Rigid Piston Research Articles

Directivity and Frequency-Dependent Effective Sensitive Element Size of Needle Hydrophones: Predictions From Four Theoretical Forms Compared With Measurements.

Directivity is a hydrophone specification that describes response as a function of angle of incidence. The goal of this study was to compare, in the context of needle hydrophones, the commonly used rigid baffle model for hydrophone directivity to three alternative models: soft baffle, unbaffled (UB), and rigid piston (RP). Directivity measurements were performed at 1, 2, 3, 4, 6, 8, and 10 MHz from ±7° in two orthogonal planes for two ceramic and two polymer needle hydrophones with nominal geometrical sensitive element diameters of 200, 400, 600, and 1000 . Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to directivity measurement data using the sensitive element radius (a) as an adjustable parameter. For > 4 (where and = wavelength), the RP model outperformed the other three models. For , the average error in estimated sensitive element radius was 7% [95% confidence interval (CI): 3%-12%] for the RP model while the lowest average error by the other three models was 46% (95% CI: 38%-54%) for the UB model.

Experimental Characterisation of a Flat Dielectric Elastomer Loudspeaker

Conventional loudspeakers are often heavy, require substantial design spaces and are hard to integrate into lightweight structures (e.g., panels). To overcome these drawbacks, this paper presents a novel extremely flat loudspeaker which uses dielectric elastomer actuators with natural rubber for the elastomeric layers and metal electrodes as transduction mechanism. To facilitate the deformation of the elastomer, the electrodes are perforated. The microscopic holes lead to a macroscopically compressible stack configuration despite the elastomer incompressibility. The design is developed and the materials are chosen to guarantee low mechanical and electrical losses and a high efficiency in the entire frequency range up to several kilohertz. The loudspeaker was designed, built and afterwards experimentally investigated and characterised. Laser measurements of the surface velocity were performed to find dynamic effects present at the diaphragm. To further characterise the device, a semi anechoic chamber as used. Sound pressure levels emitted by the device were recorded at different bias and alternating voltages to study their influence. The nonlinearity of the loudspeaker, which is inherent for this kind of actuators, was quantified considering the total harmonic distortion. Here, a dependence on the amplitude of the alternating voltage is observed. Further, the distortion decreases rapidly the higher the frequency is, which qualifies the loudspeaker concept to properly work at high frequencies. Transfer functions between supplied voltage and on-axis sound pressure were measured and showed in principle potential for high frequency application. Further, the behaviour of the diaphragm changing from rigid piston to resilient disk with respect to frequency for different configurations was observed. Additionally, the directivity of the loudspeaker was investigated at several frequencies, and was in accordance with previously found research outcomes. The results, especially in the high frequency range, prove the usability of this design concept for practical applications.

Modelling of the multi-chamber oscillating water column in regular waves at model scale

This paper studies the reliability of numerical models used for estimating multi-chamber oscillating water column (MC-OWC) response in the time-domain. The model for the internal water surface level and instantaneous pressure inside the chamber at regular waves conditions using a hybrid system of hydrodynamic and thermodynamic rigid piston models without power take-off. Reliability is assessed using experimental data obtained from a wave tank used in the model concept validation. The results show the method could be extended to describe the hydrodynamics of the MC-OWC in regular and irregular wave conditions.

Expansions for the radiation impedance of a rectangular piston in an infinite baffle.

Relatively compact analytical expressions in the form of fast-convergin g expansions are derived for the radiation resistance and reactance of a rectangular rigid piston in an infinite rigid baffle, which are computationally efficient at high frequencies or large aspect ratios and yield simple approximations (asymptotic expressions) at low frequencies. Plots of the normalized radiation resistance and reactance are shown for various aspect ratios with constant width as well as constant area. Comparisons are also made with the impedance of an elliptic piston.

The French Syrette of morphine for administration to combat casualties.

