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1.
The problem investigated is that of the response of thin plates subjected to pure tone or wide band frequency excitation. A point load is applied perpendicular to the plates. In the case of isotropic or orthotropic plates excited at high frequencies ring shaped standing wave patterns are observed. No classical explanation exists covering this phenomenon. The present analysis indicates that there are two waves radiating from the excitation point with the same frequency, but at different speeds. One wave, a bending wave, is separated from the other one, a membrane wave.  相似文献   

2.
Oscillating electrowetting on dielectrics (EWOD) with coplanar electrodes is investigated in this paper as a way to provide efficient stirring within a drop with biological content. A supporting model inspired from Ko et al. [Appl. Phys. Lett. 94, 194102 (2009)] is proposed allowing to interpret oscillating EWOD-induced drop internal flow as the result of a current streaming along the drop surface deformed by capillary waves. Current streaming behaves essentially as a surface flow generator and the momentum it sustains within the (viscous) drop is even more significant as the surface to volume ratio is small. With the circular electrode pair considered in this paper, oscillating EWOD sustains toroidal vortical flows when the experiments are conducted with aqueous drops in air as ambient phase. But when oil is used as ambient phase, it is demonstrated that the presence of an electrode gap is responsible for a change in drop shape: a pinch-off at the electrode gap yields a peanut-shaped drop and a symmetry break-up of the EWOD-induced flow pattern. Viscosity of oil is also responsible for promoting an efficient damping of the capillary waves which populate the surface of the actuated drop. As a result, the capillary network switches from one standing wave to two superimposed traveling waves of different mechanical energy, provided that actuation frequency is large enough, for instance, as large as the one commonly used in electrowetting applications (f ∼ 500 Hz and beyond). Special emphasis is put on stirring of biological samples. As a typical application, it is demonstrated how beads or cell clusters can be focused under flow either at mid-height of the drop or near the wetting plane, depending on how the nature of the capillary waves is (standing or traveling), and therefore, depending on the actuation frequency (150 Hz–1 KHz).  相似文献   

3.
The presence of maxima in the power factor-frequency and power factor-temperature curves of dielectrics has been explained by the Maxwell inhomogeneity theory and the Debye theory of polar molecular orientation. In order to ascertain the true cause of these maxima, a study has been made of the power factor of an essentially non-polar material with and without polar materials in dilute solution over extended ranges of frequency, 65 to 7.2 × 106 c.p.s., and temperature, 2.9° C. to 90° C.The non-polar solvent is a mixture of paraffins having a pour point at 55° C. Small power factor maxima, 0.0003 in value, shifting over the audio frequency range with temperature variation, are observed for this solvent alone. Adding 3 per cent. phenol gives rise to molecular orientation maxima restricted largely to frequencies above 107 c.p.s. At the lower temperatures, these maxima are greatly broadened, so that there is apparently a small contribution at power frequencies. Adding 10 per cent. stearic acid gives similar results. The failure of these maxima to shift to lower frequencies at the lower temperatures is due to the failure of the inner viscosity to increase very much in the solid state. The variation of the inner viscosity is calculated from the reciprocals of the short time conductivities since the degree of ionic dissociation is found to be essentially independent of temperature. The viscosity may be regarded as a function of particle size and varies within the medium. Where power factor maxima shift with essentially undiminished magnitude over a wide frequency range at ordinary temperatures, such as those observed here in the solvent above, it is believed that the cause is the orientation of associated or polymerized polar aggregates of such size as to be affected by the larger viscosity changes approaching the macroscopic.In the solid and amorphous states, the limited motion of ions leads to an ionic polarization as indicated by absorption curves of relatively large time constants and by high power factors in the low frequency range. The addition of organic acids greatly increases these effects and also increases the final conductivity. The true short time conductivity is largely caused by almost completely dissociated inorganic material.  相似文献   

