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Notizen: Direct numerical simulations of open-channel flow indicate that turbulence at the free surface contains large-scale persistent structures. They are "upwellings'' caused by impingement of bursts emanating from the bottom boundary; "downdrafts'' in regions where adjacent upwellings interact, and whirlpool-like "attached vortices'' which form at the edge of upwellings. The attached vortices are particularly long-lived in the sense that once formed, unless destroyed by other upwellings, they tend to interact with each other and dissipate only slowly. If turbulence generation at the bottom wall is turned off by changing the boundary condition to free slip, then the upwellings (related to bursts) and downdrafts no longer form. The dominant structures at the free surface become the attached vortices which pair, merge, and slowly dissipate. In the central regions, as expected, the structure remains three dimensional throughout the decay process. Near the free surface, the structure appears to be quasi- two dimensional, as indicated by quantitative measures such as energy spectra, interwave number energy transfer, invariants of the anisotropy tensor, and length scales. In the decaying case, the quasi-two-dimensional region increases in thickness, with decay time, though the structure in the central regions of the flow remains three dimensional. © 1995 American Institute of Physics.

Notizen: 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.

Notizen: 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.

Notizen: This paper presents the results of a numerical investigation of the effects of near-neutral density solid particles on turbulent liquid flow in a channel. Interactions of particles, in a size range about the dissipative length scale, with wall turbulence have been simulated at low volume fractions (average volume fraction less than 4×10−4). Fluid motion is calculated by directly solving the Navier-Stokes equations by a pseudo-spectral method and resolving all scales of motion. Particles are moved in a Lagrangian frame through the action of forces imposed by the fluid and gravity. Particle effects on fluid motion are fed back at each time step by calculating the velocity disturbance caused by the particles assuming the flow around them is locally Stokesian. Particle-particle interactions are not considered. The slightly heavier-than-fluid particles of the size range considered are found to preferentially accumulate in the low-speed streaks, as reported in several other investigations. It is also found that particles smaller than the dissipative length scale reduce turbulence intensities and Reynolds stress, whereas particles that are somewhat larger increase intensities and 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. A preliminary assessment of the impact of these findings on scalar transfer is made, as enhancement of transfer rate is a motivation of the overall work on this subject. For the case investigated, comparison of the calculations with an existing experiment was possible, and shows good agreement. © 1996 American Institute of Physics.

Notizen: The velocity, flux, and concentration distribution of solid particles in a turbulent boundary layer of a horizontal water flume were investigated experimentally by means of LDA and visualization techniques. The particles were of polystyrene (specific density ∼1.05). Results show that coherent wall structures are responsible for most of the characteristics of particle behavior throughout the boundary layer. Particles are often concentrated in regions of low velocity, associated with wall structures, and as a result the average particle velocity is lower than the fluid's. This was also noted previously by Rashidi et al., but not explained. The actual relative velocity between the particles and the surrounding fluid is often small. In addition, the data suggest that as the shear rate increases, the particle flux profiles asymptotically approach a shape where a strong gradient of particle flux exists in the lower part of the boundary layer (y+≤250), while it is relatively constant at higher elevations. This phenomenon may also be attributable to interactions with the wall structures. © 1995 American Institute of Physics.

Notizen: The motion of solid particles near the wall in a turbulent boundary layer was investigated experimentally in a water flume by flow visualization techniques and by LDA. The particles were of polystyrene (specific density ∼1.05) with diameters ranging from 100 to 900 μm. Results show that particle motion, as well as entrainment and deposition processes, are controlled by the action of coherent wall structures, which appear to be funnel vortices. The behavior of the particles is consistent with the motion and effects of such vortices. The vortices appear to cause the formation of particle streaks near the wall, to create suitable conditions for particle entrainment, and to assist in particle deposition by conveying them from the outer flow to the wall region. © 1995 American Institute of Physics.

Notizen: 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.

Notizen: 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.

Notizen: 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.