Computational Fluid Dynamics Problems Research Articles

Predicting urban heat island circulation using CFD

Most existing microscale computational fluid dynamics (CFD) models and mesoscale meteorological models cannot consider multi-scale urban wind flows, as neither can completely take into account the mesoscale and microscale physics, and their interactions. Here we suggest a CFD model which has good potential for development of the meso-micro scale models for predicting and designing multi-scale urban airflows. The principal idea is to use CFD numerical methods to solve a set of governing equations with appropriate boundary conditions that can govern both major mesoscale and microscale flow characteristics. Based on the coordinate transformation method proposed by Kristóf, Rácz and Balogh (2009), we derived a similar set of governing equations with some improvements and used an existing porous turbulence model for modeling the urban canopy layer. The approach was then successfully implemented using a commercial CFD package (Fluent) for studying urban heat island circulation (UHIC), which is considered to be one of the most difficult problems in CFD. Our predicted mean quantities agree well with existing data in the literature obtained from large eddy simulations, mesoscale models, and laboratory experiments. Our predicted results also reveal the effects of the different heat fluxes and urban height of a city on UHIC characteristics.

A Multi-user Performance Analysis Framework for CFD Simulations

This paper proposes a multi-user performance analysis framework for computational fluid dynamics (CFD) simulations. Independent performance datasets are given according to different requirements of computer programmers, algorithm developers and application developers. The performance analysis framework is designed based on the general procedure of CFD processing: pre-processing, problem solving and post-processing. Optimisation of the data acquisition is carried out based on data dependence to realise lightweight. A profiler is implemented in OpenFOAM based on the framework, and experiments are conducted to verify the performance information extraction and analysis. The results on a subsystem of Tianhe-1A indicate that the multiuser performance tool can probe the performance information successfully during the parallel execution. And from the performance results, we found that the direction of mesh partition turns out to be a critical factor of simulation performance for CFD problems.

The numerical analysis of wind turbine airfoils at high angles of attack

Despite decades of research, the accurate numerical simulation of severely separated flows is still one of the major problems in computational fluid dynamics. In this paper, a viscous-coupled 3D panel method is proposed for the aerodynamic analysis of wind turbine airfoils at high angles of attack. The Hess–Smith panel method is adopted for inviscid calculations, and an empirically based boundary layer analysis is performed in order to determine the separation point. The separated thick wake is then modelled as an extension of the surface geometry along which a constant pressure distribution is assumed. The wake geometry is determined iteratively, and an outer iterative loop is run to update the location of the separation point. The validity of the current method’s results is assessed by comparison with experimental and numerical results for several high thickness wind turbine airfoils, namely NACA 63-430, FFA-W3-301, FFA-W3-241, and DU 91-W2-250. At low angles of attack, pressure data predicted by the current method show excellent agreement with the experimental data, as well as the referenced numerical data. At higher angles of attack, the current method shows reasonable agreement with the experimental data, while the referenced numerical data significantly overestimate the −C p distribution along the suction surface.

A path-conservative finite volume scheme for compressible multi-phase flows with surface tension

The accurate simulation of compressible multi-phase flows with surface tension effects is currently still one of the most challenging problems in computational fluid dynamics (CFD). The basic difficulties are the capturing of the correct interface dynamics between the two fluids as well as the computation of the interface curvature. In this paper, we present a novel path-conservative finite volume discretization of the continuum surface force method (CSF) of Brackbill et al. to account for the surface tension effect due to curvature of the phase interface. This is achieved in the context of a diffuse interface approach, based on the seven equation Baer–Nunziato model of compressible multi-phase flows. Such diffuse interface methods for compressible multi-phase flows including capillary effects have first been proposed by Perigaud and Saurel. In the CSF method, the surface tension effect is replaced by a volume force, which is usually integrated as a classical volume source term. However, since this source term contains the gradient of a color function that is convected with the flow velocity, we propose to integrate the CSF source term as a non-conservative product and not simply as a source term, following the ideas on path-conservative finite volume schemes put forward by Castro and Parés.For that purpose, we use the new generalized Osher-type Riemann solver (DOT), recently proposed by Dumbser and Toro and compare it with a path-conservative Roe and Rusanov scheme. Via numerical evidence we can show that if the curvature computation is exact, our new scheme is well-balanced for a steady circular bubble in equilibrium according to the Young–Laplace law. This means that the pressure jump term across the interface in the momentum equation is exactly balanced with the surface tension force.We apply our scheme to several one- and two-dimensional test problems, including 1D Riemann problems, an oscillating droplet, as well as a deforming droplet initially attached to a wall and which is subject to gravity and surface tension forces. Finally, we also check the rising of a gas bubble due to buoyancy forces. We compare our numerical results with those published previously in the literature.

