Extreme and Topological Nonlinear Optics of Open Systems

Optics and Spectroscopy - A review of results of an investigation of the theory of optical wave packets with extreme properties with respect to a controllable pulse shape or to the complexity of...     

The Effect of a Substrate on the Current-vs-Voltage Characteristics of Thin-Film Sandwich Structures Based on Buckminsterfullerene

We analyze the current-vs-voltage (J-V) characteristics of ITO/C₆₀/Al sandwich cells, where ITO is the double indium tin oxide, C₆₀ is the buckminsterfullerene and Al is the thermally evaporated top aluminum electrode. These cells were obtained on either glass/ITO or polyethyleneterephthalate/ITO substrates (or, optionally, on glass/ZnO substrates) in a parallel process under identical conditions. However, the J-V dependences in each series behave very differently. The results are discussed in terms of the migration of admixtures from a bulk substrate material toward the top metallic electrode, thus modifying the photovoltaic properties of cells.     

The spectrometer for the GunLab experiment

The spectrometer for the GunLab experiment is described. This spectrometer incorporates a dipole magnet, a fluorescent screen, and a CCD camera and is designed to measure the momentum of electron beams in the range of 1–10 MeV/c with a resolution of 0.1%. If a transversely deflecting RF cavity is installed in front of the dipole magnet, one may investigate the longitudinal phase portrait of a beam. The spectrometer is distinctive in that a Hall sensor is placed in the magnetic field of the dipole magnet. This sensor allows one to accurately measure the magnetic field and, consequently, the momentum of an electron beam.     

Simulation of turbulent non-isothermal polydisperse bubbly flow behind a sudden tube expansion

The results of numerical simulation of the structure of non-isothermal polydisperse bubbly turbulent flow and heat transfer behind a sudden tube expansion are presented. The study was carried out at a change in the initial diameter of the air bubbles within d _m1 = 1–5 mm and their volumetric void fraction β = 0–10 %. Small bubbles are available in almost the entire cross section of the tube, while the large bubbles pass mainly through the flow core. An increase in the size of dispersed phase causes the growth of turbulence in the liquid phase due to flow turbulization, when there is a separated flow of liquid past the large bubbles. Adding the air bubbles causes a significant reduction in the length of the separation zone and heat transfer enhancement, and these effects increase with increasing bubble size and their gas volumetric flow rate ratio.     

Comparison of the Eulerian and Lagrangian approaches in studying the flow pattern and heat transfer in a separated axisymmetric turbulent gas-droplet flow

The Eulerian and Lagrangian approaches are used to perform a numerical study of the disperse phase dynamics, turbulence, and heat transfer in a turbulent gas-droplet flow in a tube with sudden expansion with the following ranges of two-phase flow parameters: initial droplet size d ₁ = 0–200 µm and mass fraction of droplets M _L1 = 0–0.1. The main difference between the Eulerian and Lagrangian approaches is the difference in the predictions of the droplet mass fraction: the Eulerian approach predicts a smaller value of M _L both in the recirculation region and in the flow core (the difference reaches 15–20%). It is demonstrated that the disperse phase mass fraction calculated by the Lagrangian approach agrees better with measured data than the corresponding value predicted by the Eulerian approach.     

Numerical investigation of flow and heat transfer in a gas-droplet separated turbulent stream using the Reynolds stress transfer model

The results of the numerical modeling of flow structure, turbulence, and heat transfer in a gas-droplet stream after sudden tube expansion on the basis of the Eulerian approach are presented. The gas phase turbulence was modeled using the Reynolds stress transfer model modified to allow for the presence of particles. The results are compared with those obtained using the two-equation k-ε model. The latter results overestimate the heat transfer in the separation flow as compared with the Reynolds stress transfer model. The heat transfer is shown to considerably increase, when evaporating droplets are incorporated in the separation flow (by a factor of more than 1.5 compared with the case of a single-phase flow at a small mass concentration of the droplets M _L1 ≤ 0.05). The addition of the disperse phase in the turbulent gas flow leads a slight increase in the recirculation zone length. Good agreement with the experimental data indicates the adequacy of the numerical model developed.     

Nonlinear-photonics devices on the basis of the coherent interaction of optical radiation with resonant media (a review)

We have examined examples of nonlinear-photonics devices that are based on the coherent interaction of light with matter. Such interaction takes place if the duration of a light pulse is shorter than the relaxation times T ₁ and T ₂ in a resonant medium and if the strength of the light field is so high that Rabi oscillations arise. Theoretical analysis shows that these systems have a number of advantages compared to similar devices that operate under incoherent interaction conditions of light with matter.     

Experimental and numerical study of heat transfer enhancement in a turbulent bubbly flow in a sudden pipe expansion

Results of an experimental and numerical simulation of heat transfer in an upward bubbly flowin a sudden pipe expansion are presented. The experimental study of the heat transfer has been performed using infrared thermography. The measurements of the bubble size before the pipe expansion area were carried out by the shadow photographymethod. The numerical simulation of the bubbly flow structure in the sudden pipe expansion has been performed using the Eulerian approach in the presence of heat transfer between the two-phase flow and the wall surface. The model uses the system of Reynolds-averaged Navier–Stokes equations in an axisymmetric approximation, written with consideration of the back effect of bubbles on the averaged and pulsation characteristics of the flow. It has been experimentally and numerically shown that addition of air bubbles causes a significant (up to 3-fold) increase in the heat transfer intensity, these effects growing with bubble concentration. The largest rise in the heat transfer has been revealed in the region of flow relaxation downstream of the flow attachment point.