Theoretical and experimental investigation of millimeter wave phase fluctuations in an absorption region

The differential phase scintillations between two neighbouring frequencies on the side of the oxygen absorption peak have been measured. The –8/3 slope of the asymptotes to the measured spectra at the lowest and highest scintillation frequencies and the spectral ratio between these asymptotes are in good agreement with the theoretical model proposed.     

Experimental investigation and numerical simulation of the spatio-temporal dynamics of the light-pulses in nanosecond optical parametric oscillators

This paper reports on the experimental investigation and the numerical simulation of the spatio-temporal dynamics of the pulse formation in nanosecond optical parametric oscillators. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD-camera. The measurements are performed for a signal-resonant nanosecond pulsed optical-parametric-oscillator (OPO) which consists of a 12 mm long, critically phase-matched beta-barium-borate (BBO)-crystal in a 40 mm long cavity of two plane mirrors. The OPO was pumped by the third harmonic of a flashlamp-pumped Nd:YAG-Laser at a repetition rate of 10 Hz. At pump energies close to the OPO threshold the emitted OPO-radiation has an almost gaussian intensity distribution throughout the entire pulse. The beam quality factor M² remains below 2.2. At high pump energies the OPO oscillation also starts with an almost gaussian beam-profile. During the build-up and parametric amplification of the pulse the fields experience, however, a spatially inhomogeneous gain, caused by walk-off in the birefringent crystal, pump-depletion, and back-conversion. The spatial intensity distribution thus becomes asymmetric and the M² value increases. The measurements are compared with the results of detailed numerical calculations. The model takes the amplification of the OPO-radiation in the nonlinear crystal, and the properties of the OPO cavity as well as the diffraction of the beams during propagation into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results. PACS 42.65.Sf; 42.65.Yj     

Investigation of toluene LIF at high pressure and high temperature in an optical engine

Toluene laser-induced fluorescence (LIF) emission spectra were acquired in an optical engine with excitation at 248 nm. Toluene was homogeneously seeded in pure nitrogen and air which were used as intake gases. Data were acquired during the compression phase without ignition leading to simultaneous increases in temperature and pressure from 20°C and 1 bar to 500°C and 23.6 bar. Compared to LIF emission spectra at high temperature and atmospheric pressure reported in the literature, the toluene-LIF emission signal shifts to longer wavelengths when temperature and pressure increase simultaneously, whereas the spectrally integrated emission intensity is slightly affected by the pressure level.     

Rectifying fluctuations in an optical lattice

We have realized a Brownian motor by using cold atoms in a dissipative optical lattice as a model system. In our experiment the optical potential is spatially symmetric and the time symmetry of the system is broken by applying appropriate zero-mean ac forces. We identify a regime of rectification of forces and a regime of rectification of fluctuations, the latter corresponding to the realization of a Brownian motor.     

Experimental investigation of the fluctuations in nonchaotic scattering in microwave billiards

We report on the experimental investigation of the properties of the eigenvalues and wavefunctions and the fluctuation properties of the scattering matrix of closed and open billiards, respectively, of which the classical dynamics undergoes a transition from integrable via almost integrable to fully chaotic. To realize such a system, we chose a billiard with a 60° sector shape of which the classical dynamics is integrable, and introduced circular scatterers of varying number, size,and position. The spectral properties of generic quantum systems of which the classical counterpart is either integrable or chaotic are universal and well understood. If, however, the classical dynamics is pseudo-integrable or almost-integrable,they exhibit a non-universal intermediate statistics, for which analytical results are known only in a few cases, e.g., if it corresponds to semi-Poisson statistics. Since the latter is, above all, clearly distinguishable from those of integrable and chaotic systems, our aim was to design a billiard with these features which indeed is achievable by adding just one scatterer of appropriate size and position to the sector billiard. We demonstrated that, while the spectral properties of almostintegrable billiards are sensitive to the classical dynamics, this is not the case for the distribution of the wavefunction components, which was analyzed in terms of the strength distribution, and the fluctuation properties of the scattering matrix which coincide with those of typical, fully chaotic systems.     

Advanced gasoline engine development using optical diagnostics and numerical modeling

