Pressure Boundary Research Articles

Estimation of pipe failure frequencies in the absence of operational experience data: A pilot study

Probabilistic failure metrics such as leak frequency and rupture frequency are commonly used to characterize piping reliability. The methodologies for calculating the failure metrics rely on a complex set of input parameters. Operating experience data and experimental data play an important role in informing the different input parameters. The paper describes results and conclusions of a coordinated research project to benchmark three different reliability models using a four-step procedure: reference case definition of relevance to advanced reactor designs, input parameter calibration, validation of results, and application of different methodologies upon completion of the calibration and validation steps. The reference case is a weld consisting of nickel-base alloy 152/52 and located within a primary pressure boundary of an advanced reactor. This alloy is a class of structural materials known to be highly resistant to stress corrosion cracking. Synergies between the different methods are noted and the importance of a multi-disciplinary approach to input parameter development is underscored. A key conclusion is that the three methods are equally suitable for estimating failure frequencies. In any specific application, a selection of the most practical or effective computational tool can be considered. The comparison of alternative models confirms and helps to gain confidence in the computed failure frequency estimates. The study was part of a coordinated research project organized by the International Atomic Energy Agency.

Laplace Transforms Method on a System of Differential Equations for Non-isothermal Chemically Reactive Flow

This study analyses Laplace transforms method on a system of partially coupled differential equations for non-isothermal chemically reactive flow through a cylindrical channel. The dimensionless governing equation for velocity, temperature and concentration was solved using Laplace Transform. Various parameters such as Temperature boundary parameter, Concentration boundary parameter, Cooling Parameter, Grashof number, pressure gradient and Magnetic field, as well as perturbation parameter had an effect on the velocity profile as well as temperature and concentration profile. The graphs were obtained with the results showing that an increase in the temperature boundary parameter resulted to an increase in the temperature of flow, an increase in perturbation parameter resulted to an increase in temperature profile of a body and an increase in Grashof temperature number results to an increase in the velocity of the body.

Development and performance of a HemeLB GPU code for human-scale blood flow simulation

In recent years, it has become increasingly common for high performance computers (HPC) to possess some level of heterogeneous architecture - typically in the form of GPU accelerators. In some machines these are isolated within a dedicated partition, whilst in others they are integral to all compute nodes - often with multiple GPUs per node - and provide the majority of a machine's compute performance. In light of this trend, it is becoming essential that codes deployed on HPC are updated to execute on accelerator hardware. In this paper we introduce a GPU implementation of the 3D blood flow simulation code HemeLB that has been developed using CUDA C++. We demonstrate how taking advantage of NVIDIA GPU hardware can achieve significant performance improvements compared to the equivalent CPU only code on which it has been built whilst retaining the excellent strong scaling characteristics that have been repeatedly demonstrated by the CPU version. With HPC positioned on the brink of the exascale era, we use HemeLB as a motivation to provide a discussion on some of the challenges that many users will face when deploying their own applications on upcoming exascale machines. Program summaryProgram Title: HemeLB (HemePure_GPU)CPC Library link to program files:https://doi.org/10.17632/jhdv4drxbx.1Developer's repository link:https://github.com/UCL-CCS/HemePure-GPULicensing provisions: LGPLv3Programming language: CUDA C++Nature of problem: The geometric characteristics of arteries and veins throughout the human body can vary considerably between individuals. This is particularly true for patients with cardiovascular disease. Accurately resolving the dynamics of blood flow within such domains requires a three-dimensional technique that can replicate such variations at high fidelity. The study of full human-scale domains for physiologically relevant timeframes further demands a tool that can be executed efficiently on the advancing technological frameworks available on modern high performance computers.Solution method: HemeLB uses the lattice Boltzmann method [1,2,3,4] to simulate blood flow in complex, three-dimensional sparse vasculatures. A single relaxation time approximation is used [5]. Solid boundaries are modelled using simple bounce-back boundary conditions [6]. Blood flow is driven by applying either velocity or pressure boundary conditions [7]. The localised solution kernels of the algorithm allow for efficient parallelisation of the method to very high core counts. This version of HemeLB outlines the conversion of the code to allow execution on NVIDIA GPUs, currently the most widely used architecture in ultra high performance computers, whilst retaining its capacity for strong scaling to very large core counts.