As French military physicians currently deployed in Theatre of Operations, we read with great interest the study by Blankenstein et al. determining the opinions of recently deployed British Army personnel with forward medical care experience on the importance of effective battlefield analgesia, the efficacy and shortcomings of current battlefield analgesia and desirable features of the ideal battlefield analgesic.1 Their results supported existing evidence for the introduction of an adjunct to intramuscular (IM) morphine. Surprisingly, in the introduction section, the authors stated that ‘The British armed forces are unique in supplying parenteral opiates to all deployed soldiers, comprising two 10 mg morphine auto-injectors designed for intramuscular (IM) administration to casualties’. We don’t totally agree with this statement since the French Army has a long experience of battlefield analgesia, using a 10 mg morphine auto-injector, also known as the Syrette of morphine. The Syrette is a device for injecting liquid through a needle, almost similar to a syringe, except that it has a closed flexible tube, instead of a rigid tube and piston. Before deployment in a combat zone, every French soldier undergoes the Forward Combat Casualty Care Level 1-courses and is trained to use his combat first aid kit, including the subcutaneous injection of 10 mg of morphine, using this Syrette.2 Finally, in the French Military Health Service policy for combat analgesia, and also suggested by the authors, other techniques are encouraged, including titration of intravenous morphine, use of ketamine and fascia-iliaca compartment block for lower limb injuries.3

Compaction of granular material inside confined geometries

In both nature and the laboratory, loosely packed granular materials are often compacted inside confined geometries. Here, we explore such behaviour in a quasi-two dimensional geometry, where parallel rigid walls provide the confinement. We use the discrete element method to investigate the stress distribution developed within the granular packing as a result of compaction due to the displacement of a rigid piston. We observe that the stress within the packing increases exponentially with the length of accumulated grains, and show an extension to current analytic models which fits the measured stress. The micromechanical behaviour is studied for a range of system parameters, and the limitations of existing analytic models are described. In particular, we show the smallest sized systems which can be treated using existing models. Additionally, the effects of increasing piston rate, and variations of the initial packing fraction, are described.

Normalized inverse characterization of sound absorbing rigid porous media.

This paper presents a methodology for the inverse characterization of sound absorbing rigid porous media, based on standard measurements of the surface acoustic impedance of a porous sample. The model parameters need to be normalized to have a robust identification procedure which fits the model-predicted impedance curves with the measured ones. Such a normalization provides a substitute set of dimensionless (normalized) parameters unambiguously related to the original model parameters. Moreover, two scaling frequencies are introduced, however, they are not additional parameters and for different, yet reasonable, assumptions of their values, the identification procedure should eventually lead to the same solution. The proposed identification technique uses measured and computed impedance curves for a porous sample not only in the standard configuration, that is, set to the rigid termination piston in an impedance tube, but also with air gaps of known thicknesses between the sample and the piston. Therefore, all necessary analytical formulas for sound propagation in double-layered media are provided. The methodology is illustrated by one numerical test and by two examples based on the experimental measurements of the acoustic impedance and absorption of porous ceramic samples of different thicknesses and a sample of polyurethane foam.

Non-contact, ultrasound-based indentation method for measuring elastic properties of biological tissues using Harmonic Motion Imaging (HMI)

Noninvasive measurement of mechanical properties of biological tissues in vivo could play a significant role in improving the current understanding of tissue biomechanics. In this study, we propose a method for measuring elastic properties non-invasively by using internal indentation as generated by harmonic motion imaging (HMI). In HMI, an oscillating acoustic radiation force is produced by a focused ultrasound transducer at the focal region, and the resulting displacements are estimated by tracking radiofrequency signals acquired by an imaging transducer. In this study, the focal spot region was modeled as a rigid cylindrical piston that exerts an oscillatory, uniform internal force to the underlying tissue. The HMI elastic modulus EHMI was defined as the ratio of the applied force to the axial strain measured by 1D ultrasound imaging. The accuracy and the precision of the EHMI estimate were assessed both numerically and experimentally in polyacrylamide tissue-mimicking phantoms. Initial feasibility of this method in soft tissues was also shown in canine liver specimens in vitro. Very good correlation and agreement was found between the measured Young’s modulus and the HMI modulus in the numerical study (r2 > 0.99, relative error <10%) and on polyacrylamide gels (r2 = 0.95, relative error <24%). The average HMI modulus on five liver samples was found to EHMI = 2.62 ± 0.41 kPa, compared to EMechTesting = 4.2 ± 2.58 kPa measured by rheometry. This study has demonstrated for the first time the initial feasibility of a non-invasive, model-independent method to estimate local elastic properties of biological tissues at a submillimeter scale using an internal indentation-like approach. Ongoing studies include in vitro experiments in a larger number of samples and feasibility testing in in vivo models as well as pathological human specimens.