A feature-based mesh adaptation for the unsteady high speed compressible flows in complex three-dimensional domains

We propose an unstructured mesh adaptation approach for unsteady high speed compressible Navier–Stokes applications involving blasts and explosions with the presence of strong shock waves propagating in three dimensional complex domains. The idea is to identify the locations of critical physics locally and then re-mesh these regions based on solution derived metrics. The approach ensures both geometry fidelity and mesh validity, especially for areas near complex geometries, a task that is always a challenge in mesh adaptation. The proposed adaptivity is applied for simulations of blast wave propagations and compared with available data in literature. The results show that the proposed method is fully robust and efficient for computational fluid dynamics (CFD) problems in complex three-dimensional domains.

Multidimensional data analysis and visualization for time-dependent CFD problems

This paper is devoted to analysis and visualization of multidimensional data in problems of computational fluid dynamics (CFD). Multidimensional data are considered as the result of parametrical search and optimizing analysis. For such types of multidimensional data, analysis is aimed to find some hidden dependencies of specific parameters. This is the main difference of the proposed approach from the general approach of data analysis, which is usually applied to solve problems of classification and clusterization. A rough approximate approach is proposed for data processing. This approach includes data visualization in the space of principal components and data approximation by geometrical primitives (in particular, planes). An example of practical application of the proposed approach is given. The possibility of elastic maps application to multidimensional data in question is discussed.

Digital Rocks: Emerging Technology to Business-Proven Capability

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170752, “Digital Rocks: Developing an Emerging Technology Through to a Proven Capability Deployed in the Business,” by J.T. Fredrich, D.L. Lakshtanov, N.M. Lane, E.B. Liu, C.S. Natarajan, D.M. Ni, and J.J. Toms, BP, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed. This paper describes the development of “digital-rocks” technology, in which high-resolution 3D image data are used in conjunction with advanced modeling and simulation methods to measure petrophysical rock properties. This technology has developed into a proven capability sanctioned for use in the operating assets of a large multinational oil company. Digital-rocks technology can improve evaluation of reservoir quality and quantification of hydrocarbons in place, and it can help inform the development of improved-recovery methods. Introduction The premise of the technology is to apply state-of-the-art high-resolution 3D imaging technologies to derive digital descriptions of reservoir rocks, typically at a resolution of approximately 1–5 Μm, or even lower for complex rocks with significant microporosity less than 1 Μm in dimension. These digital descriptions can be thought of as a numerical grid or mesh and, in conjunction with the appropriate models and numerical algorithms, can be used to perform massively parallel simulations of pore-scale processes of interest. These simulations can then be interpreted to derive macroscopic static and dynamic rock properties. Because the geometrically and topologically complex pore-scale architecture of the rock is captured in the 3D image and faithfully represented in the computational mesh, the numerical solution of the underlying physics is not compromised as long as the image is acquired with the requisite fidelity, and at a suitable resolution and over a representative elementary volume. In the field of computational physics, this general approach is known as direct numerical simulation (DNS). While DNS is often used to describe a solution method for problems in computational fluid dynamics, there exists an underlying generality and the same concept can be applied to other partial-differential equations (PDEs) of interest, thereby expanding application to various rock properties in addition to intrinsic permeability.