Twenty years ago, homogeneous-charge spark-ignition gasoline engines (using carburetion, throttle-body-, or port-fuel-injection) were the dominant automotive engines. Advanced automotive engine development remained largely empirical, and stratified-charge direct-injection gasoline-engine production was blocked by lack of robustness in its combustion process [W.G. Agnew, Proc. Combust. Inst. 20 (1984) 1-17]. Today, a wide range of direct-injection gasoline engines are in (or near) production, and combustion science is playing a direct role in advanced gasoline-engine development through the simultaneous application of advanced optical diagnostics, three-dimensional computational fluid dynamics (CFD) modeling, and traditional combustion diagnostics. This paper discusses the use of optical diagnostics and CFD in five gasoline-engine combustion systems: homogeneous spark-ignition port-fuel-injection (PFI), homogeneous spark-ignition direct-injection (DI), stratified wall-guided spark-ignition direct-injection (WG-SIDI), stratified spray-guided spark-ignition direct-injection (SG-SIDI), and homogeneous-charge compression-ignition (HCCI). The emphasis is on WG-SIDI, SG-SIDI, and HCCI engines. Key in-cylinder physical processes (e.g., sprays and vaporization, turbulent fuel-air mixing, wall wetting, ignition and early flame development, turbulent partially premixed flame propagation, and emissions formation) can be visualized, quantified, and optimized through optical engine experiments and CFD-based engine modeling. Outstanding issues for stratified engines include reducing piston wall-wetting, pool fires and smoke in WG-SIDI engines, eliminating intermittent misfires in SG-SIDI engines, and optimizing lean NO_x after-treatment systems. HCCI engines require better control of combustion timing and heat-release rate over wide speed/load operating ranges, smooth transitions between operating modes, and individual cylinder sensors and controls. Future directions in optical diagnostics and modeling are suggested to improve our fundamental understanding of important in-cylinder processes and to enhance CFD modeling capabilities.     

空间光通信的到达角起伏实验研究

2006年上半年进行了距离为3 200 m的光束传输实验,对到达角起伏进行了24 h昼夜观测。实验中,每分钟进行一次连续测量,每次以1 kHz的信号采样频率连续采集10 s,得到10 000个到达角起伏实验数据,并反映了0.2~500 Hz频段的信息。根据高采样频率下的到达角起伏昼夜观测数据,分析了到达角起伏效应,并通过到达角起伏时间平滑因子,对曝光时间的平滑作用进行了研究。昼夜观测实验结果表明：在天气阴霾、能见度低的情况下,到达角起伏会显著减小;在强湍流区,到达角起伏出现饱和效应,在弱湍流区,到达角起伏的强度随着湍流强度的增加而增强;到达角起伏幅度随着曝光时间的增加而减小,由归一化协方差拟合关系得到的时间平滑因子计算结果与实验直接分析得到的结果一致。     

空间光通信的到达角起伏实验研究

 2006年上半年进行了距离为3 200 m的光束传输实验,对到达角起伏进行了24 h昼夜观测。实验中,每分钟进行一次连续测量,每次以1 kHz的信号采样频率连续采集10 s,得到10 000个到达角起伏实验数据,并反映了0.2~500 Hz频段的信息。根据高采样频率下的到达角起伏昼夜观测数据,分析了到达角起伏效应,并通过到达角起伏时间平滑因子,对曝光时间的平滑作用进行了研究。昼夜观测实验结果表明：在天气阴霾、能见度低的情况下,到达角起伏会显著减小;在强湍流区,到达角起伏出现饱和效应,在弱湍流区,到达角起伏的强度随着湍流强度的增加而增强;到达角起伏幅度随着曝光时间的增加而减小,由归一化协方差拟合关系得到的时间平滑因子计算结果与实验直接分析得到的结果一致。     

Experimental investigation of an active open optical resonator in a turbulent atmosphere

The spectral densities of the time fluctuations of the amplitude of steady-state generation at two wavelengths ₁=0.63 and ₂=1.15 under conditions of turbulent fluctuations of the refractive index in the waveguide channel of an open optical resonator are studied. The frequency dependences of the spectral densities are analyzed at various states of atmospheric turbulence, including the state produced in an aerodynamic chamber. For wavelength ₂ the fluctuations of the water-vapor content form a random absorption field. The distinctive features of the frequency dependence of the spectral density are discussed and related to the time cross-correlation of random fields of the passive impurity of the water-vapor concentration, the temperature, and the turbulent velocity of the flow. The asymptotic behavior of the frequency dependences of the spectral density is analyzed from the standpoint of the self-similarity of the stochastic characteristics of the system consisting of the waveguide channel and the medium.Odessa Hydrometeorological Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 2, pp. 111–118, February, 1991.     

Experimental and numerical investigation of the effect of liquid temperature on the sonolytic degradation of some organic dyes in water

This paper presents a comprehensive experimental and numerical investigation of the effects of liquid temperature on the sonochemical degradation of three organic dyes, Rhodamine B (RhB), Acid orange 7 (AO7) and Malachite green (MG), largely used in the textile industry. The experiments have been carried out for an ultrasonic frequency of 300 kHz. The obtained experimental results were discussed using a new approach combining the results of single-bubble event and the number of active bubbles. The single-bubble event was predicted using a model that combines the bubble dynamics with chemical kinetics occurring inside a bubble during the strong collapse. The number of active bubbles was predicted using a method developed in our previous work. The experiments showed that the degradation rate of the three dyes increased significantly with increasing liquid temperature in the range 25–55 °C. It was predicted that the main pathway of pollutants degradation is the attack by OH radicals. The simulations showed that there exists an optimum liquid temperature of about 35 °C for the production of OH inside a bubble whereas the number of active bubbles increased sharply with the rise of the liquid temperature. It was predicted that the overall production rate of OH increased with increasing liquid temperature in the range 25–55 °C. Finally, it was concluded that the effect of liquid temperature on the sonochemical degradation of the three dyes in aqueous phase was controlled by the number of active bubbles in the range 35–55 °C and by both the number of bubbles and the single bubble yield in the range 25–35 °C.     

