Fluid Transient Research Articles

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.

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.

Nuclear Power Plants Are Not So Safe: Fluid Transients / Water Hammers, Autoignition, Explosions, Accident Predictions and Ethics

Due to misdiagnosed hydrogen explosions and ethically flawed publications, nuclear power plants are not as safe as published to be for decades. Previous publications inaccurately, unethically, and inappropriately assessed reactor safety with respect to other industries and natural disasters, where nuclear industry publications non-conservatively applied data to promote nuclear reactor safety, and published models skewed accident data to present nuclear energy as safer than it is. Major findings of this one-man study were calculated with 95% confidence. 1) Explosions caused by fluid transients accompany nuclear power plant meltdowns. 2) A meltdown similar to Three Mile Island (TMI-2) has a 50% probability before 2039, with a one in two probability of a radioactive release like Fukushima. 3) A large radioactive release like Fukushima has a 50% probability before 2067. 4) Radioactive releases can be prevented by fluid transient control with the exception of Chernobyl type accidents, which are expected far into the future. 5) Nuclear power plant accidental deaths are not significantly less than other industries as claimed for many years, but are comparable to other industries. An improved explanation of nuclear power plant explosion safety is provided here to better control dangers in the radiation business. Items 1 and 2 were previously published, but are summarized here as proof of principle to support this new research, and items 3, 4, and 5 are new to the literature. These latter concerns are the thrust for this discussion of ethics, nuclear safety, and the prevention of loss of life, property damages, and catastrophic environmental damages. A personal history of the difficulties of being the first to invent new theory and the government obstruction and unethical cover up to thwart legitimate safety concerns are intertwined with statistical proofs for nuclear reactor disaster predictions. Such disasters explode radioactive dust clouds into the air to circle our planet, and spread radioactive contamination that radioactively rains down across parts of the earth following large accidental explosions. Although the ethical path to accomplish this task has great personal costs, such as difficulty, stress, complexity of thought, long hours, impacts on family life, and money, the rewards are that imminent nuclear power plant explosions and loss of life can be stopped!

Leak Detection and Topology Identification in Pipelines Using Fluid Transients and Artificial Neural Networks

AbstractCondition assessment of water pipelines using fluid transient waves is a noninvasive technique that has been investigated for the past 25 years. Approaches to identify different anomalies a...

On the two-dimensional characteristics of laminar fluid transients in viscoelastic pipes

A two-dimensional numerical model is developed to study the behavior of laminar fluid transients in viscoelastic pipes. The present procedure couples a two-dimensional water hammer model with a single element Kelvin-Voigt model, where the latter is used to simulate the viscoelastic behavior of the pipe wall. The method of lines is implemented in the present study, where second-order accurate central difference expressions are used for the discretization of the spatial terms and the classical fourth-order accurate Runge-Kutta method is used for integrating the discretized equations in time. Through the present numerical results and dimensional analysis of the governing equations, a non-dimensional parameter is identified which governs the effect of pipe wall viscoelasticity on the fluid transient. The identified parameter shows that the effect of pipe wall viscoelasticity is more pronounced in the case of longer pipes with bigger diameters and larger wave speeds. Moreover, a parametric study is carried out to identify the effect of the non-dimensional parameter on the various characteristics of the fluid transient and on the pipe instantaneous and retarded strain components.

High vacuum measurements and calibrations, molecular flow fluid transient effects

High vacuum pressure measurements and calibrations below ≈1 × 10−6 Torr are problematic. Specifically, measurement accuracies change drastically for vacuum gauges when pressures are suddenly lowered in vacuum systems. How can gauges perform like this? To answer this question, a brief system description is first required. Calibrations were performed using a vacuum calibration chamber with attached vacuum gauges. To control chamber pressures, vacuum pumps decreased the chamber pressure while nitrogen tanks increased the chamber pressure. By balancing these opposing pressures, equilibrium in the chamber was maintained at selected set point pressures to perform calibrations. When pressures were suddenly decreased during set point adjustments, a sudden rush of gas from the chamber also caused a surge of gas from the gauges to decrease the pressures in those gauges. Gauge pressures did not return to equilibrium as fast as chamber pressures due to the sparse distribution of gas molecules in the system. This disparity in the rate of pressure changes caused the pressures in different gauges to be different than expected. This discovery of a new theory was experimentally proven to show that different gauge designs return to equilibrium at different rates, and that gauge accuracies vary for different gauge designs due to fluid transients in molecular flow.

