Standard Test Cases Research Articles

Implementation of a Modified Lid Driven Cavity in OpenFOAM

The standard lid-driven cavity test case is one of the most used validation cases in CFD. Whilst comparisons with experimental and particularly DNS simulations are possible, there is no analytical solution, and the case is ill-posed when considering the boundary conditions. A modified lid driven cavity (MLDC) case exists in the literature in which the lid velocity is non-uniform and which introduces a spatially varying body force, and for which there is a closed-form analytical solution to the Navier-Stokes equations which is a function of the Reynolds number. In this paper I present an implementation of the MLDC as a modification of the standard OpenFOAM case, using run time coding for the boundary conditions and fvOptions, and show how convergence to the solution is affected by numerical parameters ofsimpleFoam such as choice of matrix inversion. The existance of an analytical solution also allows the investigation of the relation between the solver residual and the true solution error.

EPIC: The Elliptical Parcel-In-Cell method

We present a novel approach to simulating general two-dimensional flows, which could also be applied to other areas of continuum mechanics. The approach generalises the Particle-In-Cell (PIC) method, originally used to model two-dimensional hydrodynamics, by representing fluid elements by elliptical parcels. The rotation and deformation of these parcels are calculated, and parcels split beyond a critical aspect ratio. Conversely, small parcels are eliminated by merging them with larger ones. The elliptical parcels well represent the flow deformation and have excellent conservation properties. In contrast to earlier work that combined PIC with elliptical parcels that split and merge, a vorticity-based framework is used, and accurate integration over ellipses is performed efficiently by two-point Gaussian quadrature. The small-scale mixing associated with parcel splitting and merging is shown to be strongly convergent with grid resolution. The robustness, versatility, accuracy and efficiency of the new Elliptical Parcel-In-Cell (EPIC) method is demonstrated for a variety of standard test cases, and compared with a standard pseudo-spectral method. The results indicate that EPIC is a promising, Lagrangian-based alternative to grid-based methods.

New parameters identification of Proton exchange membrane fuel cell stacks based on an improved version of African vulture optimization algorithm

Model parameters identification of the fuel cells has several applications in practice. However, due to its nonlinear dynamic and its complicated nature, solving it based on most of the classic methods is too hard. Also, using any kind of methods based on soft computing does not also provide a satisfying result which can be proved based on the no free lunch theorem. In the present study, a new methodology has been proposed for optimum parameters identification of the Proton exchange membrane fuel cell (PEMFC) stacks. Here, an improved version of African vulture optimizer (IAVO) is organized for this purpose. To prove the accuracy of the proposed method, it is applied to three standard test cases and the results have been compared with some five latest techniques. The results showed that the proposed IAVO algorithm with 1.98(±0.49), 0.03(±0.02), and 1.08(±0.47) parameter fitting value for NedStackPS6, BCS 500 W, and SR-12 500 W, respectively, provides the best results with minimum error value. This shows that the efficiency of the proposed IAVO algorithm is too better than the other methods in parameter identification of the PEMFC stacks.

Benders Subproblem Decomposition for Bilevel Problems with Convex Follower

Bilevel optimization formulates hierarchical decision-making processes that arise in many real-world applications, such as pricing, network design, and infrastructure defense planning. In this paper, we consider a class of bilevel optimization problems in which the upper level problem features some integer variables and the lower level problem enjoys strong duality. We propose a dedicated Benders decomposition method for solving this class of bilevel problems, which decomposes the Benders subproblem into two more tractable, sequentially solvable problems that can be interpreted as the upper and lower level problems. We show that the Benders subproblem decomposition carries over to an interesting extension of bilevel problems, which connects the upper level solution with the lower level dual solution, and discuss some special cases of bilevel problems that allow sequence-independent subproblem decomposition. Several novel schemes for generating numerically stable cuts, finding a good incumbent solution, and accelerating the search tree are discussed. A computational study demonstrates the computational benefits of the proposed method over a state-of-the-art, bilevel-tailored, branch-and-cut method; a commercial solver; and the standard Benders method on standard test cases and the motivating applications in sequential energy markets.

Artificial Intelligence Techniques for Power System Transient Stability Assessment

The high penetration of renewable energy sources, coupled with decommissioning of conventional power plants, leads to the reduction of power system inertia. This has negative repercussions on the transient stability of power systems. The purpose of this paper is to review the state-of-the-art regarding the application of artificial intelligence to the power system transient stability assessment, with a focus on different machine, deep, and reinforcement learning techniques. The review covers data generation processes (from measurements and simulations), data processing pipelines (features engineering, splitting strategy, dimensionality reduction), model building and training (including ensembles and hyperparameter optimization techniques), deployment, and management (with monitoring for detecting bias and drift). The review focuses, in particular, on different deep learning models that show promising results on standard benchmark test cases. The final aim of the review is to point out the advantages and disadvantages of different approaches, present current challenges with existing models, and offer a view of the possible future research opportunities.