Parallel adaptive simplical re-meshing for deforming domain CFD computations

Deforming domains occur in many fields of computational fluid dynamics (CFD), such as interface tracking, simulation of pumps and engines, and fluid/structure interaction. The deformation of the domain presents a challenge to the integrity of the computational mesh; substantial motion of the domain boundaries requires vertex motion and changes in mesh connectivity. For cases of simple boundary motion or structured meshes, predetermined changes to the mesh structure can be sufficient. However, without a priori knowledge of how the domain will change, a more robust solution is required. The present work offers a parallelized solution for simplical meshes that is well-suited to extremely complex geometry. The mesh continuously evolves without user intervention or the use of target meshes. Varying length scales imposed by evolving boundary curvatures and narrow gaps are resolved with a fast length-scale algorithm. The set of algorithms are incorporated into an object-oriented code structure that permits broad application to a range of CFD problems. The robustness and versatility of the algorithm is demonstrated in several examples, representing motion of internal and external boundaries, where the boundary motion may or may not be known a priori.

Software for numerical simulation of convection in spherical shells for hybrid CPU/GPU computing systems

This paper describes software developed by the authors for numerical 3D simulation of problems on hydrodynamics and heat transfer in spherical shells. The software was based on the OpenCL standard and intended for hybrid CPU/GPU computing systems with graphical processing units. The computations were implemented on a computing system with an Nvidia GTX Titan graphics accelerator. The presented results of the tests demonstrate the correctness of the obtained numerical solutions and the efficient use of GPUs for solving problems of computational fluid dynamics.

Evaluation of multilevel sequentially semiseparable preconditioners on computational fluid dynamics benchmark problems using Incompressible Flow and Iterative Solver Software

This paper studies a new preconditioning technique for sparse systems arising from discretized partial differential equations in computational fluid dynamics problems. This preconditioning technique exploits the multilevel sequentially semiseparable (MSSS) structure of the system matrix. MSSS matrix computations give a data‐sparse way to approximate the LU factorization of a sparse matrix from discretized partial differential equations in linear computational complexity with respect to the problem size. In contrast to the standard block diagonal and block upper‐triangular preconditioners, we exploit the global MSSS structure of the 2×2 block system from the discretized Stokes equation and linearized Navier‐Stokes equation. This avoids approximating the Schur complement explicitly, which is a big advantage over standard block preconditioners. Through numerical experiments on standard computational fluid dynamics benchmark problems in Incompressible Flow and Iterative Solver Software, we show the performance of the MSSS preconditioners. They indicate that the global MSSS preconditioner not only yields mesh size independent convergence but also gives viscosity parameter and Reynolds number independent convergence. Compared with the algebraic multigrid (AMG) method and the geometric multigrid (GMG) method for block preconditioners, the MSSS preconditioning technique is more robust than both the AMG method and GMG method, and considerably faster than the AMG method. Copyright © 2015 John Wiley & Sons, Ltd.

Molecular dynamics-based triangulation algorithm of free-form parametric surfaces for computer-aided engineering

A new computational approach for automated triangulation of Computer-Aided Design (CAD) surface models, applicable to various CFD (Computational Fluid Dynamics) problems of practical interest is proposed. The complex shaped product configurations are represented by a set of Non-Uniform Rational B-Splines (NURBS) surface patches. The suggested technique is based on the molecular dynamics method. The main idea of the approach is that the mesh nodes are considered as similarly charged interacting particles which move within the region to be meshed under the influence of internal (such as particle–particle interaction forces) and external forces as well as optional additional forces. Moreover, the particles experience a medium resistance due to which the system comes to equilibrium within a relatively short period of time. The proposed 3D surface mesh generation algorithm uses a parametric NURBS representation as initial definition of the domain boundary. This method first distributes the interacting nodes into optimal locations in the parametric domain of the NURBS surface patch using molecular dynamics simulation. Then, the well-shaped triangles can be created after connecting the nodes by Delaunay triangulation. Finally, the mapping from parametric space to 3D physical space is performed. Since the presented interactive algorithm allows to control the distance between a pair of nodes depending on the curvature of the NURBS surface, the method generates high quality triangular mesh. The algorithm enables to produce uniform mesh, as well as anisotropic adaptive mesh with refinement in the large gradient regions. The mesh generation approach has the abilities to preserve the representation accuracy of the input geometry model, create a close relationship between geometry modeling and grid generation process, be automated to a large degree. Some examples are considered in order to illustrate the method’s ability to generate a surface mesh for a complicated CAD model.