Numerical Simulation of Fluid Transients in Servo-Controlled Hydraulic Piping

His analytical model is carried out based on the one-dimensional fluid transient theory. In view of the micro-compression of the fluid can change its momentum in the transient process, an improved fluid dynamic model of pipeline is proposed. The characteristic method and the finite difference method are adopted for this simulation. In this paper, The fluid transients of the pipes after servo valve shut down are analyzed. The correctness of the simulation is approved by the comparison of calculation data with experiment data .The results showed that these models give more reasonable descriptions for water hammer and flow oscillation during servo valve starting process and could provide guidance for designs and experiments of the hydraulic pipe system

Experimental study of check valves performances in fluid transient

An experimental set-up was introduced to study the dynamic behaviour of different types of check valves under pressure transient conditions. Three types of transient comparison methods were used, with similar results being obtained from all three methods. The experimental results show that the check valves with low inertia, assisted by springs or small travelling distance/angle, gave better performance under pressure transient conditions as compared to check valves without these features. Although different amounts of air entrainment were found to affect the experimental readings, the general characteristics of each check valve remain the same when compared between valves. This study can be applied to help in the choosing of suitable check valves for a particular pumping system.

Effects of Air Entrainment on Fluid Transients in Pumping Systems

In pumping installations, fluid transient computations are necessary to achieve safety, efficiency and economy in design and operation. In some systems, where air content and air entrainment exist, such computations become highly inaccurate when constant wave speed is assumed. In this paper, a numerical model and a computational procedure have been developed to investigate the effects of air entrainment on the pressure transient in pumping systems. Free gas in the fluid and cavitation at the fluid vapour pressure were modeled in the form of variable wave speed model, which was numerically solved by the method of characteristics. This model was tested for the case of pump trips due to power failures. The pressure transient results obtained by this variable wave speed model were analyzed and compared with those results obtained by constant wave speed model and with the experimental results of other investigators.

Modelling the attenuation of laminar fluid transients in piping systems

The damping of laminar fluid transients in piping systems is studied numerically using a two-dimensional water hammer model. The numerical scheme is based on the classical fourth order Runge–Kutta method for time integration and central difference expressions for the spatial terms. The results of the present method show that the damping of transients in piping systems is governed by a non-dimensional parameter representing the ratio of the Joukowsky pressure force to the viscous force. In terms of time scales, this non-dimensional parameter represents the ratio of the viscous diffusion time scale to the pipe period. For small values of this parameter, the damping of the fluid transient becomes more pronounced while for large values, the fluid transient is subjected to insignificant damping. Moreover, the non-dimensional parameter is shown to influence other important transient phenomena such as line packing, instantaneous wall shear stress values and the Richardson annular effect.

Acoustic End Effects in Magnetohydrodynamic Submerged Vehicular Propulsors—Part 2: Solitary Waves

