Fluid Transients Research Articles

Best-estimate water hammer simulations to avoid the calculation of unrealistically high loads or unphysical pressure and force peaks

For safety-related systems fluid loads due to fluid transients have to be quantified for subsequent structural analyses to ensure their integrity or function, as required. Usually transient fluid loads in pipe systems are determined with one-dimensional water hammer software. For single-phase liquid flow, the method of characteristics (MOC) is often used that gives in this case appropriate results. For the consideration of local vapor bubbles, the MOC is combined with the discrete vapor cavity model (DVC). The DVC model may generate unrealistic pressure spikes due to the calculation of the collapse of multi-cavities in scenarios, where only one vapor bubble should actually occur. The application of a two-phase code may improve the calculation results. One requirement for the latter codes is the ability to calculate the propagation of steep gradients without suffering from numerical diffusion to exclude the underestimation of fluid loads. This is commonly attained by applying higher-order numerical schemes. However, the application of a numerical method of pure 2nd order leads to the calculation of unphysical oscillations at steep gradients causing severe problems during the solution. To exclude this, numerical methods with flux limiters can be used. With their application, the calculation of unrealistically high loads due to numerical deficiencies can be minimized. In addition, the consideration of further physical effects, that lead to the reduction of loads during transient flow processes, allows for a more realistic calculation of the loads. These are unsteady friction, widening of the pipe caused by pressure increase, fluid-structure interaction at junctions and due to friction, degassing of gas that is initially dissolved physically in a liquid and thermodynamic non-equilibrium during vapor bubble collapse. The in-house code DYVRO applies a second-order accurate scheme with flux limiters based on the Godunov method and can account for the above-described physical phenomena. It is shown by comparison of calculation results obtained by DYVRO with experimental data from literature that with modeling of these physical effects the loads can be calculated more realistically. Generally, these loads are lower than the results calculated by simplified models, which do not account for these effects. Considering that these loads are applied in subsequent structural analyses, cost-intensive oversizing of pipes and their supports can be avoided, by ensuring the necessary safety.

Composition tracking of natural gas–hydrogen mixtures in pipeline flow using high-resolution schemes

A transient pipeline flow model with gas composition tracking is solved for studying the operation of a natural gas pipeline under nonisothermal flow conditions in a hydrogen injection scenario. Two approaches to high-resolution pipeline flow modeling based on the WENO scheme are presented and compared with the implicit finite difference method. The high-resolution models are capable of capturing fast fluid transients and tracking the step changes in the composition of the transported mixture. The implicit method assumes the decoupling of the flow model components in order to enhance calculation efficiency. The validation of the composition tracking results against actual gas transmission pipeline indicates that both models exhibit good prediction performance, with normalized root mean square errors of 0.406% and 1.48%, respectively. Under nonisothermal flow conditions, the prediction response of the reduced model against a high-resolution flow model, with respect to the mass and energy linepack, is at most 3.20%.

Introducing a Newly Developed Computer Software to Analyze Fluid Transients in Pressurized Pipeline Systems

Changes in flow rates in pressurized pipeline systems, due to power loss in a pump, opening or closing of a valve, load rejection by a turbine, etc., may lead to unsteady conditions called fluid transients or water hammer in the pipeline, which, in turn, can lead to dangerous consequences. This potential phenomenon must be checked during the design phase of pipeline systems in order to prevent such dangerous situations. For this purpose, many software programs have been developed worldwide to analyze fluid transients. In this study, a computer software was developed that analyzes this type of flow in pipeline systems using C# programming language. In addition to simulating and solving problems in transient flows in pipeline systems, this will help the purpose of having a free domestic transient flow software for education and research purposes, avoiding expensive alternatives. The method of characteristics is used to solve the nonlinear, hyperbolic partial differential equations of unsteady pipe flow. The developed software was tested using various well-known benchmark cases for validation purposes. It was shown that the computed results of the new software are very similar to those obtained in the literature, proving the accuracy and reliability of the new program. In future studies, it is hoped that the program will be more comprehensive with more boundary conditions added to the program. In this paper, the developed program and some of its important features will be introduced, and validation of the program with certain benchmark studies will be provided.

