José Vazquez

Two-Dimensional Simulation of Subcritical Flow at a Combining Junction: Luxury or Necessity?

Classically, in open-channel networks, the flow is numerically approximated by the one-dimensional Saint Venant equations coupled with a junction model. In this study, a comparison between the one-dimensional (1D) and two-dimensional (2D) numerical simulations of subcritical flow in open-channel networks is presented and completely described allowing for a full comprehension of the modeling of water flow. For the 1D, the mathematical model used is the 1D Saint Venant equations to find the solution in branches. For junction, various models based on momentum or energy conservation have been developed to relate the flow variables at the junction. These models are of empirical nature due to certain parameters given by experimental results and moreover they often present a reduced field of validity. In contrast, for the 2D simulation, the junction is discretized into triangular cells and we simply apply the 2D Saint Venant equations, which are solved by a second-order finite-volume method. In order to give an answer to the question of luxury or necessity of the 2D approach, the 1D and 2D numerical results for steady flow are compared to existing experimental data.

New Approach for Predicting Flow Bifurcation at Right-Angled Open-Channel Junction

An unsteady mathematical model for predicting flow divisions at a right-angled open-channel junction is presented. Existing dividing models depend on a prior knowledge of a constant flow regime. In addition, their strong nonlinearity does not guarantee compatibility with the St. Venant solutions in the context of an internal boundary condition treatment. Assuming zero crest height at the junction region, a side weir model explicitly introduced within the one-dimensional St. Venant equations is used to cope with the two-dimensional pattern of the flow. An upwind implicit numerical solver is employed to compute the new governing equations. The performance of the proposed technique in predicting super-, trans-, and subcritical flow bifurcations is illustrated by comparing with experimental data and/or theoretical predictions. In all the tests, lateral-to-upstream discharge ratios ( Rq ) are successfully reproduced by the present technique with a maximum error magnitude of less than 9%.

Detailed Velocity and Concentration Profiles Measurement During Activated Sludge Batch Settling Using an Ultrasonic Transducer

Activated sludge settling is a complex process requiring a thorough experimental study. It is, however, difficult to obtain information on the inner behavior of the sludge blanket without disturbing it. Optical methods are inefficient due to the opacity of sludge suspensions. This paper focuses on the investigation of activated sludge batch settling using a non-invasive method based on an ultrasonic transducer. The treatment of the signal allowed obtaining data on the settling velocity and concentration profiles inside of the suspension. The different settling regimes were clearly shown by the results on the basis of the shape of the settling velocity isolines. The precision of the gathered data is adapted to the calibration and validation of numerical models. The material used in the experimental setup can be installed in actual wastewater treatment plants.

Optimization of a hydrodynamic separator using a multiscale computational fluid dynamics approach.

This article deals with the optimization of a hydrodynamic separator working on the tangential separation mechanism along a screen. The aim of this study is to optimize the shape of the device to avoid clogging. A multiscale approach is used. This methodology combines measurements and computational fluid dynamics (CFD). A local model enables us to observe the different phenomena occurring at the orifice scale, which shows the potential of expanded metal screens. A global model is used to simulate the flow within the device using a conceptual model of the screen (porous wall). After validation against the experimental measurements, the global model was used to investigate the influence of deflectors and disk plates in the structure.

Head–discharge relationship of Venturi flumes: from long to short throats

Venturi flumes are used in open-channels to determine the discharge by the measurement of a water level. The head–discharge relationship of such measurement devices can be determined following the critical flow theory, but this approach is only valid if the length of the throat is sufficiently long. Based on numerical simulations, this study determines the limit of application of this traditional critical flow theory. A discharge coefficient is introduced to take account of the effect of slope and curvature of the streamlines on the head–discharge relationship when the throat is short. An analysis of the different factors affecting this coefficient is provided. An empirical equation is proposed to evaluate this coefficient as a function of the upstream energy head, the length of the throat, the breadth of the throat and the radius of curvature of the inlet arc-shaped constriction. This correction is valid up to a deviation of 8% compared with the traditional approach and can be used in engineering practice.

Bed turbulent kinetic energy boundary conditions for trapping efficiency and spatial distribution of sediments in basins

The purpose of this study is to develop and validate a numerical tool for evaluating the performance of a settling basin regarding the trapping of suspended matter. The Euler-Lagrange approach was chosen to model the flow and sediment transport. The numerical model developed relies on the open source library OpenFOAM®, enhanced with new particle/wall interaction conditions to limit sediment deposition in zones with favourable hydrodynamic conditions (shear stress, turbulent kinetic energy). In particular, a new relation is proposed for calculating the turbulent kinetic energy threshold as a function of the properties of each particle (diameter and density). The numerical model is compared to three experimental datasets taken from the literature and collected for scale models of basins. The comparison of the numerical and experimental results permits concluding on the models capacity to predict the trapping of particles in a settling basin with an absolute error in the region of 5% when the sediment depositions occur over the entire bed. In the case of sediment depositions localised in preferential zones, their distribution is reproduced well by the model and trapping efficiency is evaluated with an absolute error in the region of 10% (excluding cases of particles with very low density).