4.
Ultrafast microfluidics using surface acoustic waves   总被引：2，自引：0，他引：2  
We demonstrate that surface acoustic waves (SAWs), nanometer amplitude Rayleigh waves driven at megahertz order frequencies propagating on the surface of a piezoelectric substrate, offer a powerful method for driving a host of extremely fast microfluidic actuation and micro∕bioparticle manipulation schemes. We show that sessile drops can be translated rapidly on planar substrates or fluid can be pumped through microchannels at 1–10 cm∕s velocities, which are typically one to two orders quicker than that afforded by current microfluidic technologies. Through symmetry-breaking, azimuthal recirculation can be induced within the drop to drive strong inertial microcentrifugation for micromixing and particle concentration or separation. Similar micromixing strategies can be induced in the same microchannel in which fluid is pumped with the SAW by merely changing the SAW frequency to rapidly switch the uniform through-flow into a chaotic oscillatory flow by exploiting superpositioning of the irradiated sound waves from the sidewalls of the microchannel. If the flow is sufficiently quiescent, the nodes of the transverse standing wave that arises across the microchannel also allow for particle aggregation, and hence, sorting on nodal lines. In addition, the SAW also facilitates other microfluidic capabilities. For example, capillary waves excited at the free surface of a sessile drop by the SAW underneath it can be exploited for micro∕nanoparticle collection and sorting at nodal points or lines at low powers. At higher powers, the large accelerations off the substrate surface as the SAW propagates across drives rapid destabilization of the drop free surface giving rise to inertial liquid jets that persist over 1–2 cm in length or atomization of the entire drop to produce 1–10 μm monodispersed aerosol droplets, which can be exploited for ink-jet printing, mass spectrometry interfacing, or pulmonary drug delivery. The atomization of polymer∕protein solutions can also be used for the rapid synthesis of 150–200 nm polymer∕protein particles or biodegradable polymeric shells in which proteins, peptides, and other therapeutic molecules are encapsulated within for controlled release drug delivery. The atomization of thin films behind a translating drop containing polymer solutions also gives rise to long-range spatial ordering of regular polymer spots whose size and spacing are dependent on the SAW frequency, thus offering a simple and powerful method for polymer patterning without requiring surface treatment or physical∕chemical templating.  相似文献   

5.
Not many years ago it was quite generally believed that iron was unable to follow rapid magnetic changes. Experiments which showed an apparent decrease in the permeability of the iron with an increase in the frequency of the magnetic cycle furnished a basis for a theory that iron was magnetically sluggish. Further and more accurate experiments proved, however, that the effects which had previously been ascribed to a peculiarity of the material were in reality caused by eddy currents in the sample. Theoretical calculations were made which demostrated that eddy currents in an iron test piece increased as the square of the frequency and that for even the lower frequencies it was necessary to use quite thin laminations in magnetic circuits in order to eliminate deleterious effects. Furthermore, it was found that due to eddy currents and the magnetic properties of iron, the magnetization in high frequency fields was confined to a thin surface layer of the piece. This “Magnetic Skin Effect” reduced the cross section of the iron which was magnetically active even though the laminations were extremely thin. Careful experimental measurements compared with theoretical calculations proved that the real permeability of iron remained unchanged at frequencies up to about 106 and that previous results had been is serious error due to neglect of the factors mentioned. This fact having been established, efforts were made to see what practical use could be made of iron in high frequency work and to that end some extensive experimental investigations of the saturation curves and core losses were made upon specimens laminated as thinly as was commercially practicable. The resulting data have furnished a basis for design.It is a demostrated fact that the permeability of all metals is unity for the magnetic cycles imposed upon them by heat and light waves. In the region between frequencies of about 106, where the true permeability of iron is practically the same as at zero frequency, and frequencies of about 1010 where the true permeability of iron approaches unity, the experimental values of μ decrease smoothly with the frequency. What happens to μ in the range of frequencies between the lingest heat waves and the shortest Hertzian waves which have yet been made is a question which has many interesting features but which has not yet yielded to the experimenter.  相似文献   

6.

Introduction:

Capillary sampling is increasingly used to obtain blood for laboratory tests in volumes as small as necessary and as non-invasively as possible. Whether capillary blood sampling is also frequent in Croatia, and whether it is performed according to international laboratory standards is unclear.