Discrete Exterior Calculus Discretization of Two-Phase Incompressible Navier-Stokes Equations with a Conservative Phase Field Method

We present a discrete exterior calculus (DEC) based discretization scheme for incompressible two-phase flows. Our physically-compatible exterior calculus discretization of single phase flow is extended to simulate immiscible two-phase flows with discontinuous changes in fluid properties such as density and viscosity across the interface. The two-phase incompressible Navier-Stokes equations and conservative phase field equation for interface capturing are first transformed into the exterior calculus framework. The discrete counter part of these smooth equations is obtained by substituting with discrete differential forms and discrete exterior operators. We prove the boundedness of the method for the first order Euler forward and predictor-corrector time integration schemes in the DEC framework. With a proper choice of two free parameters, the scheme remains phase field bounded without the requirement of any ad hoc mass redistribution. We verify the scheme against several standard test cases (for interface capturing) comprising not only the flat domains but also the curved domains, leveraging the advantage that DEC operators are independent of the coordinate system. The results show excellent properties of boundedness, mass conservation and convergence. Moreover, we demonstrate the ability of the scheme towards handling large density and viscosity ratios as well as surface tension in the simulation of various two phase flow physical phenomena on flat or curved surfaces.

Auditory-visual scenes for hearing research

While experimentation with synthetic stimuli in abstracted listening situations has a long standing and successful history in hearing research, an increased interest exists on closing the remaining gap towards real-life listening by replicating situations with high ecological validity in the lab. This is important for understanding the underlying auditory mechanisms and their relevance in real-life situations as well as for developing and evaluating increasingly sophisticated algorithms for hearing assistance. A range of ‘classical’ stimuli and paradigms have evolved to de-facto standards in psychoacoustics, which are simplistic and can be easily reproduced across laboratories. While they ideally allow for across laboratory comparisons and reproducible research, they, however, lack the acoustic stimulus complexity and the availability of visual information as observed in everyday life communication and listening situations. This contribution aims to provide and establish an extendable set of complex auditory-visual scenes for hearing research that allow for ecologically valid testing in realistic scenes while also supporting reproducibility and comparability of scientific results. Three virtual environments are provided (underground station, pub, living room), consisting of a detailed visual model, an acoustic geometry model with acoustic surface properties as well as a set of acoustic measurements in the respective real-world environments. The current data set enables i) audio–visual research in a reproducible set of environments, ii) comparison of room acoustic simulation methods with “ground truth” acoustic measurements, iii) a condensation point for future extensions and contributions for developments towards standardized test cases for ecologically valid hearing research in complex scenes.

Radiation-hydrodynamics with MPI-AMRVAC

Context. Radiation controls the dynamics and energetics of many astrophysical environments. To capture the coupling between the radiation and matter, however, is often a physically complex and computationally expensive endeavor. Aims. We sought to develop a numerical tool to perform radiation-hydrodynamics simulations in various configurations at an affordable cost. Methods. We built upon the finite volume code MPI-AMRVAC to solve the equations of hydrodynamics on multi-dimensional adaptive meshes and introduce a new module to handle the coupling with radiation. A non-equilibrium, flux-limiting diffusion approximation was used to close the radiation momentum and energy equations. The time-dependent radiation energy equation was then solved within a flexible framework, fully accounting for radiation forces and work terms and further allowing the user to adopt a variety of descriptions for the radiation-matter interaction terms (“opacities”). Results. We validated the radiation module on a set of standard test cases for which different terms of the radiative energy equation predominate. As a preliminary application to a scientific case, we calculated spherically symmetric models of the radiation-driven and optically thick supersonic outflows from massive Wolf-Rayet stars. This also demonstrates our code’s flexibility, as the illustrated simulation combines opacities typically used in static stellar structure models with a parametrized form for the enhanced line-opacity expected in supersonic flows. Conclusions. This new module provides a convenient and versatile tool for performing multi-dimensional and high-resolution radiative-hydrodynamics simulations in optically thick environments with the MPI-AMRVAC code. The code is ready to be used for a variety of astrophysical applications, where our first target is set to be multi-dimensional simulations of stellar outflows from Wolf-Rayet stars.