The Effect of Plate Discretization on Accuracy of the Sound Radiation Efficiency Measurements

Abstract This paper deals with the problem of the effect of discretization level and certain other parameters characterizing the measurement setup on accuracy of the process of determination of the sound radiation efficiency by means of the Discrete Calculation Method (DCM) described by Hashimoto (2001). The idea behind DCM consists in virtual division of an examined sound radiating structure into rectangular elements each of which is further assumed to contribute to the total radiation effect in the same way as a rigid circular piston having the surface area equal to this of the corresponding virtual element and vibrating in an infinite rigid baffle. The advantage of the method over conventional sound radiation efficiency measurement techniques consists in the fact that instead of acoustic pressure values, source (plate) vibration velocity amplitude values are measured in a selected number of regularly distributed points. In many cases, this allows to determine the sound radiation efficiency with sufficient accuracy, especially for the low frequency regime. The key part of the paper is an analysis of the effect of discretization level (i.e. the choice of the number of points at which vibration amplitude measurements are to be taken with the use of accelerometers) on results obtained with the use of the method and their accuracy. The problem of determining an optimum level of discretization for given excitation frequency range is a very important issue as the labor intensity (time-consuming aspect) of the method is one of its main flaws. As far as the technical aspect of the method is concerned, two different geometrical configurations of the measurement setup were tested.

Effects of margin-parallel shortening and density contrasts on back-arc extension during subduction: Experimental insights and possible application to Anatolia

So as to investigate the parameters influencing subduction and back-arc extension, we have done three series of laboratory experiments (32 in all) on physical models. Each model consisted of adjacent oceanic and continental plates, floating on an asthenosphere. In experiments of Series A, a wide rigid piston, moving horizontally, controlled the rate of convergence of the oceanic and continental plates, whereas, in Series B or C, a wide or narrow piston produced lateral compression, parallel to the continent-ocean boundary (COB) and perpendicular to the subduction direction. The parameters that we tested were (1) the velocity of plate convergence (Series A), (2) the width of the compressing piston (Series B and C), and (3) the density ratio between oceanic and continental plates (Series B and C). This density ratio was a key factor. For a ratio of 1.4, the amount of extension in the continental plate increased regularly throughout time; for a ratio of 1.3, the extension remained small, until the piston stopped moving laterally; and for a ratio of 1.1, there was little or no extension. The width of the compressing piston had a smaller effect, although a narrow piston provided more space, into which the continental plate could extend. One possible application of our models is to Anatolia. Despite similar geological settings, the areas north of the Hellenic and Cyprus subduction zones differ, in that extension is large in the former and much smaller in the latter. We suggest that one of the main driving forces for Aegean extension may have been a high density ratio between subducting oceanic lithosphere and a Hellenic-Balkanic upper plate.

The Research on Vibration Analysis of Blow-Down Wind Tunnel Pressure Regulating Valves

The pressure regulating valves are crucial segments to control the pressure of the stability chambers in the transonic and supersonic blow-down wind tunnels. To solve the valves vibration problem, the pistons and their supporting structures of three typical pressure regulating valves in China Aerodynamics Research and Development Center (CARDC) were analyzed based on the modal analysis theory. From the analysis, two conclusions were obtained that the supporting structures are more rigid than the pistons, and a rigid piston is of critical importance to the valves stiffness. The structural parameters comparing results of the pistons showed the same viewpoint. A new design pressure regulating valve demonstrating the successful application according to the conclusion above was presented.

Study on direct sound reduction structure for reducing noise generated by vibrating solids

A direct sound reduction structure is proposed as a new method for reducing noise generated by vibrating solids. The proposed method involves directly attaching a hard sound absorbing material with a honeycomb structure to the surfaces of vibrating solids to provide a moderate amount of back air space. Using a one-dimensional acoustic-field model, the fundamental sound reduction characteristics are investigated theoretically and experimentally, and the effectiveness of the proposed concept is verified. In the theoretical analysis, an analytical model with a sinusoidally excited rigid-frame sound reduction structure is considered. In the experiments, a one-dimensional acoustic tube is used to determine the sound reduction ratios for both stationary excitation and non-stationary impulsive excitation as a function of frequency. Furthermore, in order to verify the practical usefulness of this approach, an experimental study on sound reduction in a three-dimensional acoustic field is carried out using either a rigid piston plate surrounded by a baffle or a flexible flat plate with different vibration modes as vibrating bases. The results indicate that the sound reduction ratio has a quasi-periodic form that depends on the thickness of the back air space and the sound frequency, and a sound pressure reduction of approximately 80 percent (−14dB) is observed around the minima. In addition, the average reduction in the frequency range 0.5–5kHz is approximately 40 percent (−4.4dB). The results obtained in this study are expected to act as useful indices for designing a nearly optimum sound reduction structure if the target frequency is known in advance.

