Quantification of Uncertainty in CFD Simulation of Accidental Gas Release for O & G Quantitative Risk Assessment

Quantitative Risk Assessment (QRA) of Oil & Gas installations implies modeling accidents’ evolution. Computational Fluid Dynamics (CFD) is one way to do this, and off-the-shelf tools are available, such as FLACS developed by Gexcon US and KFX developed by DNV-GL. A recent model based on ANSYS Fluent, named SBAM (Source Box Accident Model) was proposed by the SEADOG lab at Politecnico di Torino. In this work, we address one major concern related to the use of CFD tools for accident simulation, which is the relevant computational demand that limits the number of simulations that can be performed. This brings with it the challenge of quantifying the uncertainty of the results obtained, which requires performing a large number of simulations. Here we propose a procedure for the Uncertainty Quantification (UQ) of FLACX, KFX and SBAM, and show its performance considering an accidental high-pressure methane release scenario in a realistic offshore Oil & Gas (O & G) platform deck. The novelty of the work is that the UQ of the CFD models, which is performed relying on well-consolidated approaches such as the Grid Convergence Index (GCI) method and a generalization of Richardson’s extrapolation, is originally propagated to a set of risk measures that can be used to support the decision-making process to prevent/mitigate accidental scenarios.

Computationally efficient source grid selection and source interpolation in computational aeroacoustics applied to an axial fan.

The noise generation of an axial fan is mainly caused by flow-induced noise and can therefore be extracted from its aeroacoustics. To do so, a hybrid approach separating flow and acoustics is well suited due to its low Mach number. Such a computationally efficient hybrid workflow requires a robust conservative mesh-to-mesh transformation of the acoustic sources as well as a suitable mesh refinement to guarantee good convergence behavior. This contribution focuses on the mesh-to-mesh transformation, comparing two interpolation algorithms of different complexity towards the applicability to the aeroacoustic computation of an axial fan. The basic cell-centroid approach is generally suited for fine computational acoustic (CA) meshes and low phase shift, while the more complex cut-volume method generally yields better results for coarse acoustic meshes. While the cell-centroid interpolation scheme produces source artifacts inside the propagation domain, a grid study using the grid convergence index shows monotonic convergence behavior for both interpolation methods. By selection of a proper size for the source grid and source interpolation algorithm, the computational effort of the experimentally validated simulation model could be reduced by a factor 4.06.

Response Effects Due to Polygonal Representation of Pores in Porous Media Thermal Models

Physics models — such as thermal, structural, and fluid models — of engineering systems often incorporate a geometric aspect such that the model resembles the shape of the true system that it represents. However, the physical domain of the model is only a geometric representation of the true system, where geometric features are often simplified for convenience in model construction and to avoid added computational expense to running simulations. The process of simplifying or neglecting different aspects of the system geometry is sometimes referred to as “defeaturing.” Typically, modelers will choose to remove small features from the system model, such as fillets, holes, and fasteners. This simplification process can introduce inherent error into the computational model.Asimilar event can even take place when a computational mesh is generated, where smooth, curved features are represented by jagged, sharp geometries. The geometric representation and feature fidelity in a model can play a significant role in a corresponding simulation’s computational solution. In this paper, a porous material system — represented by a single porous unit cell — is considered. The system of interest is a two-dimensional square cell with a centered circular pore, ranging in porosity from 1% to 78%. However, the circular pore was represented geometrically by a series of regular polygons with number of sides ranging from 3 to 100. The system response quantity under investigation was the dimensionless effective thermal conductivity, k*, of the porous unit cell. The results show significant change in the resulting k* value depending on the number of polygon sides used to represent the circular pore. In order to mitigate the convolution of discretization error with this type of model form error, a series of five systematically refined meshes was used for each pore representation. Using the finite element method (FEM), the heat equation was solved numerically across the porous unit cell domain. Code verification was performed using the Method of Manufactured Solutions (MMS) to assess the order of accuracy of the implemented FEM. Likewise, solution verification was performed to estimate the numerical uncertainty due to discretization in the problem of interest. Specifically, a modern grid convergence index (GCI) approach was employed to estimate the numerical uncertainty on the systematically refined meshes. The results of the analyses presented in this paper illustrate the importance of understanding the effects of geometric representation in engineering models and can help to predict some model form error introduced by the model geometry.

Asymmetrical Heat Distribution Pattern in Miniature Heat Sinks Due to Conjugate Heat Transfer

Three-dimensional numerical simulations are performed to investigate the conjugate heat transfer of water within microchannel heat sinks. Validation process is performed through comparison between obtained numerical results and experimental data. The global deviation grid convergence index (GCI) is used to conduct solution verification and calculate observed order of accuracy. Conducted numerical analyses include hydraulic diameter range of 206–330 µm, aspect ratio of 1–4 and Reynolds numbers of 300 to 850. Heat is observed to distribute non-uniformly among microchannel side and bottom walls due to conjugate heat transfer. Results show that over 93% of heat is transferred to water through microchannel side walls at the aspect ratio of 4. It is observed that the heat distribution is more non-uniform destruction while microchannel aspect ratio gets larger.

