Sauro Manenti

SPH Simulation of Sediment Flushing Induced by a Rapid Water Flow

This paper shows an advanced application of the smoothed particle hydrodynamics (SPH) method to the numerical modeling of noncohesive sediment flushing aiming at the setup of a reliable engineering tool for the prediction of the coupled water-sediment dynamics at the bottom of an artificial reservoir. Both liquid and granular materials are modeled as weakly compressible viscous fluids, whose motion results from the numerical solution of the continuity and momentum equations discretized according to standard SPH formulation. The effect of two alternative erosion criteria on the description of the failure mechanism of bottom sediments is analyzed. These criteria are based, respectively, on Mohr-Coulomb yielding criterion and Shields theory. A sensitivity analysis is performed in order to assess, for both criteria, the influence of the model parameters on the simulation of the erosion process; the method is eventually validated by comparing numerical results with the experimental data obtained in a two-dimen...

SPH Modeling of Solid Boundaries Through a Semi-Analytic Approach

Abstract: This paper presents a general semi-analytic approach for modeling solid boundaries in the SPH method: boundaries are here considered as a material continuum with a suitable distribution of velocity and pressure; their contributions to each term of the SPH mass and momentum equations can be expressed in terms of a suitable integral extended to the part of the sphere of influence of the particle delimited by the boundary surface. Analytical details with reference to a slightly compressible viscous Newtonian fluid in three dimensions are given. The validity of the method is checked by comparing the obtained numerical results with available experimental data in a benchmark flow case.

STUDY OF RELATIVE ROLES OF NONLINEARITY AND DEPTH REFRACTION IN WAVE SPECTRUM EVOLUTION IN SHALLOW WATER

Abstract For modelling a series of depth profiles covering the relative depth parameter interval 2>kph>0.3, evolution of two-dimensional gravity wave spectra is calculated in the frame of three-wave quasi-kinetic approximation derived by Zaslavskii and Polnikov (1988). The relative impact of refraction and nonlinearity on a change of two-dimensional spectra shape for gravity waves is estimated. It is shown that on the background of refraction impact on the spectrum shape, the three-wave nonlinearity results in a remarkable change of angular and frequency distribution for a wave energy spectrum. Herewith, in the spectral peak domain the nonlinearity reduces the value of the angular narrowness parameter by 20–30%, counteracting the refraction during the wave propagation into a shoal zone. In contrast to the high frequency domain of the spectrum, the angular narrowness parameter is increased due to the nonlinearity. For this reason, the nonlinearity can result in more than 10% change of wave energy in a shallow water zone with respect to the linear wave evolution case. These conclusions were checked by using the SWAN model under the same conditions. It was found that the SWAN model describes some of the main peculiarities of nonlinear waves in shallow water. Some recommendations were made to elaborate the three-wave nonlinear term in the source function of the SWAN model.

Rheology and Microbiology of Sludge from a Thermophilic Aerobic Membrane Reactor

A thermophilic aerobic membrane reactor (TAMR) treating high-strength COD liquid wastes was submitted to an integrated investigation, with the aim of characterizing the biomass and its rheological behaviour. These processes are still scarcely adopted, also because the knowledge of their biology as well as of the physical-chemical properties of the sludge needs to be improved. In this paper, samples of mixed liquor were taken from a TAMR and submitted to fluorescent in situ hybridization for the identification and quantification of main bacterial groups. Measurements were also targeted at flocs features, filamentous bacteria, and microfauna, in order to characterize the sludge. The studied rheological properties were selected as they influence significantly the performances of membrane bioreactors (MBR) and, in particular, of the TAMR systems that operate under thermophilic conditions (i.e., around 50°C) with high MLSS concentrations (up to 200 gTS L−1). The proper description of the rheological behaviour of sludge represents a useful and fundamental aspect that allows characterizing the hydrodynamics of sludge suspension devoted to the optimization of the related processes. Therefore, in this study, the effects on the sludge rheology produced by the biomass concentration, pH, temperature, and aeration were analysed.

Integrated RTD − CFD Hydrodynamic Analysis for Performance Assessment of Activated Sludge Reactors

This work shows an integrated analysis method for hydrodynamic investigation of biological reactors for wastewater treatment in order to detect the amount and position of possible defects, such as bypass and dead volume, influencing process efficiency. To reach such a goal, the proposed methodology integrates Residence Time Distribution (RTD) analysis, providing global hydrodynamic information, with Computational Fluid Dynamics (CFD) analysis, showing local flow conditions. RTD analysis was performed through a time-discretized analytical model of in-series mixed-flow reactors with dead volumes and bypass. CFD analysis was carried out with a 3D finite volume model allowing the numerical solution of turbulent incompressible isothermal flow. The method was tested on a scale activated sludge pilot plant with pre-denitrification scheme made of two in-series tanks. Hydrodynamic tests were performed carrying out the stimulus-response experiment using clear water inside the reactor and lithium chloride as a tracer. Two operating conditions of practical interest were investigated: (i) no mixing and (ii) upstream mixing. The RTD analysis of the outflow curve of lithium concentration from the experiment allowed detecting: a) no bypass for both operating conditions, and b) 5% dead volume for condition (i). These results were confirmed by the CFD analysis that allowed localizing the position of the dead volume. The integrated analysis proved to be effective for detection of both types and position of hydrodynamic defects. Therefore the proposed method can be adopted for performance assessment of activated sludge reactors and subsequent improvement of their efficiency.