Near-bed flow over a fixed gravel bed

Advances in measurement technologies allow more detailed studies of near-bed flow fields over rough surfaces. Flow measurements over a gravel-bed mold are undertaken at the Fluid Mechanics Laboratory of the University of Auckland. The flume in use is 0.45-m wide and 19-m long. Nortek's Vectrino II profiler is used to measure the flow. Flow data are obtained over a region of interest of 0.150-m length and 0.102-m width, over which 425 samples are obtained. Upstream and downstream of the region of interest, the fixed gravel bed is extended, to reduce any disturbance of the approaching uniform flow. The flow measurements cover both, the interfacial region between gravel-bed elevation troughs and crests, as well as the region beyond the roughness elements. The individual time series' are postprocessed and average flow velocity components are obtained. To showcase the potential of the applied procedure, vertical slices and a horizontal layer of near-bed flow fields are presented. It is shown that near-bed flow fields strongly interact with the gravel bed, as we already now. The advancement is in the detailed 3D representation of the near-bed flow fields, allowing any of the 14450 uniformly distributed flow data nodes to be analysed in the 3D domain. Data affected by roughness tops protruding into the sample volume can be identified with the obtained DEM. Further work is needed to compare presented data with high-resolution flow fields obtained with more widely tested instrumentation, to quantify the near-bed flow, as well as to the examine turbulent flow properties in more detail, to better understand the processes that take place at the gravel-bed boundary.