Drag determination of an array of square cylinders subjected to shear flow in a compound channel

Miltiadis Gymnopoulos,Elsa Alves,Rui Ml Ferreira,Panayotis Prinos,A Paquier,N Rivière

doi:10.1051/e3sconf/20184006020

Abstract

Overbank flow in rivers threatens integrity of built elements located in the floodplain. Elements of infrastructure close to the interface between main channel and floodplain are subjected to complex hydrodynamic actions resulting from the obstruction of the shear flow that develops in that interface. In the current paper, the drag forces and the drag coefficient of building-like structures positioned in the interface are investigated. The experimental setup in Laboratorio Nacional de Engenharia Civil (LNEC) involves the placement of an array of square cylinders on the floodplain of a straight compound channel, next to the interface with the main channel. Three-component instantaneous-velocity recordings were performed by means of Acoustic Doppler Velocimetry (ADV) within the boundaries of a considered fluid-control volume encompassing the array, while uniform-flow conditions were established in the channel. The equation of momentum conservation was applied in its integral form in the fluid control-volume towards estimation of the time-averaged drag force at a certain elevation from the floodplain. The drag coefficient is estimated accounting for the typical shear layer at the main-channel/floodplain interface and is compared with coefficients strictly valid for isolated cylinders.

Highlights

The retardation of flow caused by an obstacle inside a moving fluid is expressed by the drag force
Flow through the array is considered uniform and the drag coefficient Cd involves the parameters that refer to a single cylinder of the array, that are: the force exerted on it, the cylinder’s diameter and a streamwise velocity that characterizes the mean flow through the array
This paper presents an analytical drag-force estimation of a finite array of emergent square cylinders placed at the main-channel/floodplain interface of a laboratory compoundchannel

Summary

Introduction

The retardation of flow caused by an obstacle inside a moving fluid is expressed by the drag force. Flow through the array is considered uniform and the drag coefficient Cd involves the parameters that refer to a single cylinder of the array, that are: the force exerted on it, the cylinder’s diameter and a streamwise velocity that characterizes the mean flow through the array. The assessment of this array-averaged drag coefficient often requires the validity of the following assumptions: existence of an infinite array and steady energy slope of a uniform flow along the channel. Strong streamwise-velocity gradients in the lateral direction [4] and 3-D processes like secondary currents and vortices [5,6] mainly characterize this region, blurring the well-known image of cylinder wakes in flows with spatially uniformly-distributed bulk velocities and negligible turbulence effects

Methods

Results

Conclusion