoising

Abstract: The paper is concerned with PDE-based image restoration. A new model is introduced by hybridizing a nonconvex variant of the total variation minimization (TVM) and the motion by mean curvature (MMC) in order to deal with the mixture of the impulse and Gaussian noises reliably. We suggest the essentially nondissipative (ENoD) difference schemes for the MMC component to eliminate the impulse noise with a minimum (ideally no) introduction of dissipation. The MMC-TVM hybrid model and the ENoD schemes are applied for both gray-scale and color images. For color image denoising, we consider the chromaticity-brightness decomposition with the chromaticity formulated in the angle domain. An incomplete Crank-Nicolson alternating direction implicit time-stepping procedure is adopted to solve those differential equations efficiently. Numerical experiments have shown that the new hybrid model and the numerical schemes can remove the mixture of the impulse and Gaussian noises, efficiently and reliably, preserving edges quite satisfactorily.

Momentum Transfer for an Aerodynamic Plasma Actuator with an Imposed Boundary Layer

Abstract: In previous work at the United States Air Force Academy, the phenomenology and behavior of the aerodynamic plasma actuator (a dielectric barrier discharge (DBD)) has been investigated. In order to provide additional insight into the phenomenology associated with the transfer of momentum to air by a plasma flow actuator, the velocity distributions upstream and downstream of a plasma actuator with an induced boundary layer were measured using freestream velocities of approximately 4.6 and 6.8 m/s for a range of frequencies (5-20 kHz) and voltages (7.5-10 kV amplitude). The body forces on the air were calculated using a control volume momentum balance. The results show that the body force acts in the sub-boundary layer region. For constant voltage, the body force is proportional to frequency producing a constant impulse per cycle, and the energy dissipation per cycle and efficiency are independent of frequency. The body forces are not affected by the freestream velocity.

Temporal Force Measurements on an Aerodynamic Plasma Actuator

Abstract: In previous work at the United States Air Force Academy, the phenomenology and behavior of the aerodynamic plasma actuator (a dielectric barrier discharge (DBD)) has been investigated. In order to provide additional insight into the phenomenology associated with the transfer of momentum to the air by the plasma actuator, the time-averaged body (reaction) force and the temporal force have been measured for a range of frequencies (5-20 kHz) and voltages (7.5-10 kV amplitude). The time-averaged forces were measured with a pendulum arrangement. The time-dependent forces were measured using an accelerometer. The results show that for constant voltage, the body force is proportional to frequency and the impulse per cycle is constant. Furthermore the energy dissipation per cycle and efficiency are independent of frequency. At constant frequency the increase in body force with voltage is shown. The time-dependent measurements support the theory that the body force of the actuator consists of one large "PUSH" followed by one small "pull" during each cycle.

Performance of earthing systems of low resistivity soils

Abstract: In a previous paper , the properties of sand with different water contents were investigated under high-magnitude fast-impulse currents, and an equivalent circuit model for soil ionisation was derived. For the hemispherical test cell used, resistance values in the kilo-ohm (Omega) range were obtained. These high values of resistance limited the current magnitudes to less than 300 A before breakdown occurred in the test cell. In practice, however, the resistance of an earth electrode is usually less than 10 Omega and the transient current magnitudes can reach several tens of kiloamperes. It is, therefore, necessary to investigate the behavior of soils under high current magnitudes and compare it with low ac and dc test results. In order to achieve high impulse current magnitudes with the adopted test cell, low resistivity media were utilized as test soils. In this present paper, a new impulse test data using wet clay and sand mixed with controlled amounts of sodium chloride (NaCl) to obtain a range of low-resistivity materials, was analyzed. The characteristics of NaCl are also investigated under low-magnitude low-frequency and high-magnitude impulse currents in order to understand better its effect on sand-salt soil mixtures. Impulse currents up to 5 kA were used during the test program. These new data would be useful in understanding the characteristics of a low earth resistance value (which is a typical condition at field site) under high impulse conditions. By including the nonlinear effect of the soil under high currents, more accurate modelling of protective devices and their performance could be achieved. In addition, the consideration of the nonlinearity in soils can help the optimum design of earthing systems

