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Notes: The structural features of turbulence at the free surface of a channel flow have been experimentally investigated. The experiments were conducted in a horizontal channel of large aspect ratio in the (depth based) Reynolds number range of 2800–8800. The results indicate that the persistent structures on the free surface can be classified as upwellings, downdrafts, and spiral eddies. Upwellings are shown to be related to the bursts originating in the sheared region at the channel bottom and the eddies are seen to be generated at the edges of the upwellings. The eddies often merge if rotating in the same direction, and form "pairs" if rotating in opposite directions—though there are occasional mergers of such counter-rotating ones. The spiral eddies decay slowly and are sometimes annihilated by fresh upwellings. The population densities and the persistence times of the various structures were measured for different flow conditions. The resulting data show that the physical parameters characterizing the structures at the interface, scale with a mix of inner (wall shear stress and viscosity) and outer variables. Measurement of the streamwise and spanwise velocities at the free-surface were made by particle imaging velocimetry (PIV) and the surface normal velocity near the free-surface estimated by continuity. The results indicate that the upwellings and spiral eddy regions would be expected to dominate scalar transport rates at high Prandtl/Schmidt numbers. The one-dimensional energy spectra of the flow field at the free-surface compare well with direct numerical simulations and show a region with −5/3 slope at low wave numbers. This experimentally confirms a previous result regarding the two-dimensionality of turbulence near the free surface, based on numerical simulations by Pan and Banerjee [Phys. Fluids 7, 1649 (1995)]. © 1998 American Institute of Physics.

Notes: A method for particle image velocimetry (PIV) is presented which improves upon the accuracy, computational efficiency and dynamic range (i.e., the difference between the largest and smallest resolvable particle displacement vectors) of conventional PIV techniques. The technique is applied to free-surface turbulence to resolve energy spectra for motions with a wide dynamic range. The methodology—based on multi-grid image processing algorithms for rigid body motion analysis, estimates the displacement vectors at discrete particle locations. The essence of this technique is to estimate large scale motions from image intensity patterns of low spatial frequencies and small scale motions from intensity patterns of high spatial frequencies. Cross-correlation between a pair of time separated particle images is implemented by the hierarchical computational scheme of Burt ["Fast filter transforms for image processing," Int. J. Comput. Vision 16, 20 (1981)]. Each image is convolved with a series of band-pass filters and subsampled to obtain a set of images progressively decreasing in resolution and size. A coarse estimate of the displacement field obtained from pairs of lower resolution images are used to obtain more accurate estimates at the next (finer) level. Processing starts at the level of lowest resolution and stops at the highest resolution level, which contains the original image pair. Due to subsampling of low resolution images, the match template size can be kept constant for all stages of computation, thus eliminating the dependence of the largest resolvable displacement on the size of match template. In the present work, the search area at each level is kept constant at 3×3 pixels and the match template size at 5×5 pixels for all levels of computation. The algorithm has been implemented using simple thresholding based on the confidence level of an estimated displacement vector, as suggested by Anandan ["A computational framework and an algorithm for measurement of visual motion," Int. J. Comput. Vision, 2, 283, (1987)]. However, the confidence-level-based smoothing technique for rigid body motions (continuous velocity fields) could not be applied to displacement estimates obtained at discrete points i.e., the particle locations. Instead, smoothing was performed over the area covered by each particle. The algorithm has been tested against direct numerical simulations of turbulent flows when the flow field is known and particle images have been generated from these with the addition of noise. Both the accuracy of motion estimation and the computation time are seen to improve as compared to conventional PIV methods. Finally, video images taken of particle motion on the free-surface of a channel flow have been used to determine the capabilities of the technique in an experimental study. The resulting spectra show a quasi-two-dimensional character of the free-surface turbulent flow field, which corresponds well with the direct numerical simulations. © 1998 American Institute of Physics.

Notes: Silver particles of average diameters in the range 10.3–25.7 nm have been grown within a gel medium by an electrodeposition technique. Detailed optical absorption characteristics in the wavelength range 250–600 nm have been investigated for nanocomposites comprising these particles dispersed in a polystyrene matrix. Absorption maximum occurs at a wavelength around 350 nm, which increases as the metal particle size is increased. Mie theory with the incorporation of a distribution of particle size gives remarkable agreement with the experimental data. The electrical conductivity as extracted from the theoretical analysis for particles with diameters ∼3 nm is found to be less than Mott's minimum metallic conductivity. This indicates the possibility of a metal insulator transition in this system, which appears to be consistent with earlier electrical conductivity measurements. © 1998 American Institute of Physics.

Notes: Composites containing copper particles with nanometer dimensions in a silica gel medium have been synthesized by an electrodeposition technique. The precursor composition of the gel was in the system Cu(NO3)2–SiO2 and the copper particle diameters were in the range of 3.2–11.4 nm. The dc electrical resistivity of pellets obtained from the nanocomposite powders was measured in the temperature range of 110–300 K. A temperature dependence with a fractional exponent of 0.25 was observed. This behavior has been explained on the basis of a variable range hopping mechanism. © 1998 American Institute of Physics.

Notes: Composites of nanometer-sized copper metal with diameters varying from 3.2 to 11.4 nm dispersed in a silica gel medium were synthesized by an electrodeposition method. The ac conductivity and dielectric dispersion of these nanocomposites were measured over the frequency range 0.2 kHz–1.5 MHz at temperatures varying from 150 to 300 K. The ac conductivity showed a frequency dependence of ∝ωn where ω is the angular frequency and n∼0.62 the latter being temperature independent. The quantum mechanical tunneling model was used to explain this result. The dielectric modulus data were analyzed on the basis of a stretched exponential relaxation function. The values of the exponent β as extracted from such analysis were found to be in the range 0.31–0.42 and were temperature independent for different gel compositions. The activation energies were estimated from the temperature variation of frequency at which the imaginary part of the dielectric modulus was maximum. The activation energy value ∼0.24 eV could be explained satisfactorily on the basis of an electron tunneling mechanism. © 1998 American Institute of Physics.

