超声速三维空腔流气动噪声被动控制

抑制超声速武器舱空腔流噪声是航空领域中一项重要课题。大量研究表明在空腔前缘采用主/被动控制技术可以在一定程度上抑制腔内噪声水平。利用大涡模拟(large eddy simulation, LES)技术计算分析了Mach 1.4开式矩形方腔及波形、弧形两种前后壁几何修形后空腔的流动及噪声, 探索超声速来流条件下几何修形被动控制技术对开式方腔流噪声的抑制能力。计算结果表明波形和弧形空腔对腔内噪声均具有一定的抑制作用, 且波形空腔噪声控制效果更优。分析认为空腔几何修形能够改变空腔上方剪切层及腔内大尺度涡结构的发展演化, 进而实现对腔内噪声的控制。此外, 还应用LES方法计算分析了增厚的来流边界层条件下超声速方腔流, 发现来流边界层增厚可显著降低腔内噪声水平。      

Semiclassical theory of conductance and noise in open chaotic cavities

Conductance and shot noise of an open cavity with diffusive boundary scattering are calculated within the Boltzmann-Langevin approach. In particular, conductance contains a nonuniversal geometric contribution, originating from the presence of open contacts. Subsequently, universal expressions for multiterminal conductance and noise, valid for all chaotic cavities, are obtained classically, based on the fact that the distribution function in the cavity depends only on energy, and using the principle of minimal correlations.     

A novel multi-cavity Helmholtz muffler

A novel multi-cavity Helmholtz muffler is proposed. The multi-cavity Helmholtz muffler is composed of steel structures and silicone membranes. With suitable construction, the Helmholtz muffler can be designed to exhibit negative mass density in low frequency, and the muffling frequency can be adjusted when we change the internal structure of the cavity,which will be very attractive for noise control. In this paper, we investigate the influence of the membranes and the cavities on noise reduction characteristics with theoretical calculations and simulations. The results show that the numbers of membranes and the volumes of the cavities can have a great effect on the position of the muffling frequency. The number of cavities can have a great effect on the width of the muffling frequency(reduce the noise by 10 dB). With different combinations of the membranes and cavities, we can get different muffling frequencies, which can meet different muffling demands in practical applications and is more flexible than the traditional Helmholtz cavity.     

Investigation of higher spanwise Helmholtz resonance modes in slender covered cavities

Cavity aeroacoustic noise is relevant for aerospace and automotive industries and widely investigated since the 1950s. Most investigations so far consider cavities where opening length and width are of similar scale. The present investigation focuses on a less investigated setup, namely cavities that resemble the door gaps of automobiles. These cavities are both slender (width much greater than length or depth) and partially covered. Furthermore they are under influence of a low Mach number flow with a relatively thick boundary layer. Under certain conditions, these gaps can produce tonal noise. The present investigation attempts to reveal the aeroacoustic mechanism of this tonal noise for higher resonance modes. Experiments have been conducted on a simplified geometry, where unsteady internal pressures have been measured at different spanwise locations. With increasing velocity, several resonance modes occur. In order to obtain higher mode shapes, the cavity acoustic response is simulated and compared with experiment. Using the frequency-filtered simulation pressure field, the higher modes shapes are retrieved. The mode shapes can be interpreted as the slender cavity self-organizing into separate Helmholtz resonators that interact with each other. Based on this, an analytical model is derived that shows good agreement with the simulations and experimental results.     

Passive noise control by enhancing aeroacoustic interference due to structural discontinuities in close proximity

In-duct devices are commonly installed in flow ducts for various flow management purposes. The structural construction of these devices indispensably creates disruption to smooth flow through duct passages so they exist as structural discontinuities in duct flow. The presence of these discontinuities provides additional possibility of noise generation. In real practice, in-duct devices do not exist alone in any duct system. Even though each in-duct device would generate its own noise, it might be possible that these devices could be properly arranged so as to strengthen the interference between individual noise; thus giving rise to an overall reduction of noise radiation in the in-duct far field. This concept of passive noise control is investigated by considering different configurations of two structural discontinuities of simple form (i.e., a cavity) in tandem in an unconfined flow and in opposing setting within a flow duct. It is known that noise generated by a cavity in unconfined domain (unconfined cavity) is strongly dependent on flow-resonant behavior within the cavity so the interference it produces is merely aeroacoustic. The objective of the present study is to verify the concept of passive noise reduction through enhancement of aeroacoustic interference due to two cavities by considering laminar flow only. A two-dimensional approach is adopted for the direct aeroacoustic calculations using a direct numerical simulation (DNS) technique. The position and geometries of the cavities and the Mach number are varied; the resultant aeroacoustic behavior and acoustic power are calculated. The numerical results are compared with a single cavity case to highlight the effect of introducing additional cavities to the aeroacoustic problem. Resonant flow oscillations occur when two unconfined cavities are very close and the associated acoustic field is very intense with no noise reduction possible. However, for duct aeroacoustics, it is found that a 7.9 db reduction of acoustic power in the downstream side of the duct or a total reduction of ∼6 db is possible with opposing cavities having an offset of half a cavity length. In addition, the reduction is shown to be free from lock-on with trapped modes of the ducts with cavities.     