Model for the Performance of Airbreathing Pulse-Detonation Engines

Abstract: A simplified flowpath analysis of a single-tube airbreathing pulse detonation engine is described. The configuration consists of a steady supersonic inlet, a large plenum, a valve, and a straight detonation tube (no exit nozzle). The interaction of the filling process with the detonation is studied, and it is shown how the flow in the plenum is coupled with the flow in the detonation tube. This coupling results in total pressure losses and pressure oscillations in the plenum caused by the unsteadiness of the flow. Moreover, the filling process generates a moving flow into which the detonation has to initiate and propagate. An analytical model is developed for predicting the flow and estimating performance based on an open-system control volume analysis and gasdynamics. The existing single-cycle impulse model is extended to include the effect of filling velocity on detonation tube impulse. Based on this, the engine thrust is found to be the sum of the contributions of detonation tube impulse, momentum, and pressure terms. Performance calculations for pulse detonation engines operating with stoichiometric hydrogen–air and JP10–air are presented and compared to the performance of the ideal ramjet over a range of Mach numbers.

Modeling and Analog Realization of the Fundamental Linear Fractional Order Differential Equation

Abstract: This paper provides a rational function approximation of the irrational transfer function of the fundamental linear fra- ctional order differential equation, namely, $$(\tau _0)^m \frac{{d^m x(t)}}{{dt^m }} + x(t) = e(t)$$ whose transfer function is given by $$G(s) = \frac{{X(s)}}{{E(s)}} = \frac{1}{{[1 {+} (\tau _0 s)^m ]}}$ for $0 < m < 2$$ for 0

Dynamic friction force in a carbon peapod oscillator

Abstract: We investigate a new generation of fullerene nano-oscillators: a single-walled carbon nanotube with one buckyball inside with an operating frequency in the tens-of-gigahertz range. A quantitative characterization of energy dissipation channels in the peapod pair has been performed via molecular dynamics simulation. Edge effects are found to be the dominant cause of dynamic friction in the carbon-peapod oscillators. A comparative study on the energy dissipation also reveals the significant impact of temperature and impulse velocity on the frictional force.

Characteristic investigation of ablative laser propulsion driven by nanosecond laser pulses

Abstract: The momentum transfer and the specific impulse of the ablative laser propulsion of nanosecond laser irradiation on copper, lead, aluminum and graphite targets are investigated. The effects of the ambient pressure and laser focal spot sizes on the target momentum are measured. The results show that the target momentum strongly relates to the ambient pressure and target property. The highest target momentum about 2.28 g center dot cm/s is obtained on lead targets under 1 atmospheric pressure. With the increase of the focal spot sizes, the specific impulse decreases. The highest specific impulse in vacuum is about 950 s on copper targets.

Optimal unity-magnitude input shaper duration analysis

Abstract: Command profiles are required to move a dynamical system from rest to rest without residual vibration in a minimum time. The unity-magnitude (UM) input shaper is one of the faster input shaping techniques. However, analytical solution of the impulse time locations is impossible due to the dependent residual vibration constraint equations. Many researchers solve these by using curve fitting. In this paper, the characteristics of the UM shaping coefficients as a function of system parameters are investigated. An analysis procedure to obtain UM input shaper impulse time sequences is presented. Finally, the proposed technique is compared to the zero vibration (ZV) shaper in both response and robustness to modeling error.

Characteristic overpressure–impulse–distance curves for the detonation of explosives, pyrotechnics or unstable substances

Abstract: A number of models have been proposed to calculate overpressure and impulse from accidental industrial explosions. When the blast is produced by explosives, pyrotechnics or unstable substances, the TNT equivalent model is widely used. From the curves given by this model, data are fitted to obtain equations showing the relationship between overpressure, impulse and distance. These equations, referred to here as characteristic curves, can be fitted by means of power equations, which depend on the TNT equivalent mass. Characteristic curves allow determination of overpressure and impulse at each distance.

