A high-precision current measurement system for laboratory flume systems: a case study around a circular cylinder

For detailed flow studies around benthic organisms in flumes there is need for a precise velocity sensor that can be positioned accurately. In this pa per an automated positioning unit, combined with a velocity sensor, is desc ribed for a recirculating salt water flume in which natural sediment cores can be introduced. The spatial resolution of the positioning system is 0.21 mm in all 3 dimensions. It is equipped with a Sontek acoustic-Doppler velo cimeter which has a small sampling volume (0.25 mm(3)) located 5 cm below t he sensor head. A computer controls both the coupled positioning and sampli ng procedures. Hence it can follow even complex flow patterns or record the flow regime in a complete 3-D grid. Results from flow measurements around a single polychaete tube mimic in the flume show a characteristic vortex pa ttern with high spatial resolution and reveal the effects on particle fluxe s in the benthic boundary-layer. Thus it confirms the findings of previous publications on polychaete tube induced flow patterns and adds new resoluti on through direct flow measurements around the object.