In a magnetohydrodynamic (MHD) seawater propulsion system for a submerged vehicle, a region of high-uniform magnetic induction (magnetic flux density) inside the MHD propulsion channel is separated from the region of essentially zero magnetic induction outside the channel in the seaway by a region of nonuniform, fringing magnetic induction at each end of the channel. This paper treats the propagation of an a periodic fluid transient (solitary wave) which is produced by an arbitrary unit impulse in velocity or pressure at any cross section in the uniform-field region and which propagates through either fringing field region and into the zero-field region outside the channel. The time scale for the transients is sufficiently short that compressive effects are important, so that the fluid transients are acoustic waves. The channel is a straight, rectangular duct with electrically insulating walls and highly conducting walls perpendicular and parallel to the magnetic induction, respectively. The linearized acoustic equations are averaged over each cross section of the channel to obtain a pair of coupled equations governing the average pressure and average axial velocity as functions of the axial coordinate and time. Together these equations represent a simple wave equation with a retarding force which is proportional to the square of the local magnetic flux density. Results are presented for three values of the acoustic interaction parameter N, which is the characteristic ratio of the electromagnetic body force opposing motions across magnetic-induction lines to the inertial "force" in the fluid transients. An abrupt change in velocity produces a wave front which travels at the speed of sound. Without MHD effects, the entire change in velocity or pressure occurs suddenly as the wave front passes. With MHD effects, only part of each change occurs suddenly as the wave front passes, followed by a gradual evolution to reach the entire change. The split between the abrupt and gradual fractions of the entire change depends on N. In previous work, the authors treated the fundamentally different MHD acoustic problem of the transmission of periodic waves from the channel and the previous paper (Walker et al 1992) is Part 1 of the present work.

Influence of Gas Diffusion on Fluid Transient Phenomena Associated with Column Separation Generated during Decompression Operation.

A mathematical model is presented for numerical simulation of the fluid transients in the return line during the decompression operation of a hydraulic system. Particular attention has been paid to clarification of the influence of gas diffusion on the fluid transients because in such a system, part of the separation cavity consists of a gas-liquid two-phase flow containing a number of minute gas (air) bubbles. Experiments using sump oil and deaerated oil as a test fluid showed that the generation mechanism of the separation cavity and the fluid transients are significantly affected by the gas diffusion.

Fluid Transient Phenomena Associated with Column Separation in Fluid Power Pipelines. 4th Report. Influence of Gas Diffusion on Fluid Transients during Decompression Operation.

A mathematical model is presented for numerical simulation of the fluid transients in the return line during the decompression operation of any hydraulic system. Particular attention has been paid to clarification of the influence of gas diffusion on the fluid transients because part of the separation cavity consists of a gas-liquid two-phase flow containing a number of minute gas (air) bubbles in such a system. Experiments, using sump oil and deaerated oil as a test fluid, showed that the generation mechanism of the separation cavity and the fluid transients are significantly affected by the gas diffusion. Values calculated from the proposed bubble-diffusion model, which takes into consideration the effects of gas diffusion as well as the amount of gas bubbles released from the liquid flowing out of the valve before column separation occurs, a change of state of gas and the surface tension applied to the gas bubble, agreed well with the measured values over a wide range of tests.

低圧戻り管路におけるサージ圧力の低減

To increase the operating speed of machines which use hydraulics, it is necessary to increase the flow rate of hydraulic systems. As flow rate becomes larger, it becomes increasingly important at the design stage to take into account fluid transients in the fluid lines. In particular, column separation in the low-pressure return line generates a high surge pressure causing an undesirable noise and possible damage to components.A number of studies have already been made of this phenomena. In the actual development of hydraulic systems however, there are problems in which the check valves are installed in the return line, or one of a variety of valve stroke patterns is used. These problems have not been sufficiently dealt with in the previous studies.To analyze fluid transient phenomena in the fluid lines, a simulation program which combines the method of characteristics with numerical integration was developed. The results of the calculations with the program were verified by a comparison with the experimental results. Using this simulation program, the effect of various parameters on the surge pressure in the low-pressure return line investigated. These parameters include line dimensions, void ratio, check valve position, pressure loss, and timing of the valve control. Furthermore, to reduce the surge pressure, the use of two check valves mounted in a circuit was proposed. The effect of this proposed idea was verified with the numerical method and by experiments.

Some problems on fluid transient phenomena.

For the analysis and design of fluid systems, especially whose systems in which fluid transients in a pipe play an important role, numerical calculation by the method of characteristics is a very useful tool. Because the method of characteristics is suitable for computer programing, even fluid transients in complicated pipelines can be analyzed with less effort. Using this method, the surge pressure and transient flow rate in a pipeline can be accurately analyzed. Due to the reliance on computer simulations, it becomes more and more important and necessary to fully comprehend physical phenomena through experiments and to make use of the results of the design and control of fluid systems.