Materials and methods:

All medical laboratories that participate in the Croatian National External Quality Assessment Program (N = 204) were surveyed on-line to collect information about the laboratory’s parent institution, patient population, types and frequencies of laboratory tests based on capillary blood samples, choice of reference intervals, and policies and procedures specifically related to capillary sampling. Sampling practices were compared with guidelines from the Clinical and Laboratory Standards Institute (CLSI) and the World Health Organization (WHO).

Results:

Of the 204 laboratories surveyed, 174 (85%) responded with complete questionnaires. Among the 174 respondents, 155 (89%) reported that they routinely perform capillary sampling, which is carried out by laboratory staff in 118 laboratories (76%). Nearly half of respondent laboratories (48%) do not have a written protocol including order of draw for multiple sampling. A single puncture site is used to provide capillary blood for up to two samples at 43% of laboratories that occasionally or regularly perform such sampling. Most respondents (88%) never perform arterialisation prior to capillary blood sampling.

Conclusions:

Capillary blood sampling is highly prevalent in Croatia across different types of clinical facilities and patient populations. Capillary sampling procedures are not standardised in the country, and the rate of laboratory compliance with CLSI and WHO guidelines is low.  相似文献   

7.
Since being postulated more than a decade ago, inerters have been successfully employed to enhance the dynamic performance of mechanical systems in several applications. Their ability to lend a high dynamic mass presence to systems that employ them with only a relatively small static device mass makes them unique among mechanical elements. This study explores the mechanical wave manipulation characteristics of nonlinear inertant acoustic metamaterial (NLIAM) configurations using analysis and simulations for their one-dimensional discrete element lattice representations. Firstly, based on notional concepts for nonlinear inertant devices, potential frequency-dependent and acceleration-dependent nonlinear inertant models are identified. Using an effective mass model for the NLIAM with frequency-dependent inertance in the local resonator attachment, the dispersion characteristics of inverse square law and power law inertance models are examined and contrasted with those for an acoustic metamaterial with frequency invariant inertance. While a tuned inverse square law inertance model ensures the existence of a band gap over almost the entire frequency bandwidth of interest even encompassing the extremely low frequency regime, the low and high frequency limits for this inertance law would not be realizable in practice. A potentially more practical power law approximation is proposed and shown to deliver a widening of the band gap by more than 100% towards frequencies below the lower bound of the band gap for the acoustic metamaterial with frequency invariant inertance. Further, drawing inspiration from the Duffing-type stiffness, an acceleration-dependent cubically nonlinear inertance model is proposed. First order corrections to the dispersion characteristics are obtained for an NLIAM with acceleration-dependent inertance using a perturbation approach. For weakly nonlinear cases, excitation amplitude-activated shifts in the dispersion curves are found to enable this NLIAM to act as a passive adaptive filter for mechanical waves based solely on their excitation amplitude. Practical manifestations of such NLIAM could therefore provide a means to realize extraordinary wave manipulation capabilities especially suitable for low frequency structural dynamic applications.  相似文献   

8.
We report a modeling and experimental study of techniques to acoustically focus particles flowing through a microfluidic channel. Our theoretical model differs from prior works in that we solve an approximate 2-D wave transmission model that accounts for wave propagation in both the solid and fluid phases. Our simulations indicate that particles can be effectively focused at driving frequencies as high as 10% off of the resonant condition. This conclusion is supported by experiments on the acoustic focusing of particles in nearly square microchannels, which are studied for different flow rates, driving frequencies and placements of the lead zirconate titanate transducer, either underneath the microchannel or underneath a parallel trough. The relative acoustic potential energy and the resultant velocity fields for particles with positive acoustic contrast coefficients are estimated in the 2-D limit. Confocal microscopy was used to observe the spatial distribution of the flowing microparticles in three dimensions. Through these studies, we show that a single driving frequency from a single piezoelectric actuator can induce the 2-D concentration of particles in a microchannel with a nearly square cross section, and we correlate these behaviors with theoretical predictions. We also show that it is possible to control the extent of focusing of the microparticles, and that it is possible to decouple the focusing of microparticles in the vertical direction from the lateral direction in rectangular channels with anisotropic cross sections. This study provides guidelines to design and operate microchip-based acoustofluidic devices for precise control over the spatial arrangement of microparticles for applications such as flow cytometry and cellular sorting.  相似文献   

