Statistical analysis methods for transient flows – the dam-break case

Chanson, Hubert

doi:10.1080/00221686.2020.1729266

Cited by 8 publications

(5 citation statements)

References 18 publications

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“…The results for both tips showed PDFs with similar behaviors, including a strong skewness, deviating from the Gaussian bell-shape and resulting in different values between mode, median, and mean. This is consistent with hydraulic jump data (Chanson, 2007a;Chachereau and Chanson, 2011), and the median was used for further analysis since this value was less affected by extreme values (Chanson, 2020). It is acknowledged that the intrusive nature of the phase-detection probes might affect the statistical distribution of the chord/length data, particularly for bubble sizes lesser than the probe's diameter.…”

Section: B Bubble Chord Time and Pseudo-chord Lengthsupporting

confidence: 67%

“…For breaking bores, long-duration measurements were physically meaningless because of the non-stationary nature of the flow. Instead, a large number of (Chanson, 2020). Herein, all data were synchronized based on the bore arrival time at the left tip of the reference probe (i.e., first air-to-water detection corresponded to T = 0), and the results were ensemble averaged or mediated over all available repetitions.…”

Section: Signal Processing and Methodologymentioning

confidence: 99%

“…Previous results on unsteady bores showed that at least 20 repetitions were necessary for accurate measurements of free-surface properties, velocity components, and tangential stresses in clear-water flows (Leng and Chanson, 2017;Chanson, 2020). However, to date, no sensitivity analysis was ever performed on the number or repetitions necessary to obtain meaningful and reliable air-water properties in unsteady flows.…”

Section: Articlementioning

confidence: 99%

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Ensemble-statistical approach in the measurement of air–water flow properties in highly unsteady breaking bores

Wüthrich

Shi

Chanson

2022

Review of Scientific Instruments

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Breaking bores are commonly observed in a number of natural processes, often associated with the presence of a transient mixture of air and water, with intense recirculation, air bubble entrainment, and splashing. Two-phase flow measurements in such highly unsteady flows cannot be based on long-duration measurements and require novel ensemble-statistical approaches based on multiple repetitions. Detailed measurements of air–water flow properties were then conducted in a breaking bore with Fr1 = 2.4 using an array of multiple dual-tip phase-detection probes. Based on an extensive experimental program, inclusive of 2000 tests at a single position and 100 tests at multiple elevations, a detailed sensitivity analysis was conducted on the necessary number of repetitions to obtain physically meaningful and statistically reliable air–water flow properties. The results led to a robust methodology to estimate ensemble-statistical values, including confidence intervals and residual error. In addition, these results provided a detailed characterization of the behavior of air–water flow properties in highly unsteady flows, including void fraction, number of interfaces, and bubble chord time/length. Despite the transient nature, all physical processes showed consistent behaviors with theoretical models and other stationary flows, including hydraulic jumps and plunging jets. Overall, this study provided two-phase flow characteristics that go beyond the limitations imposed by the unsteady nature of the flow, proving thoroughly the importance of large datasets for the estimation of air–water flow properties in highly unsteady flows.

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Section: B Bubble Chord Time and Pseudo-chord Lengthsupporting

confidence: 67%

Section: Signal Processing and Methodologymentioning

confidence: 99%

Section: Articlementioning

confidence: 99%

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Ensemble-statistical approach in the measurement of air–water flow properties in highly unsteady breaking bores

Wüthrich

Shi

Chanson

2022

Review of Scientific Instruments

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“…For a limited number of cases, the experiments were repeated 25 times in the current study and an ensemble statistical analysis was conducted to extract mean and fluctuating quantities. Such a rigorous methodology is difficult to achieve high quality data sets (Chanson 2020). In turn, the approach restricted the spatial coverage of the data, and the secondary flow maps could not be generated with the current metrology.…”

