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1.
Impact of a drop on a water surface is accompanied by a series of sound pulses propagating in air and underwater. Depending
on the falling mode (drop size and initial velocity), pulses substantially differ in amplitude, duration, and modulation frequency.
We study falling modes in which in addition to conventional sound packets—the shock pulse and single resonance sound packets—several
packets are observed. Experiments were conducted with simultaneous recording of sound in air and underwater and were accompanied
by synchronous video depiction of currents in the drop impact region. Comparison of videograms and phonograms demonstrate
that the sources of sound packets are gas cavities of arbitrary shape detached from the underwater cavern under the action
of large accelerations (several km/s2) during a sharp change in its surface area, which gradually achieve equilibrial elliptical and spherical shapes. 相似文献
2.
Devos A Poinsotte F Groenen J Dehaese O Bertru N Ponchet A 《Physical review letters》2007,98(20):207402
We present experimental results obtained in two-color pump-probe experiments performed in semiconductor self-assembled quantum dot (QD) layers. The sample reflectivities present several acoustic contributions, among which are strong acoustic phonon wave packets. A comparison between one- and two-color experiments and a fine analysis of the echo shape attest that a high magnitude phonon pulse emerges from each single QD layer. This conclusion is supported by a numerical modeling which perfectly reproduces our experimental signals only if we introduce a strong generation in each QD layer. We explain such a strong emission thanks to an efficient capture of the carriers by the QDs. 相似文献
3.
Yu. D. Chashechkin V. E. Prokhorov 《Journal of Experimental and Theoretical Physics》2016,122(4):748-758
Consistent optical and acoustic techniques have been used to study the structure of hydrodynamic disturbances and acoustic signals generated as a free falling drop penetrates water. The relationship between the structures of hydrodynamic and acoustic perturbations arising as a result of a falling drop contacting with the water surface and subsequent immersion into water is traced. The primary acoustic signal is characterized, in addition to stably reproduced features (steep leading edge followed by long decay with local pressure maxima), by irregular high-frequency packets, which are studied for the first time. Reproducible experimental data are used to recognize constant and variable components of the primary acoustic signal. 相似文献
4.
Emission of sound signals accompanying the fall of a water drop onto a free liquid surface is investigated in a laboratory
bath by high-speed video filming and underwater acoustics. Emission of high-frequency sound is detected during the initial
contact of the drop with the surface. Low-frequency sound packets are observed after a prolonged silence interval with the
formation of nonequilibrium-shaped gas cavities with sharpenings during their reconstruction into spherical bubbles. By virtue
of complexity of the flow pattern at the stages of detachment and coalescence of gas cavities, the frequency and intensity
of the emitted sound varied in wide ranges even under invariable initial experimental conditions. 相似文献
5.
6.
Mustafa Erol 《Czechoslovak Journal of Physics》2000,50(7):851-855
In this paper, we report hot carrier energy relaxation processes studied by acoustic phonon emission in wurtzite GaN epilayers,
using the heat pulse technique. In this method, the carriers were heated up by means of short (≈ 10 ns) voltage pulses and
emitted phonons were detected by Al bolometers biased at their superconducting transition. Obtained phonon signals indicate
that the optical phonon emission threshold has not been reached and longitudinal acoustic and transverse acoustic modes can
be clearly resolved. This paper specifically concentrates on the electron temperature dependence of the energy relaxation
rates and compares the experimental results with the existing theory. 相似文献
7.
V. E. Prokhorov 《Journal of Experimental and Theoretical Physics》2018,126(4):479-484
The dynamics of an impact pressure pulse and the evolution of the coalescence bridge between a drop and the surface of water are investigated in experiments on the impact of the drop on water surface in the range of low impact velocities U. Experimental sequences of radii r i of the bridge, which are approximated by a function of the form t1/2, are extrapolated to the instant of contact and are compared with radii r k of the outer contour of the cross section formed by the bottom part of the drop with the surface. The impact pressure pulse exhibits the critical dependence on ratio \({\varepsilon _{\text{i}}}(U) = {\dot r_i}/{\dot r_k}\) of the velocities of spreading. The value of ε = 1 determines the velocity threshold below (above) which the pressure is hydrodynamic (hydroacoustic) by nature. 相似文献
8.
S. Residori P. K. Buah-Bassuah F. T. Arecchi 《The European physical journal. Special topics》2007,146(1):357-374
We review here a series of experiments on the fragmentation instabilities that a liquid drop undergoes as it falls inside
a fluid with which it is miscible, so called the solvent. Motivated by the original experiments initiated by Thomson and Newall
in 1885, we started to investigate this subject more than one decade ago, encountering up to date a number of challenging
problems in hydrodynamical instabilities, complicated by the presence of transient interfaces between the drop and the solvent.
In particular, we have shown that when a drop of liquid is deposited
over the surface of the same liquid, it falls down inside the
solvent because the energy associated to its surface tension
against air is instantaneously converted into kinetic energy.
As a consequence, a very fast fluid injection takes place as
the drop touches the free surface of the solvent and the drop enters inside the solvent. Due to the hydrodynamical instabilities
related to the large velocity gradients, it develops into a ring that expands radially. The ring continues to go downwards
inside the solvent until it stops at a certain height due to viscous dissipation.
