A. B. Almeida

Dynamic orifice model on waterhammer analysis of high or medium heads of small hydropower schemes

The most severe hydropower transients are induced in long hydraulic circuits due to extreme operating conditions. A computational model was developed in order to on one hand ensure waterhammer system control and, on the other hand, provide a more reliable and easier analysis for different specific speed turbines and alternative solutions of the system as a whole, through interaction between different hydraulic components. In reaction turbines, runaway conditions and guide vane closure cause significant discharge variations and pressure fluctuations that can affect the design of conveyance systems. A new approach for groups modelling as dynamic orifices concept was developed enabling the characterisation of the integrated system. The simulation results were compared with laboratory tests. This model can be used in the initial stages of civil works design as an efficient way to better characterise the hydrodynamic behaviour of the system when equipped with reaction turbines.

A computational model of rockfill dam breaching caused by overtopping (RoDaB)

The outflow hydrograph from a dam failure is a boundary condition of a dam break flood model used on the downstream valleys risk management. Limited research has been made on the rockfill dams breaching process and there are no specific models to this type of structures yet. This paper describes a lumped model for the compulation of the outflow hydrograph due to a Rockfill Dam Breaking named RoDaB. The methodology is based on the governing equations of reservoir routing and depletion, and breach erosion. Results obtained f r om experimental tests performed in a laboratory flume were considered to fulfil the phenomenological aspects to which does not exist any analytical approach so far. The erosion process from the dam breach is modelled as a function of two erosion parameters and of the breach final geometry dimensions obtained from the experiments. Finally, the model RoDaB is applied to a rockfill dam, with characteristics typical of this type of structure, and the results are compared with the ones fro...

Water Pipe System Diagnosis by Transient Pressure Signals

Hydraulic transients that naturally occur in pressurised pipe systems propagate back and forth and carry information about different features of the systems, like bursts or leaks, changes of pipe diameter or material, tee-junctions, local head losses, air pockets, closed valves or pipe branches. The aim of the current paper is the report and analysis of different characteristic pressure traces associated with different pipe system features, whose detection, in the transient pressure signal, allows its identification and approximate location. Transient events analysed herein were artificially generated in different experimental facilities and real life systems by the fast closure of side discharge or in-line valves. Transient pressure data were collected at several locations, with and without bursts, air pockets, changes in pipe diameter/material and branches. Different transient pressure traces were observed and are reported: a burst creates a pressure drop; a dead-end reflects totally incident waves; a diameter decrease induces a pressure peak; an air pocket creates a pressure drop followed by an increase; different pipe materials have different pressure wave shapes and damping. Analyses carried out were aided by two diagnosis support tools: a hydraulic transient simulator and an inverse transient solver. Collected data were compared with the results of numerical simulations obtained by running a transient solver incorporating the accurate description of these phenomena in multi-pipe systems, and an excellent agreement was obtained. An inverse transient solver was applied for the evaluation of the location and size of bursts and air pockets, and it successfully estimated them, providing that an accurate transient solver was used, the evaluation was carried out simultaneously with calibration, and the transient event was created by a fast linear flow change. Conclusions are drawn concerning the importance of considering these phenomena since the design phase until system operation and future expansion. Transient-based techniques are particularly important for the diagnosis, monitoring and control of existing water supply systems, not only to identify abnormal features and estimate their approximate locations, but also for a better understanding of the causes of pipe bursts induced by transient events. This paper was presented at the 8th Annual Water Distribution Systems Analysis Symposium which was held with the generous support of Awwa Research Foundation (AwwaRF).

Modeling of washout of dams

Dam breach modelling still constitutes a challenge for the scientific community at it is evidenced by the relative accuracy of the simulation models (CADAM, 2000). The work from Andrey L. Rozov is a very important and valuable contribution: the topic and the numerical scheme presented can be a useful tool for practical and scientific purposes. The methodology is well structured and is sustained by experimental results. Recently the discussers (M.J. Franca and A.B. Almeida) were involved in a project on dam breaching of rockfill dams, based on laboratory experiments similar to the ones shown here (Franca and Almeida, 2002), and developed a similar computational model, applicable to rockfill dams (Franca and Almeida, 2004), where the breach simulation is based on a modified Exner erosion equation applied only in one breach cross-section. A later version of the model expanded its application scope for embankment dam ruptures caused by overtopping (Franca et ai, 2004). The use of the Exner equation for the breaching simulation seems to be very appropriated for use in dam breaching lumped models (Singh, 1996). Its simple formulation permits an easy implementation and small dependency on empirical parameters; the erosion constants can be related with the dam material allowing a quasi-rational approach of the problem. We allowed ourselves to comment A.L. Rozovs work in some points, hoping that this will contribute to improve further improvements of the authors work. We will divide our discussion in four parts, which includes: general comments on the paper, breach development, model application, and conclusions.

Closure to “Conceptual analogy for modelling entrapped air action in hydraulic systems” by Sandra C. Martins, Helena M. Ramos and António B. Almeida

The key points under discussion are the values of the empirical coefficient n extracted from an analysis of experimental data, through the simplified model proposed by the Authors (based on a conceptual analogy) for simulating the dynamic behaviour of an isolated entrapped air pocket in a confined pipe system. The consideration of a constant damping coefficient is also called into question. Based on a theoretical model, the Discussers suggest that the air thermodynamic behaviour in the Authors’ experiments is better represented as adiabatic and recommend that the Authors refine their conclusions. The Discussers also consider the Authors’ conceptual model as somewhat limited for practical hydraulic engineering applications, since a prediction of maximum pressure is often of high importance. The answers to the raised questions are presented below.