DETAILED PIV MEASUREMENTS ON PARTICLE-TURBULENCE INTERACTION IN OSCILLATORY SHEET FLOW: IN DILUTE REGIME

Abstract

An experimental investigation of sediment transport mechanisms under oscillatory sheet flow condition is conducted. Focus is placed upon the dilute regime of solid-liquid transport with volume concentrations C=0.01, where significant fluid turbulence is present, strong particle-turbulence interaction occurs and inter-particle collisions can be neglected. Understanding the coupling dynamics between phases is critical for the validation and improvement of the existing numerical models. Simultaneous determination of the dynamics of each phase is often prohibitively expensive to acquire by direct numerical simulation and poses significant challenges to experimental measurements. In our experiment, a U-shaped water tunnel is used to create highly repeatable oscillatory sheet flow conditions over a mobile bed. The test section of the tunnel is 375 cm in length, with a cross-sectional area of 30X45 cm^2. The sediment is modeled using narrowly sorted spherical soda lime glass beads with a mean diameter of d=240 \mu m and a specific gravity of s=2.5.

The efforts have been made in two directions: the measurement technique development and the application of the developed technique to an oscillatory sheet flow. First, a novel measurement technique, based upon Particle Image Velocimetry (PIV), was developed and validated, that aims for a simultaneous measurement of both phases. For the sediment phase measurement, the multi-camera single-plane (MCSP) method was developed to reconstruct particle's instantaneous 3D positions towards a higher concentration. This was followed by Lagrangian particle tracking (LPT) to link the reconstructed particles over successive frames, with an in-house developed algorithm based upon shake-the-box (STB, Schanz et al. Experiments in Fluids 2016). For the carrier phase measurement, stereoscopic PIV (SPIV) was implemented. In order to reduce the cross-talk errors due to the presence of sediment particles, the apertured filter method was developed to produce adequate image quality of both phases allowing for a reliable extraction of each phase independently.

Second, the developed measurement technique was applied in a sinusoidal oscillatory sheet flow (period, T=5 s, and peak free stream velocity, Uo,p=1 m/s) to provide a whole field, phase-locked time-resolved, particle resolved and concurrent measurement of both the fluid and the sediment phase in the dilute regime (C<0.01). Such detailed measurements in sheet flow have never been reported before to the author's best knowledge. The analysis of the acquired data focused upon three phase angles when the external flow is reversing the direction. It has been found that during flow reversal, 1) distinct particle suspension mechanisms are identified in the upper dilute regime (y>11 mm) and the lower dilute regime (2<y<4 mm), where y represents the vertical distance from the static sediment bed; 2) sediment particles show strong preferential sampling in flow regions with ejection events (Q2) and high turbulent fluctuations for y>11 mm, while the over-sampling diminishes towards the bed; and 3) the particle velocity and their ambient fluid velocity show a strong correlation for y>11 mm and the correlation becomes weaker as approaching the bed.