On an investigation of slow frequency fluctuations

The possibilities of an experimental investigation of the frequency fluctuations of the oscillations are considered by means of a digital electronic frequency meter. A comparison is made of the results of the measurements with the spectrum of the fluctuations, and an analysis of the errors of the measurements is likewise carried out.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 15, No. 4, pp. 573–579, April, 1972.     

Experimental and numerical investigation of the unsteady flow field and tone generation in an isolated centrifugal fan impeller

In spite of a low circumferential Mach number the sound of isolated centrifugal fan impellers is sometimes dominated by distinctive tones at blade passing frequency (BPF) and integer multiples. This paper reports on an experimental and numerical investigation intended to unveil the tone generating mechanism. The sound spectra from three impellers operating at a large range of speed were measured and decomposed into Strouhal and Helmholtz number dependent functions. This led to the preliminary conclusion that the BPF related tones are exclusively flow-induced. Based on hot-wire and blade pressure fluctuation measurements and a subsequent correlation analysis, coherent flow structures different from those associated with the principal azimuthal flow pattern due to the blades were detected. Eventually a numerical three-dimensional unsteady flow simulation revealed an inlet vortex. It takes on a helical form, with the vortex core slowly varying its position with respect to the impeller center. As the blades cut through that quasi-stationary helical vortex they encounter blade force fluctuations, producing the BPF tones. Slow spin of the vortex core and slow variation of vortex strength were identified as the reasons for amplitude modulation of the BPF tone.     

Experimental and numerical investigation of microscale hydrogen diffusion flames

Characteristics of microscale hydrogen diffusion flames produced from sub-millimeter diameter (d = 0.2 and 0.48 mm) tubes are investigated using non-intrusive UV Raman scattering coupled with LIPF technique. Simultaneous, temporally and spatially resolved point measurements of temperature, major species concentrations (O₂, N₂, H₂O, and H₂), and absolute hydroxyl radical concentration (OH) are made in the microflames for the first time. The probe volume is 0.02 × 0.04 × 0.04 mm³. In addition, photographs and 2-D OH imaging techniques are employed to illustrate the flame shapes and reaction zones. Several important features are identified from the detailed measurements of microflames. Qualitative 2-D OH imaging indicates that a spherical flame is formed with a radius of about 1 mm as the tube diameter is reduced to 0.2 mm. Raman/LIPF measurements show that the coupled effect of ambient air leakage and pre-heating enhanced thermal diffusion of H₂ leads to lean-burn conditions for the flame. The calculated characteristic features and properties indicate that the buoyancy effect is minor while the flames are in the convection–diffusion controlled regime because of low Peclet number. Also, the effect of Peclet number on the flame shape is minor as the flame is in the convection–diffusion controlled regime. Comparisons between the predicted and measured data indicate that the trends of temperature, major species, and OH distributions are properly modeled. However, the code does not properly predict the air entrainment and pre-heating enhanced thermal-diffusive effects. Therefore, thermal diffusion for light species and different combustion models might need to be considered in the simulation of microflame structure.     

Quantum fluctuations of a vortex in an optical lattice

Using a variational ansatz for the wave function of the Bose-Einstein condensate, we develop a quantum theory of vortices and quadrupole modes in a one-dimensional optical lattice. We study the coupling between the quadrupole modes and Kelvin modes, which turns out to be formally analogous to the theory of parametric processes in quantum optics. This leads to the possibility of squeezing vortices. We solve the quantum multimode problem for the Kelvin modes and quadrupole modes numerically and find properties that cannot be explained with a simple linear-response theory.     

Experimental investigation of deuteron photodisintegration in the δ-resonance region

Differential cross sections for the deuteron photodisintegration process were measured for photon energies between 200 and 440 MeV using the tagged photon beam facility of the Bonn 500 MeV synchrotron. At eight angles between 18° and 145° charged particles were detected simultaneously in time-of-flight spectrometers consisting of scintillation counters. Above the resonance region the measured cross sections agree fairly well with earlier results, whereas there are larger discrepancies at low photon energies.     

Flame propagation monitoring in an engine by an optical technique

A Schlieren laser technique has been used to investigate the transit time of the flame front at selected locations inside a running automotive engine. Thus we get information on the flame speed and the delay between ignition and formation of the front in a very simple fashion without disturbing the combustion itself. The only obvious requirement is that the engine has optical access, and the proposed technique can thus be used whenever other optical diagnostic techniques are used. We also present some examples of the results we obtained.