Notes: This article discusses the electrical characterization of low-temperature intrinsic Si films deposited by remote plasma-enhanced chemical vapor deposition. Metal-oxide-semiconductor (MOS) capacitors were fabricated on films deposited over a range of temperatures. Conventional MOS measurements such as capacitance versus voltage, breakdown voltage, Zerbst plot, and charge-to-breakdown were used to analyze the capacitors. The results of these measurements not only yielded information about the electrical properties of the films, but also led to conclusions regarding structural quality and the presence of metal contamination. This, coupled with the fact that capacitor fabrication requires only a simple, moderate-thermal budget process, makes MOS capacitor measurements an attractive technique for the characterization of low temperature epitaxial Si films. © 1997 American Institute of Physics.

Notes: We report on the effects of the wet and dry oxidation processes on the interfacial roughness and time dependent dielectric breakdown (TDDB) characteristics of the poly-Si/SiO2/Si(100) trilayer. The interface roughness of the oxide layers buried under a thick poly-Si electrode has been investigated using an x-ray reflectivity technique. Analysis of x-ray reflectivity data for the trilayer samples and for a bare oxide film shows that interface roughness of poly-Si electrode/SiO2 interfaces depends on oxidation process while oxide layers have smooth SiO2/Si-subtstrate interfaces. TDDB of the SiO2 layer has also been observed to depend on the oxidation process, indicating that the interface roughness is a crucial factor affecting the TDDB characteristics. The wet oxidized SiO2 film is more stable to dielectric breakdown and has smoother interfaces than the dry oxidized sample. © 1998 American Institute of Physics.

Notes: The reactions of atomic hydrogen with boron-doped Si(100) were studied using temperature programmed desorption (TPD). In addition to adsorbing at surface sites, hydrogen penetrates into boron-doped Si(100) samples and gets trapped by forming subsurface boron–hydrogen complexes. H2-TPD spectra, taken after exposure to atomic hydrogen, showed, in addition to the well known dihydride (680 K) and monohydride (795 K) desorption features, two peaks at 600 and 630 K due to decomposition of subsurface boron–hydrogen complexes. Increasing total hydrogen uptake with increasing dosing temperature (1.7 ML at 300 K, 4.2 ML at 500 K), suggests an activation barrier for subsurface hydrogen uptake. A quantitative correlation between boron concentration and subsurface hydrogen uptake is shown. © 1997 American Institute of Physics.

Notes: The Navier–Stokes equations have been solved, by a pseudospectral method, for pressure-driven flows between a no-slip wavy wall and a slip flat wall. Periodic boundary conditions were used in the streamwise and spanwise directions. The physical domain is mapped into a computational domain that is a rectangular parallelepiped using a nonorthogonal transformation. The pseudospectral solution procedure employed in previous studies, for example, Lam and Banerjee [Phys. Fluids A 4, 306 (1992)], eliminated the pressure and solved for the wall–normal velocity and vorticity. The other velocity components were calculated using the definition of vorticity, and the continuity equation. This procedure leads to oscillations in the pressure field when solutions were attempted in the mapped computational domain. To overcome the problem, the procedure had to be modified and the pressure solved for directly using a fractional time step technique. For the cases examined here, these modifications resulted in spectral accuracy being maintained. Flow over sinusoidal wave trains has been simulated and the results compare well with available experiments. The simulations show significant effects of the wavy boundary on the mean flow and the turbulence statistics. The mean velocity profile differs substantially from the profile for the flat-wall case, particularly in the buffer region where the fluid is under the influence of both the wavy wall and the slip boundary. The velocity fluctuations in the streamwise direction decrease in the buffer region. This effect becomes more pronounced when the wave amplitude increases. Most of the redistribution of energy, from the streamwise direction to the spanwise and wall–normal directions, occurs in a thin layer close to the boundary, downstream of the wave troughs. The energy primarily redistributes into spanwise fluctuations. High shear stress regions form downstream of the wave troughs, and streaky structures and quasi-streamwise vortices are also seen to initiate in these regions. The length of the streaks, and the extent of the quasi-streamwise vortices, scale with wave length for the two cases investigated. © 1997 American Institute of Physics.

Notes: Particle-laden turbulent flows, at average volume fraction less than 4×10−4, in open channels are numerically simulated by using a pseudospectral method. The motion of particles, that are large compared with the dissipative length scale, is coupled to the fluid motion by a method that generates a "virtual" no-slip boundary on the particle surface by imposition of an external force field on the grid-points enclosed by the particle. Cases for both moving and stationary particles, lying on the wall, are simulated. The investigations focus on particle-turbulence interaction. It is found that particles increase turbulence intensities and Reynolds stress. By examining higher order turbulence statistics and doing a quadrant analysis of the Reynolds stress, it is found that the ejection-sweep cycle is affected—primarily through suppression of sweeps by the smaller particles and enhancement of sweep activity by the larger particles. An assessment of the impact of these findings on scalar transfer is made, as enhancement of wall heat/mass transfer rates is a motivation of the overall work on this subject. In the cases considered, comparison of the calculations with an existing experiment was possible, and shows good agreement. At present, due to limitations in available computational resources, this method cannot be used when the particle diameter is smaller than the smallest turbulence scale (e.g. the Kolmogorov length scale) and the volume fraction is of the same order as studied in this paper, i.e. between 10−3 and 10−4. © 1997 American Institute of Physics.