Double-averaged open-channel flow over regular rough beds
The majority of analysis of hydraulically rough open-channel flows has been founded on the temporally-averaged momentum equations (RANS). However, the structure of shallow open-channel flows over hydraulically rough beds, together with the near-bed region of deeper open-channel flows with rough beds awaits clarification. The purpose of this project was to conduct an extensive programme of laboratory-based experiments to gauge the applicability of the double-averaged momentum equations for shallow open-channel flows over simple rough beds (transverse square bars with varying centre-to-centre pitch). Proper assessment of the double-averaged equations requires comprehensive measurement of fluid velocities. Therefore, detailed Particle Image Velocimetry (PIV) measurements have been obtained of the streamwise and bed-normal velocity components, including their variation in both temporal and spatial domains. The results show that double-averaging is a powerful tool for the analysis of hydraulically rough flows. For a range of isolated flow types, the vertical distribution of the double-averaged streamwise velocity follows a linear trend between bars. Quadrant analysis has been applied to the spatial, rather than temporal, fluctuations of velocity components for the first time, and double-averaging analysis has revealed areas of intense local momentum transfer despite negligible global momentum exchange over the averaging window. This thesis also reports the discovery of an instability at the transition between wake interference and isolated roughness flows at which the overall properties of the flow are dramatically altered.