Fast Meshless Solution With Lumped Friction for Laminar Fluid Transients

Abstract Fluid transients without friction in pipelines can be solved by a time-domain exact solution, using a simple recursive process without computational grid. However, the calculation time cost of this approach is very high because of the recursion algorithm. Its improved method, named as the fast meshless solution (FMS), was developed to speed up the computation by introducing time-line interpolation. However, when the friction is considered, the conventional distributed friction model cannot be employed directly for the FMS is meshless. To address this problem, the fluid friction is lumped at pipe ends, and both steady and unsteady laminar flow are investigated. Three kinds of lumped models are proposed here, and compared with a numerical case of the water hammer. The laminar fluid transients can be calculated quickly by the present method, with a little reduced accuracy. This method may be of interest in a quick assessment of the piping fluid transients.

Modeling of Pipe Whip Phenomenon Induced by Fast Transients Based on Fluid–Structure Interaction Method Using a Coupled 1D/3D Modeling Approach

The sudden increase in the operating pressure of nuclear power plants (NPPs) is due to the water hammer phenomenon, which tends to produce a whipping effect that causes serious damage to the pipes and their surroundings. The mechanical response of these pipelines under the influence of such fast fluid transients can be estimated using the fluid–structure interaction (FSI) method. The computational time and expense are predominantly dependent on the number of finite elements developed in the model. Hence, an effective modeling technique with limited and efficient nodes and elements is desired to obtain the closest possible results. A coupled 1D/3D finite element modeling approach using the FSI method is proposed to determine the influence of fast transients on the mechanical pipe whipping behavior of gas pipelines in NPPs. The geometric coupled modeling approach utilizes the presence of both the 3D solid elements and the 1D beam elements sharing a local conjunction. The computational model is modelled for a pipe-to-wall impact test scenario taken from the previously conducted French Commissariat a l’Energie Atomique (CEA) pipe whip experiments. The results of displacement, stresses, and impact velocity at the 3D section featuring the elbow are compared for the change in the 3D solid length varied at the juncture of the elbow. The computed results from the Ansys FSI coupling method using the Fluent and Transient Structural modules provides fair validation with the previously conducted experimental results and correlates with the CEA pipe whip tests on pipe-to-wall impact models. Thus, the 1D/3D coupled modeling approach, which minimizes the area of the solid region by constricting it to the impact area with appropriate contact modeling at the junctures, can be considered in the future for decreasing the computational time and the creation of finite elements.

An analysis of fluid–structure interaction coupling mechanisms in liquid-filled viscoelastic pipes subject to fast transients

An extension of a recently developed quasi-2D flow model for fluid transients in viscoelastic pipes to handle fluid–structure interaction mechanisms is presented. In a context in which the fluid flow is devised as a structured pseudo-mixture and the pipe’s viscoelasticity is rooted in an internal variable theory, the axial movement of the pipe wall is allowed to occur, giving rise to friction, Poisson, and junction coupling mechanisms. The resulting governing equations of the model form a quasi-linear hyperbolic system of partial differential equations, which approximated solution is achieved by means of the method of characteristics. The proposed approach is validated against pressure traces acquired from a reservoir–pipe–valve experimental setup found in the literature. In the course of the validating process, different pipe anchoring conditions are employed to study the system responses. Focus is given to pipe–fluid interface interactions, energy dissipation, and transfer of energy between both media.

