On the use of the integral momentum-balance to calculate drag on a square cylinder in a compound-channel

Abstract

River flooding, threatens nearby infrastructure, as overbank flow occupies the adjacent berms (floodplains) and poses significant drag loads on the existing structures. The drag coefficient of such structures is possible to be influenced by the strong shear-layer formed at the interface of the main channel and the floodplain. Herein, this assumption is investigated in an experimental configuration involving the placement of an emergent cylinder at the main-channel/floodplain interface. The drag force on the cylinder at a certain distance from the floodplain bed is assessed through the application of the momentum-balance equation, in its integral form. The method is based on local measurements of the mean flow and turbulence characteristics. Drag is expressed as counteraction to the force on the flow in a control volume and is estimated as the residual in the momentum-balance equation. The experiment was conducted in the straight compound-channel facility of Laboratório Nacional de Engenharia Civil (LNEC), Lisbon. Uniform-flow conditions were set in the channel for a relative flow-depth hr=hfp/hmc=0.31 (hfp is the floodplain flow-depth and hmc is the main-channel flow-depth). A square cylinder was placed in one of the floodplains right next to the main-channel/floodplain interface. An Acoustic Doppler Velocimeter (ADV) was used for measuring the three-component instantaneous velocities at sequential positions on the surfaces of a fluid control-volume. The terms of the momentum-balance equation were estimated. Then the drag coefficient emerged from the respective drag force and the characteristic velocity U0 that accounts for the existence of the compound-channel-flow shear layer. The same calculations were applied to the case in which a cylinder is found in flow with uniform upstream velocities. This reference case is represented by placement of the cylinder in the middle of the floodplain in the same facility. The effect of the shear flow is assessed through comparison of the corresponding terms of the momentum-balance equation and the drag coefficients.