Abstract
There are no studies specifically aimed at characterizing and quantifying drag forces on finite cylinder arrays in the mixing layer of compound channel flows. Addressing this research gap, the current study is aimed at characterizing experimentally drag forces and drag coefficients on a square-cylinder array placed near the main-channel/floodplain interface, where a mixing layer develops. Testing conditions comprise two values of relative submergence of the floodplain and similar ranges of Froude and bulk Reynolds numbers. Time-averaged hydrodynamic drag forces are calculated from an integral analysis: the Reynolds-averaged integral momentum (RAIM) conservation equations are applied to a control volume to compute the drag force, with all other terms in the RAIM equations directly estimated from velocity or depth measurements. This investigation revealed that, for both tested conditions, the values of the array-averaged drag coefficient are smaller than those of cylinders in tandem or side by side. It is argued that momentum exchanges between the flow in the main channel and the flow in front of the array contributes to reduce the pressure difference on cylinders closer to the interface. The observed drag reduction does not scale with the normalized shear rate or the relative submersion. It is proposed that the value of the drag coefficient is inversely proportional to a Reynolds number based on the velocity difference between the main-channel and the array and on cylinder spacing.
Highlights
The present study addresses the problem of determining drag forces and drag coefficients on finite arrays of emergent cylinders located at the interface between floodplain and the main-channel, in flood events
Single cylinders in compound channel mixing layers are subjected to lower drag forces, but the drag reduction does not show the same trends of the linear shear flow
This study was aimed at experimentally assessing the bulk drag force on arrays of cylinders in compound channel mixing layers and the corresponding array-averaged drag coefficients
Summary
Floodplains are formed when the river becomes able to store its excess sediment load outside the main channel [1]. They extend from the riverbanks to the edge of the valley, forming, in each cross-section, a two-stage geometry. Floodplains are periodically flooded, which occurs when the conveyance of the main channel is exceeded. Floodplains are generally composed of fertile soil where a variety of natural vegetation, from meadows to shrubs and forests, can develop [2]. The combination of fertile land and waterways encouraged early humans settle in floodplains. In spite of the flooding risk, many of todays’
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