Linking and Comparison of the Damping of Fluid Transients and Frequency Response Diagram Methods for Pipe Leak and Burst Detection and Localization

Linking and Comparison of the Damping of Fluid Transients and Frequency Response Diagram Methods for Pipe Leak and Burst Detection and Localization

Effect of Hydraulic Characteristics on Fluid Transients Analysis under Different Types of Control Valves

Effect of Hydraulic Characteristics on Fluid Transients Analysis under Different Types of Control Valves

Fluid transients in viscoelastic pipes via an internal variable constitutive theory

This work presents a fluid transient model capable of handling the viscoelastic behavior of the pipe. A previously developed quasi-2D flow model is employed as a base, and the viscoelastic behavior of the pipe is incorporated by considering constitutive equations formulated in a thermodynamically consistent framework of an internal variable theory. Such an approach straightforwardly provides expressions for computing the rates of energy dissipation in the fluid and pipe accurately and separately. This novel feature discerns the local and overall impacts of the energy dissipation on the pressure oscillations caused by each medium. The governing equations of the model form a hyperbolic system of partial differential equations whose approximated solutions are obtained by the method of characteristics. Taking as reference pressure signals obtained by a classic reservoir-pipe-valve experiment found in the literature, it is shown that the model predictions are fully consistent. A comparison between the pressure responses of viscoelastic and elastic pipes reveals that in addition to delaying the pressure oscillations, the viscoelastic behavior causes a faster attenuation of them. The rates of energy dissipation in the viscoelastic pipe attain significant magnitudes during the first moments of the fluid transient and alters the hydrodynamical behavior of the flow. Such interference is exposed by comparing the responses of the same experimental setup when two different viscoelastic pipe materials are considered. It is also shown that the knowledge of the parcels of energy dissipated in the fluid and pipe individually can improve the comprehension of the phenomenon and be utilized for theoretical and applied research in the field.

A new model for fluid transients in piping systems taking into account the fluid–structure interaction

The present work extends a recently developed quasi-2D flow model for fluid transients in elastic pipes to accommodate fluid–structure interaction mechanisms. In this context, the primary goal of the proposed approach is to analyze energy transfer and dissipative effects in the fluid–pipe system. The fluid–structure interaction couples the flow dynamics with the axial movement of the tube, giving rise to friction, Poisson, and junction coupling mechanisms to take place. The mechanical model mathematical structure forms a quasi-linear hyperbolic system of partial differential equations for which approximated solutions are sought by employing the method of characteristics. The proposed model reproduces classical benchmark solutions and presents a good agreement with experimental data. In addition, the whole thermomechanical consistent framework in which the model is established allows an accurate description of the flow’s internal structure. This feature allows a better comprehension of the phenomenon when compared to the classic frictionless and quasi-steady-friction-based four-equation models. The present approach allows a better description of the friction coupling mechanism and unveils uneven shear stress distributions in the unsteady flow. As a result, the energy dissipation in the fluid is captured with accuracy. Due to the absence or limited ability to describe these dispersive and dissipative effects, the traditional approaches are proved to lose their accuracy as the fluid transient goes. The coupled and uncoupled models are also analyzed and are proven to respond differently due to localized pipe–fluid interface effects in addition to the bulk mechanisms of transfer of energy that occur differently in both approaches.

Unified modelling framework of flowslide triggering and runout

Hydro-mechanical forcing, including sediment deposition, seismic loading, rainfall events and phreatic level rise, can alter the pore water pressure regime and trigger ground failure. In loose sands, such phenomena can manifest with rapid downslope movements displaying signatures of liquefaction and commonly referred to as ‘flowslides’. These can have catastrophic outcomes, yet the use of constitutive models for soils able to quantify the likelihood of liquefaction is usually restricted to triggering analyses, while the quantification of runout distance and velocity tends to be conducted through computationally expensive models based on rheological laws for fluids. To enable a seamless transition from the triggering to the runout stage, in this paper, a spatially condensed modelling framework of low computational cost is proposed, which allows for straightforward use of soil constitutive models in the analysis of the entire life cycle of a flowslide. The proposed framework couples the dynamic motion of a landslide mass with the pore fluid transients taking place within a pre-defined liquefiable zone. The constitutive law modulates the feedback between pore fluid diffusion and landslide movement, encompassing elastoplastic and viscoplastic soil models within the same formulation. Simulations show that partial drainage in the liquefiable zone has considerable effects on the triggering stresses initiating a flowslide. Specifically, the results imply that liquefaction risks are sharply suppressed when the ratio between the characteristic times of loading and pore pressure diffusion falls above a critical threshold influenced by the inelastic deformation response of the soil. Furthermore, it is shown that all the dynamics of flowslide propagation are dominated by the ratio between the characteristic timescales related to fluid consolidation, wave propagation and soil reaction time (i.e. soil viscosity). Although viscosity is not necessary to carry out propagation analyses, ignoring it leads to unrealistic estimates of peak velocity and runout distance. As a result, the model predicts that sand viscosity reflecting the emergence of a microscopic inertial regime is required to explain the flowslide propagation characteristics reported in the literature.