General Circulation Model Errors Are Variable across Exoclimate Parameter Spaces

Abstract General circulation models (GCMs) are often used to explore exoclimate parameter spaces and classify atmospheric circulation regimes. Models are tuned to give reasonable climate states for standard test cases, such as the Held–Suarez test, and then used to simulate diverse exoclimates by varying input parameters such as rotation rates, instellation, atmospheric optical properties, frictional timescales, and so on. In such studies, there is an implicit assumption that the model works reasonably well for the standard test case will be credible at all points in an arbitrarily wide parameter space. Here, we test this assumption using the open-source GCM THOR to simulate atmospheric circulation on tidally locked Earth-like planets with rotation periods of 0.1–100 days. We find that the model error, as quantified by the ratio between physical and spurious numerical contributions to the angular momentum balance, is extremely variable across this range of rotation periods with some cases where numerical errors are the dominant component. Increasing model grid resolution does improve errors, but using a higher-order numerical diffusion scheme can sometimes magnify errors for finite-volume dynamical solvers. We further show that to minimize error and make the angular momentum balance more physical within our model, the surface friction timescale must be smaller than the rotational timescale.

Effect of Crossflow Transition on the Pressure-Sensitive-Paint Rotor in Hover

The amplification factor transport transition model has been extended to include crossflow transition and applied to a rotor in hover. Validation of the crossflow model implementation has been performed using standard test cases. The pressure-sensitive-paint rotor is simulated in hover for various collective pitch angles as a test case for the transition model. The OVERFLOW 2.2n solver is used with laminar–turbulent transition modeling and hybrid Reynolds–averaged Navier–Stokes/large-eddy simulation turbulence modeling enabled. Grid generation, numerical methods, and transition modeling are described. The effects of crossflow transition are qualitatively and quantitatively assessed through comparisons to experiments, to a transition model without crossflow, and to a fully turbulent model. Additionally, several techniques for evaluating unsteady boundary-layer transition are described and are applied to solutions for the pressure-sensitive-paint rotor in hover.

A Coscaling Grid for Athena++

Abstract We present a coscaling-grid formalism and its implementation in the magnetohydrodynamics code Athena++. The formalism relies on flow symmetries in astrophysical problems involving expansion, contraction, and center-of-mass motion. The grid is evolved at the same time order as the fluid variables. The user specifies grid evolution laws, which can be independent of the fluid motion. Applying our implementation to standard hydrodynamic test cases leads to improved results and higher efficiency, compared to the fixed-grid solutions.

Online Probabilistic Power Flow with Tie-line Power Transfer: A Stacked Denoising Auto-Encoders Method

Based on stacked denoising auto-encoders (SDAE) and combined with the Monte-Carlo simulation (MCS) method, this paper proposes a hybrid algorithm (SDAE-Based-PPF, SPPF) for online calculation of probabilistic power flow (PPF) with tie-line power transfer. By incorporating the deep structure and reconstructive strategy of SDAE, this paper extracts high-level features of nonlinear power flow equations, thereby constructing SDAE-based power flow models. Then, the parameters of SDAE-based power flow models, with improved training speed and accuracy, are optimized by mini-batch gradient descent algorithm, momentum learning rate and introduced cross-entropy loss function. The SDAE-based power flow models are developed to approximate power flow equations with unidirectional extracted high-level features, so as to enable non-iterative solvability judgment and calculation of power flow, therefore fast and accurate. Benefiting from the parallelizability, speed and accuracy of SDAE, the proposed SDAE-based power flow models, with unsolved random samples generated by the MCS method, are able to calculate all samples simultaneously, and this enables PPF to be calculated online. Finally, numerical results are implemented on three standard IEEE test cases to verify the effectiveness of proposed SDAE-based power flow models and SPPF method.

A linear stability analysis of compressible hybrid lattice Boltzmann methods

An original spectral study of the compressible hybrid lattice Boltzmann method (HLBM) on standard lattice is proposed. In this framework, the mass and momentum equations are addressed using the lattice Boltzmann method (LBM), while finite difference (FD) schemes solve an energy equation. Both systems are coupled with each other thanks to an ideal gas equation of state. This work aims at answering some questions regarding the numerical stability of such models, which strongly depends on the choice of numerical parameters. To this extent, several one- and two-dimensional HLBM classes based on different energy variables, formulations (primitive or conservative), collision terms and numerical schemes are scrutinized. Once appropriate corrective terms introduced, it is shown that all continuous HLBM classes recover the Navier-Stokes-Fourier behavior in the linear approximation. However, striking differences arise between HLBM classes when their discrete counterparts are analyzed. Multiple instability mechanisms arising at relatively high Mach number are pointed out and two exhaustive stabilization strategies are introduced: (1) decreasing the time step by changing the reference temperature Tr and (2) introducing a controllable numerical dissipation σ via the collision operator. A complete parametric study reveals that only HLBM classes based on the primitive and conservative entropy equations are found usable for compressible applications. Finally, an innovative study of the macroscopic modal composition of the entropy classes is conducted. Through this study, two original phenomena, referred to as shear-to-entropy and entropy-to-shear transfers, are highlighted and confirmed on standard two-dimensional test cases.

