1,720,967 research outputs found
A linearized model for estimating the performance of sea wave energy converters (REWEC)
No full textA realistic performance analysis of oscillating water column wave energy converters (WECs) addresses to a set of non-linear differential equations that need to be integrated in time, by using a stochastic approach, under the hypothesis of random wind-generated sea waves, for all the sea states which characterize the location of the system. Non-linearities of the differential equations have several origins: •minor and major losses of the unsteady flow of water and air;•compressibility of air and heat exchange with the walls of the air chamber;•non-linear characteristics of the turbine. Under the hypothesis of random sea waves with Gaussian distribution, the authors propose an original methodology for linearizing the differential equations that describe the flow motion inside a wholly submerged WEC. Under such hypothesis, the linearized model can be used for predicting the power output by means of the calculations in the frequency domain and for control design. The developed methodology has been applied to the estimation of the performance of the new "resonant sea wave energy converters", called REWEC, patented by Boccotti in 1998, and consisting of several caissons, characterized by a structure similar to the caissons of the traditional breakwaters and placed on the seabed, close one to each other, to form a submerged breakwater. Each caisson is connected to a vertical duct wholly beneath the sea level, where a hydraulic Wells turbine is placed. The matching between turbine and resonance characteristic of the system is carefully analysed in order to maximize the energy conversion efficiency. Some results, given for a small installation in the Mediterranean sea, confirm that the REWEC system is able to absorb a large share of the incident wave energy due to a very simple regulation system which permits the tuning on sea states with different significant heights
Experimental verification of the stochastic model for predicting the performance of Oscillating Water Colums devices
No full textStochastic models are often used to evaluate the
average power output of an oscillating-water-column
(OWC) wave power plant equipped with a Wells
turbine and for defining optimal control criteria of the
system. The application of the stochastic model to
OWC devices is based on the hypothesis that the
dynamic behavior of the system can be modeled by a
set of linear differential equations and that the sea
surface elevation, acting as an input, has a Gaussian
probability density function. Under such hypotheses,
from the theory of the random processes of the linear
systems, it comes that the outputs of the system, such
as the pressure in the chamber and the turbine flow
coefficient, have a gaussian distribution. Actually, there
are several non-linear phenomena that can alter the
linear behavior of a OWC device:
- minor and major losses of the unsteady flow of
water and air;
- compressibility of air and heat exchange with the
walls of the air chamber;
- non-linear characteristics of the turbine.
The stochastic model can be applied if such nonlinearities
have, on the whole, limited effects or if a
specific procedure able to take them into account is
adopted, as suggested by the authors in previous
papers.
In the Authors’ knowledge, no experimental
validation of the application of the stochastic model to
OWC devices are present in the open literature.
This work, making use of data gathered during the
experiment on a 1:10 scale model of a ocean OWC
breakwater, put at the sea off the beach of Reggio
Calabria, aims at verifying that the energy conversion
process inside the OWC can be actually described as a
gaussian process. To this purpouse, the frequency
distribution of the main physical parameters, relevant to
the system dynamics, are evaluated. Moreover, in order
to characterize the behavior of the Wells turbine, the
experimental values of the time averaged turbine torque
and pressure drop are evaluated as a function of the variance of the flow coefficient. The results show a
very high level of correlation and a very good
agreement with those that can be obtained from the
application of the stochastic model, using as an input
the characteristic curves of the turbine, yelded in the
unsteady flow
Solitary wave loads on submerged breakwater: Laboratory tests
No full textA solitary wave almost halves its propagation speed passing over a submerged breakwater for coast defense. This is the basic assumption made by Filianoti & Piscopo (2008) for their calculation of horizontal wave loads on the breakwater. They calculated this slowing down, through a BEM model, on assuming that it has the same value both for
a periodic and a solitary wave. Once estimated the speed slowing down, it is straightforward to obtain solitary wave loads thorugh the calculation of the Froude-Krilov force.
