The hydrodynamics of a recirculating (O-tube) flume

An O-tube flume is a horizontal closed-circuit flume that can be driven by an inline impellertype pump to produce steady and/or oscillatory flow over a mobile seabed. An O-tube has the ability to reproduce large combined wave and current conditions near the seabed, typical of (cyclonic) storm conditions. In this paper, we investigate the hydrodynamics of an O-tube and show applications of this technology. First, we derive a dynamic equation to explain the coupling between pressure and flow rate within the O-tube. This result can be thought of as an extension of the classical hydrodynamic equations developed for U-tubes and allows for improved control of the flow within the O-tube by providing a prediction of the non-linear interaction between steady flow (i.e. currents) and unsteady flow (i.e. waves). We demonstrate this improved control by comparing measurements of flow rate taken from an O-tube with the dynamic equation. Secondly, we present velocity measurements to give a detailed description of the flow field within the O-tube, including mean flow profiles and seabed shear stress. Finally, we conclude the paper by providing two example applications of the facility to study sediment transport and scour.