Simultaneous Velocity and Pressure Measurements using PIV and Multi Layer Pressure Sensor Arrays in Gravel Bed Flows

To investigate hydrodynamic processes above and within river beds, laboratory experiments have been performed to quantify the interaction between turbulent flow and pressure fluctuations at a porous gravel bed. A 2-D Particle Image Velocimetry (PIV) system measuring in streamwise vertical or horizontal planes above the gravel layer was used, synchronized with a pressure sensor array of 16 miniaturized piezoelectric pressure sensors (MPPS). This setup enables the visualization and quantification of instantaneous, simultaneous velocity and pressure fields. Results under stable bed conditions show alternating large-scale wedge-like structures of uniform momentum, inclined at an angle of 10− 25° to the bed in flow direction. The measurements show a significant pressure drop in regions where fluid with high speed interacts with fluid of lower velocity in the sense of a sweep event. This characteristic pressure drop can lead to the initial lift that is needed for the entrainment of single grains.