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.
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.
Particle-laden turbulent flows, at average volume fraction less than 4ϫ10 Ϫ4 , 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 scale͒ and the volume fraction is of the same order as studied in this paper, i.e. between 10 Ϫ3 and 10 Ϫ4 .
Gas-liquid bubbly flow in 2-D bubble columns was studied by numerical simulation. A Eulerian-Eulerian two-fluid model
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