9.
Bond graphs are used to generate the equations of motion of a whirling flywheel. The formulation is shown to be a natural one when control forces and moments are included for active control of the whirling modes. Critical frequencies are interpreted as rotational speeds at which non-zero equilibrium configurations exist for displacements (as opposed to the more familiar momenta) and arise when the system dynamics matrix of the complete whirl motion has two zero eigenvalues. In addition, oscillatory modes corresponding to non-zero eigenvalues are examined. Time simulations and other numerical results are given for an example flywheel system which has been proposed for electric utility energy storage.  相似文献   

10.
利用集集地震余震记录分析场地地震动反应   总被引：1，自引：0，他引：1  
利用1999年台湾地区"921集集大地震"的余震记录,用传统谱比法、线性反演法、S波接收函数法和遗传算法对比分析了车笼埔断层近场区域的场地反应.通过对比4种方法的卓越频率和场地放大因子,发现各种方法能给出较一致的卓越频率,但每种方法的场地反应放大值不完全相同.结果表明,线性反演法与传统谱比法的结果受到参考场自身场地反应的影响;遗传算法作为一种绝对场地反应计算方法其结果是可行的,用遗传算法计算得到的参考场结果对线性反演法结果进行校正,校正后的线性反演法与遗传算法结果非常吻合,也证明了遗传算法结果的可行性,建议推广遗传算法来计算场地反应;接收函数法的结果则同其他3种方法的结果相差较大,考虑到运用强震记录进行H/V方法计算的物理机制仍有争议,建议在实际应用中,慎重考虑应用S波H/V的方法来估计场地放大.最后给出车笼埔断层研究区0.4~20Hz频率范围内S波品质因子值为Qs=69.7f?0.73.  相似文献   

11.
基于ERA-Interim再分析数据的OE-W01区块波浪能资源评估   总被引：3，自引：0，他引：3  
万勇  张杰  孟俊敏  王晶 《资源科学》2014,36(6):1278-1287
波浪能是一种重要的海洋可再生能源,开发利用波浪能资源可以有效地缓解常规能源短缺所带来的能源问题,波浪能资源分布状况的可靠评估是开发利用波浪能资源的前提条件。利用ECMWF ERA-Interim 1992-2011年0.75°×0.75°的较高精度的海浪场数据,计算了OE-W01区块(中国渤海、黄海及东海的部分区域)的波浪能流密度以及波能资源的可用度频率、富集度频率、变化指数等评价指标。对该海域的波浪能资源进行了全面评估。研究发现:渤海海域波浪能资源呈现非常明显的季节分布特征,秋冬季较丰富、春夏季较贫乏,秋冬季为波浪能资源的主要利用期,波浪能资源的相对富集区集中在渤海海峡以及渤海和黄海相连通的区域;黄海海域(包括东海部分海域)波浪能资源也呈现季节分布特征,与渤海不同,该海域90%的区域年平均波浪能流密度均在2kW/m以上,波浪能资源的可用范围较大,时间较长,波浪能资源的富集区集中在长江口东部海域以及舟山群岛东南部海域。在此基础上确定了波浪能资源的重点开发利用区,可为该海域的波浪能资源的开发利用提供参考依据。  相似文献   