Validation of transition modeling techniques for a simplified fuselage configuration

A Reynolds-averaged Navier-Stokes solver has been equipped with two different transition prediction and modeling techniques for automatic transition prediction on general three-dimensional geometrical configurations. The first technique is a streamline-based approach that applies an automated local linear stability code, the eN-method and a simplified two-N-factor strategy and is applicable with any kind of turbulence model. The second technique is a transition transport equation approach and uses the γ-Reθ,t model. The standard γ-Reθ,t model has been extended by a modeling approach that captures transition due to crossflow instabilities. The extended model can be applied to three-dimensional configuration exhibiting crossflow transition whereas the original γ-Reθ,t model only captures streamwise transition mechanisms. The paper focuses on a three-dimensional inclined 6:1 prolate spheroid which can be considered as a simplified fuselage configuration. For a number of years this configuration has become a standard basic three-dimensional test case for transition prediction approaches and has been used in a number of international workshops and research activities. The focus of the paper is the validation of the two transition modeling approaches for six different angles of attack using two different turbulence models, a standard two-equation eddy-viscosity model and a differential Reynolds-stress model. The simulation results from the two approaches have been compared against each other and to the available experimental data. The comparisons highlight as many qualitative and quantitative similarities and agreements as they reveal quantitative differences and deviations of specific details.

A comparative accuracy and convergence study of eigenerosion and phase-field models of fracture

We compare the accuracy, convergence rate and computational cost of eigenerosion (EE) and phase-field (PF) methods. For purposes of comparison, we specifically consider the standard test case of a center-crack panel loaded in biaxial tension and assess the convergence of the energy error as the length scale parameter and mesh size tend to zero simultaneously. The panel is discretized by means of a regular mesh consisting of standard bilinear or Q1 elements. The exact stresses from the known analytical linear elastic solution are applied to the boundary. All element integrals over the interior and the boundary of the domain are evaluated exactly using the symbolic computation program Mathematica. When the EE inelastic energy is enhanced by means of Richardson extrapolation, EE is found to converge at twice the rate of PF and to exhibit much better accuracy. In addition, EE affords a one-order-of-magnitude computational speed-up over PF.

Additional Yield of Active Case Finding Through House Hold Survey in Selected District of Kabul, Afghanistan, 2018

Every year, 10.4 million people get sick with Tuberculosis (TB) and 40 percent of them do not receive treatment care - they are “missed” to identify by the health systems. Missing TB cases is major challenge in fighting the disease; pose a serious threat to global health security. As per World Health organization estimation, in 2018, Afghanistan missed 27 percent TB case notification. Thus we conducted an active case finding through house hold survey from June – November, 2018 in Kabul city, districts of Afghanistan.
 This descriptive cross sectional study included all the residence of 21 sub-districts in Kabul City Afghanistan. After obtaining written consent, data was collected using a structured questionnaire from all presumptive TB cases. Sputum was collected from eligible participants and tested for AFB, Gene-Xpart and Chest X-ray was done. Standard test algorithm and case categorization was done according to WHO recommendations. Descriptive analysis of demographic, clinical and laboratory data was done.
 A total of 22,596 participants were included and among them 6740 were eligible. Among them 1614(24.3%) individuals were screened positive to have presumptive TB and 105 (6.5%) had laboratory confirmed TB. Thus the case notification rate was 465 in 100,000 populations. Among the presumptive TB cases, mean age was 38.6 + 18.5 years; 74% were female; 72% were housewives/unemployed; 2.6% were smokers.
 The case notification rate and the point prevalence showed increase numbers than the actual reports by the Afghanistan National TB program, which signifies the program was missing significant numbers of TB cases. Hence active TB case finding might be an effective approach in terms of case notification in Afghanistan. We recommend extending active community TB surveillance of the country.