Laboratory tests carried out on a small scale model of submerged breakwater interacting with solitary waves, permit us to experimentally reproduce the phenomenon, to check whether the speed slowing down exists, and to measure it
The FLO Diffusive 1D-2D Model for Simulation of River Flooding
Full text linkAn integrated 1D-2D model for the solution of the diffusive approximation of the shallow water equations, named FLO, is proposed in the present paper. Governing equations are solved using the MArching in Space and Time (MAST) approach. The 2D floodplain domain is discretized using a triangular mesh, and standard river sections are used for modeling 1D flow inside the section width occurring with low or standard discharges. 1D elements, inside the 1D domain, are quadrilaterals bounded by the trace of two consecutive sections and by the sides connecting their extreme points. The water level is assumed to vary linearly inside each quadrilateral along the flow direction, but to remain constant along the direction normal to the flow. The computational cell can share zero, one or two nodes with triangles of the 2D domain when lateral coupling occurs and more than two nodes in the case of frontal coupling, if the corresponding section is at one end of the 1D channel. No boundary condition at the transition between the 1D-2D domain has to be solved, and no additional variable has to be introduced. Discontinuities arising between 1D and 2D domains at 1D sections with a top width smaller than the trace of the section are properly solved without any special restriction on the time step
Large eddy simulation within the smoothed particle hydrodynamics: Applications to multiphase flows
No full textIn this paper, the large eddy simulation (LES) model introduced in the smoothed particle hydrodynamics (SPH) by Di Mascio et al. [Phys. Fluids 29, 035102 (2017)] and called d-LES-SPH, is extended to treat multiphase flows. This is achieved by modifying the multiphase d-SPH by Hammani et al. [Comput. Methods Appl. Mech. Eng. 368, 113189 (2020)] by switching the viscous and density diffusion constants to dynamic variables evaluated as turbulence closure terms. The equation for energy conservation is also written for the presented model. The validation is performed for two-dimensional problems, by comparison with other established SPH solvers, with a finite volume method solver based on the turbulence closure corresponding to that adopted for the Lagrangian scheme, and with experimental data. The first test case investigated is a modified Taylor-Green vortex in which the introduction of macro-bubbles of a lighter fluid phase inside the domain is considered. In the second test case, a more violent problem involving wave breaking and splashing dynamics is analyzed. In the final test, the dynamic of a sloshing problem is reproduced. An analysis of turbulence resolution is conducted by considering modeled and resolved turbulent kinetic energies, as well as viscous dissipation and turbulent viscosity dissipation
A Banki-Michell turbine for in-line water supply systems
No full textThe design of a novel Banki-Michell type turbine, to be located in existing water pipelines, is proposed. The turbine has a very efficient diffuser which allows the turbine to be compact and, most important, to have in-line flanges for minimal piping modifications at existing sites. This turbine combines a simple geometry with stable efficiency in a wide range of water discharges. The design procedure estimates the outer diameter of the impeller, its width and the geometry of the diffuser. A series of experimental tests has been carried out to measure the efficiency of the proposed turbine prototype. The turbine was tested in two different configurations, with and without rotational velocity regulation. The results of the tests showed that rotational velocity adaptation improves turbine efficiency in a wide range of flow rates. A significant reduction of the optimal velocity ratio, with respect to the predicted two values, is likely due to 3D effects not accounted for in the design procedure. A simple way to roughly estimate this extra energy dissipation is derived from experimental data
Fluid dynamics inside a U-shaped oscillating water column (OWC): 1D vs. 2D CFD model
No full textSeveral mathematical models have been proposed in the last decades, focusing on peculiar aspects of dynamic behaviors of OWCs Wave Energy Converters (WEC). Computational fluid dynamics (CFD) studies have been widely diffused in the recent past, thanks to the strong increasing of computing power. Numerical wave flumes, both 2D or 3D, are become refined tools of investigations. However, there are tasks performed effectively and conveniently through 1D mathematical models, which require much less time of setup and running, in respect to CFD. Tuning the eigenperiod of a plant with most energetic waves is the most important of these tasks. It requires several trials, changing the size of some geometric parts of the device. In this work, we improved a conventional 1D mathematical model, by removing the assumption of polytropic transformation of the air mass inside the plenum, which is physically not consistent, and considering the actual asymmetry of thermodynamics during inhalation and exhalation phases. The results were validated through a numerical experiment carried out in a 2D numerical wave flume, considering a U-OWC at a full scale, achieving an excellent agreement of pressure and temperature variations in the plenum chamber
Coupled Hydraulic and Electronic Regulation for Banki Turbines
No full textThe potential benefit of coupling hydraulic and electronic regulation to maximize the energy production of a Bank turbine in hydraulic plants is analyzed and computed with reference to a specific case. Design criteria of the Banki turbine inside hydraulic plants are first summarized, along with the use of hydraulic regulation in the case of constant water head and variable discharge at the end of aqueducts feeding water distribution systems. Optimal turbine impeller rotational speed is derived and traditional, as well as innovative systems for electricity production according to controlled rotational speed of the generator are presented. The study case at the purification plant named Risalaimi, in Italy, is analyzed, and the potential production of energy along the year is computed according to the known monthly average demand and two possible choices: the choice of hydraulic regulation only, called CFT1, and the choice of coupled hydraulic and electric regulations, called CFT2. The Return time of Capital Investment (RCI) is then computed for both the CFT1 and CFT2 cases. The result is that the CFT2 choice provides an increment of the total produced energy, along with an increment of about 30% of the corresponding RCI
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