A Multi-Fluid Model for Water and Methanol Transport in a Direct Methanol Fuel Cell

Direct-methanol fuel cell (DMFC) systems are comparatively simple, sometimes just requiring a fuel cartridge and a fuel cell stack with appropriate control devices. The key challenge in these systems is the accurate determination and control of the flow rates and the appropriate mixture of methanol and water, and fundamental understanding can be gained by computational fluid dynamics. In this work, a three-dimensional, steady-state, two-phase, multi-component and non-isothermal DMFC model is presented. The model is based on the Eulerian approach, and it can account for gas and liquid transport in porous media subject to mixed wettability, i.e., the simultaneous presence of hydrophilic and hydrophobic pores. Other phenomena considered are variations in surface tension due to water–methanol mixing and the capillary pressure at the gas diffusion layer–channel interface. Another important aspect of DMFC modeling is the transport of methanol and water across the membrane. In this model, non-equilibrium sorption–desorption, diffusion and electro-osmotic drag of both species are included. The DMFC model is validated against experimental measurements, and it is used to study the interaction between volume porosity of the anode gas diffusion layer and the capillary pressure boundary condition at the anode, and how it affects performance and limiting current density.

Two pressure boundary conditions for multi-component multiphase flow simulations using the pseudo-potential lattice Boltzmann model

In this study, two boundary condition (BC) schemes are proposed for the pseudo-potential (PP) lattice Boltzmann method (LBM). The two BC schemes include a modified Convective Boundary Condition (mCBC) and a modified Non-equilibrium Bounceback (mNEBB), and their development is aimed at treating pressure-driven outflow problems for fluids with similar and different density ratios in multi-component multiphase (MCMP) environments. It is the first attempt to incorporate interaction force at boundaries for MCMP fluids. Key limitations of existing LBM boundary schemes for MCMP flow are alleviated. In particular, the constraint of zero-velocity specification at tangential direction is removed by applying the proposed mCBC scheme. In addition, it is demonstrated that multiphase outflow performance is improved with the application of either scheme, while alleviating the nonphysical distortions introduced by the available BC schemes. Three numerical case studies have been conducted to test the proposed scheme. Results have been validated using uncut-off extended domains. These cases are designed to test the induced distortion to the fluids exiting boundaries, examine the scheme’s capability to treat the pressure boundary condition, and explore whether the model can make physically meaningful predictions when these modified BCs are applied. The mCBC scheme is shown to be more capable to correctly treat such pressure specified boundary conditions.

Deep-learning-based surrogate model for fast and accurate simulation in pipeline transport

A new deep-learning-based surrogate model is developed and applied for predicting dynamic temperature, pressure, gas rate, oil rate, and water rate with different boundary conditions in pipeline flow. The surrogate model is based on the multilayer perceptron (MLP), batch normalization and Parametric Rectified Linear Unit techniques. In training, the loss function for data mismatch is considered to optimize the model parameters with means absolute error (MAE). In addition, we also use the dynamic weights, calculated by the input data value, to increase the contribution of smaller inputs and avoid errors caused by large values eating small values in total loss. Finally, the surrogate model is applied to simulate a complex pipeline flow in the eastern part of the South China Sea. We use flow and pressure boundary as the input data in the numerical experiment. A total of 215690 high-fidelity training simulations are performed in the offline stage with commercial software LeadFlow, in which 172552 simulation runs are used for training the surrogate model, which takes about 240 min on an RTX2060 graphics processing unit. Then the trained model is used to provide pipeline flow forecasts under various boundary conduction. As a result, it is consistent with those obtained from the high-fidelity simulations (e.g., the media of relative error for temperature is 0.56%, pressure is 0.79%, the gas rate is 1.02%, and oil rate is 1.85%, and water is 0.80%, respectively). The online computations from our surrogate model, about 0.008 s per run, achieve speedups of over 1,250 relative to the high-fidelity simulations, about 10 s per run. Overall, this model provides reliable and fast predictions of the dynamic flow along the pipeline.

