Dynamic Instability Analysis of a Spring-Loaded Pressure Safety Valve Connected to a Pipe by Using CFD Method

As the ultimate protection of a pressure system, pressure safety valves (PSV) can respond in an instable manner such as flutter and chatter which will affect service life, reliability and performance. In order to study the dynamic instability caused by multi-source forces including the flow force, the spring compressing force and the pressure wave force, a more realistic CFD model containing a PSV and different connected pipes as well as the pressure vessel is developed, in which advanced techniques in Fluent such as User Defined Function (UDF) and Dynamic Layering method are combined to allow the PSV to operate. Based on this model, the process of the valve’s opening and reclosing is monitored to examine the influence of design parameters on the dynamic instability of the PSV. Specifically, the propagation of pressure waves along the connecting pipes is successfully captured, which is of great help to explain the instability mechanism and optimize the design and setup of pressure relief systems.

Prediction of Elbow Flow Dynamics Using Correlated Wall Pressure Data

The present study focuses on the analysis of the flow-induced vibration phenomenon typically encountered on piping systems containing an elbow. The correlation between the turbulent flow through the elbow and the dynamic forcing it yields on the piping walls was assessed experimentally. A closed water loop containing a transparent elbow was designed in order to develop fully turbulent duct flow condition. Particle Image Velocimetry (PIV) was applied in the transparent zone in order to provide unsteady data on the flow dynamics through the elbow; simultaneously, wall pressure fluctuations were measured on and around the elbow. Several flow configurations were tested in order to obtain a large coupled database linking the flow features to the resulting dynamic excitation on the walls. Finally, Partial Least Square Regression (PLSR) was applied in order to harvest the correlated information contained in multiple pressure signals at multiple time-delays and build a relationship capable of estimating the temporal evolution of the velocity field using a set of measured wall pressure signals.

A Simplified Approach to Estimate Flow-Induced Vibrations on an Elbowed Piping System

A mixed experimental and numerical approach was undertaken in order to develop a data-based model of the flow-induced vibration levels attained in a piping system containing a 90° elbow. A closed water loop was used to provide unsteady flow data as well as wall pressure and vibration measurements. In parallel, the unsteady water flow through the elbow was computed using an incompressible Large-Eddy Simulation (LES). Proper Orthogonal Decomposition (POD) and Partial Least Squares Regression (PLSR) were used in order to build a relationship between the flow properties and the resulting excitation. This relationship was then used to estimate the evolution of the spatially distributed loadings, which were finally applied to a finite element model of the piping structure. The results consisted of an estimation of the vibration levels. The estimated vibrations were then compared to measurements in order to validate the proposed modeling strategy.

Fluid-Structure Interaction Approach for Numerical Investigation of a Flexible Hydrofoil Deformations in Turbulent Fluid Flow

The study deals with a 3D Fluid-Structure Interaction (FSI) numerical model of a rectangular cantilevered flexible hydrofoil subjected to a turbulent fluid flow regime. The structural response and dynamic deformations are studied by analyzing the oscillations frequencies and amplitudes, under a hydrodynamics loads. The obtained numerical results are confronted with experimental ones, for validation. The numerical model is performed in the same geometric, physical and material conditions as the experimental set-up carried out in a hydrodynamic tunnel. A polyacetal (POM) flexible hydrofoil NACA0015 with an angle of attack of 8° is considered to be immersed in a fluid flow at a Reynold number of 3 × 105. The structure is initially at rest and then moved by the action of the fluid flow. The numerical model is based on a strong coupling procedure for solving the Fluid-Structure Interaction problem. The Arbitrary Lagrangian-Eulerian (ALE) formulation of the Navier-Stokes equations is used and an anisotropic diffusion equation is solved to compute the fluid mesh velocity and position at each time step. The finite volume method is used for the numerical resolution of the fluid dynamics equations. The structure deformations are described by the linear elasticity equation which is solved by the finite elements method. The Fluid-Structure coupled problem is solved by using the partitioned FSI implicit algorithm. A good agreement between numerical and experimental results for the hydrodynamics coefficients and hydrofoil deformations, maximum deflection and frequencies is obtained. The added mass and damping are analyzed and then the FSI effect on the dynamic deformations of the structure is highlighted.

Modeling Hypervelocity Impacts Using Smoothed Particle Hydrodynamics

Hypervelocity impacts occur in outer space where debris and micrometeorites with a velocity of 2 km/s endanger spacecraft and satellites. A proper shield design, e.g. a laminated structure, is necessary to increase the protection capabilities. High velocities result in massive damages. The resulting large deformations can hardly be tackled with mesh based discretization methods. Smoothed Particle Hydrodynamics (SPH), a Lagrangian meshless scheme, can resolve large topological changes whereas it still follows the continuous formulation. Derived by variational principles, SPH is able to capture large density fluctuations associated with hypervelocity impacts correctly. Although the impact region is locally limited, a much bigger domain has to be discretized because of strong outgoing pressure waves. A truncation of the computational domain is preferable to save computational power, but this leads to artificial reflections which influence the real physics. In this paper, hypervelocity impact (HVI) is modelled by means of basic conservation assumptions leading to the Euler equations of fluid dynamics accompanied by the Mie-Grueneisen equation of state. The newly developed simulation tool SPHlab presented in this work utilizes the discretization method smoothed particle hydrodynamics (SPH) to capture large deformations. The model is validated through a number of test cases. Different approaches are presented for non-reflecting boundaries in order to tackle artificial reflections on a computational truncated domain. To simulate an HVI, the leading continuous equations are derived and the simulation tool SPHlab is developed. The method of characteristics allows to define proper boundary fluxes by removing the inwards travelling information. One- and two-dimensional model problems are examined which show excellent absorption behaviour. An hypervelocity impact into a laminated shield is simulated and analysed and a simple damage model is introduced to model a spallation failure mode.

