Best practices for velocity estimations in highly aerated flows with dual-tip phase-detection probes

Abstract

Dual-tip phase-detection probes can be used to measure flow properties in gas-liquid flows. Traditionally, time-averaged interfacial velocities have been obtained through cross-correlation analysis of long time-series of phase fraction signals. Using small groups of detected particles, a recently developed adaptive window cross-correlation (AWCC) technique enables the computation of pseudo-instantaneous interfacial velocities and turbulence quantities in highly aerated flows, albeit subject to some smoothing which is due to the use of a finite window duration. This manuscript provides guidance on the selection of optimum processing parameters for the AWCC technique, additionally addressing shortcomings such as velocity bias correction in turbulent flows and extrapolation of turbulence levels to single particles. The presented technique was tested for three highly turbulent air-water flows: smooth and rough-wall boundary layers (tunnel chute and stepped spillway), as well as breaking shear layer flows of a hydraulic jump. Robust estimations of mean velocities and velocity fluctuations were obtained for all flow situations, either using dual-tip conductivity or fiber optical probe data. The computation of integral time scales and velocity spectra is currently limited by the data rate and must be treated with caution.

Free surface aeration and development dependence in chute flows

“…The present study proposes an analysis of air-water turbulent diffusion based on a theoretical model. Recent experimental and theoretical analyses confirm that a reasonable model for particle groups in air-water flows can result in reliable two-phase properties, including time-average mixture-velocity and air concentration distributions 44 . According to the present analysis results, the air-water mixture in high-speed chute flows is a result of the interaction of air and water under turbulence.…”

Comparison Of Signal Processing Algorithms For Multi-Sensor Intrusive Phase-Detection Probes

“…Synthetically generated signal times series have been used to test air-water flow instrumentation in recent studies (e.g., Kramer et al, 2019Kramer et al, , 2020aValero et al, 2019). Herein, we extend these approaches to 3-D velocity fields with correlations between stream-and spanwise components to model shearing.…”

“…Instead, dual-tip intrusive phase-detection probes in combination with cross-correlation based signal processing have been the preferred method over several decades for the retrieval of time averaged velocities (Neal and Bankoff, 1963;Wood, 1983) and other air-water flow properties including void fraction, particle frequencies and particle chord lengths (Straub and Anderson, 1958;Boes and Hager, 2003;Felder and Chanson, 2014). The recently proposed adaptive window cross-correlation (AWCC) signal processing algorithm (Kramer et al, 2019(Kramer et al, , 2020a allows for robust measurements of local pseudo-instantaneous 1-D velocities of signal windows Proceedings of the 39th IAHR World Congress 19-24 June 2022, Granada, Spain consisting of a discrete number of particles (bubbles or droplets) and significantly improved measurement accuracy of turbulent statistics in highly aerated flows compared to previous methods (Chanson and Toombes, 2002). However, the applied window averaging still reduces the data rate in comparison to eventdetection algorithms (Kramer et al, 2020b), thereby introducing an averaging effect on turbulence quantities and limiting the retrieval of time-resolving turbulence statistics.…”

“…However, the applied window averaging still reduces the data rate in comparison to eventdetection algorithms (Kramer et al, 2020b), thereby introducing an averaging effect on turbulence quantities and limiting the retrieval of time-resolving turbulence statistics. While Kramer et al (2020a) proposed a method to overcome the smoothing effect as a result of window averaging by extrapolation of turbulence levels to = 1, this approach requires calibration. In any case, information from dual-tip probes is limited to streamwise velocities, prohibiting the measurement of vertical velocity fluctuations which are of special interest in the context of air-water flows.…”

Air–Water Properties in Rectangular Free-Falling Jets

“…According to our previous analyses, at least 30 s are needed in order to obtain a relative uncertainty of about ±1% in the local void fraction. Other authors such as Kramer et al [23] suggest registers of at least 45 s to obtain accurate measurements. To err on the side of caution, in this study, each point was measured using a sampling rate of 20 kHz and a sampling duration of 45 s.…”