Decentralized Low-Rank State Estimation for Power Distribution Systems

This article considers the low-observability state estimation problem in power distribution networks and develops a decentralized state estimation algorithm leveraging the matrix completion methodology. Matrix completion has been shown to be an effective technique in state estimation that exploits the low dimensionality of the power system measurements to recover missing information. This technique can utilize an approximate (linear) load flow model, or it can be used with no physical models in a network where no information about the topology or line admittance is available. The direct application of matrix completion algorithms requires solving a semi-definite programming (SDP) problem, which becomes computationally challenging for large networks. We therefore develop a decentralized algorithm that capitalizes on the popular proximal alternating direction method of multipliers (proximal ADMM). The method allows us to distribute the computation among different areas of the network, leading to a scalable algorithm. By doing all computations at individual control areas and only communicating with neighboring areas, the algorithm eliminates the need for data to be sent to a central processing unit and thus increases efficiency and contributes to the goal of autonomous control of distribution networks. We illustrate the advantages of the proposed algorithm numerically using standard IEEE test cases.

Exact spatial and temporal balance of energy exchanges within a horizontally explicit/vertically implicit non-hydrostatic atmosphere

A new horizontally explicit/vertically implicit (HEVI) time splitting scheme for atmospheric modelling is introduced, for which the horizontal divergence terms are applied within the implicit vertical substep. The new HEVI scheme is implemented in conjunction with a mixed mimetic spectral element spatial discretisation and semi-implicit vertical time stepping scheme that both preserve the skew-symmetric structure of the non-canonical Hamiltonian form of the equations of motion. Within this context the new HEVI scheme allows for the exact balance of all energetic exchanges in space and time. However since the choice of horizontal fluxes for which this balance is satisfied is not consistent with the horizontal velocity at the end of the time level the scheme still admits a temporal energy conservation error. Linearised eigenvalue analysis shows that similar to a fully implicit method, the new HEVI scheme is neutrally stable for all buoyancy modes, and unlike a second order trapezoidal HEVI scheme is stable for all acoustic modes below a certain horizontal CFL number. The scheme is validated against standard test cases for both planetary and non-hydrostatic regimes. For the planetary scale baroclinic instability test case, the new formulation exhibits a secondary oscillation in the potential to kinetic energy power exchanges, with a temporal frequency approximately four times that exhibited by a horizontally third order, vertically second order trapezoidal scheme. For the non-hydrostatic test case, the vertical upwinding of the potential temperature diagnostic equation is shown to reduce spurious oscillations without altering the energetics of the solution, since this upwinding is performed in an energetically consistent manner. For this test case, which is configured on an affine geometry, the exact balance of energy exchanges allows the model to run stably without any form of dissipation.

Simulation of non-Newtonian viscoplastic flows with a unified first order hyperbolic model and a structure-preserving semi-implicit scheme

We discuss the applicability of a unified hyperbolic model for continuum fluid and solid mechanics to modeling non-Newtonian flows and in particular to modeling the stress-driven solid-fluid transformations in flows of viscoplastic fluids, also called yield-stress fluids. In contrast to the conventional approaches relying on the non-linear viscosity concept of the Navier-Stokes theory and representation of the solid state as an infinitely rigid non-deformable solid, the solid state in our theory is deformable and the fluid state is considered rather as a “melted” solid via a certain procedure of relaxation of tangential stresses similar to Maxwell’s visco-elasticity theory. The model is formulated as a system of first-order hyperbolic partial differential equations with possibly stiff non-linear relaxation source terms. The computational strategy is based on a staggered semi-implicit scheme which can be applied in particular to low-Mach number flows as usually required for flows of non-Newtonian fluids. The applicability of the model and numerical scheme is demonstrated on a few standard benchmark test cases such as Couette, Hagen-Poiseuille, and lid-driven cavity flows. The numerical solution is compared with analytical or numerical solutions of the Navier-Stokes theory with the Herschel-Bulkley constitutive model for nonlinear viscosity.

An original Pitot-static probe design for subsonic boundary layer investigation

This work presents a Pitot-static probe designed and developed to help in some boundary layer (BL) measurements, as for instance when average velocity profiles are required around models already available for wind tunnel tests but not provided with static pressure taps. The probe reads both total and static pressures at the desired location and by traversing it through the BL leads to the desired velocity measurements on the body under test, without having to rebuild the model and without using more expensive and complicated techniques. The probe is presented by describing its geometry in detail and completing the information by the relevant operating instructions. The probe response is then characterized by reporting its directional sensitivity and a validation for a standard test case. The specific probe presented here is suitable for a certain range of BL thicknesses, however the design can be easily scaled to different ranges.