Source mechanism of volcanic tremor

Low‐frequency (<10 Hz) volcanic earthquakes originate at a wide range of depths and occur before, during, and after magmatic eruptions. The characteristics of these earthquakes suggest that they are not typical tectonic events. Physically analogous processes occur in hydraulic fracturing of rock formations, low‐frequency icequakes in temperate glaciers, and autoresonance in hydroelectric power stations. We propose that unsteady fluid flow in volcanic conduits is the common source mechanism of low‐frequency volcanic earthquakes (tremor). The fluid dynamic source mechanism explains low‐frequency earthquakes of arbitrary duration, magnitude, and depth of origin, as unsteady flow is independent of physical properties of the fluid and conduit. Fluid transients occur in both low‐viscosity gases and high‐viscosity liquids. A fluid transient analysis can be formulated as generally as is warranted by knowledge of the composition and physical properties of the fluid, material properties, geometry and roughness of the conduit, and boundary conditions. To demonstrate the analytical potential of the fluid dynamic theory, we consider a single‐phase fluid, a melt of Mount Hood andesite at 1250°C, in which significant pressure and velocity variations occur only in the longitudinal direction. Further simplification of the conservation of mass and momentum equations presents an eigenvalue problem that is solved to determine the natural frequencies and associated damping of flow and pressure oscillations. For a simple, constant pressure reservoir‐conduit orifice fluid system, a change in orifice size can cause the source signal duration due to a single disturbance to change from several seconds to in excess of an hour. Fluid kinematic viscosity (0.412 m2/s) is both included and neglected in the analysis to illustrate its effect upon system damping. Tremor magnitude is related to the magnitude of the conduit wall motion occurring in response to oscillating fluid pressures. The entire volcanic fluid system does not generally experience a transient flow condition at a given time. Closed end and constant pressure fluid system boundaries reflect pressure waves and isolate components of the fluid system. Total fluid movement may not be accurately monitored by tremor data alone, since steady state and slowly changing flows are aseismic.

TRANS2A: An Unconditionally Stable Code for Thermal Transient Stress Analysis in Piping

A technique for numerical integration of the finite difference (matrix) formulation of the unsteady heat transfer equation has been applied to the thermal stress analysis requirements of ASME B&PV Section III, Article NB-3650. This technique, with its properties of unconditional solution stability, has been incorporated into a new computer program, TRANS2A, which has been designed totally around the needs of the stress analyst. To be of maximum aid to the analyst, in addition to the necessary output of thermal gradients (ΔT2 and ΔT2) and average temperatures (Ta and Tb), TRANS2A provides a complete set of thermal stress histories and tables of thermal stress extrema. Values of the thermal stresses are output at maxima of the thermal gradient terms (with or without adjacent sections), in addition to the extrema of the secondary and secondary plus peak stresses and time of occurrence. Each solution is performed for a set of seven general and three optional stress indices. The process allows a strict and simple data interface to the combined stress analaysis computation without excessive approximations. Data may also be stored so that sections need not require repeated analyses. All computational output, from the detailed heat transfer solution to the stress summaries, may be requested or deleted at the option of the analyst. For generality, TRANS2A includes a complete set of temperature-dependent material properties for all current piping materials and a complete set of fluid properties for water, steam, and sodium. Fluid transient data are input using phase and temperature, and a choice of four flow rate specifications. Accepted heat transfer correlations for laminar and turbulent flow in liquids and gases are included, with smoothing at two-phase excursions. Samples of the TRANS2A benchmark problems are included, with discussions on data interface and sensitivity for erratic fluid transients.

A Finite Difference Method for Thermally Expandable Fluid Transients

In this paper we discuss equations that define a thermally expandable fluid transient. We then present a set of finite difference equations for the numerical solution of such transients, discuss their solvability, and investigate certain aspects involving the convergence of the numerical solution. Finally, we apply these ideas to a problem involving flow in the preheater section of a steam generator.