Tullio Tucciarelli

Cost-Benefit Analysis for Hydropower Production in Water Distribution Networks by a Pump as Turbine

AbstractThe use of microhydroelectric plants in urban pipe networks, based on the combination of a pump as turbine (PAT), two regulating valves, and two pressure meters, is proposed along with simple automation rules. Its economic benefit is tested on a small pipe network, where the network geometry as well as the demand coefficient variation in time and space have been inferred from previously collected data and existing analysis. A similar analysis has been also carried out for different scenarios in which the reduction of pipe installation cost due to a diameter reduction is compared with the increased benefit in energy production. The case study shows that a small increment of the pipe installation cost, with respect to the minimum required by the nodes minimum pressure, can lead to a larger benefit for energy production.

A 3-D finite element conjugate gradient model of subsurface flow with automatic mesh generation

Abstract The 3-D flow modelling of groundwater systems of realistic size generally requires a big effort for the preparation of the input data as well as large computational costs. A numerical finite element model (MAITHREE) is developed for the efficient analysis of the steady and unsteady behaviour of natural confined 3-D basins. Starting from an initial triangular grid the code automatically generates a set of tetrahedral elements in each of the geologic units or subunits specified by the user in the vertical profile. The original element incidences list is then rearranged in order to provide conforming 3-D elements throughout the domain. The model is designed with a view to saving much of the labour involved in setting a 3-D grid and to providing flexibility as well as economical convenience through a high computational efficiency. The latter task is achieved by the aid of a solver based on the modified conjugate gradient (MCG) method which has proved to be an excellent technique for the solution of large linear finite element sets of sparse 3-D subsurface equations. Some examples derived from both hypothetical and real-world situations are discussed to illustrate the innovative features of MAITHREE and its computational performance.

A methodology to determine optimal transmissivity measurement locations in groundwater quality management models with scarce field information

A new criterion to determine transmissivity measurement location for groundwater quality management models is proposed. The transmissivity is represented as a random field with known mean and covariance function, conditioned to existing measurements values. The criterion is the minimization of the coefficient of variation of the optimal remediation cost distribution. This distribution is computed numerically by means of Monte Carlo simulations in which each optimal remediation cost is evaluated by solving a deterministic management problem, where the transmissivity is a single realization of the random field. The coefficient of variation is equivalent to the relative error in the prediction of the optimal cost and is shown to be invariant with respect to the actual (unknown) measurement value according to some limiting conditions. In the more general case the coefficient of variation of the optimal cost is less sensitive than the expected optimal cost to the actual measurement value. The proposed criterion becomes a useful tool to determine the “best” transmissivity measurement location when only few measurements have been performed.

A marching in space and time (MAST) solver of the shallow water equations. Part I: The 1D model

A new approach is presented for the numerical solution of the complete 1D and 2D Saint-Venant equations. At each time step, the governing system of Partial Differential Equations (PDEs) is split, using a fractional time step methodology, into a convective prediction system and a diffusive correction system. Convective prediction system is further split into a convective prediction and a convective correction system, according to a specified approximated potential. If a scalar exact potential of the flow field exists, correction vanishes and the solution of the convective correction system is the same solution of the prediction system. A MArching in Space and Time (MAST) technique is used for the solution of the two systems. MAST solves a system of two (in the 1D case) or three (in the 2D case) Ordinary Differential Equations (ODEs) in each computational cell, using for the time discretization a self-adjusting fraction of the original time step. The computational cells are ordered and solved according to the decreasing value of the potential in the convective prediction step and to the increasing value of the same potential in the convective correction step. The diffusive correction system is solved using an implicit scheme, that leads to the solution of a large linear system, with the same order of the cell number, but sparse, symmetric and well conditioned. The numerical model shows unconditional stability with regard of the Courant number

Cross-Flow Turbine Design For Energy Production And Discharge Regulation

AbstractCross-flow turbines are very efficient and cheap turbines that allow a very good cost/benefit ratio for energy production located at the end of conduits carrying water from a water source to a tank. In this paper, a new design procedure for a cross-flow turbine working with a variable flow rate is proposed. The regulation of the head immediately upstream the turbine is faced by adopting a shaped semicircular segment moving around the impeller. The maximum efficiency of the turbine is attained by setting the velocity of the particles entering the impeller at about 2× the velocity of the rotating system at the impeller inlet. If energy losses along the pipe are negligible, the semicircular segment allows always a constant hydraulic head and a constant velocity at the impeller inlet, even with variable flow rate. The decrease of the turbine efficiency along with the inlet surface reduction is first investigated; a design methodology, using also computational fluid dynamics simulations, is then propos...

