86,776 research outputs found
Study and interpretation of the cavitation in a low pressure loop system equipped with water hammer control device
No full textA water hammer protection device in lifting systems, especially in low pressure ones, has been experimentally studied by Giustolisi et al.(1,2,3). Water hammer, in the experimental tests of the analyzed device, is caused by a fast interception of the water flow by means of a spherical valve located just downstream of the hydraulic system pump. Such a device comprises a generic check valve and a concentrated head loss located at one end of a bypass; the latter must be installed between the upstream reservoir and the delivery pipe of the hydraulic system, downstream of the section in which the water flow cut-off has been generated, with the valve close to the reservoir. The valve allows water to flow from the reservoir to the delivery pipe through the bypass. The positive experimental results regarding the efficiency of this device bypass - check valve - concentrated head loss in protecting low pressure lifting systems from water hammer, have been the starting point for studies which have been directed towards the creation of a numerical model of the hydraulic system.
The tests presented in Giustolisi ct al.(1,3) show that a numerical model, simulating the water hammer in a system equipped with the protection device under investigation, should be based upon models simulating cavitation. A numerical model that takes into account the occurrence of cavitation was developed in Giustolisi et al.(4,5). The further step of the study is to verify the simulation of cavitation in the numerical model, by repeating the experimental tests changing the material of the bypass. The bypass, an iron pipe in the previous experimental tests, has been substituted with a plexiglas pipe to film the cavitation during water hammer and to compare the cavitation model with the physical phenomenon
Modelling a Protection Device in a Low Pressure Lifting System
No full textA water hammer protection device in lifting systems has been experimentally studied, especially low pressure ones [1][2][3][7] (see figures n°1 and n°2, pages n°5 and n°6 of the paper). Water hammer, in the experimental tests of the analyzed device, has been caused by the fast water flow interception with a spherical valve located just downstream from the pump of the hydraulic system. Such a device is composed of a generic check valve and a concentrated head loss located at one end of a bypass; the latter must be installed between the upstream reservoir and the delivery pipe of the hydraulic system, just downstream from the section in which water flow cutoff has been generated, with the valve close to the reservoir. The valve allows water to flow from the reservoir to the delivery pipe through the bypass. The positive experimental results regarding the efficiency of this device bypass-check valve-concentrated head loss in protecting low pressure lifting systems from water hammer, have been the starting point for studies which have been directed towards the creation of a numerical model of the hydraulic system. From the trials presented in [1][3], the authors have noted that a numerical model, simulating water hammer in a system equipped with the studying protection device, must use theoretical studies from the cavitation and models reproducing it. Previously [2], a simulating model, without considering the possibility that cavitation occurs, was realized. As the new experimental trials show [1][3], having been carried out starting from steady velocities much higher than older ones [2], the previous simulating model can he applied only when cavitation does not occur
A more realistic simulation of a real water distribution system based on an enhanced demand driven model
No full textThe simulation of a water distribution system is quite usually based on the working hypothesis of dividing the demand along the pipe into two equal parts which are then lumped into pipe's end nodes. This constitutes a simplification of the actual network topology which allows for a huge reduction of the number of network's nodes. However, this can introduce head loss errors due to the poor accuracy in the solution of the energy balance equation along the pipes. This paper shows the effect of this approximation comparing the scenarios returned by the application of the classical Global Gradient Algorithm and by an enhanced Global Gradient Algorithm. The enhanced Global Gradient Algorithm allows for the effective introduction of the lumped nodal demands, and without deteriorating the correct pipe head loss, by means of a pipe hydraulic resistance correction. The enhanced Global Gradient Algorithm and its classical solution are tested on a real water distribution network, where position and number of the connections to users/properties and their actual average demand are available
A biasing approach to design Ultra-Low-Power Standard-Cell-Based Analog Building Blocks for Nanometer SoCs
No full textThis paper presents an approach to design analog building blocks for nanometer systems on a chip (SoCs) that are based on digital standard cells. The proposed approach guarantees that all the CMOS inverters, taken from a standard-cell library, operate with well-defined quiescent current and output voltage, thus allowing the implementation of analog circuits with good robustness against PVT variations. The approach is based on an Analog Body Bias Generator (ABBG) reusable block, similar to the ones adopted in digital applications to cope with process variations, and exploits the bulk terminals of both the p-channel and n-channel MOS transistors of the standard-cell inverter as current and voltage control inputs. The bulk voltages generated by the ABBG are routed to all the standard-cell inverters used for analog functions and allow to set the quiescent current of each cell to a multiple of a reference current and the static output voltage of each cell to half the supply voltage. The full custom design of the ABBG is presented, as well as the design flow to allow the automatic place and route of the proposed standard-cell based analog building blocks. We finally give an example of application to the design of a fully synthesizable four-stage-gain low-power operational transconductance amplifier (OTA). Both the body bias generator and the OTA have been implemented in a 65-nm CMOS technology. The OTA nominal current consumption is 1.75 μA with 0.41-μA standard deviation. Good robustness against supply and temperature variations is also found
Deterministic vs. Stochastic Design of Water Distribution Networks
No full textThe paper describes a procedure for the robust design of water distribution networks which incorporates the un-certainty of nodal water demands and pipes roughness in a multi-objective optimization scheme aimed at mini-mizing costs and maximizing hydraulic reliability. The methodology begins with a deterministic system design in order to generate a set of optimal networks that serves as the initial population for subsequent multi-objective stochastic design. This approach does not depend on the choice of multi-objective optimizer (for example, a multi-objective Genetic Algorithm is used here) and can drastically reduce the number of “stochastic” runs needed for searching robust solutions. A collection of probability density functions based on the beta-function is introduced and applied to modeling variable uncertainty according to different physical requirements. The ap-proach is tested in a case study involving a real network, illustrating its computational advantages
An evolutionary multiobjective strategy for the effective management of groundwater resources
No full textThis paper introduces a modelling approach aimed at the management of groundwater re-sources based on a hybrid multiobjective paradigm, namely the Evolutionary Polynomial Re-gression. Multiobjective modelling in hybrid evolutionary computing enables the user (a) to find a set of feasible symbolic models, (b) to make a robust choice of models and (c) to improve the computational efficiency developing simultaneously a set of models with diverse structural parsimony levels. Moreover the methodology here presented proves to be particularly fit to those cases where the input to the process and the boundary conditions are not easily accessible. The multiobjective approach is based on the Pareto dominance criterion and it is fully integrated into the Evolutionary Polynomial Regression paradigm. This approach proves to be effective for modelling groundwater systems, which usually requires (a) accurate analyses of the underlying physical phenomena, (b) reliable forecasts under different hypothetical sce-narios and (c) good generalisation features of the models identified. For these reasons it is important to construct easily interpretable models which are specialized for well defined pur-poses. The introduced methodology is tested on a case study concerned with the determination of the dynamical relationship between rainfall height and groundwater levels for a shallow unconfined aquifer located in southeast of Italy, which is climatically a Mediterranean zone
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