The Performance of a Gradient-Based Method to Estimate the Discretization Error in Computational Fluid Dynamics

Although the grid convergence index is a widely used for the estimation of discretization error in computational fluid dynamics, it still has some problems. These problems are mainly rooted in the usage of the order of a convergence variable within the model which is a fundamental variable that the model is built upon. To improve the model, a new perspective must be taken. By analyzing the behavior of the gradient within simulation data, a gradient-based model was created. The performance of this model is tested on its accuracy, precision, and how it will affect a computational time of a simulation. The testing is conducted on a dataset of 36 simulated variables, simulated using the method of manufactured solutions, with an average of 26.5 meshes/case. The result shows the new gradient based method is more accurate and more precise then the grid convergence index(GCI). This allows for the usage of a coarser mesh for its analysis, thus it has the potential to reduce the overall computational by at least by 25% and also makes the discretization error analysis more available for general usage.

Natural Convection Analysis Around Elliptical Shapes With Sinusoidal Heat Flux Using CFD

Heat transfer is important as technology becomes more compact, thereby increasing the heat flux and consequently the need for cooling. This paper will investigate natural convection on an elliptical shape with sinusoidal heat flux input. Natural convection was analysed using CFD simulations on an ellipse, with minor- to major axis ratio b/a = 0.6 and an inclination angle of α = 90°. The sinusoidal heat flux was non-dimensionalised by a modified Grashof number Gr* = g · β · (∂T/∂n) · (Lc)4/v2 with a mean value of 2 × 107, amplitude of up to 2 × 107, and dimensionless angular frequency ω* = ω · Lc2/v = 24, 36, and 72. All simulations were made with a Prandtl number of Pr = 7.0. To ensure reliable results a Grid Convergence Index analysis was carried out. Validation was made by comparing the obtained surface averaged Nusselt numbers to previous studies and results from performed experiments. An experiment using Particle Image Velocimetry, PIV, measured the flow field around an ellipse. The results from the sinusoidal heat flux showed that the difference in accounting for the sinusoidal Grashof function was up to 10% for the time-surface averaged temperature and time-surface averaged Nusselt number. Generally, the amplitude would increase the temperature, while the effect of the dimensionless angular frequency was dependent on the given amplitude.

Application of grid convergence index to shock wave validated with LS-DYNA and ProsAir

The discretization error is not always calculated, even though it is essential for the studies of computational solid mechanics. However, it is well known that an error committed by the mesh used can be as large as the measured variable, which greatly invalidates the results obtained. The grid convergence index (GCI) method makes possible to determine on a solid basis, the order of convergence and the asymptotic solution. This method seems to be a suitable estimator despite further research is needed in the context of blast situations and finite element (FE) calculations. For this purpose, field trials were performed consisting in the detonation of a spherical hanging load of homemade explosive. The pressure generated by the shock wave was measured in different positions at two distances. With these data, a TNT equivalent has been obtained and used to calculate the shock propagation with the solvers LS-DYNA and ProsAir. This work aims to verify the GCI method by comparing its results with field data along with the simulations carried out. The comparison also seeks to validate the methodology used to obtain the TNT equivalent.This research shows that the GCI gives good results for both solvers despite the complexity of the physical problem. Besides, LS-DYNA displays better correlation with the experimental data than the ProsAir results, with an error of less than 10% in all values.

Design and hydrodynamic analysis of horizontal-axis hydrokinetic turbines with three different hydrofoils by CFD

The conversion of kinetic energy that comes from low-head water currents to electrical energy has gained importance in recent years due to its low environmental and social impact. Horizontal axis hydrokinetic turbines are one of the most used devices for the conversion of this type of energy [1], being an emerging technology more studies are required to improve the understanding and functioning of these devices. In this context, the hydrodynamic study to obtain the characteristic curves of the turbines are fundamental. This article presents the design and hydrodynamic analysis for three horizontal axis tri-blade hydrokinetic turbine rotors with commercial profiles (NACA 4412, EPPLER E817 and NRELS802). The Blade Element Momentum (BEM) was used to design three rotors. The DesignModeler, Meshing and CFX modules from the ANSYS® commercial package were used to discretize the control volumes and configure the numerical study. In addition, Grid Convergence Index (GCI) analysis was performed to evaluate the precision of the results. The computational fluid dynamics (CFD) was used to observe the behavior of the fluid by varying the speed of rotation of the turbines from 0.1 rad s-1 to 40 rad s-1, obtaining power coefficient of 0.390 to 0.435. For a maximum shaft power of 105W. In addition, it is evident that for the same conditions the rotor designed with the EPPLER E817 profile presents better performance than built with the NACA4412 and NREL S802.

A Fuzzy Logic Approach for the Reduction of MeshInduced Error in CFD Analysis: A Case Study of an Impinging Jet

A crucial step in any computational fluid dynamics (CFD) analysis is the discretization of the domain because it influences truncation errors, numerical stability, and the convergence of the model. Therefore, the appropriate selection of numerical mesh parameters crucially contributes to the reliability of the obtained results. Therefore, an innovative approach to reducing the mesh-induced error in CFD analysis of an impinging jet using fuzzy logic is proposed within the paper. The flow parameters were obtained using the Reynolds-averaged Navier–Stokes calculations, based on the mesh parameters obtained using the grid convergence index and fuzzy logic, were compared to each other and to experimental research results. The fuzzy logic approach to define mesh parameters turned out to be a very promising method as it allowed us to obtain results that are qualitatively and quantitatively comparable to commonly used but far more time-consuming methods.