摘要:

Our empirical results show that theI‐Vrelationship of flash lamps operating in the preionized mode with peak current density higher than 2 kA/cm2follows Marotta’s formulaV=K0i0.85, and reduces to Gonz’s equationV=K0i0.5for current density lower than 0.6 k A/cm2. At a high‐current, short‐pulse system, the inductanceLfand the equivalent negative resistancedLf/dtacross the flash tube becomes more significant than the inductance of the external circuit. The plasmaz‐pinch equation can successfully solve these time‐variation terms.

ISSN:0021-8979    

DOI:10.1063/1.328331

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Nonlinear imaging, in which object acoustic illumination is at a frequency &ohgr; and the output is detected at 2&ohgr;, has been complicated by competing nonlinear generation from the propagation medium between the object and the output lens. However, recent results with a nonspecular oblique incidence acoustic reflection microscope show interesting object nonlinear generation from polyvinylidine fluoride (PVF)2and mica in granite. These micrographs will be presented and discussed.

ISSN:0021-8979    

DOI:10.1063/1.328332

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The authors report here recent advances in reflection acoustic microscopy using an obliquely incident focused acoustic beam. Results show that, in addition to exaggerated surface contouring on objects such as integrated circuits and blood cells, a contrast enhancement is observed which facilitates the study of thin‐film polymers such as MylarX (Dupont). A theoretical explanation is presented which describes these results and provides a means of predicting which objects are best studied in the normal incidence and oblique incidence reflection acoustic microscopes.

ISSN:0021-8979    

DOI:10.1063/1.328333

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Nonlinear interactions of two ultrasonic waves, propagating collinearly in a dispersionless liquid medium, are investigated experimentally for both degenerate and nondegenerate cases. Results are described of the first experimental observation of parametric amplification of the signal wave for the degenerate case. For the nondegenerate case, no such amplification of the amplitude of the signal wave is observed. These experimental results are found to be in good agreement with the results of the theoretical analysis presented in the paper.

ISSN:0021-8979    

DOI:10.1063/1.328334

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The erosion of solids caused by cavitating liquids is a result of the concerted collapse of clusters of cavities. In vibratory cavitation equipment the clusters grow and collapse adjacent to a solid surface and are typically of hemispherical or cylindrical form. In the present paper the collapse process of these clusters is described and the collapse equations are developed and solved. The theoretical results are compared with results from high‐speed photography of the clusters and with the initial stages of cavitation erosion on metal specimens. Experimental and theoretical results show that the collapse of a cavity cluster is driven by the ambient pressure and the collapse proceeds from the outer boundary of the cluster towards its center. During the collapse the pressure at the inward‐moving cluster boundary increases continuously, and at the cluster center it rises significantly above the ambient pressure. Therefore the collapse velocity of the individual cavities increases towards the cluster center, which explains that the erosion, being caused by the individual cavities, occurs predominantly in this region. Likewise, the pressure increase at the cluster boundary explains why materials of even high strength can be eroded by collapse of cavity clusters at atmospheric ambient pressure.

ISSN:0021-8979    

DOI:10.1063/1.328335

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

The first temperature derivatives of the fundamental elastic constants of quartz are employed along with the thermally induced biasing strains in the equation for the first perturbation of the eigenvalue for the linear electroelastic equations for small fields superposed on a bias to calculate the resulting change in surface‐wave velocity with temperature. Since the description employed is referred to afixedreference state, the geometry does not change and the temperature coefficient of velocity is the negative of the temperature coefficient of delay. In all earlier work, a variable temperature‐dependent geometric state was considered, but the attendant shearing or skewing of the coordinate axes was not properly included. The temperature dependence of the velocity of surface waves propagating in various directions on various cuts of quartz is obtained. For cases in which measurements are available, the calculations are shown to be in substantially better agreement with the measurements than previous calculations.

ISSN:0021-8979    

DOI:10.1063/1.328336

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Solutions are given for a number of one‐dimensional problems in heat conduction arising in connection with extensions of the flash method of measuring thermal diffusivity: conduction through slabs of one or two materials, with flash or step irradiation of the front surface, with and without thermal losses from the surfaces, with and without interior contact thermal resistances. The treatment involves application of Laplace transform methods, but with the usual inversion of the transform by integration in the complex plane replaced by a convenient expansion of the transform that permits term‐by‐term inversion by use of standard tables. This yields the temperature rise on the back face of the slab as a series of terms that converges particularly well at low values of the time, in contrast to the commonly used solutions. In many cases one to three terms suffice to give good results, even for times well in excess of the half‐rise time for the temperature on the back face. The advantage of using step irradiation instead of flash irradiation is indicated briefly.

ISSN:0021-8979    

DOI:10.1063/1.328337

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Vibration properties of PVF2cantilevers were studied in vacuum to exclude air loading. Young’s modulus calculated from the resonant frequency at strain levels below 10−4was 4–8×1010dyn/cm2, a significantly higher value than hitherto reported with measurements at higher strain levels. As the vibration amplitude and strain was increased, resonance shifted to lower frequencies and hysteresis effects appeared. These effects are attributed to a decrease in Young’s modulus. The elastic loss factor &eegr; andQvalue, measured from the resonance curve, yielded &eegr;=0.025–0.045 andQ=22–40 for the frequency range 8–200 Hz.

ISSN:0021-8979    

DOI:10.1063/1.328338

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

Recently Drucker has provided an alternative theoretical analysis of the Rayleigh‐Taylor experiments on flat metal plates that we reported on earlier. His analysis is at variance with our conjecture in that it explains the small growth observed in one of our experiments on an initial amplitude, rather than an initial wavelength, basis. In this note we report the results of an experiment designed to test these two competing explanations; Drucker’s conclusions appear to be confirmed.

ISSN:0021-8979    

DOI:10.1063/1.328339

出版商:AIP    

年代:1980

数据来源: AIP

摘要:

A result frequently quoted in engineering handbooks is that one‐dimensional fluid flow theory places an upper limit on windmill efficiency at 16/27 or 59.3%. An error in the development invalidates this conclusion. One‐dimensional flow analysis places no restrictions on realizable windmill output.

ISSN:0021-8979    

DOI:10.1063/1.328340

出版商:AIP    

年代:1980

数据来源: AIP