The Application Of Apertured Filter Method To The Simultaneous Measurement Of Particle-Turbulence Interaction In Dilute Suspension Layer Under Oscillatory Sheet Flows

One of the challenges confronted in particle image velocimetry (PIV) measurements applied in multi-phase flows is the discrimination of signals from different phases simultaneously registered in each single monochromatic camera. In the current sheet flow study, the sediment particles with a mean diameter of 240 μm are used and the diameter of tracer particles ranges from 25~50 μm. Under conventional imaging, the disparity in particle sizes would inevitably cause highly imbalanced reflected signals from the two phases and the image quality of one of the phases typically must be sacrificed. This manuscript introduces a low-cost and exceedingly convenient solution, named as the apertured filter method, which is characterized by the combined use of fluorescent tracer particles and an optical band pass filter with a round hole drilled in the center, with the hole diameter comparable to the smallest lens aperture size. It is demonstrated that the apertured filter method can provide balanced image intensities among the sediment and tracer particles and preserve the identifiability and separability of both phases. To compute the carrier phase velocity, median filter subtraction (Kiger & Pan, 2000) is applied to the acquired images, followed by direct implementation of stereoscopic PIV (SPIV) process. The cross-talk PIV errors due to the presence of sediment particles are assessed and found to be within 0.3 pixels up to a sediment concentration of C=1%. Some guidance on the fabrication of the apertured filter are also addressed to assist potential users. Enabled by the apertured filter method and the multi-camera single-plane (MCSP) method developed by Liu & Kiger (2022), a whole field, phase-locked time-resolved, particle-resolved and concurrent measurement of both the fluid and the sediment phase within the dilute regime (C≤1%) in a sinusoidal oscillatory sheet flow (period, T=5s, and peak free stream velocity, U_(o,p)=1 m/s) is achieved. 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. The phase averaged sediment concentration, mean velocities for each phase and slip velocities between the two phases are evaluated.