Ekman boundary layers in a large-scale gravity current experiment

Date & time

11am–12pm 18 July 2017

Location

Hales Room, Jaeger 7

Speakers

Dr Matt Wells (University of Toronto)

Oceanic density currents such as contourite currents and turbidity currents can be strongly influenced by Coriolis forces. Coriolis forces lead to Ekman boundary layers, which result in transverse secondary flows of as much as 10% of the primary flow [Cossu et al., 2010]. For flows in sinuous channels these Ekman boundary layers can either act with, or against, the secondary flows driven by centrifugal forces. These secondary near-bed flows are responsible for many of the details of sediment erosion and deposition in contourites, as well as in larger channels formed by turbidity currents [Cossu et al., 2015]. The magnitude and direction of flows in the Ekman boundary layers can be defined by a Rossby number, RoW =U/Wf, where U is the mean downstream velocity, W is the width of the channel and f is the Coriolis parameter. Flows with Rossby numbers less than 1 are dominated by Coriolis forces and have strong Ekman boundary layers dominating secondary circulation. A new finding is that even when the Rossby number is of order 10, there is an appreciable influence of Coriolis forces, supporting empirical observations of straighter turbidite channels when |RoW| < 10 [Wells et al., 2013]. We will present new results of changes in velocity structure in a very large-scale gravity current as from a series of laboratory experiments in a sinuous channel on the large Coriolis platform in Grenoble, France.

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