Water Hammers Exploded the Nuclear Power Plants at Fukushima Daiichi

Abstract A fundamental contemporary theory, the Leishear explosion theory, explains major explosions at Three Mile Island and Fukushima Daiichi. Focusing on the Fukushima Daiichi explosions as well as many small nuclear power plant explosions, auto-ignited explosions hammered the largest seismic response at Unit 1 on Mar. 12, 2011. At Unit 3 on Mar. 14, a visibly larger explosion ignited with an observed fireball and smoke cloud but lower seismic forces. On Mar. 15, a Unit 2 reactor system explosion ignited hydrogen in the Unit 4 reactor building to cause damages following ignition, and seismic responses were negligible. Note that a Unit 2 reactor building explosion did not occur, and this fact is questionably attributed to the destructive removal of one of the walls of Unit 2 due to the earlier Unit 1 explosion. All of these explosions were ignited by fluid transients that exploded flammable hydrogen that was created during nuclear reactor core meltdowns, which were initiated by loss of power due to a tsunami. The conclusions presented here build upon earlier publications, where fluid transients auto-ignite hydrogen to explode buildings. In addition, research from Argonne National Laboratory provides background to explain this common cause for nuclear power plant explosions. Although different than the Argonne report conclusions, conclusions here are consistent with observations provided by the Argonne report. New ideas challenge existing beliefs, but the stakes are high since nuclear reactor safety is important to prevent loss of life and catastrophic environmental damages. The next nuclear power plant explosion can be stopped!

Laboratory and field experiment validations on the use of hydraulic transients for estimating buried water pipeline deterioration

This paper investigates the validity of using fluid transients as a rapid screening tool to augment existing methods for assessing the condition of buried water systems. In particular, the paper provides one of the first detailed investigations on the impact of pipe wall deterioration on the characteristic of transient waves in both laboratory and field experiments. Laboratory pipeline sections were deteriorated by an accelerated corrosion process using controlled electrolytic cell reactions, and three cases were considered: where corrosion is limited to the internal wall, limited to the external wall, and where both internal and external walls are corroded. Hydraulic transient tests were carried out to measure the transient wave speed in these corroded pipe sections, and the measurements are found to be consistent with the theoretical predictions using the observed wall thickness loss, confirming that wave speed can be used as an indicator of pipe wall deterioration. Field experiments were carried out in the water supply network in the Waimakariri District, New Zealand, to validate the performance of this pipe condition diagnostic methodology. Transient tests were conducted on selected 60-year-old sections of asbestos cement (AC) pipelines and sensors were connected to standard fire hydrants to measure the wave speeds and wave reflections. Compared with the theoretical calculated wave speed of intact pipelines, a 140–300 m/s wave speed decrease was observed in most of the tested sections which is in line with the expectation for pipelines of this age. The wave speeds were used to predict the in-situ thickness of the pipe wall and these predictions were found to match with computed tomography X-ray scan results of the excavated pipe with good accuracy. The same methodology was also used to correctly detect an unrecorded plastic pipe section in the AC pipe network.