Effects of azimuthal temperature distribution and rod internal gas energy on ballooning deformation and rupture opening formation of a 17 × 17 type PWR fuel cladding tube under LOCA-simulated burst conditions

ABSTRACT In order to contribute to better modeling and evaluation of fuel fragmentation, relocation, and dispersal expected under loss of coolant accident (LOCA) conditions, LOCA-simulated cladding burst experiments were performed on as-received nonirradiated 17 × 17 type Zircaloy-4 cladding specimens that were internally pressurized. The experiments were designed to terminate at burst occurrence to focus on ballooning and rupture opening formation and to investigate the effects of various factors. The postburst cladding hoop strain decreased with the increase in azimuthal temperature difference (ATD) of the cladding, as found previously. The rupture opening size increased with the increase in ATD and the increase in energy of the pressurized gas stored inside the pressure boundary of the test sample system. Comparison with the existing database, which included tests on irradiated rods containing fuel pellets, suggested that formation of the rupture opening was influenced by the characteristic behavior of high burnup fuels, such as limited gas migration in the cladding tube due to fuel–cladding bonding and interaction of the ejected fuel fragments with the cladding tube.

Influence of MRI-based boundary conditions on type B aortic dissection simulations in false lumen with or without abdominal aorta involvement.

Most computational hemodynamic studies of aortic dissections rely on idealized or general boundary conditions. However, numerical simulations that ignore the characteristics of the abdominal branch arteries may not be conducive to accurately observing the hemodynamic changes below the branch arteries. In the present study, two men (M-I and M-II) with type B aortic dissection (TBAD) underwent arterial-phase computed tomography angiography and four-dimensional flow magnetic resonance imaging (MRI) before and after thoracic endovascular aortic repair (TEVAR). The finite element method was used to simulate the computational fluid dynamic parameters of TBAD [false lumen (FL) with or without visceral artery involvement] under MRI-specific and three idealized boundary conditions in one cardiac cycle. Compared to the results of zero pressure and outflow boundary conditions, the simulations with MRI boundary conditions were closer to the initial MRI data. The pressure difference between true lumen and FL after TEVAR under the other three boundary conditions was lower than that of the MRI-specific results. The results of the outflow boundary conditions could not characterize the effect of the increased wall pressure near the left renal artery caused by the impact of Tear-1, which raised concerns about the distal organ and limb perfused by FL. After TEVAR, the flow velocity and wall pressure in the FL and the distribution areas of high time average wall shear stress and oscillating shear index were reduced. The difference between the calculation results for different boundary conditions was lower in M-II, wherein FL did not involve the abdominal aorta branches than in M-I. The boundary conditions of the abdominal branch arteries from MRI data might be valuable in elucidating the hemodynamic changes of the descending aorta in TBAD patients before and after treatment, especially those with FL involving the branch arteries.

Development of fabrication technologies for the first wall of the HCCR blanket

The helium-cooled ceramic reflector (HCCR) concept is a primary candidate among demonstration fusion power reactor (DEMO) blanket options in Korea. The HCCR blanket is composed of a first wall (FW), a breeding zone, side cover plates and a back manifold. This study focuses on fabrication technologies for the FW using laser-beam welding (LBW) and hot isostatic pressure (HIP) bonding. Permanent backing strips in the pressure boundaries of the HCCR blanket are not allowed according to regulations related to pressure vessels and nuclear pressure vessels because high-pressure helium gas at 8 MPa is used as the coolant. LBW conditions and procedures for the fabrication of a FW without backing strips were investigated using small mock-ups with an advanced reduced-activation alloy called ARAA, which was developed in Korea. The laser beam power, thickness of the cover plate, and width of the backing strips were considered as parameters. The LBW conditions and procedures were optimized to remove the backing strip and achieve uniform and continuous back beads. After LBW, the fabrication of the FW with small mock-ups was completed by joining the back plate with HIP bonding. The small mock-ups were subject to very high temperatures exceeding 1000°C during the LBW and HIP bonding processes, which led to changes in the microstructures and hardness levels. The microstructures and hardness levels were recovered through post-heat treatments performed after HIP bonding. The microstructures of the HIP bonding interface after the heat treatments were observed. A partial HIP bonding line was confirmed and transverse tensile tests were carried out to investigate the impact of the partial HIP bonding line on the mechanical properties of the HIP bonding interface. The tensile strength of the HIP bonding interface was approx. 2% lower than that of the base metal, indicating that the partial HIP bonding line caused a slight decrease in the tensile strength, but the impact on the HCCR blanket design would be negligible.

