Numerical comparison of pipe-column-separation models

Results comparing six column-separation numerical models for simulating localized vapor cavities and distributed vaporous cavitation in pipelines are presented. The discrete vapor-cavity model (DVCM) is shown to be quite sensitive to selected input parameters. For short pipeline systems, the maximum pressure rise following column separation can vary markedly for small changes in wave speed, friction factor, diameter, initial velocity, length of pipe, or pipe slope. Of the six numerical models, three perform consistently over a broad number of reaches. One of them, the discrete gas-cavity model, is recommended for general use as it is least sensitive to input parameters or to the selected discretization of the pipeline. Three models provide inconsistent estimates of the maximum pressure rise as the number of reaches is increased; however, these models do give consistent results provided the ratio of maximum cavity size to reach volume is kept below 10%.Angus R. Simpson and Anton Bergan

Dynamic Behaviour of Air Valves in a Large-Scale Pipeline Apparatus

[EN] This paper describes an experimental programme on the dynamic behaviour of air valves performed in a large-scale pipeline apparatus. Dynamic flow tests were performed at large (full) scale, since previous quasi-steady flow tests at small scale did not lead to realistic results. Investigations in a large-scale pipeline apparatus lead to a better understanding of the physical processes associated with the dynamic performance of air valves. Float type air valves of nominal diameter of 50 and 100 mm were tested in geometrically similar 200 and 500 mm test sections, to allow for the assessment of dynamic scale effects and the development of dimensionless parameter groups and dynamic scale laws. The approach in the determination of the dynamic performance of air valves was to measure their response to flow acceleration/ decelerations, which are imposed upon the valve. In this way, the air valve behaviour following events like system start-up, pump trip and pipe rupture is simulated. Key results of the dynamic flow tests, including air release tests (valve slam) and column separation tests (effect of air valve on surge suppression), are presented and discussed.The authors gratefully acknowledge the support of the European Commission for their funding of the Transnational Access to Major Research Infrastructure activity within the Improving Human Potential (IHP) Programme.Bergant, A.; Kruisbrink, A.; Arregui De La Cruz, F. (2012). Dynamic Behaviour of Air Valves in a Large-Scale Pipeline Apparatus. Strojniški vestnik ¿ Journal of Mechanical Engineering. 58(4):225-237. doi:10.5545/sv-jme.2011.032S22523758

DESIGN OF WATER HAMMER CONTROL STRATEGIES IN HYDROPOWER PLANTS

Hydropower plants play an important role in the growth of the renewable energy sector. The main objective of the paper is to present, discuss and assess critical parameters which may cause unacceptable water hammer loads in hydropower plants. Water hammer is caused by flow disturbances in a conduit from one steady state to another. It induces pressure rise or drop in hydraulic systems, rotational speed variation in hydraulic turbomachinery and level fluctuation in surge tanks and air chambers. Design principles of water hammer control strategies (mitigation of excessive loads) are outlined including operational scenarios (closing and opening laws), surge control devices (flywheel, surge tank, regulating valve, air valve, etc.) or redesign of the pipeline components. Water hammer models and solutions are briefly discussed in the light of their capability. Case studies include hydropower plants with long fluid conveying systems (open channels, tunnels) and water hammer control devices (surge tank, regulating valve)

Modeling of Unsteady Friction and Viscoelastic Damping in Piping Systems

In real systems, the phenomena, such as pipe-wall viscoelasticity, unsteady friction or fluid structure interaction induce additional damping and dispersion of transient pressure waves than that defined by classical waterhammer. In this paper, unsteady friction models and viscoelastic damping models will be presented and a theoretical formulation of the viscoelastic damping in piping systems without cavitation will be developed. Firstly, the friction factor will be presented as the sum of the quasi-steady part and the unsteady part related to the instantaneous local acceleration and instantaneous convective acceleration. This unsteady friction model has been incorporated into the method of characteristic algorithm (MOC). Secondly, the damping will be defined in terms of viscoelastic effect attributed to a second viscosity µ’. This model is solved using the Finite Difference Method. Finally, numerical results from the unsteady friction and viscoelastic models are compared with results of laboratory measurements for waterhammer cases with low Reynolds number turbulent flows. This comparison validates the new viscoelastic model