Numerical investigation of two-phase fluid-transient induced cavitation in the cryogenic propellant feedlines

Rapid changes in fluid-flow conditions, whether purposeful or accidental, may result in generation of a pressure surge in the fluid handling equipment followed by oscillations of pressure, which is known as the fluid transient (or fluid hammer). Fluid transient occurs very often during the filling of the dry liquid lines as well as during the valving operation in the cryogenic propellant feed systems. The maximum amplitude of fluid transient induced pressure oscillations may exceed the strength of the material, which results in the rupture of the flow components. Sometimes, due to its oscillatory nature, when the pressure wave amplitude falls below the vapour pressure of the fluid it may lead to cavity generation resulting in two-phase fluid transients. The pressure generated due to the collapse of the cavity will be higher than the pressure in the single-phase fluid transients that may lead to the structural failure of the fluid handling equipment.In this study, a one–dimensional numerical model has been proposed using the Method of Characteristics (MOC) scheme to investigate the fluid transient induced cavitation due to sudden closure of the valve in the propellant feedlines. Different two-phase models namely Discrete Vapour Cavity Model (DVCM) and Discrete Gas Cavity Model (DGCM) have been used to predict the vapour volume in the pipe. The applicability of the developed model has been evaluated by validating its predictions with the results available in the literature. It has been observed that both the models are in good agreement with experiments, but DGCM can be preferred due to its ability to handle larger void fractions as compared to DVCM.

Stochastic Resonance Enhancement for Leak Detection in Pipelines Using Fluid Transients and Convolutional Neural Networks

Water losses through leakage represent a significant problem for asset management in water distribution systems. The interpretation of fluid transient pressure waves after the generation of a transient event has been previously used as a technique to locate and characterize leaks, but existing approaches are often both model-driven and limited to the existing knowledge of the system. The potential of using artificial neural networks (ANN) and fluid transient waves to detect, locate, and characterize anomalies in water pipelines has recently been proposed. However, its application in more realistic conditions (e.g., in the presence of background pressure fluctuations) has proven challenging. To address this, one alternative to enhance the response of any nonlinear system includes the introduction of artificial noise, a phenomenon known as stochastic resonance. In this paper, the enhanced detection of leaks in pressurized pipelines via the deployment of stochastic resonance is demonstrated. This paper harnesses this approach by presenting a methodology for the active inspection of pipelines using convolutional neural networks (CNNs). This methodology finds the optimal artificial noise intensity to be introduced into the training dataset for a set of CNNs. The methodology has been applied to a real pipeline in a laboratory at the University of Adelaide in which 14 transient experimental tests were conducted. The results indicated that the addition of noise to the transient pressure head training samples significantly enhances the CNN predictions for the leak location highlighting the existence of an optimum noise intensity to obtain both accurate and reliable results. When trained with the optimum noise intensity, the CNNs were able to locate leaks with an average error of 0.59% in terms of the actual location (in a 37.24-m long pipeline), demonstrating the promising potential of developing techniques based on CNNs to detect leaks and anomalies in water pipelines.

Numerical validation of pressure and flow characteristics across a control valve in a feed line

This work is intended to understand the variation of pressure and flow at the pump inlet of liquid rocket engine. The opening and closure of the valve upstream of the pump features complex phenomenon. The opening and closing of the valve cause pressure and flow variations at the pump inlet which may lead to combustion instabilities in combustion chamber of engine, hydraulic transients in feedlines, and off-design operation of turbo-pumps which are fundamental to the efficient testing and operation of engine. A numerical model to predict the pressure and flow transients across a control valve for different rate of opening in fluid feed systems has been developed using first-order finite difference technique. In case of flow in pipes, the velocity and pressure is governed by momentum and continuity equations. A computer code for the prediction of fluid transients is developed based on method of characteristics for one-dimensional fluid flow in pipelines and compared with test data for validation. The control valve is considered to be in-line with the feed line and modeled based on the valve coefficient vs. percent opening of valve. This model can subsequently be used to predict the effect of opening/closing time of the valve on pressure surges across the control valve and corresponding flow rate in the feedline for different opening of the valve.