Analysis of Space-time Structures Appearance for Non-Stationary CFD Problems

The paper presents a combined approach to finding conditions for space-time structures appearance in non-stationary flows for CFD (computational fluid dynamics) problems. We consider different types of space-time structures, for instance, such as boundary layer separation, vortex zone appearance, appearance of oscillating regimes, transfer from Mach reflection to regular one for shock waves, etc. The approach combines numerical solutions of inverse problems and parametric studies. Parallel numerical solutions are implemented. This approach is intended for fast approximate estimation for dependence of unsteady flow structures on characteristic parameters (or determining parameters) in a certain class of problems. The numerical results are presented in a form of multidimensional data volumes. To find out hidden dependencies in the volumes some multidimensional data processing and visualizing methods should be applied. The approach is organized in a pipeline fashion. For certain classes of problems the approach allows obtaining the sought-for dependence in a quasi-analytical form. The proposed approach can be considered to provide some kind of generalized numerical experiment environment. Examples of its application to a series of practical problems are given. The approach can be applied to CFD problems with ambiguities.

Benchmarking multi-GPU communication using the shallow water equations

The shallow water model equations provide a simple yet realistic benchmark problem in computational fluid dynamics (CFD) that can be implemented on a variety of computational platforms. Graphical processing units (GPUs) can be used to accelerate such applications on a single device using a data parallel decompositional scheme or with multiple devices using a domain decompositional approach. A shallow water equation simulation is implemented on a range of modern GPU architectures and multi-GPU systems. The typical performance of these systems using the two main device-device communication methods is reported on for single- and double-precision calculations.

Numerical investigation on two-dimensional boundary layer flow with transition

As a basic problem in many engineering applications, transition from laminar to turbulence still remains a difficult problem in computational fluid dynamics (CFD). A numerical study of one transitional flow in two-dimensional is conducted by Reynolds averaged numerical simulation (RANS) in this paper. Turbulence model plays a significant role in the complex flows’ simulation, and four advanced turbulence models are evaluated. Numerical solution of frictional resistance coefficient is compared with the measured one in the transitional zone, which indicates that Wilcox (2006) k-ω model with correction is the best candidate. Comparisons of numerical and analytical solutions for dimensionless velocity show that averaged streamwise dimensionless velocity profiles correct the shape rapidly in transitional region. Furthermore, turbulence quantities such as turbulence kinetic energy, eddy viscosity, and Reynolds stress are also studied, which are helpful to learn the transition’s behavior.

English

In this project we describe the complete process of modeling and simulation of computational fluid dynamics (CFD) problems that occur in engineering practice. We focus mainly on the simulation of the airflow around the aircraft. The fluid flow simulations are obtained with the CFX software package ANSYS. We use the solver based on the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE). The important part is the preparation of the model with the software ANSYS. We will describe the preprocessing including the creation and modification of the surface mesh in ANSYS and the three-dimensional volume grid generation. We discuss the generation of the threedimensional grid by the snappy Hex Mesh tool, which is included in the ANSYS package. Further, we present a way of analyzing the results and some of the outputs of the simulations and following analysis. The CFD simulations were performed on the computational model of the commercial aircraft. The computations were performed for different model settings and computational grids. It means that we considered laminar and turbulent flow and several combinations of the angle of attack and inlet velocity. This case study aims to perform a CFD analysis on an aircraft model using CFX solver. While performing the simulations, meshing techniques, pre-processing and post processing sections and evaluation of a simulation is being learnt. Coefficient of lift and drag were also recorded as a user input data. These values were also compared by running two different simulations with one change of input parameter i.e. angle of attack and inlet velocity

Richardson Extrapolation-Based Discretization Uncertainty Estimation for Computational Fluid Dynamics

