Author(s): Andrea Zampiron; Stuart Cameron; Mark Stewart; Vladimir Nikora
Keywords: Open-channel flows; Hydraulic flumes; Experimental hydraulics; Turbulence
Abstract: The knowledge of flow development is of critical importance to assure that appropriate experimental conditions are recreated. In open-channel flows, one of the main assumptions in most experimental studies is the flow uniformity, which means that flow parameters do not change in the streamwise direction. The question then is what distance from the flume entrance is required to achieve flow uniformity? The available data are not sufficient to properly answer this question and researchers often employ some rather intuitive approximations instead of tailored preliminary measurements to assess the required distance for the establishment of fully developed uniform flow conditions. Therefore, some experimentally-justified guidance regarding flow development in hydraulic flumes would be highly useful for experimental designs. It is generally assumed that once the flow depth is entirely occupied by the internal boundary layer, formed after the flow enters the flume, the flow can be considered as ‘fully developed’. However, this approach seems quite superficial as it neglects specific evolutions of large-scale (LSMs) and very-large-scale (VLSMs) turbulent motions, secondary currents (SCs), water surface effects and any possible interactions between them. In general, the distances from the flume entrance to achieve full development for LSMs, VLSMs, SCs and associated velocity statistics may not be the same and thus the whole definition of the term ‘flow development’ may need some revision. Stereoscopic particle-image velocimetry and long-duration acoustic Doppler velocimetry measurements were used to study development of open-channel flows for a range of bed roughness types and flow conditions. Particular attention was given to tracking the development of velocity statistics and velocity spectra along the flow, starting from the flume entrance. Based on the bulk velocity statistics, the internal boundary layer was found to reach the water surface at about 40H (H is flow depth), with no appreciable differences for the examined range of flow conditions and bed roughness types observed. At ~40H, the time-averaged surface velocity reaches its maximum, while becoming fully streamwise-homogeneous only at distances >100H (similar trends can be observed for higher statistical moments). These trends in velocity statistics can be explained by the differences in streamwise evolution of LSMs, VLSMs, and SCs. These results suggest that a condition of a turbulent boundary layer occupying the entire flow depth is not a sufficient condition for a ‘fully developed flow’ as the development of its key components such as LSM, VLSM, and SCs shows, requiring in some cases up to ~200H to be established. We conclude this study with a set of specific recommendations that may help in experimental designs.