12.
Alternating current (AC) dielectrophoresis (DEP) experiments for biological particles in microdevices are typically done at a fixed frequency. Reconstructing the DEP response curve from static frequency experiments is laborious, but essential to ascertain differences in dielectric properties of biological particles. Our lab explored the concept of sweeping the frequency as a function of time to rapidly determine the DEP response curve from fewer experiments. For the purpose of determining an ideal sweep rate, homogeneous 6.08 μm polystyrene (PS) beads were used as a model system. Translatability of the sweep rate approach to ∼7 μm red blood cells (RBC) was then verified. An Au/Ti quadrapole electrode microfluidic device was used to separately subject particles and cells to 10Vpp AC electric fields at frequencies ranging from 0.010 to 2.0 MHz over sweep rates from 0.00080 to 0.17 MHz/s. PS beads exhibited negative DEP assembly over the frequencies explored due to Maxwell-Wagner interfacial polarizations. Results demonstrate that frequency sweep rates must be slower than particle polarization timescales to achieve reliable incremental polarizations; sweep rates near 0.00080 MHz/s yielded DEP behaviors very consistent with static frequency DEP responses for both PS beads and RBCs.  相似文献   

13.
The problem of calculating the resonance frequency and power pulsation of a synchronous motor direct connected to reciprocating apparatus is comparatively easy, provided the machine is operated from an infinite system, i.e., a system having constant voltage and frequency.In this paper a method is developed for calculating the resonance frequency of two synchronous machines electrically in parallel. It is also shown that in a system of n machines in parallel, there are (n - I) resonance frequencies. A general method of calculating these is given. Systems of three and four machines are treated as special examples of the general case. The theory is also given for calculating the actual power pulsations which occur in systems of two and three synchronous machines connected to reciprocating apparatus. With some approximation the theory is extended to four machines and a general method for n machines is suggested.Simple methods of calculating the correct sizes of flywheels necessary for the successful parallel operation of engine-driven generators are given. These are calculated directly from a knowledge of the resonance frequencies involved.In general the thesis is a mathematical treatment of the phenomena which occur in connection with hunting of synchronous machines, electrically in parallel and direct connected to reciprocating apparatus. Specific problems have been treated and illustrative examples presented which make quite clear not only the nature and causes of hunting, but also the methods of overcoming it.  相似文献   

14.
A general procedure is outlined for obtaining single or coupled transmission line models to represent the propagation of surface wave modes in conductively unshielded dielectric waveguides. The procedure uses a homogeneous electrically or magnetically walled waveguide having the same dimensions as the dielectric of the surface waveguide, to produce a set of orthogonal eigenfunctions. These eigenfunctions are projected upon Maxwell's equations resulting in a system of transmission lines coupled together through a wave immittance, which represents the ratio of a longitudinal and a transverse field component at the dielectric-air interface. Examples are given for various modes of the dielectric slab and the dielectric rod, in particular the HE1n modes for the latter. The transmission line models derived for these examples consist of a single trasmission line found directly by projection or reduced from a coupled transmission line model by port elimination, or of two transmission lines coupled together. All circuit models derived preserve the basic properties of surface waves (e.g. no solution below cut-off), and any of the single line models can be solved to give explicit approximate algebraic formulae for the propagation constant as a function of frequency. Numerical results show that the dispersion curves calculated from the models versus exact values are generally excellent over the entire frequency spectrum.  相似文献   

15.
以刻划著名的Belousov Zhabotinsky化学反应的俄勒冈振子为数学模型 ,研究解决了激励介质中一些悬而未决的理论问题 (如波的存在性和稳定性等 ) ,进一步完善了激励介质的非线性波型动力学的理论体系 .通过Painlev啨分析 ,B¨acklund变换和奇异摄动方法 ,分析地给出了一些常见的波型解 (如行波 ,螺旋波 ,靶型波 ,V 型波 ,涡卷波等 ) .在波前的邻域内 ,通过引进新的运动坐标系 ,获得了波在直角坐标下的运动方程 .特别是定量地给出了刻划小幅波的组织中心沿轴向和径向运动的规律 ,并由此可判定波的组织中心何时沿径向呈现膨胀或收缩 ,何时沿轴向有正向或反向漂移 .这一结果很好地与实验和数值模拟结果相吻合 .此外 ,研究了耦合的俄勒冈振子 ,解决了Tyson于 1 979年提出的一个猜想 (即对耦合的俄勒冈振子 ,稳定的回声波可以和稳定的正定态共存 ) ,提出了一套解决类似问题的一般方法 .  相似文献   

