TY - JOUR A1 - Zhang, G. A1 - Valero, Daniel A1 - Bung, Daniel Bernhard A1 - Chanson, H. T1 - On the estimation of free-surface turbulence using ultrasonic sensors JF - Flow Measurement and Instrumentation N2 - Accurate determination of free-surface dynamics has attracted much research attention during the past decade and has important applications in many environmental and water related areas. In this study, the free-surface dynamics in several turbulent flows commonly found in nature were investigated using a synchronised setup consisting of an ultrasonic sensor and a high-speed video camera. Basic sensor capabilities were examined in dry conditions to allow for a better characterisation of the present sensor model. The ultrasonic sensor was found to adequately reproduce free-surface dynamics up to the second order, especially in two-dimensional scenarios with the most energetic modes in the low frequency range. The sensor frequency response was satisfactory in the sub-20 Hz band, and its signal quality may be further improved by low-pass filtering prior to digitisation. The application of the USS to characterise entrapped air in high-velocity flows is also discussed. Y1 - 2018 U6 - https://doi.org/10.1016/j.flowmeasinst.2018.02.009 SN - 0955-5986 VL - 60 SP - 171 EP - 184 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Valero, Daniel A1 - Chanson, Hubert A1 - Bung, Daniel Bernhard T1 - Robust estimators for turbulence properties assessment Y1 - 2019 SP - 1 EP - 24 ER - TY - JOUR A1 - Kerpen, Nils B. A1 - Bung, Daniel Bernhard A1 - Valero, Daniel A1 - Schlurmann, Torsten T1 - Energy dissipation within the wave run-up at stepped revetments JF - Journal of Ocean University of China Y1 - 2017 U6 - https://doi.org/10.1007/s11802-017-3355-z SN - 1993-5021 VL - 16 IS - 4 SP - 649 EP - 654 PB - Springer CY - Berlin ER - TY - JOUR A1 - Valero, Daniel A1 - Bung, Daniel Bernhard T1 - Development of the interfacial air layer in the non-aerated region of high-velocity spillway flows: Instabilities growth, entrapped air and influence on the self-aeration onset JF - International Journal of Multiphase Flow N2 - Self-aeration is traditionally explained by the water turbulent boundary layer outer edge intersection with the free surface. This paper presents a discussion on the commonly accepted hypothesis behind the computation of the critical point of self-aeration in spillway flows and a new formulation is proposed based on the existence of a developing air flow over the free surface. Upstream of the inception point of self-aeration, some surface roughening has been often reported in previous studies which consequently implies some entrapped air transport and air–water flows coupling. Such air flow is proven in this study by presenting measured air velocities and computing the air boundary layer thickness for a 1V:2H smooth chute flow. Additionally, the growth rate of free surface waves has been analysed by means of Ultrasonic Sensors measurements, obtaining also the entrapped air concentration. High-speed camera imaging has been used for qualitative study of the flow perturbations. Y1 - 2016 U6 - https://doi.org/10.1016/j.ijmultiphaseflow.2016.04.012 SN - 0301-9322 VL - 84 SP - 66 EP - 74 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Valero, Daniel A1 - Bung, Daniel Bernhard T1 - Reformulating self-aeration in hydraulic structures: Turbulent growth of free surface perturbations leading to air entrainment JF - International Journal of Multiphase Flow N2 - A new formulation for the prediction of free surface dynamics related to the turbulence occurring nearby is proposed. This formulation, altogether with a breakup criterion, can be used to compute the inception of self-aeration in high velocity flows like those occurring in hydraulic structures. Assuming a simple perturbation geometry, a kinematic and a non-linear momentum-based dynamic equation are formulated and forces acting on a control volume are approximated. Limiting steepness is proposed as an adequate breakup criterion. Role of the velocity fluctuations normal to the free surface is shown to be the main turbulence quantity related to self-aeration and the role of the scales contained in the turbulence spectrum are depicted. Surface tension force is integrated accounting for large displacements by using differential geometry for the curvature estimation. Gravity and pressure effects are also contemplated in the proposed formulation. The obtained equations can be numerically integrated for each wavelength, hence resulting in different growth rates and allowing computation of the free surface roughness wavelength distribution. Application to a prototype scale spillway (at the Aviemore dam) revealed that most unstable wavelength was close to the Taylor lengthscale. Amplitude distributions have been also obtained observing different scaling for perturbations stabilized by gravity or surface tension. The proposed theoretical framework represents a new conceptualization of self-aeration which explains the characteristic rough surface at the non-aerated region as well as other previous experimental observations which remained unresolved for several decades. Y1 - 2018 U6 - https://doi.org/10.1016/j.ijmultiphaseflow.2017.12.011 SN - 0301-9322 VL - 100 SP - 127 EP - 142 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Valero, Daniel A1 - Bung, Daniel Bernhard A1 - Erpicum, Sebastien A1 - Peltier, Yann A1 - Dewals, Benjamin T1 - Unsteady shallow meandering flows in rectangular reservoirs: a modal analysis of URANS modelling JF - Journal of Hydro-environment Research N2 - Shallow flows are common in natural and