2 results
Laboratory experiments simulating a coastal river inflow
- ALEXANDER R. HORNER-DEVINE, DEREK A. FONG, STEPHEN G. MONISMITH, TONY MAXWORTHY
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- Journal:
- Journal of Fluid Mechanics / Volume 555 / 25 May 2006
- Published online by Cambridge University Press:
- 11 May 2006, pp. 203-232
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The dynamics of buoyant water entering a rotating basin are studied using a series of laboratory experiments designed to elucidate the alongshore transport mechanisms in river plumes. Inflowing water, which is discharged perpendicular to the tank wall, is observed to form a growing anticyclonic bulge and a coastal current downstream of the bulge. Detailed simultaneous measurements of the velocity and buoyancy fields in the plume confirm that the bulge momentum is in a gradient–wind balance and the coastal current is geostrophic. The growth of the bulge and accumulation of fluid within it coincides with a reduction in coastal current transport to approximately 50% of the inflow discharge. The bulge is characterized by a depth scale, $h$, which is proportional to the geostrophic depth, $h_{g}$, and two time-dependent horizontal length scales, $y_{c}$, the displacement of the bulge centre from the wall, and $r_{b}$, the effective radius of the bulge. These two length scales are proportional to the inertial radius, $L_{i}$, and the local Rossby radius, $L_{b}$, respectively. When $r_{b}\gg y_{c}$, the bulge is held tightly to the wall, and a relatively large fraction of the inflow discharge is forced into the coastal current. For plumes with $y_{c}$ approaching $r_{b}$, the bulge is further from the wall, and the coastal current flux is reduced. Once ${y_{c}}/{r_{b}}\,{>}\,0.7$, the bulge separates from the wall causing flow into the coastal current to cease and the bulge to become unstable. In this state, the bulge periodically detaches from and re-attaches to the wall, resulting in pulsing transport in the coastal current. Scaling of the bulge growth based on $h_{g}$, $L_{i}$ and $L_{b}$ predicts that it will increase as $\hbox{\it Ro}^{1/4}$, where $\hbox{\it Ro}$ is the inflow Rossby number. The bulge growth, inferred from direct measurements of the coastal current transport, is proportional to $\hbox{\it Ro}^{0.32}$ and agrees with the predicted dependence within the experimental error.
Horizontal dispersion of a near-bed coastal plume
- DEREK A. FONG, MARK T. STACEY
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- Journal:
- Journal of Fluid Mechanics / Volume 489 / 25 July 2003
- Published online by Cambridge University Press:
- 30 July 2003, pp. 239-267
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The transport of scalars in the coastal ocean is considered through the analysis of a vertically constrained plume which disperses laterally. Observations of the plume are made using an autonomous underwater vehicle (AUV) operating in two modes: (i) repeated transects of the plume at a fixed distance from the source; and (ii) a large-scale mapping of the plume development. Together, these measurements define both the variability in the plume centreline (i.e. the meandering) and the growth of the plume around the centreline (i.e. the relative dispersion). The analysis of the measurements suggests that the meandering is well-described by a spatially uniform but temporally variable velocity field, indicating that large-scale flow structures dominate the centreline variability. The growth of the plume downstream is seen to follow a scale-dependent dispersion law, most likely of a compound structure: a 4/3-law in the near field, and a scale-squared law in the far field. This transition between dispersion laws is consistent with the transition from three-dimensional turbulence structures to two-dimensional eddies, which is due to the constraints imposed on the vertical dimension at the site. Comparing the two dispersion processes, the effective dispersion created by meandering is found to be comparable with or larger than the relative dispersion in the near field; but in the far field, the relative dispersion is found to dominate considerations of overall dispersion.