16.
We report on reversible electroporation of cells in a flow-through microfluidic device, whereby the required electric field is delivered through a set of integrated microcapillaries between a centre stream of cells and side streams of liquid electrolytes. The electrolytes are applied with a sine wave voltage and cells flow by the microcapillary openings encounter a burst of ac field with a duration and strength determined by their average speed and spatial proximity to the microcapillary openings, respectively. Effectiveness of the approach is presented through numerical simulations and empirical results on electroporation efficiency and cell viability against various flow rates (exposure time to the field) as well as frequencies and root-mean-square (rms) intensities of the field. High frequencies (80–400 kHz) and high intensities (e.g., 1.6 kV/cm, rms) are identified with increased electroporation efficiency 61% and viability 86% on average. These results suggest that the device demonstrated here with a simple design and robust operation offers a viable platform for flow-through electroporation.  相似文献   

17.
Electrorotation is widely used for characterization of biological cells and materials using a rotating electric field. Generally, multiphase AC electric fields and quadrupolar electrode configuration are needed to create a rotating electric field for electrorotation. In this study, we demonstrate a simple method to rotate dielectrophoretically trapped microparticles using a stationary AC electric field. Coplanar interdigitated electrodes are used to create a linearly polarized nonuniform AC electric field. This nonuniform electric field is employed for dielectrophoretic trapping of microparticles as well as for generating electroosmotic flow in the vicinity of the electrodes resulting in rotation of microparticles in a microfluidic device. The rotation of barium titanate microparticles is observed in 2-propanol and methanol solvent at a frequency below 1 kHz. A particle rotation rate as high as 240 revolutions per minute is observed. It is demonstrated that precise manipulation (both rotation rate and equilibrium position) of the particles is possible by controlling the frequency of the applied electric field. At low frequency range, the equilibrium positions of the microparticles are observed between the electrode edge and electrode center. This method of particle manipulation is different from electrorotation as it uses induced AC electroosmosis instead of electric torque as in the case of electrorotation. Moreover, it has been shown that a microparticle can be rotated along its own axis without any translational motion.  相似文献   

18.
舰载机惯导系统传递对准时,航母或舰船由于受风浪的影响会产生一定幅值和频率的摇摆运动,进而对传递对准的精度和时间产生一定的影响。本文利用三种仿真环境。研究不同的摇摆模型对舰载机惯导系统传递对准时的精度和时间的影响。结果表明,摇摆运动在一定程度内可以提高传递对准的精度和缩短对准时间,单轴方向上的幅值和频率的增大可以提高其他轴向的估计精度和缩短对准时间,但对该轴向的估计精度影响不大。  相似文献   

19.
The problem of reduced-order modelling is considered in connection with the design of restricted complexity controllers. The suggested reduction method develops in two phases: (i) a simple frequency response of the overall feedback control system is determined according to the design specifications; (ii) a reduced-order transference of the controlled plant is obtained by solving a linear set of equations in such a way that its behaviour approximates that of the original plant at frequencies which are meaningful for the overall transfer function derived in the first step (e.g. resonance and cutoff frequencies). An example shows how the procedure yields a reduced-order model suitable for designing robust controllers whereas other standard methods, based on properties of the plant only, fail.  相似文献   

20.
Using a composite sample of over 3600 index terms drawn from 11 different machine-readable bibliographic data bases, estimates were made of the spelling error frequencies of each of these data bases, as well as the frequency of posting to misspelled terms. The terms studied included assigned index terms as well as some terms from titles and abstracts. The frequency of index term misspellings ranged from a high of almost 23% for one data base to a low of less than 12% for another data base. The frequency of posting to misspelled terms ranged from about one posting in 8000 citations for one data base, to about one posting in 160 citations in another data base. The impact of these error rates is discussed for the tape supplier, tape user and end user. Some suggestions are given regarding search strategry.  相似文献   

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