186,172 research outputs found
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
In field measurements on small scale OWC device
No full textA 1:10 scale model of an ocean OWC breakwater was realised and put at the sea off the beach of Reggio Calabria (Italy). The breakwater (about 16 m long and 3.5 m height) was placed on a 2.1 m bottom depth, and embodied the REWEC wave energy converter (Boccotti et al.,2007, J. of Ocean Engineering, 34, 820-841). A small scale Wells turbine was installed onto the
central caisson of the breakwater. Two different experiments were carried out. The first one aimed to test the energy absorption capabilities of the system and to analyse the waves-absorber interaction mechanism. The second experiment aimed to test the Wells turbine under oscillating randomly varying flows produced into the plant by sea waves. The goals of these experiments involved some measurements made critical because ofthe harsh environment, the high variability of randomly oscillating flows and unavoidable scale effects. Here,
we show how these measurement difficulties were overcome by developing “ ad hoc” measurements techniques. In particular, in this paper some critical measurements are described: the measurement of the water discharge through the plant, necessary to estimate the absorbed power; the instantaneous value of the torque exerted by the air flow on the turbine, necessary to analyse the actual unsteady behaviour of the turbine. Both measurements were carried out by using two different techniques, in order to make them robust. Water discharge were calculated by integrating the acceleration of the water column, by recording the pressure simultaneously in different point along the streamline. This punctual measurement was compared with measurements carried out by an ultrasonic probe located on the roof of the plenum chamber. Because of the width of the sound beam, this measurement is space
averaged. Torque measurements were carried out by using the DC motor coupled with the Wells turbine as an electro-mechanical transducer. The procedure proved to be accurate once calibrated. A not trivial task was to separate, in the unsteady randomly varying reverse flow, the share of power transferred to the blade by the air current, from the shaft power driving the turbine, being necessary to sustain the rotation of the wheel many times in the cycle, because of high incidence of friction power losses in the small scale model
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
Behaviour of a small Wells turbine under randomly varying oscillating flow
No full textA monoplane Wells turbine was tested during the
experiments conducted at sea on a small scale model of
a REWEC (Resonant Wave Energy Converter)
breakwater. Tests aimed at analyzing the behaviour of
the turbine subjected to randomly varying oscillating
air flow, variable according to the intensity and spectral
characteristics of the sea. During the experimental
campaign, 261 records (sea states) were acquired in
order to characterize the behaviour of both the plant
and turbine. Thanks to the measurement techniques ad
hoc developed for tests at sea and described in a
companion paper, it was possible to determine the
values of torque coefficient T* and pressure coefficient
Dp* as a function of the flow coefficient, f. Because
during each sea state lasting five minutes, data on
dozens of cycles of oscillation were recorded, it was
possible to perform a statistical analysis of all the
available data, with regard to the sign of f and of its
derivative. The results were classified by maximum
oscillation amplitude and peak frequency of the
spectrum. The paper presents the results of the
statistical analysis carried out by highlighting the
effects on the stall condition at high values of flow
coefficient and on the hysteresis between the phases in
which the flow rate is growing and those where the
flow rate is decreasing. Finally, the influence of the
spectral components at higher frequencies on the
hysteresis phenomenon was highlighted
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
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
The FLO Diffusive 1D-2D Model for Simulation of River Flooding
No full textAn 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
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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