Section: Experimental Flow Conditionsmentioning

confidence: 99%

Transient secondary currents behind a compression wave in an irregular channel

2020

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A compression wave is an unsteady rapidly-varied open channel flow motion, characterised by an increase in water depth. A detailed investigation was conducted in a prismatic asymmetrical channel, to better understand the physical processes observed in tidal/tsunami-bore affected estuaries with irregular topography. The prismatic channel was equipped with a 1V:5H transverse slope across the full channel width. The free-surface data presented three-dimensional unsteady flow features, and the velocity measurements showed a drastic impact of the surge on the flow field, with strong three-dimensional features.With the arrival of the surge front, the transverse velocity component became large in the shallow-water section, indicating some unsteady secondary motion and recirculation during and shortly after the passage of the surge. The Reynolds shear stress data were associated with large turbulent stresses and shear stress fluctuations. For d 1 /D < 1, both velocity and shear stress data showed a transient longitudinal x-z shear plane, about y/W  0.5-0.6, with conjugate transverse flow reversal for 0.5-0.6 < y/W < 1. In the transient shear plane, high Reynolds stress magnitudes and shear stress fluctuations were observed behind the surge front, up to one to two orders of magnitude larger than boundary shear stress levels observed in steady flows in compound channels.

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“…The empirical mode decomposition method has recently been developed by Huang et al [12], who used it extensively to analyze nonstationary and nonlinear time-series data for different geophysical applications [13]. Although the ensemble average method is the most reliable approach for extracting time-varying average flow quantities [14], due to the practical and technical limitations, time constraints, and limited resources [15], this method is seldom applied in studies on unsteady open-channel flow. For the other three methods, there is a weakness that likely causes errors in the estimation of the time-varying mean flow quantities.…”

Section: Introductionmentioning

confidence: 99%

Hydrodynamic Characteristics of Strong, Unsteady Open-Channel Flow

Hu,

Huang

et al. 2023

Sustainability

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Due to climate change, the intensity of extreme rainfall has been observed to increase with a shorter duration, causing flash floods (strong, unsteady flow) that lead to serious loss of life and economic damage all over the world. In this study, by repeating the same flume experiments twenty times over a bare bed or with a submerged vane installed, the hydrodynamic characteristics of a strong, unsteady open-channel flow were investigated. Acoustic Doppler velocimetry (ADV) was used to measure the instantaneous three-dimensional velocity, and the ensemble average method was then adopted to obtain the time-varying mean flow velocities. Reynolds decomposition was applied to disintegrate the instantaneous velocity to time-varying average velocity and fluctuating velocity. Turbulence characteristics such as turbulent intensity, turbulent bursting, and power spectral density (PSD) were analyzed against water depth variations. The results show that the loop pattern of the streamwise velocity against the water depth variations could significantly affect the turbulence characteristics of unsteady flow. Near the bed, the peaks of the turbulent intensity and the TKE lag behind the peak of the water depth. The PSD revealed that the turbulent energy increases at the rising and falling stages were due to the generation of small-scale turbulence vortices or eddies. As a submerged vane was present, the increase in the angle of attack caused the increase in the turbulent intensity and TKE, which means that the induced vortex became stronger and the wake region was larger. When the angle of attack was equal to 20°, the TKE abruptly enlarged in the falling stages, implying the breaking-up of the induced vortex. The PSD of the transverse fluctuation velocity showed multiple spikes at the high-frequency part, possibly denoting the shedding frequency from the induced vortex. Further downstream, behind the submerged vane, the peak frequencies of the PSD became imperceptible, likely because of the induced vortex decays or other factors such as the turbulence generated from the free surface or the channel bed mixing with the turbulence from the induced vortex.

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Statistical analysis methods for transient flows – the dam-break case

Cited by 8 publications

References 18 publications

Ensemble-statistical approach in the measurement of air–water flow properties in highly unsteady breaking bores

Ensemble-statistical approach in the measurement of air–water flow properties in highly unsteady breaking bores

Transient secondary currents behind a compression wave in an irregular channel

Hydrodynamic Characteristics of Strong, Unsteady Open-Channel Flow

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