Brown University School of Engineering

Fluids Seminar: Similarity & Instability in Flows Over Permeable Layers

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Tuesday, April 01, 2014 3:00pm - 4:00pm

Center for Fluid Mechanics, Division of Applied Mathematics Fluids and Thermal Systems, School of Engineering Joint Seminar Series TUESDAY – APRIL 1, 2014 3:00pm Barus & Holley, Room 190 Marco Ghisalberti School of Environmental Systems Engineering, University of Western Australia. email: SIMILARITY & INSTABILITY IN FLOWS OVER PERMEABLE LAYERS Permeable obstructions (such as seagrass meadows) are prevalent in the benthic region of freshwater and coastal environments. Their impact on the near-bed flow, turbulence and vertical transport is profound. Here, I use particle imaging and point velocity measurements in both steady and oscillatory flows to demonstrate three salient features of environmental flows over permeable layers: (1) A framework developed for vegetation canopies has the capacity to predict flow, turbulence and mixing properties over a wide range of permeable layers (from sediment beds to coral reefs to 'urban' canopies to ancient rangeomorph communities). (2) Steady flows are characterized by the development of a Kelvin-Helmholtz-type instability at the interface between the permeable layer and the free flow. These coherent structures dominate vertical mixing at the interface and generate regular oscillations in flow and transport. The height of the permeable layer relative to its drag length scale defines three regimes of shear flow. (3) Such instability is also observed in oscillatory flow when both the Reynolds (Re) and Keulegan-Carpenter (KC) numbers exceed threshold values. This is important in the prediction of residence time in ecologically-significant benthic habitats that exist in shallow (and therefore, typically, wave-dominated) coastal regions.