@article{LeandroBungCarvalho2014,
  author    = {Leandro, J. and Bung, Daniel B. and Carvalho, R.},
  title     = {Measuring void fraction and velocity fields of a stepped spillway for skimming flow using non-intrusive methods},
  series = {Experiments in fluids},
  journal   = {Experiments in fluids},
  number    = {55},
  publisher = {Springer Nature},
  address   = {Heidelberg},
  issn      = {0723-4864 (Print) ; 1432-1114 (Online)},
  doi       = {10.1007/s00348-014-1732-6},
  pages     = {Art. 1732},
  year      = {2014},
  language  = {en}
}
@article{Bung2013,
  author    = {Bung, Daniel B.},
  title     = {Non-intrusive detection of air-water surface roughness in self-aerated chute flows},
  series = {Journal of hydraulic research},
  volume    = {Vol. 51},
  journal   = {Journal of hydraulic research},
  number    = {Iss. 3},
  publisher = {Taylor \& Francis},
  address   = {London},
  issn      = {1814-2079 (E-Journal); 0022-1686 (Print)},
  pages     = {322 -- 329},
  year      = {2013},
  language  = {en}
}
@article{OertelBung2015,
  author    = {Oertel, Mario and Bung, Daniel B.},
  title     = {Numerische Str{\"o}mungssimulationen von Fließgew{\"a}ssern : Praxisanwendungen und zuk{\"u}nftige Entwicklungen},
  series = {Korrespondenz Wasserwirtschaft : KW},
  volume    = {8},
  journal   = {Korrespondenz Wasserwirtschaft : KW},
  number    = {H. 3},
  publisher = {Gesellschaft zur F{\"o}rderung der Abwassertechnik},
  address   = {Hennef},
  issn      = {1616-430X},
  pages     = {177 -- 182},
  year      = {2015},
  language  = {de}
}
@article{ZhangValeroBungetal.2018,
  author    = {Zhang, G. and Valero, Daniel and Bung, Daniel B. and Chanson, H.},
  title     = {On the estimation of free-surface turbulence using ultrasonic sensors},
  series = {Flow Measurement and Instrumentation},
  volume    = {60},
  journal   = {Flow Measurement and Instrumentation},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0955-5986},
  doi       = {10.1016/j.flowmeasinst.2018.02.009},
  pages     = {171 -- 184},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{BungValero2016,
  author    = {Bung, Daniel B. and Valero, Daniel},
  title     = {Optical flow estimation in aerated flows},
  series = {Journal of Hydraulic Research},
  volume    = {54},
  journal   = {Journal of Hydraulic Research},
  number    = {5},
  publisher = {Taylor \& Francis},
  address   = {London},
  doi       = {10.1080/00221686.2016.1173600},
  pages     = {575 -- 580},
  year      = {2016},
  abstract  = {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.},
  language  = {en}
}
@article{ValeroSchalkoFriedrichetal.2021,
  author    = {Valero, Daniel and Schalko, Isabella and Friedrich, Heide and Abad, Jorge D. and Bung, Daniel B. and Donchyts, Gennadii and Felder, Stefan and Ferreira, Rui M. L. and Hohermuth, Benjamin and Kramer, Matthias and Li, Danxun and Mendes, Luis and Moreno-Rodenas, Antonio and Nones, Michael and Paron, Paolo and Ruiz-Villanueva, Virginia and Wang, Ruo-Qian and Franca, Mario J.},
  title     = {Pathways towards democratization of hydro-environment observations and data},
  series = {Iahr White Paper Series},
  journal   = {Iahr White Paper Series},
  number    = {1},
  publisher = {International Association for Hydro-Environment Engineering and Research (IAHR)},
  pages     = {1 -- 9},
  year      = {2021},
  language  = {en}
}
@article{BungValero2018,
  author    = {Bung, Daniel B. and Valero, Daniel},
  title     = {Re-aeration on stepped spillways with special consideration of entrained and entrapped air},
  series = {Geosciences},
  volume    = {8},
  journal   = {Geosciences},
  number    = {9},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2076-3263},
  pages     = {Article number 333},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{ValeroBung2018,
  author    = {Valero, Daniel and Bung, Daniel B.},
  title     = {Reformulating self-aeration in hydraulic structures: Turbulent growth of free surface perturbations leading to air entrainment},
  series = {International Journal of Multiphase Flow},
  volume    = {100},
  journal   = {International Journal of Multiphase Flow},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0301-9322},
  doi       = {10.1016/j.ijmultiphaseflow.2017.12.011},
  pages     = {127 -- 142},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{ValeroChansonBung2020,
  author    = {Valero, Daniel and Chanson, Hubert and Bung, Daniel B.},
  title     = {Robust estimators for free surface turbulence characterization: A stepped spillway application},
  series = {Flow Measurement and Instrumentation},
  volume    = {76},
  journal   = {Flow Measurement and Instrumentation},
  number    = {Art. 101809},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0955-5986},
  doi       = {10.1016/j.flowmeasinst.2020.101809},
  year      = {2020},
  abstract  = {Robust estimators are parameters insensitive to the presence of outliers. However, they presume the shape of the variables' probability density function. This study exemplifies the sensitivity of turbulent quantities to the use of classic and robust estimators and the presence of outliers in turbulent flow depth time series. A wide range of turbulence quantities was analysed based upon a stepped spillway case study, using flow depths sampled with Acoustic Displacement Meters as the flow variable of interest. The studied parameters include: the expected free surface level, the expected fluctuation intensity, the depth skewness, the autocorrelation timescales, the vertical velocity fluctuation intensity, the perturbations celerity and the one-dimensional free surface turbulence spectrum. Three levels of filtering were utilised prior to applying classic and robust estimators, showing that comparable robustness can be obtained either using classic estimators together with an intermediate filtering technique or using robust estimators instead, without any filtering technique.},
  language  = {en}
}
@article{ValeroChansonBung2019,
  author    = {Valero, Daniel and Chanson, Hubert and Bung, Daniel B.},
  title     = {Robust estimators for turbulence properties assessment},
  pages     = {1 -- 24},
  year      = {2019},
  language  = {en}
}