Experiments were conducted to study confinement and friction effects on a shallow recirculating flow that was formed in a lateral expansion of an open-channel flow on a smooth bed. Dye was introduced to the flow as tracer. The instantaneous dye concentration in the flow was captured by video simultaneously across the width and along the length of the flume. The mean and the root-mean-square values of the dye-concentration fluctuations at selected cross sections along the flow are analyzed and compared with a previous series of data that were collected using a probe to measure the concentration one point at a time. The previous experiment was conducted in a small flume on a roughen bed. The new flume in the present experiment was 2.35 times wider. The large width of the new flume facilitates the study of the same friction effect on a smooth bed over a greater range of the confinement effect. In comparison with the previous experiments, the dye concentration in the present experiments is higher for the same range of bed-friction numbers. The difference in the results is tentatively attributed to the bed-generated turbulence. Key words: recirculating flow, shallow open-channel flow, quasi-two-dimensional turbulent flow, horizontal turbulence, bed-generated turbulence, friction effects, tracer concentration measurements, video imaging method.

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Simulations of turbulent interfaces produced by positive and negative buoyancy are conducted by moving blocks of fluid in the direction of the flow. The second moment of the blocks increases at a rate proportional to the diffusivity. The block simulation is free of numerical oscillations. Unlike most classical methods, the error associated with Lagrangian block simulation is not cumulative. Artificial diffusion error is negligible. The non-diffusive Lagrangian block simulations have provided reliable data to evaluate the performance of (i) sub-grid scale modelling and (ii) K-ε modelling of turbulent flow under the opposing influence of buoyancy. INTRODUCTIONMost computational fluid-dynamics codes are developed using the Eulerian description. To find the numerical solution, fluxes are estimated on the surface of the finite volume using a truncation series. Spurious numerical oscillations and artificial numerical diffusion are consequences, particularly in regions across flow discontinuities. Diffusion often is introduced synthetically in many schemes to gain computational stability. Occasional switching to a diffusive upwind scheme is one classic strategy to manage the numerical oscillations [1,2,3,4,5]. Lagrangian block simulation (LBS) offers an alternative that could eliminate the spurious numerical oscillations and false diffusive error [6,7]. The blocks move in the direction of the flow. The squares of the block widths expand in proportion to the diffusivities. The block simulation procedure consists of three steps: (i) Lagrangian advection and diffusion, (ii) division into portions, and (iii) reassembly of the portions into new blocks. The blocks are renewed in each time increment to prevent excessive distortion. In this paper simulation of buoyancy at turbulence interfaces has been carried out using the LBS method. In one series of simulations, the Kelvin-Helmholtz instabilities initiate turbulence across

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