Nonperiodic Eddy Pulsations

Abstract

Recirculating flow in lateral separation eddies is typically weaker than main stem flow and provides an effective environment for trapping sediment. Observations of recirculating flow and sedimentary structures demonstrate that eddies pulsate in size and in flow velocity even when main stem flow is steady. Time series measurements of flow velocity and location of the reattachment point indicate that these pulsations are nonperiodic. Nonperiodic flow in the lee of a channel margin constriction is grossly different from the periodic flow in the lee of a cylinder that is isolated in a flow. Our experiments demonstrate that placing a flow-parallel plate adjacent to a cylinder is sutficient to cause the leeside flow to change from a periodic sequence of vortices to a nonperiodically pulsating lateral separation eddy, even if flow conditions are otherwise unchanged. Two processes cause the leeside flow to become nonperiodic when the plate is added. First, vortices that are shed from the cylinder deform and become irregular as they impact the plate or interfere with remnants of other vortices near the reattachment point. Second, these deformed vortices and other flow structures are recirculated in the lateral separation eddy, thereby influencing the future state (pressure and momentum distribution) of the recirculating flow. The vortex deformation process was confirmed experimentally by documenting spatial differences in leeside flow; vortex shedding that is evident near the separation point is undetectable near the reattachment point. Nonlinear forecasting techniques were used in an attempt to distinguish among several possible kinds of nonperiodic flows. The computational techniques were unable to demonstrate that any of the nonperiodic flows result from low-dimensional nonlinear processes.

Processes Governing Hydrochory along Rivers: Hydraulics, Hydrology, and Dispersal Phenology

“…6). Although there is some pulsation and irregularity in the zone of flow separation even under steady flow conditions (Rubin and McDonald 1995), observations in the flume suggest that this instability is amplified under ascending and rapidly changing flow conditions. 7).…”

“…Microturbulent waves were observed to deposit seeds slightly above the water surface even in the most turbulent (RE Ͼ 25 000) fluvial environments in the flume such as cut banks and constrictions (Fig. Instability in the zone of flow separation, which occurs when vortices are shed from the point of separation (Rubin and McDonald 1995), was observed to negatively affect the numbers of seeds trapped, retained, and deposited in eddies. These results suggest that the hydraulic factors governing seed deposition act over two spatial scales: (1) macroscale hydraulics such as eddies and expansions which detain and recirculate seeds and (2) microscale hydraulics such as small waves produced by bursting phenomenon at higher flows and hydraulics regulated by channel microtopography, boundary roughness, and bedform irregularities.…”

Of sand and mud: Sedimentological criteria for identifying the turbidity maximum zone in a tidally influenced river

“…Finally, flow separation along the channel margins has been observed throughout the lower Fraser River. Based on the work of Leeder & Bridges (1975), Rubin et al (1990) and Rubin & McDonald (1995), as well as the morphology of the Fraser River, it is likely that flow separation results from the seaward widening of the channel, the sharp break in slope between the channel thalweg and the channel margins, and/or flow around meander bends. Where flow separation occurs on the channel margins, reverse eddies develop and assist in the upstream transport of fine-grained sediment (Czarnecki et al, 2014), and deposition of mud in a zone that extends landward of the SWW incursion.…”

Unsteady two-dimensional paleohydraulic reconstruction of extreme floods over the last 4000yr in Segura River, southeast Spain

“…At the largest spatial scale is the upstream migration direction of climbing ripples. The deposits are interpreted as slackwater fluvial deposits where flow reversals have been linked to the evolution of eddies (e.g., passage of small eddies or instabilities in the main eddy or eddy pulsations) during high discharge stage changes in the margin of modern river channels (Rubin and McDonald, 1995). The alternation of symmetric and asymmetric ripples, and ripple-crest zigzags on alternating flanks of the vertical ripples, demonstrate an uneven flow.…”