Jorge Matos

Skimming Flow in the Nonaerated Region of Stepped Spillways over Embankment Dams

Traditionally, research on stepped spillway hydraulics has been focused on the air-water flow region but for the hydraulic design of small embankment dams experiencing relatively large overtopping flows, the nonaerated region can be very important. Empirical formulas are presented for predicting skimming flow properties upstream of the point of inception of air entrainment for 1V:2H sloping stepped spillways, and the location and flow depth at the point of inception. Particular emphasis is placed on the clear-water depth, velocity distribution, and the energy dissipation characteristics in the developing nonaerated flow region. The velocity distribution is well described by a power law. The normalized clear-water depth and the normalized specific energy varied with the relative distance along the spillway and the effect of the normalized critical depth was negligible. Finally, the rate of energy dissipation was small, which has direct implications for the design of the downstream energy dissipator.

Cavitation Potential of Flow on Stepped Spillways

AbstractCavitation on spillways has proven to be an undesirable condition. The formation of vapor-filled bubbles and cavities and their eminent collapse has led to significant damage to major spillway components and appurtenant structures worldwide. Although stepped spillways are thought to be less prone to cavitation damage than smooth spillways, designers continue conservative practices regarding specifying stepped spillways at many sites. Using laboratory experiments in a specialized reduced ambient pressure chamber, cavitation was shown to form on stepped geometries that are representative of typical stepped spillways currently in service. Experiments in a nonaerated closed conduit revealed the strength and extents of the highly intense shear layer that forms above the step tips, and the friction characteristics were determined and compared with results from previous researchers. Advanced techniques for detecting cavitation characteristics along with high-speed videography have given additional insigh...

On the Performance of Velocity Measurement Techniques in Air-Water Flows

Note: [362] Reference LCH-CONF-2002-013 Record created on 2007-04-24, modified on 2016-08-08

Air entrainment onset in skimming flows on steep stepped spillways: an analysis

ABSTRACT We discuss, apply and validate several physics-based criteria for air entrainment into flows on steep stepped spillways, using laboratory observations and numerical results published elsewhere. To the best of our knowledge, this is the first time a validation of these criteria is undertaken for any chute flow in general, and for the skimming flow in steep stepped spillways in particular. To undertake the validation, we employed experimental and numerical data covering a considerable range of volume flow rates and step heights. We observed an overall good performance of most of the criteria, especially taking into account the intrinsic difficulties in defining the time-averaged location and depth of the inception point of air entrainment experimentally. Finally, we present two novel non-dimensional numbers designed to facilitate the physical interpretation of the location of the inception point of air entrainment.

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.

Robust tracking of vessels in oceanographic airborne images

In this paper we present and evaluate an algorithm for tracking vessels in oceanographic airborne image sequences on the visible spectrum. Such sequences are challenging due to sun reflections, wakes, wave crests and fast motions, which significantly degrade the performance of general purpose tracking algorithms. The proposed method is based on state-of-the-art correlation filter tracking complemented with an image segmentation and blob analysis stage. The purpose of this later stage is to re-center the target in the tracking window to compensate for drifts in the correlation filter. We evaluate our proposal using a known benchmark in the field and compare it with general purpose tracking algorithms. Results show that our method beats the general purpose state-of-the-art tracking algorithms in the airborne maritime scenario both in performance and in computation time.

Experimental repetitions and blockage of large stems at ogee crested spillways with piers

ABSTRACT Large wood is often transported by rivers into reservoirs during heavy rainfall events. When a critical section like a spillway is blocked and discharge capacity reduced, an uncontrolled increase of the reservoir water level may occur. This study aims to statistically analyse the importance of repetitions for the accuracy of experimental campaigns when studying blocking probabilities at ogee crested spillways equipped with piers. Systematic and reliable estimations based on physical models are critical for developing preventive measures against large wood blockage. Two statistical methods have been described and applied to calculate confidence intervals. A minimum number of repetitions for a maximum acceptable error is recommended for blocking probabilities. The minimum number of experimental repetitions has been statistically justified in accordance with a reasonable use of resources for experimental campaigns. In addition, a maximum acceptable level of error is proposed as a common metric of accuracy in large wood studies.

Numerical modelling of air-water flows in sewer drops

The presence of dissolved oxygen (DO) in water flows is an important factor to ensure the aerobic conditions recognised as beneficial to preventing the occurrence of detrimental effects. The incorporation of DO in wastewater flowing in sewer systems is a process widely investigated in order to quantify the effect of continuous reaeration through the air-liquid interface or air entrained due to the presence of singularities such as drops or junctions. The location of sewer drops to enhance air entrainment and subsequently reaeration is an effective practice to promote aerobic conditions in sewers. In the present paper, vertical drops, backdrops and stepped drops were modelled using the computational fluid dynamics (CFD) code FLOW-3D® to evaluate the air-water flows due to the turbulence induced by the presence of these types of structures. An assessment of the hydraulic variables and an analysis of the air entrainment based on the available experimental studies were carried out. The results of the CFD models for these structures were validated using measurements of discharge, pressure head and water depth obtained in the corresponding physical models. A very good fit was obtained for the hydraulic behaviour. After validation of numerical models, analysis of the air entrainment was carried out.

A Dataset for Airborne Maritime Surveillance Environments

This paper presents a data set with surveillance imagery over the sea captured by a small size UAV. This data set presents the object examples ranging from cargo ships, small boats, life rafts to hydrocarbon slick. The video sequences were captured using different types of cameras, at different heights, and different perspectives. The data set also contains thousands of labels with positions of objects of interest. This was only possible to achieve with the labeling tool also described in this paper. Additionally, using standard evaluation frameworks, we establish a baseline of results using algorithms developed by the authors, which are better adapted to the maritime environment.

Performance of a Combined Three-Hole Conductivity Probe for Measuring Velocity in Air-Water Jets

A three-hole pressure probe with back-flushing, combined with an electrical conductivity probe, was developed to perform the simultaneous measurement of the magnitude and direction of the velocity vector in complex air-water flows, where the flow direction is not known prior to data acquisition. Such situation typically occurs inside and around the rotor of Cross-Flow turbines. A calibration rig was also built aiming at the calibration of the combined probe (both conductivity and three-hole pressure components), placed in a position similar to that adopted for flow measurements in a Cross-Flow turbine rotor. After concluding the combined probe calibration procedure, the probe was tested in the outside region of the turbine rotor. This paper addresses the probe development and calibration, with particular emphasis on the velocity measurements. The experimental results are encouraging towards the use of the combined probe in complex air-water flows, namely in Cross-Flow turbines.