Numerical analysis of sound propagation in rooms using the finite difference time domain method

Abstract: Impulse response is an important quantity for room acoustics. To predict the impulse response, several types of calculation methods based on wave theory, such as the boundary element method (BEM), finite element method (FEM) and finite difference method (FDM), are being investigated. Among them, the authors are exploring application of the finite difference time domain (FDTD) method to calculate room sound fields. Using the method, by which the acoustical quantities at discrete grid points of a closed field are successively solved step‐by‐step according to vector operations, the impulse responses can be obtained directly in a time domain using little computer memory storage. Furthermore, changes of spatial distributions of room sound fields in time can be obtained easily by storing the sound pressure at grid points and displaying them by animation. In this presentation, several examples of the FDTD analyses of room impulse responses are reported and computer animation of their room sound fields is introduced. In addition, examination of the applicability of prediction of the impulse responses by comparison with in situ measurement of a small hall is discussed.

Development of Aeroservoelastic Analytical Models and Gust Load Alleviation Control Laws of a SensorCraft Wind-Tunnel Model Using Measured Data

Abstract: Aeroservoelastic (ASE) analytical models of a SensorCraft wind-tunnel model are generated using measured data. The data was acquired during the ASE wind-tunnel test of the HiLDA (High Lift-to-Drag Active) Wing model, tested in the NASA Langley Transonic Dynamics Tunnel (TDT) in late 2004. Two time-domain system identification techniques are applied to the development of the ASE analytical models: impulse response (IR) method and the Generalized Predictive Control (GPC) method. Using measured control surface inputs (frequency sweeps) and associated sensor responses, the IR method is used to extract corresponding input/output impulse response pairs. These impulse responses are then transformed into state-space models for use in ASE analyses. Similarly, the GPC method transforms measured random control surface inputs and associated sensor responses into an AutoRegressive with eXogenous input (ARX) model. The ARX model is then used to develop the gust load alleviation (GLA) control law. For the IR method, comparison of measured with simulated responses are presented to investigate the accuracy of the ASE analytical models developed. For the GPC method, comparison of simulated open-loop and closed-loop (GLA) time histories are presented.

Lightning impulse performances of grounding devices covered with low-resistivity materials

Abstract: The low-resistivity material (LRM) is widely used to decrease the power-frequency grounding resistances of grounding devices for transmission lines in the regions with high soil resistivity. The study about the influence of the LRM on impulse performances of grounding devices cannot be found in literature. Experimental results of lightning impulse properties of grounding devices covered with LRM were presented in this paper. The influences of impulse current, geometrical dimensions of grounding devices, and soil resistivity on impulse grounding resistance and impulse coefficient of grounding devices with the LRM coverings are still the same with those without the LRM coverings. The impulse grounding resistance decreases from about 25% to 45% when the LRM covering is used. The fitting formulae to calculate the reducing ratio of impulse grounding resistance of different grounding devices are provided. On the other hand, the influence of LRM on the effective impulse length of grounding electrodes is discussed.

Edge-diffraction impulse responses near specular-zone and shadow-zone boundaries

Abstract: Many methods for the computation of edge-diffraction impulse responses are based on the Biot-Tolstoy solution, an explicit, continuous-time expression for diffraction by an infinite wedge. This expression contains two singularities at the onset of the impulse response: one which is present for all source-receiver combinations, and a second which occurs only when a receiver moves across a specular-zone or shadow-zone boundary, i.e. a boundary where a geometrical-acoustics component has a discontinuity. For the calculation of discrete-time impulse responses, such a continuous-time analytical expression must be numerically integrated, and the singularities demand special attention. Svensson et al. [U. P. Svensson et al., J. Acoust. Soc. Am. 106, 2331-2344 (1999)] presented an analytic, secondary-source model of edge diffraction based on the Biot-Tolstoy expression in which the first singularity was eliminated by reformulating the expression as an integral along the edge. In this paper, analytical approximations for the model presented by Svensson et al. are described which address the second type of singularity and thus allow for accurate numerical computations for receivers at or near zone boundaries. Implementation details are also provided, along with example calculations.