Possibility of suppressing quantum light fluctuations when excess photon fluctuations occur inside a cavity

Using the optical excitation of a high-Q cavity as an example, it is shown that when light is observed at the output of this cavity, effective suppression of the photocurrent shot noise below the quantum limit is in general independent of the parameters of the stationary state of the field oscillator (in particular, it is independent of the rms photon fluctuations) inside the cavity and can occur not only at any allowed negative value but even at a positive value of the Mandel parameter. It was assumed in solving the problem that the cavity is optically excited by superimposing the radiation of a sub-Poisson laser and a laser with excess photon noise. A formal solution was obtained in terms of the kinetic equation for the density matrix of the actual fields (inside the laser cavities and the empty cavity), which is derived here on the basis of the Heisenberg-Langevin quantum equations, taking into account directed propagation of the field from the laser cavities inside the empty cavity. The resulting kinetic equation can also be used to solve other physical problems, since it is applicable to optical systems that contain, in principle, an arbitrary number of coupled cavities and interference mixers. Zh. éksp. Teor. Fiz. 111, 1579–1600 (May 1997)     

Measurement of frequency noise spectra of frequency-stabilized LD-pumped Nd:YAG laser by using a cavity with separately suspended mirrors

We have developed a frequency stability system for a commercial LD-pumped Nd:YAG laser in a nonplanar ring oscillator geometry (MISER) which is used for our 20-m prototype gravitational wave detector. The frequency of the laser is locked to the rigid cavity resonance frequency, and the relative frequency noise is suppressed down to 3 × 10⁻⁴ Hz/Hz^1/2 the shot-noise limited level at below 1 kHz. We are successful in evaluating the frequency noise level more accurately by means of a separately suspended reirror type cavity (4-m mode-cleaner); the noise level is 2 × 10⁻² Hz/Hz^1/2. As compared to a frequency noise spectrum which is locked to separately suspended mirror type cavities, the frequency noise is lower at a frequency below 400 Hz.     

Giant excess noise and transient gain in misaligned laser cavities

The excess noise factor is calculated analytically for a very general class of optical cavities, and is shown to have a superexponential dependence on cavity misalignment, easily attaining values of order 10(10). The physical basis is shown to be "ransient gain" associated with amplified spontaneous emission. Similarly dramatic effects of symmetry breaking can be expected in other physical systems with non-normal modes.     

Frequency scales for current statistics of mesoscopic conductors

We calculate the third cumulant of current in a chaotic cavity with contacts of arbitrary transparency as a function of frequency. Its frequency dependence drastically differs from that of the conventional noise. In addition to a dispersion at the inverse RC time characteristic of charge relaxation, it has a low-frequency dispersion at the inverse dwell time of electrons in the cavity. This effect is suppressed if both contacts have either large or small transparencies.     

Displacement-noise-free gravitational-wave detection with two Fabry-Perot cavities

We propose two detuned Fabry-Perot cavities, each pumped through both the mirrors, positioned in line as a toy model of the gravitational-wave (GW) detector free from displacement noise of the test masses. It is demonstrated that the noise of cavity mirrors can be completely excluded in a proper linear combination of the cavities output signals. This model is illustrated by a simplified round trip model (without Fabry-Perot cavities). We show that in low-frequency region the obtained displacement-noise-free response signal is stronger than the one of the interferometer recently proposed by S. Kawamura and Y. Chen.     

Entanglement transfer from two-mode squeezed vacuum light to spatially separated mechanical oscillators via dissipative optomechanical coupling

In this paper, we propose a scheme to generate an entangled state between two spatially separated movable mirrors by injecting the two-mode squeezed optical reservoir to the dissipative optomechanics, in which the movable mirrors can modulate the linewidth of the cavity modes. When the coupling between the mirrors and the corresponding cavity modes is weak, the two driven cavity fields can respectively behave as the squeezed-vacuum reservoir for the two movable mirrors by utilizing the effect of completely destructive interference of quantum noise. Thus the mechanical modes are prepared in a two-mode squeezed vacuum state. Moreover,when the coupling between the two mirrors and the cavities modes is strong, the entanglement between the two movable mirrors decreases because photonic excitation can preclude the completely destructive interference of quantum noise, but the movable mirrors are still entangled.     

Radiation noise in a three-mirror system

The effect of noise on the output parameters of a three-mirror system is investigated under the conditions when one of the cavities is filled with an amplifying medium and the other is empty. For weak coupling between the cavities, the amplitude noise at the outlet of the empty cavity decreases sharply, and the emission band becomes narrower than the natural bandwidth. Expressions are given for analyzing a more general problem on the statistical characteristics of laser radiation emitted by the three-mirror system with a weak noise.     

