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ABSTRACT Connecting real-time measurements of current–bed interactions to the temporal evolution of submarine channels can be extremely challenging in natural settings. We present a suite of physical experiments that offer insight into the spectrum of interactions between turbidity currents and their channels, from i) detachment-limited erosion to ii) transport-limited erosion to iii) pure deposition. In all three cases channel sinuosity influenced patterns of erosion and deposition; the outsides of bends displayed the highest erosion rates in the first two cases but showed the highest deposition rates in the third. We connect the evolution of these channels to the turbulence of the near-bed boundary layer. In the erosional experiments the beds of both channels roughened through time, developing erosional bedforms or trains of ripples. Reynolds estimates of boundary-layer roughness indicate that, in both erosional cases, the near-bed boundary layer roughened from smooth or transitionally rough to rough, whereas the depositional channel appears to have remained consistently smooth. Our results suggest that, in the absence of any changes from upstream, erosion in submarine channels is a self-reinforcing mechanism whereby developing bed roughness increases turbulence at the boundary layer, thereby inhibiting deposition, promoting sediment entrainment, and enhancing channel relief; deposition occurs in submarine channels when the boundary layer remains smooth, promoting aggradation and loss of channel relief.
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Boundary condition induced passive chaotic mixing in straight microchannels
When fluids flow through straight channels sustained turbulence occurs only at high Reynolds numbers [typically Re∼O(1000)]. It is difficult to mix multiple fluids flowing through a straight channel in the low Reynolds number laminar regime [Re<O(100)] because in the absence of turbulence, mixing between the component fluids occurs primarily via the slow molecular diffusion process. This Letter reports a simple way to significantly enhance the low Reynolds number (in our case Re≤10) passive microfluidic flow mixing in a straight microchannel by introducing asymmetric wetting boundary conditions on the floor of the channel. We show experimentally and numerically that by creating carefully chosen two-dimensional hydrophobic slip patterns on the floor of the channels, we can introduce stretching, folding, and/or recirculation in the flowing fluid volume, the essential elements to achieve mixing in the absence of turbulence. We also show that there are two distinctive pathways to produce homogeneous mixing in microchannels induced by the inhomogeneity of the boundary conditions. It can be achieved either by (1) introducing stretching, folding and twisting of fluid volumes, i.e., via a horse-shoe type transformation map, or (2) by creating chaotic advection, achieved through manipulation of the hydrophobic boundary patterns on the floor of the channels. We have also shown that by superposing stretching and folding with chaotic advection, mixing can be optimized in terms of significantly reducing mixing length, thereby opening up new design opportunities for simple yet efficient passive microfluidic reactors.
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- Award ID(s):
- 1719425
- PAR ID:
- 10589137
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Physics of Fluids
- Volume:
- 34
- Issue:
- 5
- ISSN:
- 1070-6631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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