Prompt toroidal momentum balance with collisionless neutral beam injected torque in DIII-D

Abstract: In electron cyclotron heated (ECH) H-mode discharges with neutral beam injection (NBI) pulses that are short compared with the fast ion scattering or slowing times, and it is observed that the plasma stores all the angular momentum delivered by the NBI torque impulse. The pulse length is also much shorter than the momentum confinement time of the plasma. Source computations with the Monte Carlo code TRANSP [R. J. Goldston, D. C. McCune, H. H. Towner et al., J. Comput. Phys. 43, 61 (1981)] show that during a pulse approximately 90% of this torque impulse is delivered via the collisionless fast radial current injection process, so that the plasma acquires the balancing toroidal acceleration through ion drift motion in the increasing ∂E∕∂t where E is the electric field normal to the flux surfaces. The measured radial profile of the toroidal momentum increase matches the source, i.e., the computed torque impulse profile. We measure the bulk ion toroidal acceleration in helium discharges, as well as that of th...

Ultra-wide Band Channel Calculation by SBR/Image Techniques for Indoor Communication

Abstract: In this paper, we use ray tracing techniques and inverse Fourier transform to compute the impulse responses for ultra-wide band (UWB) indoor communication. First, the frequency responses of different indoor environments for any transmitter-receiver location are computed by shooting and bouncing ray/image (SBR/Image) techniques in UWB frequency ranges. The frequency dependence utilized in the structure on the indoor channel is accounted for in the channel calculation. Then, the impulse responses of channel is obtained by applying inverse Fourier transform to transform the frequency domain data into time domain. A realistic complex environment is simulated by our proposed method. By using the obtained impulse responses of these multi-path channels, the impact of metallic furniture to indoor multi-path environment is investigated. Numerical results have shown that the multi-path effect with metallic cabinets is more severe than that without metallic cabinets. Moreover, it is also found that when the receiver...

Selective switching median filter for the removal of salt & pepper impulse noise

Abstract: In this paper, a new median based filtering algorithm is presented for the removal of impulse noise from digital images. A good analysis of the limitations of the top ranking median filters, the progressive switching median filter, PSMF and the rank-order based adaptive median filter, RAMF is made and are overcome very effectively by the proposed filter which cleans the impulse corruptions of a digital image in two distinct phases of impulse detection and impulse correction. The detection phase identifies the corrupted pixels into a flag image by a spatial rank ordered approach and the correction phase modifies the corrupted pixels identified in the flag image by a more suitable rank ordered value by considering the neighboring features. The filter shows better performance than most other impulse filters, and in particular, that of the top ranking RAMF and PSMF in terms of computational aspect and objectively. The filtering technique gives good results with less patchy effects from images corrupted at higher noise levels and the results are more appealing with good fidelity at lower noise levels.

Impulse-Response Analysis in Markov Switching Vector Autoregressive Models

Abstract: By utilizing the state-space representation of Markov-switching vector autoregressive models, we develop impulse response functions with regard to shocks to variables of the system and shifts in regime. The proposed analysis is related to the concept of generalized impulse responses introduced by Koop, Pesaran and Potter (1996) but characterizes the properties of the model dynamics in a more concise form. In contrast to the impulse response functions proposed by Ehrmann, Ellison and Valla (2003) the analysis here fully reflects the Markov prop erty of the switching regimes. Empirical illustrations of the approach suggested here include the un ivariate Hamilton (1989) model of the US business cycle and two Markov-switching vector autoregressions of US output growth, employment growth and the term structure.