Developments in unsteady pipe flow friction modelling

This paper reviews a number of unsteady friction models for transient pipe flow. Two distinct unsteady friction models, the Zielke and the Brunone models, are investigated in detail. The Zielke model, originally developed for transient laminar flow, has been selected to verify its effectiveness for "low Reynolds number" transient turbulent flow. The Brunone model combines local inertia and wall friction unsteadiness. This model is verified using the Vardy's analytically deduced shear decay coefficient C* to predict the Brunone's friction coefficient k rather than use the traditional trial and error method for estimating k. The two unsteady friction models have been incorporated into the method of characteristics water hammer algorithm. Numerical results from the quasi-steady friction model and the Zielke and the Brunone unsteady friction models are compared with results of laboratory measurements for water hammer cases with laminar and low Reynolds number turbulent flows. Conclusions about the range of validity for the three friction models are drawn. In addition, the convergence and stability of these models are addressed.Anton Bergant, Angus Ross Simpson, John Vìtkovsk

Skalak's extended theory of water hammer

Half a century ago Richard Skalak [see T.C. Skalak, A dedication in memoriam of Dr. Richard Skalak, Annual Review of Biomedical Engineering 1 (1999) 1-18] published a paper with the title "An extension of the theory of water hammer" [R. Skalak, An Extension of the Theory of Water Hammer, PhD Thesis, Faculty of Pure Science, Columbia University, New York, USA, 1954; R. Skalak, An extension of the theory of water hammer, Water Power 7/8 (1955/1956) 458-462/17-22; R. Skalak, An extension of the theory of water hammer, Transactions of the ASME 78 (1956) 105-116], which has been the basis of much subsequent work on hydraulic transients with fluid-structure interaction (FSI). The paper considers the propagation of pressure waves in liquid-filled pipes and the coupled radial/axial response of the pipe walls. In a tribute to Skalak's work, his paper is revisited and some of his less-known results are used to assess the dispersion of pressure waves in long-distance pipelines. Skalak's theory predicts that the spreading of wave fronts due to FSI is small, at most of the order of 10 pipe diameters. © 2007 Elsevier Ltd. All rights reserved.Arris S. Tijsseling, Martin F. Lambert, Angus R. Simpson, Mark L. Stephens, John P. Vítkovský, and Anton Berganthttp://www.elsevier.com/wps/find/journaldescription.cws_home/622899/description#descriptio

Systematic evaluation of one-dimensional unsteady friction models in simple pipelines

In this paper, basic unsteady flow types and transient event types are categorized, and then unsteady friction models are tested for each type of transient event. One important feature of any unsteady friction model is its ability to correctly model frictional dissipation in unsteady flow conditions under a wide a range of possible transient event types. This is of importance to the simulation of transients in pipe networks or pipelines with various devices in which a complex series of unsteady flow types are common. Two common one-dimensional unsteady friction models are considered, namely, the constant coefficient instantaneous acceleration-based model and the convolution-based model. The modified instantaneous acceleration-based model, although an improvement, is shown to fail for certain transient event types. Additionally, numerical errors arising from the approximate implementation of the instantaneous acceleration-based model are determined, suggesting some previous good fits with experimental data are due to numerical error rather than the unsteady friction model. The convolution-based model is successful for all transient event types. Both approaches are tested against experimental data from a laboratory pipeline.John P. Vítkovský, Anton Bergant, Angus R. Simpson and Martin F. Lamber

Parameters affecting water-hammer wave attenuation, shape and timing. Part 2: Case studies

This two-part paper investigates parameters that may significantly affect water-hammer wave attenuation, shape and timing. Possible sources that may affect the waveform predicted by classical water-hammer theory include unsteady friction, cavitation (including column separation and trapped air pockets), a number of fluid–structure interaction effects, viscoelastic behaviour of the pipe-wall material, leakages and blockages. Part 1 of this two-part paper presents the mathematical tools needed to model these sources. Part 2 of the paper presents a number of case studies showing how these modelled sources affect pressure traces in a simple reservoir-pipeline-valve system. Each case study compares the obtained results with the standard (classical) water-hammer model, from which conclusions are drawn concerning the transient behaviour of real systems.Anton Bergant, Arris S. Tijsseling, John P. Vítkovský, Dídia I. C. Covas, Angus R. Simpson and Martin F. Lamber

Parameters Affecting Water Hammer Wave Attenuation, Shape and Timing

This paper investigates parameters that may affect water hammer wave attenuation, shape and timing. Possible sources that may affect the waveform predicted by the classical water hammer theory include unsteady friction, cavitation, and a number of fluid-structure interaction (FSI) effects. The discrepancies originate from assumptions in the development of the classical water hammer equations. Mathematical tools for modelling of unsteady friction effects, vaporous and gaseous cavitation, and fluid-structure interaction are presented. The method of characteristics is used as a basic tool. The paper concludes with a number of case studies showing how these parameters affect pressure traces in a simple reservoir-pipeline-valve system. 1