human-made environments. Even for simple rectangular shallow reservoirs, recent laboratory experiments show that the developing flow fields are particularly complex, involving large-scale turbulent structures. For specific combinations of reservoir size and hydraulic conditions, a meandering jet can be observed. While some aspects of this pseudo-2D flow pattern can be reproduced using a 2D numerical model, new 3D simulations, based on the unsteady Reynolds-Averaged Navier-Stokes equations, show consistent advantages as presented herein. A Proper Orthogonal Decomposition was used to characterize the four most energetic modes of the meandering jet at the free surface level, allowing comparison against experimental data and 2D (depth-averaged) numerical results. Three different isotropic eddy viscosity models (RNG k-ε, k-ε, k-ω) were tested. The 3D models accurately predicted the frequency of the modes, whereas the amplitudes of the modes and associated energy were damped for the friction-dominant cases and augmented for non-frictional ones. The performance of the three turbulence models remained essentially similar, with slightly better predictions by RNG k-ε model in the case with the highest Reynolds number. Finally, the Q-criterion was used to identify vortices and study their dynamics, assisting on the identification of the differences between: i) the three-dimensional phenomenon (here reproduced), ii) its two-dimensional footprint in the free surface (experimental observations) and iii) the depth-averaged case (represented by 2D models). KW - coherent structures KW - hydraulic modelling KW - model performance KW - Proper Orthogonal Decomposition KW - Q-criterion Y1 - 2022 U6 - https://doi.org/10.1016/j.jher.2022.03.002 SN - 1570-6443 IS - In Press PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Bung, Daniel Bernhard A1 - Valero, Daniel T1 - Re-aeration on stepped spillways with special consideration of entrained and entrapped air JF - Geosciences N2 - As with most high-velocity free-surface flows, stepped spillway flows become self-aerated when the drop height exceeds a critical value. Due to the step-induced macro-roughness, the flow field becomes more turbulent than on a similar smooth-invert chute. For this reason, cascades are oftentimes used as re-aeration structures in wastewater treatment. However, for stepped spillways as flood release structures downstream of deoxygenated reservoirs, gas transfer is also of crucial significance to meet ecological requirements. Prediction of mass transfer velocities becomes challenging, as the flow regime differs from typical previously studied flow conditions. In this paper, detailed air-water flow measurements are conducted on stepped spillway models with different geometry, with the aim to estimate the specific air-water interface. Re-aeration performances are determined by applying the absorption method. In contrast to earlier studies, the aerated water body is considered a continuous mixture up to a level where 75% air concentration is reached. Above this level, a homogenous surface wave field is considered, which is found to significantly affect the total air-water interface available for mass transfer. Geometrical characteristics of these surface waves are obtained from high-speed camera investigations. The results show that both the mean air concentration and the mean flow velocity have influence on the mass transfer. Finally, an empirical relationship for the mass transfer on stepped spillway models is proposed. Y1 - 2018 SN - 2076-3263 VL - 8 IS - 9 SP - Article number 333 PB - MDPI CY - Basel ER - TY - JOUR A1 - Kramer, Matthias A1 - Valero, Daniel A1 - Chanson, Hubert A1 - Bung, Daniel Bernhard T1 - Towards reliable turbulence estimations with phase-detection probes: an adaptive window cross-correlation technique JF - Experiments in Fluids Y1 - 2019 U6 - https://doi.org/10.1007/s00348-018-2650-9 SN - 1432-1114 VL - 60 EP - Article number 2 PB - Springer CY - Berlin ER - TY - JOUR A1 - Bung, Daniel Bernhard A1 - Crookston, Brian M. A1 - Valero, Daniel T1 - Turbulent free-surface monitoring with an RGB-D sensor: the hydraulic jump case JF - Journal of Hydraulic Research Y1 - 2020 U6 - https://doi.org/10.1080/00221686.2020.1844810 SN - 1814-2079 PB - Taylor & Francis CY - London ER - TY - JOUR A1 - Bung, Daniel Bernhard A1 - Valero, Daniel T1 - Optical flow estimation in aerated flows JF - Journal of Hydraulic Research N2 - Optical flow estimation is known from Computer Vision where it is used to determine obstacle movements through a sequence of images following an assumption of brightness conservation. This paper presents the first study on application of the optical flow method to aerated stepped spillway flows. For this purpose, the flow is captured with a high-speed camera and illuminated with a synchronized LED light source. The flow velocities, obtained using a basic Horn–Schunck method for estimation of the optical flow coupled with an image pyramid multi-resolution approach for image filtering, compare well with data from intrusive conductivity probe measurements. Application of the Horn–Schunck method yields densely populated flow field data sets with velocity information for every pixel. It is found that the image pyramid approach has the most significant effect on the accuracy compared to other image processing techniques. However, the final results show some dependency on the pixel intensity distribution, with better accuracy found for grey values between 100 and 150. Y1 - 2016 U6 - https://doi.org/10.1080/00221686.2016.1173600 VL - 54 IS - 5 SP - 575 EP - 580 PB - Taylor & Francis CY - London ER -