Discharge estimation in open channels by means of water level hydrograph analysis

A new methodology, based on the synchronous measurement of stage hydrographs in two river sections located some kilometres from each other, was developed to estimate the discharge hydrograph in the upstream section. The methodology is based on the one-parameter calibration of a numerical flow routing algorithm, solving the Saint-Venant equations in diffusive or complete form. The methodology was validated using results of laboratory experiments carried out at the Polytechnic of Bari University. A known discharge hydrograph was generated in the upstream tank of a rectangular flume, where two water level sensors were located. Two different bed materials have been used to account for different roughness coefficients. Eight measured discharge hydrographs have been compared with the hydrographs computed using both a diffusive and a fully dynamic model. The diffusive model provides a good estimate of the measured discharge in the experiments with the highest roughness value.

A new formulation for transmissivity estimation with improved global convergence properties

A new formulation is proposed for the estimation of transmissivity in confined aquifers based on head measurements. This formulation consists of a formulation which minimizes a weighted residual norm of the gradient of the measured and observed head. The weighting is the square of the estimated transmissivity. We show by theoretical analysis and numerical experimentation the improved global convergence properties and characteristics of stability under head measurement uncertainty exhibited by the new formulation. We conclude that the new formulation may be superior to conventional head residual formulations or may be useful as an added term to conventional formulations. The formulation is further demonstrated by a field scale application to the Friuli Aquifer in Italy.

A new algorithm for a robust solution of the fully dynamic Saint-Venant equations

A new procedure for the numerical solution of the fully dynamic shallow water equations is presented. The procedure is a fractional step methodology where the original system is split into two sequential ones. The first system differs from the original one because of the head gradient term, that is treated as constant and equal to the value computed at the end of the previous time step. The solution of this system, called kinematic, is computed in each element using a spatial zero order approximation for both the heads and the flow rates by means of integration of single ODEs. The second system is called diffusive, contains in the momentum equations only the complementary terms and can be easily solved using implicit methods. The major advantages of the methodology are: (1) it guarantees mass conservation; (2) it shows unconditional stability with respect to the Courant number; (3) it can be applied to initially dry domains; (4) it can be applied to closed conduits without the help of the Preissman approximation.

Numerical and experimental investigation of a cross-flow water turbine

ABSTRACT A numerical and experimental study was carried out for validation of a previously proposed design criterion for a cross-flow turbine and a new semi-empirical formula linking inlet velocity to inlet pressure. An experimental test stand was designed to conduct a series of experiments and to measure the efficiency of the turbine designed based on the proposed criterion. The experimental efficiency was compared to that from numerical simulations performed using a RANS model with a shear stress transport (SST) turbulence closure. The proposed semi-empirical velocity formula was also validated against the numerical solutions for cross-flow turbines with different geometries and boundary conditions. The results confirmed the previous hydrodynamic analysis and thus can be employed in the design of the cross-flow turbines as well as for reducing the number of simulations needed to optimize the turbine geometry.

The MAST FV/FE scheme for the simulation of two-dimensional thermohaline processes in variable-density saturated porous media

A novel methodology for the simulation of 2D thermohaline double diffusive processes, driven by heterogeneous temperature and concentration fields in variable-density saturated porous media, is presented. The stream function is used to describe the flow field and it is defined in terms of mass flux. The partial differential equations governing system is given by the mass conservation equation of the fluid phase written in terms of the mass-based stream function, as well as by the advection-diffusion transport equations of the contaminant concentration and of the heat. The unknown variables are the stream function, the contaminant concentration and the temperature. The governing equations system is solved using a fractional time step procedure, splitting the convective components from the diffusive ones. In the case of existing scalar potential of the flow field, the convective components are solved using a finite volume marching in space and time (MAST) procedure; this solves a sequence of small systems of ordinary differential equations, one for each computational cell, according to the decreasing value of the scalar potential. In the case of variable-density groundwater transport problem, where a scalar potential of the flow field does not exist, a second MAST procedure has to be applied to solve again the ODEs according to the increasing value of a new function, called approximated potential. The diffusive components are solved using a standard Galerkin finite element method. The numerical scheme is validated using literature tests.