This study investigates the accuracy of various Richardson extrapolation-based discretization error and uncertainty estimators for problems in computational fluid dynamics (CFD). Richardson extrapolation uses two solutions on systematically refined grids to estimate the exact solution to the partial differential equations (PDEs) and is accurate only in the asymptotic range (i.e., when the grids are sufficiently fine). The uncertainty estimators investigated are variations of the grid convergence index and include a globally averaged observed order of accuracy, the factor of safety method, the correction factor method, and least-squares methods. Several 2D and 3D applications to the Euler, Navier–Stokes, and Reynolds-Averaged Navier–Stokes (RANS) with exact solutions and a 2D turbulent flat plate with a numerical benchmark are used to evaluate the uncertainty estimators. Local solution quantities (e.g., density, velocity, and pressure) have much slower grid convergence on coarser meshes than global quantities, resulting in nonasymptotic solutions and inaccurate Richardson extrapolation error estimates; however, an uncertainty estimate may still be required. The uncertainty estimators are applied to local solution quantities to evaluate accuracy for all possible types of convergence rates. Extensions were added where necessary for treatment of cases where the local convergence rate is oscillatory or divergent. The conservativeness and effectivity of the discretization uncertainty estimators are used to assess the relative merits of the different approaches.

The efficient implementation of correction procedure via reconstruction with graphics processing unit computing

Computational fluid dynamics (CFD) has long been a useful tool to model fluid flow problems across many engineering disciplines, and while problem size, complexity, and difficulty continue to expand, the demands for robustness and accuracy grow. Furthermore, generating high-order accurate solutions has escalated the required computational resources, and as problems continue to increase in complexity, so will computational needs such as memory requirements and calculation time for accurate flow field prediction. To improve upon computational time, vast amounts of computational power and resources are employed, but even over dozens to hundreds of central processing units (CPUs), the required computational time to compute solutions can be weeks, months, or longer, which is particularly true when generating high-order accurate solutions over large computational domains. One response to lower the computational time for CFD problems is to implement current CFD solvers on graphical processing units (GPUs). GPUs have illustrated the ability to solve problems orders of magnitude faster than their CPU counterparts with identical accuracy. The goal of the presented work is to combine a CFD solver and GPU computing with the intent to solve complex problems at a high-order of accuracy while lowering the computational time required to generate the solution. The CFD solver should have high-order spacial capabilities to evaluate small fluctuations and fluid structures not generally captured by lower-order methods (2nd and 1st order) and be efficient for the GPU architecture. This research combines the high-order Correction Procedure via Reconstruction (CPR) method for explicit time-stepping with compute unified device architecture (CUDA) from NVIDIA to reach these goals. In addition, the study demonstrates accuracy of the developed solver by comparing results with other solvers and exact solutions. Finally, to illustrate speed-ups, a single core Intel Xeon X5650 at 2.67GHz is compared against a Tesla C2070 and a K20x, where a maximum speed-up of nearly 70x is demonstrated.

In-plane mesh regularization for node-based shape optimization problems

In this contribution a global and linear method for controlling the surface mesh quality during node-based shape optimization is presented. In this method, an artificial stress is applied on the surface mesh and the global linear system of equations is solved for the equilibrium state by finite elements. The applied stress adapts each element towards a desired predefined template geometry and at the end a globally smooth mesh is achieved. In this way both the shape and the size of each element is controlled. Some examples, as well as possible fields of application are shown. At the end, the strength of the method in shape optimization of computational fluid dynamics problems is described.

Current visualization trends in CFD problems

The paper presents authors view at the current trends of visualization in CFD (computational fluid dynamics) problems. The main factors of influence defining these trends are considered. Some examples of methods synthesis for better insight of numerical experiments data are enclosed. Also the prospects of such synthesis applying to construction of generalized experiment are discussed. Mathematics Subject Classification: 76N25, 76M27, 65Y05

On the finite-element-based lattice Boltzmann scheme

The finite-element-based lattice Boltzmann scheme for the computations on unstructured meshes is proposed. The scheme is based on the weighted residuals method with Galerkin approximation. The scheme is reduced to the solution of the linear algebraic system at every time step. The possibilities of application of the developed scheme for practical problems are demonstrated on the applications to two test problems of computational fluid dynamics. The investigation of the equilibrium boundary conditions for distribution functions is performed. The obtained results demonstrate that this type of boundary conditions can produced a results which are in good agreement with the results obtained from the solution of hydrodynamical equations at Re ≥ 20. The algorithm of the proposed scheme is realized in GNU license software freefem++ v.3.20. Mathematics Subject Classification: 76-04; 76M10; 65N30