Optical transfer functions of Kerr nonlinear cavities and interferometers

We present the optical transfer functions for third-order nonlinear cavities that involve an optical carrier frequency and its modulation sideband fields. Our approach is based on linearized transformations and provides a convenient tool to calculate squeezed light sources as well as complex interferometer topologies, containing subsystems that involve intensity dependent phase shifts, i.e., optical Kerr media. As the result we present the noise spectral density of a Michelson interferometer with Kerr nonlinear arm cavities and resonant sideband extraction and find that quantum noise can be squeezed by arbitrary amounts even outside the cavity linewidth. Such a system might apply for future gravitational wave detectors or simply for a continuous wave source of squeezed states.     

Entangled light from white noise

An atom that couples to two distinct leaky optical cavities is driven by an external optical white noise field. We describe how entanglement between the light fields sustained by two optical cavities arises in such a situation. The entanglement is maximized for intermediate values of the cavity damping rates and the intensity of the white noise field, vanishing both for small and for large values of these parameters and thus exhibiting a stochastic-resonancelike behavior. This example illustrates the possibility of generating entanglement by exclusively incoherent means and sheds new light on the constructive role noise may play in certain tasks of interest for quantum information processing.     

Spontaneous emission of an atom in a cavity with nonorthogonal eigenmodes

The spontaneous emission of an excited atom in a lossy cavity with nonorthogonal eigenmodes is analyzed. The quantum Langevin formalism is used to describe the dynamics of the spontaneous decay. The analysis shows that the spontaneous decay is modified by the Q value and the effective mode volume factor of each cavity eigenmode. The effective mode volume is generalized for cavities with nonorthogonal modes, which can be a very significant modification in the microcavity regime. It is shown that the spontaneous decay is not enhanced by the excess noise factor as claimed by other analyses.     

Observation of self-oscillation in ring cavities with sodium vapor phase conjugate mirrors

We present two kinds of ring cavities with phase conjugate mirrors. One cavity requires a tuning condition while the other one can oscillate independently of the cavity length. Both cavities have been experimentally studied using a sodium vapor phase conjugate mirror. Oscillations occur without any other gain medium inside the cavity. A strong correlation between the beams propagating in opposite directions in the cavity is observed.     

Cavity cluster approach for quantification of cavitational intensity in sonochemical reactors

The mechanism involved in the spectacular effects from cavitation phenomenon is very complex and there have been several proposed theories to explain the observed results. The experimental as well as the visual observations indicate that a single collapsing cavity is also influenced by the dynamics of the surrounding cavities, which are very near to the collapsing cavity. The observed effects and erosion patterns cannot be explained properly on the basis of a single cavity collapse and hence in this study a cavity cluster (group of cavities) has been considered to understand the mechanism of cavitational effects. The effect of intensity, frequency of ultrasound, initial size of the cluster and the fraction of energy transferred from the collapsing cavities to the surrounding cavities on the cavitational intensity quantified in terms of the pressure pulse generated at the collapse of cavities as well as the active zone of cavitation has been investigated using bubble/cavity dynamics equations, numerically. On the basis of the trends obtained, empirical correlations estimating the collapse pressure and active volume of cavitation, have been developed.     

Cavity enhanced wavelength modulation spectrometry for application in chemical analysis

A simple and robust absorption technique is developed which combines cavity enhanced absorption spectroscopy and wavelength modulation spectroscopy and measures the integrated output of unlocked cavities. The detection power of the technique is affected by peculiarities of the laser injection into a cavity causing excess noise which exceeds the shot noise. The noise and ways for its reduction are discussed. The new method is demonstrated by absorption measurements of excited carbon atoms in a microwave induced plasma. Preliminary detection limits equivalent to optical depths below 10^-6 were obtained. PACS 42.55.Px; 42.62.Fi; 42.60.Mi     

A theory of complex open resonators in the form of cavity chain

The previous theory ((2,6)) of microwave open resonators has been improved, which can be applicable not only to simple cavities, but also to complex cavities. The comparison between calculations and experimental results shows tha the diffraction Q value of an open resonator may be obtained more precisely from the modified theory than one from the previous theory. According to the theory suggested here, many complex cavities, e.g. a complex cavity with a fourcavity chain, have been analyzed and calculated. It is seen that a variety of field profile forms can exist in complex cavities by adjustment of their cavity geometry.     

Short-term frequency stabilization of diode-laser-pumped Nd:YAG lasers using double-pendulum suspended cavities

We describe the improvement of short-term frequency stability of diode-laser-pumped Nd:YAG lasers. To improve the vibrational isolation of reference cavities, the reference cavities were suspended by a double pendulum with magnetic damping. The frequency noise was reduced to lower than 1 Hz/Hz at Fourier frequencies higher than 5 Hz and the minimum noise of 7 × 10^–3 Hz/Hz was recorded. The minimum root Allan variance was about 10^–14 for the sampling time of 0.01 s. Heating of the reference cavity by absorbed laser power caused the thermal drift of cavity resonance frequencies. It resulted in the laser linewidth in the range of 30–50 Hz.