In the first stages of the ring expansion, a fluid membrane remains attached to the ring, so-called “turban"
for its shape, which is concave or convex depending on the sign of the density difference between the drop and the solvent.
When a small density difference is introduced between the drop and the solvent, the ring becomes unstable because of density
gradients and it fragments into smaller droplets. On their turn, the secondary droplets may undergo the same instability
and may fragment again, so that a cascade of fragmentation takes place. If the density difference is positive, that is, the
drop is heavier than the solvent, the
secondary droplets continue to go down deeper inside the solvent, until the whole process is washed out by the slow diffusion
of the concentration gradients. When the density difference between the drop and the solvent is negative, then the secondary
droplets rise up to the free surface of the solvent, where they are distorted by the equivalent of an impact with a rigid
wall. Universal scaling laws for the cascade of fragmentation and for the dynamical behavior of the drop have been derived
and accompany the experimental observations. 相似文献
9.
10.
The acoustic response of silicon single crystals to the action of a millisecond laser pulse, with an excitation wavelength
1.06 μm, has been investigated. It has been discovered for the first time that additional acoustic emission, delayed in time
with respect to the photoacoustic response, is observed for laser energy flux density above the threshold corresponding to
surface melting. The delay time depends linearly on the laser radiation power and varies from one to tens of milliseconds.
It is shown, by comparing the parameters of the acoustic emission with the dynamical development of thermoelastic stresses
in the laser action zone as well as with the kinetics of melting of the irradiated surface, that crack formation under the
action of the thermoelastic stresses is the source of the additional acoustic emission.
Fiz. Tverd. Tela (St. Petersburg) 39, 505–509 (March 1997) 相似文献
11.
为研究超高速撞击声发射信号经过载人航天器加筋结构后的传播规律,分别在平板结构和加筋结构上模拟高速撞击实验,利用传感器阵列采集高速撞击产生的声发射信号。结合小波和傅里叶分析方法从板波模态、频域以及时域三方面分析加筋结构对声发射信号传播特性的影响,并研究成坑和穿孔损伤模式下声发射信号的传播规律。结果表明:加筋板中的信号高频部分比平板中高频部分能量少,筋体对信号高频部分有滤波效果。加筋结构受高速撞击产生穿孔损伤时,S0模态声波的能量增多。研究成果可为载人航天器结构的高速撞击感知与定位技术提供有利参考。 相似文献
12.
13.
V. V. Koshovy 《Radiophysics and Quantum Electronics》1999,42(8):691-704
We present results of preliminary studies aimed at detection of weak ionospheric disturbances induced by acoustic emission
of a ground-based controlled transmitter. Radio astronomical and radiophysical facilities based on the decameter radio telescope
URAN-3 were used in these experiments. Three methods were applied in this study: occultation of the disturbed region by radio
emission from discrete cosmic sources, scattering of sounding radio waves from the perturbed ionospheric region, and weakly-oblique
sounding of the ionosphere. It is shown that the parameters of transmitted, scattered, and reflected signals are well correlated
with the parameters of the acoustic radiation, and that the weak ionospheric disturbances detected in our experiments are
actually induced by the acoustic radiation.
G. V. Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Khar'kov, Ukraine. Translated
from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 785–798, August 1999. 相似文献
14.
A. G. Rousskikh A. V. Shishlov A. S. Zhigalin V. I. Oreshkin S. A. Chaikovsky R. B. Baksht 《Technical Physics》2010,55(11):1619-1627
The results of the experiments on the formation of a plasma emitter with small spatial dimensions for pulsed radiography in
the soft X-ray spectral range are presented. Emitting hot plasma was formed as a result of compression of the plasma jet by
a current pulse with amplitude I
m
= 215 kA and rise time T
fr = 200 ns. For the jet formation, we used a plasma gun based on the arc discharge (I
m
= 8.5 kA and T
fr = 6 μs) initiated by breakdown over the surface of a dielectric in vacuum. The experiments were carried out with aluminum,
tin, copper, and iron plasma jets. A single emitter, i.e., point Z-pinch (PZ-pinch), was formed when an interelectrode gap of a high current generator of 1.3–1.5 mm was used. The smallest spatial dimensions
of the emitting region were obtained with the use of aluminum and tin. For a tin jet, the diameter of the emitting region
was 7 ± 2 μm and its height was 17 ± 2 μm. The emission pulse duration at half-height was 2–3 ns. The total emission yield
per pulse in the spectral range 1.56–1.90 keV was 30–50 mJ for the aluminum pinch and 10–30 mJ for the tin pinch. The developed
method makes it possible to carry out radiographic examination of microobjects (including biological ones) 1–1000 μm in thickness,
with spatial (10–20 μm) and time (2–3 ns) resolution. 相似文献
15.