Acoustic radiation efficiency of a periodically corrugated rigid piston

The radiation of sound by a periodically corrugated rigid piston is explored using theoretical and numerical approaches and compared with the radiation of flat rigid piston. The depth and the period of the corrugation are considered to be comparable with the wavelength in the surrounding fluid. Radiation enhancement is predicted due to cavity resonances and coherent diffraction. In addition, broad regions of low radiation efficiency are observed. Both effects could have an impact in acoustic transducers technology, either to increase the piston radiated power or to create a source of evanescent acoustic waves. The possibilities offered by this strategy in the nonlinear acoustic regime are also briefly discussed.

PARTICLE ACCELERATION AT A FLARE TERMINATION SHOCK: EFFECT OF LARGE-SCALE MAGNETIC TURBULENCE

We investigate the acceleration of charged particles (both electrons and protons) at collisionless shocks predicted to exist in the vicinity of solar flares. The existence of standing termination shocks has been examined by flare models and numerical simulations e.g., Shibata,Forbes. We study electron energization by numerically integrating the equations of motion of a large number of test-particle electrons in the time-dependent two-dimensional electric and magnetic fields generated from hybrid simulations (kinetic ions and fluid electron) using parameters typical of the solar flare plasma environment. The shock is produced by injecting plasma flow toward a rigid piston. Large-scale magnetic fluctuations -- known to exist in plasmas and known to have important effects on the nonthermal electron acceleration at shocks -- are also included in our simulations. For the parameters characteristic of the flaring region, our calculations suggest that the termination shock formed in the reconnection outflow region (above post-flare loops) could accelerate electrons to a kinetic energy of a few MeV within 100 ion cyclotron periods, which is of the order of a millisecond. Given a sufficient turbulence amplitude level ($\delta B^2/B_0^2 \sim 0.3$), about 10% of thermal test-particle electrons are accelerated to more than 15 keV. We find that protons are also accelerated, but not to as high energy in the available time and the energy spectra are considerably steeper than that of the electrons for the parameters used in our simulations. Our results are qualitatively consistent with the observed hard X-ray emissions in solar flares.

Sound Radiation from a Loudspeaker, from a Spherical Pole Cap, and from a Piston in an Infinite Baffle

Loudspeakers are often modelled as a rigid piston in an infinite baffle. As a model for real loudspeakers, this approach is limited in two ways. One issue is that a loudspeaker cone is not rigid, and a second issue is that a loudspeaker is mostly used in a cabinet. Both issues are addressed here by developing the velocity of the radiator in terms of orthogonal polynomials known from optical diffraction theory as Zernike circle polynomials. Using these polynomials we develop semi-analytic expressions for the sound pressure from the radiator in two different cases: as a flexible flat radiator mounted in an infinite baffle, and as the cap of a rigid sphere. In the latter case the comparison is done not only for the pressure but also for other quantities viz. the baffle-step response, sound power and directivity, and the acoustic centre of the radiator. These quantities are compared with those from a real loudspeaker. Finally, in the case of the baffled-piston radiation the spatial impulse response is presented.

A hearing protector model for predicting impulsive noise hazard.

Reduction in impulsive noise hearing hazard by earplugs and earmuffs is calculated with an electroacoustic lumped-parameter model of HPD attenuation using real ear attenuation at threshold (REAT) data. Energy flows into the occluded volume along three paths, each considered as a piston: (1) the rigid protector mass moving against the skin, (2) leakage at the support, and (3) transmission through the protector material (a second piston within the rigid piston). Circuit elements are adjusted so loss matches REAT data assuming path 1 is important at low, 2 at middle, and 3 at high REAT frequencies. Applying the model to 384 REAT data sets for ANSI S12.6 method B naive users gives statistical frequency distributions of occluded volume and leakage elements. For a given free-field impulsive noise, the model pressure predictions under the protector are compared to measurements on human and acoustical manikin ears to check validity of assumptions. The hearing hazards of the measured waveforms and the predicted waveforms are calculated with our previously developed auditory hazard analysis algorithm for humans ear model. The result is a cumulative frequency distribution of hazard based on user fit data useful in finding the best protector for a given impulsive noise.