Evaluation of sampling techniques for probabilistic structural integrity assessment of steam generator tubing

Steam generator (SG) tubes for nuclear rectors serve as both pressure and containment boundaries. Rupture of a SG tube is considered a significant event from the perspective of safety and potential for release of radiation. Therefore, SG tubes are designed and operated at high reliability to maintain the pressure boundary's structural integrity. Whereas probabilistic approaches become more widely used in the operational assessment to adequately account for uncertainties from various sources, the high reliability level exerts a high demand for computational efforts, which sometimes may be beyond present computational capabilities. Using the crude Monte Carlo simulation as the baseline approach, this paper investigates several variance reduction techniques that reduce the computational effort for the desired small failure probability of a SG tube rupture. It is shown that Latin hypercubic sampling, directional simulation and subset simulation can improve the accuracy of the failure probability estimate, and directional simulation can improve both accuracy and efficiency dramatically. The probabilistic operational assessment of a specific example shows the calculated failure frequency satisfies the probabilistic structural integrity performance criteria (SIPC) under the normal operating condition, which is consistent with the results when deterministic SIPC is applied.

Power Dynamics within Icelandic Nursing: Walking the Fine Line

Like other Nordic nations, Iceland has a reputation of gender-equality, despite 98% of the country’s nursing profession being women. This paper examines power dynamics within the profession. Fifteen semi-structured interviews with nurses were analyzed with a thematic analysis. Our theoretical framework draws on an ecological perspective highlighting nurses’ vulnerability to power dynamics, and Allen’s work on organizational labor and the invisibility of nurses’ ‘glue work’. The findings reveal that the nurses experience power imbalances when their autonomy is restricted in cooperation with other professionals, demanding their time and disrespecting their professional workspace, and they miss support from their supervisors. They feel their professionalism is belittled, and that the gender imbalance hinders equality. For coping and meeting norms and expectations, the nurses use silencing, which with time pressure and unclear boundaries preserve and enhance stereotypical images. Attracting more male nurses could enhance equality, but additional effort at multiple levels is needed.

Generalized and efficient wall boundary condition treatment in GPU-accelerated smoothed particle hydrodynamics

This paper presents a generalized and efficient wall boundary treatment in the smoothed particle hydrodynamics (SPH) method for 3-D complex and arbitrary geometries with single- and multi-phase flows to be executed on graphics processing units (GPUs). Using a force balance between the wall and fluid particles with a novel penalty method, a pressure boundary condition is applied on the wall dummy particles which effectively prevents non-physical particle penetration into the wall also in highly violent impacts and multi-phase flows with high density ratios. A new density reinitialization scheme is also presented to enhance the accuracy and robustness. The proposed method is simple in comparison with previous wall boundary formulations on GPUs and enforces no additional memory caching and thus is well suited for massively parallel architecture of GPUs. The method is validated in various test cases involving inviscid violent single- and multi-phase flows, demonstrating very good robustness, accuracy and performance. The new wall boundary treatment is able to improve the high accuracy of its previous version by Adami et al. [1] also in 3-D and multi-phase problems, while it is efficiently executable on GPUs. Therefore, the method can be a reliable solution for the long-lasting wall boundary condition challenge in SPH for a broad range of problems.