Complexity of Deep Low‐Frequency Earthquake Activity in Shikoku (Japan) Imaged From the Analysis of Continuous Seismic Data

AbstractWe derive an extensive catalog of low‐frequency earthquakes (LFEs) in western and central Shikoku applying an automatic, coherency‐based detection and location method to 4‐year long continuous data from selected seismic stations. Our catalog is the first LFE catalog in this area that does not rely on a priory templates and results from a homogeneous data processing scheme. It allows investigating in detail main characteristics of LFE activity in space and time. We observe clear heterogeneity of the LFE space distribution in the along‐strike direction of the subducting Philippine Sea plate corresponding to segments with different sizes, some of which are highly productive in LFE generation. More detailed statistical and correlation‐based analysis of LFE occurrence patterns allows quantification of the along‐strike segmentation and examination of the migration and event interaction during the LFE sequences and inter‐sequence periods. The analysis indicates that a strong interaction exists among LFE sources during slow slip events, but it otherwise varies significantly among the along‐strike segments. We suggest that the observed segmentation of LFE activity is related to static heterogeneity, such as structural property variations along the subduction interface, or dynamic heterogeneity, corresponding to memory‐dependent stress variations or possible fluid transients. We also confirm that regions with the highest LFE productivity correspond to spots of tectonic tremor triggered by teleseismic earthquakes' surface waves supporting a localized fluid‐rich environment with possible fluid transients. Our results illustrate how high‐resolution LFE catalog can contribute to the characterization and quantification of slow earthquake processes through detailed statistical analysis of their activity.

On the Hierarchy of Models for Pipe Transients: From Quasi-Two-Dimensional Water Hammer Models to Full Three-Dimensional Computational Fluid Dynamics Models

Abstract A hierarchy of models exists in the literature for the simulation of pipe transients. One-dimensional (1D) water hammer models provide a cost-effective tool for the analysis of such transients. Traditional 1D models implement a quasi-steady approximation of the frictional term, which results in poor modeling of the attenuation of the transient. To improve the modeling of the attenuation phenomenon, alternative unsteady friction models were developed for the 1D water hammer formulation. Moreover, quasi-two-dimensional (quasi-2D) water hammer models were introduced, which allow the computation of the unsteady velocity profile and hence provide improved modeling of the attenuation phenomenon. Recently, interest has developed in the use of computational fluid dynamics (CFD) models based on the Navier–Stokes equations in the simulation of fluid transients. Both axisymmetric and full three-dimensional (3D) CFD models are used in this regard. The aim of the current paper is to carry out a comparative study between the performance of quasi-2D water hammer models, axisymmetric CFD models, and full 3D CFD models. Numerical computations using the three models are performed for both laminar and turbulent flow cases. Present results show that the quasi-2D water hammer model and the axisymmetric CFD model provide near identical results in terms of computing the magnitude, phase, and attenuation of the transient. Reported results also demonstrate the computational efficiency of the quasi-2D model, which provides results that agree reasonably well with the full 3D CFD model results while using a grid density, which is an order of magnitude lower than the grid requirements for the full 3D CFD model.

Approach for Near-Real-Time Pipe Burst Detection, Localization, and Quantification with Low Data Transmission and Sampling Rates

The purpose of the paper is to present an approach for pipe burst detection, localization, and cross-sectional area quantification based on the changes in the discrete harmonic spectrogram and analysis of damping of fluid transients. The amplitude of each resonance response of the burst-induced transient wave in each analyzed time window in the spectrogram is damped differently because of the presence of the burst. A pressure signal processing algorithm for pipe burst detection and estimation has been developed by the authors to explore this specific type of analysis, in conjunction with a predefined gap between each window. Because the window gap can be set to be equal to the data transmission and sampling rates in the analysis to detect and estimate the burst, it can be applied during real-time data monitoring. Unlike other hydraulic transient-based detection methods, the proposed algorithm only requires the Nyquist frequency of the third resonance response as the data transmission and sampling rates, which are significantly lower than the same quantities in commonly used data acquisition systems. The algorithm has been verified both numerically and experimentally.

Numerical Modeling of Fluid Transients with Structural Compliance

AbstractThis paper presents fluid transients in a pipeline with the sudden opening of a valve using a network flow solver based on the finite-volume method. There is entrapped air downstream of the...