Nonlinear modeling of an immersed transmitting capacitive micromachined ultrasonic transducer for harmonic balance analysis

Finite element method (FEM) is used for transient dynamic analysis of capacitive micromachined ultrasonic transducers (CMUT) and is particularly useful when the membranes are driven in the nonlinear regime. One major disadvantage of FEM is the excessive time required for simulation. Harmonic balance (HB) analysis, on the other hand, provides an accurate estimate of the steady-state response of nonlinear circuits very quickly. It is common to use Mason's equivalent circuit to model the mechanical section of CMUT. However, it is not appropriate to terminate Mason's mechanical LC section by a rigid piston's radiation impedance, especially for an immersed CMUT. We studied the membrane behavior using a transient FEM analysis and found out that for a wide range of harmonics around the series resonance, the membrane displacement can be modeled as a clamped radiator. We considered the root mean square of the velocity distribution on the membrane surface as the circuit variable rather than the average velocity. With this definition, the kinetic energy of the membrane mass is the same as that in the model. We derived the force and current equations for a clamped radiator and implemented them using a commercial HB simulator. We observed much better agreement between FEM and the proposed equivalent model, compared with the conventional model.

On-axis and far-field sound radiation from resilient flat and dome-shaped radiators

On-axis and far-field series expansions are developed for the sound pressure due to an arbitrary, circular symmetric velocity distribution on a flat radiator in an infinite baffle. These expansions are obtained by expanding the velocity distributions in terms of orthogonal polynomials R(2n) (0)(sigma/a)=P(n)(2(sigma/a)(2)-1) with P(n) the Legendre polynomials. The terms R(2n) (0) give rise to a closed-form expression for the pressure on-axis as well as for the far-field pressure. Furthermore, for a large number of velocity profiles, including those associated with the rigid piston, the simply supported radiator, and the clamped radiators as well as Gaussian radiators, there are closed-form expressions for the required expansion coefficients. In particular, for the rigid, simply supported, and clamped radiators, this results in explicit finite-series expressions for both the on-axis and far-field pressures. In the reverse direction, a method of estimating velocity distributions from (measured) on-axis pressures by matching in terms of expansion coefficients is proposed. Together with the forward far-field computation scheme, this yields a method for far-field loudspeaker assessment from on-axis data (generalized Keele scheme). The forward computation scheme is extended to dome-shaped radiators with arbitrary velocity distributions.

366 減音構造による騒音低減効果の特性に関する研究

The present paper presented the concept of, Sound Reduction Structure, in which hard sound absorbing materials are directly installed on the surface of vibrating solid body with some back air spaces. First, by the study assuming one-dimensional acoustic field, it was ascertained theoretically that the sound reduction effects are actually obtainable by the Structures with moderate back air space, showing the good agreements with the experimental results. Then, for the cases of vibrating rigid piston plates or flexible rectangular flat plates with vibrating modes, sound reduction characteristics in the three-dimensional acoustic fields were experimentally clarified in detail.

Hotspot Formation in Shock-Induced Void Collapse

We present results from molecular dynamics simulations of shock-induced hydrodynamic void collapse in a model energetic crystal. During void collapse, hotspot formation is observed that leads to subsequent detonation. The hotspot formation mechanism is identified as shock energy focusing via jetting. There is another initiation mechanism that arises from the interaction of reflected shock waves with the rigid piston, which is considered to be an artifact. Such artifact can be eliminated by altering the location of the void. The detonation threshold as a function of the velocity of the driven piston is determined for various void geometries. It is found that a system containing a void has a lower detonation threshold than that of a perfect energetic crystal. The amount of reduction of the detonation threshold depends on the geometry of the void. For square voids, there exists a minimum size above which reduction of the detonation threshold occurs. Among voids that have an equal volume, the void that is elongated along the shock direction gives the lowest detonation threshold.