Spatiotemporal evolution of seismicity during the cyclic operation of the Hutubi underground gas storage, Xinjiang, China

Underground gas storages (UGSs) are important large-scale industrial facilities used to bridge the gap between natural gas consumption and supply. The cyclic operation of the UGS may alter the subsurface stresses and local seismicity. We examined seismicity around the Hutubi UGS from 2011 to 2019 using the matched filter technique (MFT) and double-difference location methods. More than 1300 earthquakes were detected with seismicity around the UGS showing a remarkable increase since the start of its operation and showing a clear correlation to seasonal gas production. About 684 detected earthquakes were located, most of them occurred within 6 km of the reservoir. The events can be grouped into two clusters. Both clusters initiated around the gas pressure boundary. The first cluster extinct after the first injection period. While the second cluster diffused upward along a pre-existing fault. We speculate that strain localization caused by non-uniform gas injection contributes to the initiation of seismicity clusters around the UGS, and the trapped crude oil/gas played an important role in the migration of the second surge. The revealed seismicity pattern contributes to a better understanding of the mechanism of induced seismic events and emphasizes the importance of seismic monitoring in the UGS region.

An improved dislocation density reliant model to address the creep deformation of reduced activation ferritic martensitic steel

The blanket module of nuclear fusion reactor is proposed to be made of reduced activation ferritic martensitic (RAFM) steel. Thus, in order to prevent the unprecedented failure, the creep response of RAFM steel must be studied meticulously. In that direction, physical based creep curve modeling is one of the available choices to obtain the insights about the substructural evolution during the creep deformation, hence enabling to enlighten about material weakening mechanisms. In this paper, experimental creep curves of RAFM steel at varying conditions are simulated employing an improved dislocation density reliant physical model. The microstructure based internal variables are considered for addressing the mechanisms such as hardening, recovery, coarsening of precipitates and creep cavitation. All the simulated creep curves are found to be in reasonable agreement with the experimental creep curves. Furthermore, the evolution of involved eighteen parameters, i.e., dislocation glide velocity, climb velocity, internal stress, climb stress, effective stress, mobile dislocation density, dipole dislocation density, boundary dislocation density, subgrain radius, boundary pressure, boundary dislocation spacing, subgrain mobility and dipole capture spacing is demonstrated and discussed thoroughly.

A coupled hybrid numerical study of tunnel longitudinal ventilation under fire conditions

This paper presents the validation of a coupled hybrid (1D/3D-CFD) modelling methodology, using FDS version 6.7.5, for the first time, with full-scale fire tests. Real fire conditions of the Runehamar tests with maximum heat release rates of 6 MW, 66 MW, and 119 MW are compared by assessing temperature profiles, centreline velocity, backlayering lengths, and maximum temperatures at different locations both upstream and downstream from the fire source. An expression to evaluate the length of the 3D domain where the fire is located is successfully assessed. Also, a pressure boundary condition at one of the portals is suggested to predict more precisely the inner flow conditions. The temperature profiles are accurately predicted with time-averaged differences lower than 20% beyond 40 m downstream from the fire source in the three tests. Furthermore, with the coupled hybrid approach, the backlayering length is estimated accurately with the fire of 66 MW and underestimated in the 6 MW and 119 MW fires, i.e. a maximum difference lower than 4% of the total tunnel length. The validated proposed methodology allows accurate predictions of temperature for tunnel fires and reduces the computational cost between 27% and 75% with respect to a full-CFD numerical model.

Rayleigh waves in thermoelasticity: Triple porous media in local thermal non-equilibrium

Assuming the hypothesis of local thermal non-equilibrium, this work investigates the propagation of Rayleigh surface waves in a thermoelastic half-space, isotropic, homogeneous and structured with a triple level of porosity. Its surface is supposed to be stress-free, thermally insulated and characterized by null pressure boundary conditions. A class of wave solutions is highlighted for the differential system of the model, each solution satisfying suitable asymptotic conditions in the depth of the considered half-space. Then, the Rayleigh wave solution is sought as a linear combination of the elements of this class, and, moreover, by means of the selected boundary conditions, the associated secular equation is found. By solving the secular equation, the characteristics of the wave solution are determined: the propagation speed as well as the damping in time. With the purpose of clearly highlighting the characteristics of the model, the secular equation is solved numerically and significant graphical representations are provided, using the software packages Mathematica and MATLAB. This suggests an increase of the Rayleigh wave propagation speed, corroborated with the appearance of the damping in time of its amplitude.

