10th Pacific Symposium on Flow Visualization and Image Processing Naples, Italy, 15-18 June, 2015 Paper ID: 25 1 Visualization of Shear Layers in Compound Channel Flows Saad Mulahasan*, Professor Thorsten Stoesser and Fernando Alvarez Hydro-environmental Research Centre, School of Engineering, Cardiff University, Cardiff, UK *corresponding author: mulahasansh@cf.ac.uk Abstract This paper explains the process of visualizing shear layer formation by thermal video sequences captured at the interface between the main channel and the floodplain in a shallow flow compound channel. Hot water was added at the water surface using a small tube at ambient condition and at the interface between main- channel and floodplain. A thermal camera (SC640) operating at 5Hz frequency and 640x480 pixels resolution was mounted vertically at 0.6m above a shallow flow compound channel at the interface between the main channel and the floodplain at a section where a fully developed flow had been achieved. This set-up enabled the capturing of the flow temperature differences at 5 locations longitudinally and at 3 locations transversely. Flir’s image processing software and Matlab were used to extract and analyse captured water surface temperature data. Image processing is done, first for five locations at the junction edge of the floodplain adjacent to the main channel with longitudinal overlaps between any two successive images, thus creating a part of the shear layer at the centre. The other five locations on either side of the centre were similarly carried with longitudinal overlaps to create the second and third parts of the shear layer. At the end three parts were creating that represent the entire shear layer. In order to visualize the final shear layer (from these three parts), a horizontal overlap was done for the left and the centre part firstly, and then overlap between the right and the centre parts was achieved secondly. The created shear layer image was then exported into Tec-plot software. The analysis showed that the shear layer at the interface between the main channel and the floodplain is well captured and quantified by this technique. Keywords: Shear layer, Compound channel, Shallow flow, Image processing, Thermal technique 1 Introduction Shallow flows are defined as open channel flows with transverse velocity gradient and is shallow because their mixing layer width larger than the water depth [1]. Shallow flow examples in nature are of different types such as lakes, bays, estuaries, lowland flows, and river confluences. Lowland rivers and their floodplain are examples of a compound channel quasi uniform flow which consists of main channel and one or two floodplains. Floodplains are usually rougher than main channels due to growing different types of vegetation in different alignment and configurations such as one line vegetation and wholly vegetated floodplain. When the flow exceeds the main channel, the faster flow in the main channel interacts with the slower flow velocity on the floodplain generates mixing shear layers (turbulent structures) near to the interface between the main channel and the floodplain which generates extra resistance. Mass and momentum transfer due to the velocity difference between the main channel and the floodplain(s). Shear layers were visualized experimentally by a number of authors using different techniques such as a dye tracer, Particle Image Velocimetry (PIV), Laser Doppler Anemometer (LDA) and numerically using such as large eddy simulations (LES). Sellin [2] injected aluminium powder on the water surface between the main channel and the floodplain in a straight compound channel flow. He visualized a strong vortex structure at the interface between the main channel and the floodplain. These vortices were recorded by a photography means. Pasche and Rouve [3] investigated experimentally the flow structures in compound channel flows with and without vegetated floodplain using Laser-Doppler Velocimetry (LDV) and Priston-tube techniques. They visualized the formation of eddies at the interface between the main channel and the floodplain. Tamai et al. [4] observed periodic large eddies (vortices) at the water surface at the interface between the main channel and the floodplain of a uniform compound channel flow using flow visualization method as hydrogen bubble method. These vortices generated by the local shear at the interface. Tominaga and Nezu [5] presented the interaction between the main-channel and floodplain flow in fully developed compound open channel flows using a fibre-optic laser Doppler anemometer. The contribution of secondary currents on momentum transport is very large near the junction, in which strong inclined secondary currents associated with a pair of longitudinal vortices on both sides of the inclined up-flow are generated from the junction edge toward the free surface. The effects of channel geometry and bed roughness on turbulent structure are examined. As the relative depth “which is defined as water depth on floodplain/ water depth in the main channel” decreases, strong vortices appear near the free surface of the floodplain, and the main channel