In this paper we investigated the surface morphology and emission property of polymer velvet in a cathode test system powered by a ∼400 ns, ∼400 kV pulsed generator. After a series of pulse shots, the velvet surface exhibited an obvious decrease in the amount of emitters, namely, the smoothing of microprotrusions, indicating a lower field enhancement factor or a higher turn-on electric field than that for no shots. As the velvet cathode lifetime proceeded, the beam degradation was observed in terms of the voltage pulse length, maximum emission current, and rise time of diode current. Further, the average current density significantly decreased during a 100 pulse shot test, from 280 to 160 A/cm2. The surface discharge caused many plasma spots on the velvet surface. The cathode plasma expands towards the anode, directly leading to the diode gap closure. The degradation in the velvet performance after high-current emission may be related to this behavior of cathode plasma. Finally, the electron emission mechanisms, how to affect the surface morphology of velvet, are presented. 相似文献
16.
Lhermite J Sabourdy D Desfarges-Berthelemot A Kermene V Barthelemy A Oudar JL 《Optics letters》2007,32(12):1734-1736
We report on a mode-locked erbium-doped fiber laser suited to the generation of high-repetition-rate pulse trains or packets. The two-arm interferometer resonator performs a spectral filtering leading to the emission of short pulse packets with a high repetition rate, adjustable up to 200 GHz. A continuous pulse train with a repetition rate of up to 6.9 GHz was obtained by adding a recirculation loop inside the cavity to fill the spaces between all pulse packets. 相似文献
17.
The paper presents results of laboratory experiments on studying the characteristics of acoustic emission generated by a flow of compressed air, which is filtered by porous pumice samples with and without partial fluid saturation. The construction features of the laboratory setup and details of the experiments are described. Porous samples with dry and partially fluid-filled pores are used. The visual patterns of the acoustic emission spectrum, which occurs under stationary filtration of the compressed air, are presented, and its amplitude-frequency distribution characteristic for different sample porosities and different degrees of their fluid saturation is shown. It is demonstrated that the relaxation times of the emission noise level differ. This is revealed during the sharp elimination of the drop in pressure from such samples, i.e., in the nonstationary filtration mode. 相似文献
18.
Juha?M.?Saarela Torbj?rn?L?fqvist Kerstin?Ramser Per?Gren Erik?Olson Jan?Niemi Mikael?Sj?dahl 《Central European Journal of Physics》2010,8(2):235-241
This study is focused on exploring the feasibility of an all-optic surface scanning method in determining the size and position
of a submerged, laser generated, optoacoustic (OA) source. The optoacoustic effect in this case was generated when the absorption
of a short electromagnetic pulse in matter caused a dielectric breakdown, a plasma emission flash and a subsequent acoustic
wave. In the experiment, a laser pulse with λ = 1064 nm and 12 ns pulse length was aimed at a volume of deionized water. When
the laser beam was focused by a f = 16 mm lens, a single dielectric breakdown spot occurred. When a f = 40 mm was used several breakdowns in a row were induced. The breakdowns were photographed using a double shutter camera.
The acoustic wave generated by the dielectric breakdowns were detected at a point on the water surface using a laser Doppler
vibrometer (LDV). First, the LDV signal was used to calculate the speed of sound with an accuracy of 10 m/s. Secondly, the
location and length of the dielectric breakdown was calculated with an accuracy of 1 mm. The calculated position matched the
breakdown location recorded by a camera. The results show that it is possible to use LDV surface measurements from a single
spot to determine both the position and length of the OA source as well as the speed of sound in the medium. Furthermore,
the LDV measurements also show a secondary peak that originates from the OA source. To unravel the origin and properties of
this interesting feature, further investigations are necessary 相似文献
19.
This paper represents the continuation of our research on built-in piezoelectric sensor for structural health monitoring of composite materials. Experimental research is focused on examining the effects of the embedded sensors on the structural integrity of composite laminates subjected to mechanical tests. A series of composite specimens with and without embedded sensor are tested in fatigue loading while constantly monitoring the response by acoustic emission technique. The acoustic signals are analysed using the classification k-means method in order to identify the different damage mechanisms and to follow the evolution of these mechanisms for both types of composite materials (with and without sensor). The mechanical behaviour of composites with and without embedded sensor shows no difference in the form. The incorporation of piezoelectric sensor causes low degradation of mechanical properties of composites. Comparing embedded sensor to sensor mounted on the surface, the embedded sensor showed a much higher sensitivity. It is thus verified that the embedded acoustic emission sensor had great potential for acoustic emission monitoring in fibre reinforced composite structures. 相似文献
20.
Diffraction effects and features of acoustic wave propagation in elastic media with the surface-breaking microcrack were investigated in detail for the pulse probing signals. The crack’s plane was oriented along the direction of longitudinal ultrasonic wave incidence in such a way that the detection of the crack with such an unfavorable spatial location was difficult by means of traditional acoustic techniques. Using laser Doppler interferometer a set of instantaneous pictures of acoustic field on the specimen’s surface, corresponding to the different moments of time was obtained. This allowed us to investigate and visualize the diffraction effects of acoustic field in dynamics. Using numerical modeling of diffraction processes of acoustic waves on the crack’s edge and top for pulse signals the origin of V-like structures on the snapshots of acoustic fields was explained and analyzed. 相似文献