Flow structure of pitched and skewed microjet vortex generator array

To understand the micro-vortex structure of Microjet Vortex Generators, a plate with pitched and skewed jet holes was established. Simulations using SST-DDES method based on WENO scheme were conducted to analyse the effect of jet pressure, hole interval and incoming boundary layer thickness on the flow structure. The pitch angle and skew angle are 55 degrees and 45 degrees respectively. The total pressure of microjets ranges from 2.5 to 3.5 times the mainstream pressure. The interval of microjets ranges from 6.75 to 13.5 diameters of the hole, and the thickness of incoming boundary layer ranges from 2 to 4 diameters of the hole. The flow field is dominated by three vortices, including a main vortex and two secondary vortices which are significantly influenced by jet pressure and interval. The interaction between secondary vortices and main vortex plays an important role in the process of boundary layer mixing.

An efficient partial-differential-equation-based method to compute pressure boundary conditions in regional geodynamic models

Abstract. Modelling the pressure in the Earth's interior is a common problem in Earth sciences. In this study we propose a method based on the conservation of the momentum of a fluid by using a hydrostatic scenario or a uniformly moving fluid to approximate the pressure. This results in a partial differential equation (PDE) that can be solved using classical numerical methods. In hydrostatic cases, the computed pressure is the lithostatic pressure. In non-hydrostatic cases, we show that this PDE-based approach better approximates the total pressure than the classical 1D depth-integrated approach. To illustrate the performance of this PDE-based formulation we present several hydrostatic and non-hydrostatic 2D models in which we compute the lithostatic pressure or an approximation of the total pressure, respectively. Moreover, we also present a 3D rift model that uses that approximated pressure as a time-dependent boundary condition to simulate far-field normal stresses. This model shows a high degree of non-cylindrical deformation, resulting from the stress boundary condition, that is accommodated by strike-slip shear zones. We compare the result of this numerical model with a traditional rift model employing free-slip boundary conditions to demonstrate the first-order implications of considering “open” boundary conditions in 3D thermo-mechanical rift models.

An improved recoil pressure boundary condition for the simulation of deep penetration laser beam welding using the SPH method

Deep penetration laser beam welding is simulated using the mesh-free Lagrangian Smoothed Particle Hydrodynamics (SPH) method. The physical phenomena modeled are the elastic material behavior of the solid phase, the fluid dynamics of the liquid phase including temperature-dependent surface tension, heat transfer by heat conduction, and the phase transitions melting, solidification, and evaporation. The energy input of the laser beam is determined by a ray-tracing scheme that calculates the absorbed irradiance distribution in the capillary. Based on the locally absorbed irradiance, an improved recoil pressure model is introduced and compared with a temperature-dependent model frequently used in the literature. To transfer the momentum of the recoil pressure to the surrounding melt, two boundary conditions based on either dynamically detected boundary particles or surface forces are presented and validated using simple examples. The results show that the recoil pressure is a key factor for the formation of a deep capillary and the acceleration of the melt around the capillary. Comparison of capillary geometry and melt pool size from simulation and experiment shows good agreement for the irradiance-dependent recoil pressure model.

Investigation on energy conversion instability of pump mode in hydro-pneumatic energy storage system

The pump mode of hydro-pneumatic energy storage (HPES) system often experiences off-design conditions due to the boundary pressure rises, and the resultant energy conversion instability has an adverse effect on the system operation. However, the evolutionary process of this instability and the corresponding flow events are still not fully understood. Experimental and numerical simulation studies of a centrifugal pump with vaned diffuser were conducted in a wide operating range, and the following notable results were obtained. The energy conversion of pump mode shows three typical stages as the pump head increases, stable work→stumbled work→shaky work, corresponding to the inner flow pattern development of stall-free→stall inception→stall deepening. In this evolutionary process, the attenuation of rotor blade loading gradually spreads from the inlet shroud side to the entire blade. This strengthens the natural adverse potential rothalpy gradient and induces the energetic vortical structures including leading backflow vortices, dynamic stall cells and trailing helical vortices. A strong coupling between large-scale vortical motions and energy conversion is observed and this yields a Logistic growth of the shaft power and pump head fluctuations. The critical condition for the energy conversion instability is the diagnostic function SI = 1, and this is recommended for determining the stability limit of the pump mode.