Daniel B. Bung

Developing flow in skimming flow regime on embankment stepped spillways

In hydraulic engineering, stepped spillways are effective structures in regard to both energy dissipation and re-aeration processes. It is well known that flow depths and flow velocities generally depend on the spillway geometry in terms of slope and step-induced macro-roughness height on the one hand and the discharge on the other hand. Moreover, these parameters affect the air entrainment process and hence, the amount and the size of entrained air bubbles which are directly linked to the specific air–water interface being of particular significance for oxygen transfer. In this study, physical model investigations on a stepped spillway scaled 1:10 of varying step height, chute angle and discharge are conducted with the objective to analyse the development of these air–water flow properties in the non-uniform flow region. Measurements of air–water mixture velocity and air concentration are carried out by the use of an intrusive double-tip conductivity probe. It is found that the step-induced macro-roughness has a differing effect on particular flow characteristics.

Non-intrusive detection of air–water surface roughness in self-aerated chute flows

Stepped spillways are known to enhance the energy dissipation potential when compared with common smooth invert chutes. In the skimming flow regime, the self-aerated flow becomes more chaotic and surface waves (referred to as air–water surface roughness) are enhanced when the discharge decreases. In this study, experiments on smooth invert and stepped chutes models with a slope of 1: 2 were conducted to characterize this surface roughness by the use of a high-speed camera and ultrasonic sensor. It was found that the amount of entrapped air – at the water level where the air concentration is 90% – is reduced when a stepped spillway is considered. With decrease in step height (and on smooth invert chutes), entrapped air became more relevant. Wave heights increased with increase in step height and exceeded significantly the characteristic water level with 90% air concentration which was detected by a conductivity probe. A wide range of wave frequencies indicated the turbulent structure of stepped spillway flows.

Initial stage of two-dimensional dam-break waves: laboratory versus VOF

Since several decades, dam-break waves have been of main research interest. Mathematical approaches have been developed by analytical, physical and numerical models within the past 120 years. During the past 10 years, the number of research investigations has increased due to improved measurement techniques as well as significantly increased computer memories and performances. In this context, the present research deals with the initial stage of two-dimensional dam-break waves by comparing physical and numerical model results as well as analytical approaches. High-speed images and resulting particle image velocimetry calculations are thereby compared with the numerical volume-of-fluid (VOF) method, included in the commercial code FLOW-3D. Wave profiles and drag forces on placed obstacles are analysed in detail. Generally, a good agreement between the laboratory and VOF results is found.

Optical flow estimation in aerated flows

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.

Numerical investigation of USBR type III stilling basin performance downstream of smooth and stepped spillways

Recent systematic studies on air-water flows have included stepped spillways. However, to date, little has been investigated about how the hydraulic conditions on the stepped spillway may affect the design of traditional energy dissipation structures. In this paper, both smooth chute and stepped chute configurations terminating with the USBR type III stilling basin are tested by means of numerical modelling, allowing a qualitative comparison. Unsteady RANS equations have been employed together with VOF and RNG k-e for free surface tracking and turbulence modeling, respectively. Eight different Froude numbers (F) ranging from 3.1 to 9.5 have been analyzed for a type III basin designed for F = 8, following recent studies conducted in a physical model by Reclamation. The basin flow structure is discussed for both smooth chute and stepped chute cases. Additionally, the modelled basin has been tested for design and adverse hydraulic conditions, obtaining a detailed insight of the role of each basin element and their adapting roles when insufficient tail water conditions exist.

Application of the Optical Flow Method to Velocity Determination in Hydraulic Structure Models

Optical flow estimation is used in Computer Vision for detection of moving obstacles in a sequence of images. The optical flow (OF) is defined as the displacement of brightness patterns between two sequent images. In this paper, this method is applied to high-speed images taken in hydraulic structure models for determination of velocity fields. Water is seeded with tracer particles in two cases and is self-aerated in a third case. It will be shown that the OF method gives valuable results that compare well to other velocity measurements, e.g. by Particle Image Velocimetry. The OF method is generally more time-consuming compared to PIV, but an advantage is given by the density of information when using a so-called global method, as velocity information is obtained at every pixel location.