Dynamics of a Free Heaving and Prescribed Pitching Hydrofoil in a Turbulent Flow, With a Fluid-Structure Interaction Approach

This paper deals with the dynamics of an oscillating foil, describing a free heaving (vertical displacement) and prescribed pitching (rotational displacement) movement which is computed from its position in two different ways. A fluid-structure interaction approach is chosen, as the physics of the flow and the structure are strongly coupled. The flow is unsteady, turbulent and incompressible. The pressure/velocity problem is solved using SIMPLEC scheme. First, the pitching movement is considered as a given continuous function of the hydrofoil heaving position. Second, the pitching motion is performed alternately at the end of each heave cycle. For each case, two maximum angles of attack and one heaving amplitudes are studied. Preliminary results showed that a high maximum angle of attack generates more lift hydrodynamics force, but also requires more energy to perform the rotation of pitch.

Study on the Simplified Method of Special Supporting Structure for Steam Generator Tubes

The flow induced vibration of tubes in the steam generator gradually attracts great attention. The natural frequency of the tube is the basic parameter for vibration analysis. The supporting structures of the tube bundle in steam generator are the cinquefoil orifice-baffle and the anti-vibration bar which is different from the common baffle plate. The issue that researchers focus on is how these supporting structures affect the natural frequency. A simplified method of the special support in the tube bundle was studied based on the numerical simulation. According to the characteristics of supporting structures, the effect of stiffness of the supporting structures on the natural frequency was studied by the spring element constraints. The results show that when the stiffness of the support structure is larger than the magnitude of 105 N/m, the stiffness has no influence on the natural frequency. The frictional force of the circumferential constraint inside the plane is too weak to constrain the tube so we need to pay more attention on the vibration inside the plane. Through changing the contact length of the support component and tubes, the effect of the contact condition on the natural frequency is studied. The results show that the contact condition has a certain effect on the natural frequency. When the support is simplified as simple support, the influence on natural frequency is small and the deviation is less than 1.5%. And there is a certain safety margin under the simplified method. In the calculation process, the special support can be simplified as simple support and the calculation results are relatively accurate and conservative.

A Proposed Guideline for Applying Waterhammer Predictions Under Transient Cavitation Conditions: Part 1 — Pressures

Waterhammer analysis (herein referred to as Hydraulic Transient Analysis or simply “HTA”) becomes more complicated when transient cavitation occurs (also known as liquid column separation). While standard HTA transient cavitation models used with analysis based on the Method of Characteristics show good correlation when compared to known test/field data, the great majority of test/field data are for simple systems experiencing a single transient. Transient cavitation in more complicated systems or from two or more independently initiated transients have not been validated against data. Part 1 of this paper describes the various safety factors already provided by ASME B31.3 for pressure containment, provides criteria for accepting the results of HTA calculations that show the presence of transient cavitation, and makes recommendations where the user should include additional safety factors based on the transient cavitation results. Situations are discussed where waterhammer abatement is recommended to reduce hydraulic transient pressures and forces, and for increasing confidence in HTA results in specific cases. The result is a proposed comprehensive and pragmatic guideline which practicing engineers can use to perform waterhammer analysis and apply pressure predictions to pipe stress analysis.

Importance of Accounting for Finite Particle Size in CFD-Based Erosion Prediction

Being capable in predicting the removal of material from a surface subjected to the impingements of solid particles within a carrier liquid is of considerable industrial interest. This phenomenon, called impact erosion, is of concern in many applications due to its severe technical and economic consequences. The use of Computational Fluid Dynamics (CFD) techniques for impact erosion prediction is a challenging approach to avoid the cost and complexity of laboratory testing. A well-established methodology exists for CFD-based erosion estimation, consisting in the simulation of the slurry flow by an Eulerian–Lagrangian two-phase model followed by the application of an empirical erosion correlation to estimate the loss of material produced by each particle-wall impact. One of the main assumptions of this approach is that the solids are treated as massive point particles, even if, from a theoretical point of view, this approximation may be too simplistic, as it requires the particle size to be infinitesimal. The objective of the present study was, primarily, to assess how the point–particle treatment of the dispersed phase may affect the accuracy of CFD-based erosion prediction models. Based on these findings, numerical strategies were proposed in order to correct for the induced error without the need of resorting to a fully-resolved description of the slurry flow, which would not be affordable in practical applications due to its excessive computational burden. As a first step, reference was made to the benchmark case of slurry abrasive jet impingement test. The obtained results will open the way for addressing more complex flows in future research.

Development of Structural Integrity Assessment Method for Flow-Induced Vibration of Reactor Internals in PWR

For verifying the structural integrity of the reactor internals (RIs) in a pressurized water reactor (PWR) plant, it is important to estimate the vibration response of the core barrel (CB) due to flow turbulence. Instead of scale model test, the computational fluid dynamics (CFD) has been expected as a method to predict the turbulence forcing function for the response analysis of the CB. In this article, a hybrid approach combining empirical equations based on flow test and CFD analysis is proposed in order to predict the turbulence forcing function. The scale model test of new RIs, which were developed by Mitsubishi Heavy Industries, Ltd., was conducted, and the pressure fluctuations for the turbulence forcing function and the vibration response of the CB were measured. The pressure fluctuations were calculated by CFD analysis, and the vibration analysis using the turbulence forcing function determined from the calculated pressure fluctuations was performed. This article provides the scale model test data and the empirical equations of the turbulence forcing functions, and validation results of the proposed method to predict the turbulence forcing function using CFD.