Optical flow estimations in aerated spillway flows: Filtering and discussion on sampling parameters

Abstract

Image-based velocimetry has captured great interest in physical modelling of highly-aerated flows, for example in stepped spillways. The present study investigated the performance of a novel filtering technique based upon a detailed systematic comparison with intrusive phase-detection probe data, recorded in a large-size stepped spillway. A sensitivity analysis provided recommendations in terms of optimum sampling and processing parameters for optical flow measurements in high-velocity air-water flows.

Strong free-surface turbulence in breaking bores: a physical study on the free-surface dynamics and air–water interfacial features

“…These techniques are well established in the field of computer vision, and only recently applied to fluid dynamics. In the present study, the ultra-high-speed video images were post-processed using the Gunnar–Farneback (GF) OF technique (Farneback 2002, 2003), previously used to characterise turbulent properties in aerated stepped spillway flows (Bung & Valero 2016; Zhang & Chanson 2018; Kramer & Chanson 2019).…”

“…These techniques are well established in the field of computer vision, and only recently applied to fluid dynamics. In the present study, the ultra-high-speed video images were post-processed using the Gunnar–Farneback (GF) OF technique (Farneback 2002, 2003), previously used to characterise turbulent properties in aerated stepped spillway flows (Bung & Valero 2016; Zhang & Chanson 2018; Kramer & Chanson 2019). The GF technique proposed that the brightness data followed a quadratic polynomial, where is the pixel coordinate vector in the image plane, is a systematic coefficient matrix for second-order terms, is a coefficient vector for first-order terms and is a constant.…”

“…The built-in GF algorithm provided a multi-resolution image pyramid to detect large displacement, and a Gaussian filter to remove image noise. Several input parameters were selected based on the work of Kramer & Chanson (2019) and Shi et al. (2020).…”

Applications of optical flow technique in air-water flows

“…The present OF results were around 10 % less than the centreline phase-detection probe data. Several works (Kramer and Chanson, 2019;Zhang and Chanson, 2018) reported 10-30 % reduction of the velocity between OF data and centreline data. The inconsistencies were likely caused by the sidewall effects.…”

“…For all experiments, the video signals were sampled at 10,000 fps with full HD resolution. For the post-processing, the ultra-high-speed videos were subsampled to 5,000 fps following Kramer and Chanson (2019). A strong LED array (GS Vitec MultiLED), which was specifically manufactured for the Phantom camera, was used to provide a constant illumination during sampling.…”