Sensitivity of turbulent Schmidt number and turbulence model to simulations of jets in crossflow

Environmental discharges have been traditionally designed by means of cost-intensive and time-consuming experimental studies. Some extensively validated models based on an integral approach have been often employed for water quality problems, as recommended by USEPA (i.e.: CORMIX). In this study, FLOW-3D is employed for a full 3D RANS modelling of two turbulent jet-to-crossflow cases, including free surface jet impingement. Results are compared to both physical modelling and CORMIX to better assess model performance. Turbulence measurements have been collected for a better understanding of turbulent diffusions parameter sensitivity. Although both studied models are generally able to reproduce jet trajectory, jet separation downstream of the impingement has been reproduced only by RANS modelling. Additionally, concentrations are better reproduced by FLOW-3D when the proper turbulent Schmidt number is used. This study provides a recommendation on the selection of the turbulence model and the turbulent Schmidt number for future outfall structures design studies. Accuracy assessment of FLOW-3D and CORMIX for 3D modelling of complex environmental discharges.RNG kź-źe and kź-źω turbulence model performance analysis for a jet to crossflow study including free surface impingement.Sensitivity analysis of turbulent Schmidt number for contaminant modelling with RANS equations.High-speed camera analysis of Zone of Flow Establishment and usage of imaging techniques.

Laboratory Models of Free-Surface Flows

Hydraulic modeling is the classical approach to investigate and describe complex fluid motion. Many empirical formulas in the literature used for the hydraulic design of river training measures and structures have been developed using experimental data from the laboratory. Although computer capacities have increased to a high level which allows to run complex numerical simulations on standard workstation nowadays, non-standard design of structures may still raise the need to perform physical model investigations. These investigations deliver insight into details of flow patterns and the effect of varying boundary conditions. Data from hydraulic model tests may be used for calibration of numerical models as well. As the field of hydraulic modeling is very complex, this chapter intends to give a short overview on capacities and limits of hydraulic modeling in regard to river flows and hydraulic structures only. The reader shall get a first idea of modeling principles and basic considerations. More detailed information can be found in the references.

Hybrid Investigation on the Hydraulic Performance of a New Trapezoidal Fishway

The current study presents a new type of vertical slot fishway. The main difference of this trapezoidal fishway compared to the standard design of a vertical slot fishway remains in the separation of the pools into two zones: the migration corridor and the energy dissipation zone. The structure is first investigated in a physical model to optimize the training walls and slot geometry in order to avoid recirculation of the flow. Velocity and flow depth data from experimental flow measurements is later compared to the three-dimensional numerical model which provides a deeper insight into the flow field. The proposed design is found to avoid large vortices within the migration corridor. Moreover, uniform flow conditions are also found within the energy dissipation zone, thus providing an alternative corridor for fish passage.

Three-dimensional Flow Structure Inside the Cavity of a Non-aerated Stepped Chute

Accurate energy dissipation estimation and improved knowledge on stepped spillways and stepped revetments flow structure may allow safer design of hydraulic and coastal structures. In this study, an ADV Vectrino Profiler has been used to obtain dense observations of the three-dimensional flow structure occurring inside a cavity of 20 cm to 10 cm (length to height) for four flow cases. The obtained friction factors show a strong inverse dependence on the Reynolds number. The displacement length also shows a reduction with increasing Reynolds number, which may indicate that the flow “feels” the cavity more at smaller streamwise velocities. Streamwise and normalwise velocities reveal both a turbulent boundary layer type of flow (upper flow region) and a jet impact and recirculation inside of the cavity. Spanwise median velocities allowed insight on the uncertainty levels of the ADV Vectrino Profiler measurements.