Boundary Layer Mesh Research Articles

Analysis of flow characteristics of a pump turbine under the transient condition of 75% load rejection

Abstract In this study, the flow calculations for a pump turbine under the transient condition of 75% load rejection was conducted. The model incorporated a boundary layer on the surface of the guide vane, with the node coordinates of each element in the boundary layer region being updated. This approach ensured the stability of the boundary layer mesh. Experimental measurements provided parameter curves for inlet pressure, outlet pressure, runner speed, and guide vane opening over time. The pressure fluctuation at each monitoring point was compared with the experimental data, and a good agreement was found. Additionally, the variation curve of the axial hydraulic thrust of the head cover and the pressure distribution nephogram of the head cover at typical moments were analysed. The results of this study could be used as a reference for the safe and stable operation of pumped-storage power stations.

Study on Resistance Loss of Fly Ash Slurry Multistage High‐Pressure Grouting Pipeline Based on Fluent

In order to understand the resistance loss along the way during multistage high‐pressure slurry transportation, the flow state of fly ash slurry in the pipeline was simulated by Fluent software in this paper, and the effects of pipe diameter D, pipe transportation flow rate Q, and fly ash mass concentration Cw on the resistance loss along the pipeline were studied. The fly ash slurry is a non‐Newtonian Bingham fluid that moves in a turbulent state in a pipeline. When simulating the flow of fly ash slurry using Fluent software, the mesh type is a mixed mesh of hexahedron and wedge shapes, and the viscous model is selected as realizable k‐ε Turbulence Model, with Enhanced‐Wall Function (EWF) selected as the wall function, combined with a four‐layer boundary layer mesh, which can more accurately capture the details of velocity changes at the wall, thereby improving the accuracy of the model. The inlet of the model is the velocity inlet, and the outlet is the pressure outlet. The coupled algorithm is chosen as the solution method. Under these conditions, the model converges quickly and the calculation accuracy is high. The results show that the resistance loss along the pipeline decreases as a power function with the increase of pipeline diameter, and there is a polynomial relationship between the pipeline flow and the resistance loss along the pipeline, while the mass concentration of fly ash slurry changes linearly with the resistance loss along the pipeline. In addition, three friction coefficient models, namely Blasius formula, Colebrook–White equation, and Wilson–Thomas model, were selected according to the flow characteristics of fly ash slurry. Based on the Blasius formula with the smallest relative calculation error, the Blasius formula was modified by multiple linear regression analysis to improve the accuracy of the frictional resistance coefficient model and to provide help for the design and use of separate layer grouting conveying system.

Harnessing CFD Modeling Techniques for Optimizing Automotive Streamlines and Reducing Aerodynamic Drag

Computational fluid dynamics (CFD) has emerged as an indispensable tool in automotive design, significantly enhancing vehicle performance through aerodynamic optimization. This paper offers a comprehensive exploration of the application of CFD modeling techniques in refining automotive streamline designs, with the overarching goal of mitigating aerodynamic drag and amplifying vehicle efficiency. To contextualize the discussion, the author commences with a historical overview of fluid dynamics, segueing into a detailed exposition of core CFD algorithms, numerical methodologies, and advanced modeling approaches encompassing boundary layer dynamics, turbulence modeling, meshing strategies, and coupling techniques. Through illustrative case studies, the author elucidates the transformative impact of CFD in streamlining the design process and achieving tangible drag reduction outcomes. The discourse extends to the implications of CFD on fuel economy, vehicular speed, stability, and holistic design considerations. In conclusion, the author addresses the challenges inherent in transposing CFD insights to tangible automotive applications and charts prospective trajectories for CFD's continued evolution in automotive engineering, underscoring its pivotal role in steering the future of vehicular design.

Measurements and Prediction of Ash Deposition in a Cyclone-Fired Boiler Operating under Variable Load Conditions

Measurements of ash deposition rates were made between the secondary superheater and reheater sections of a 450 MW cyclone-fired lignite boiler as the operational load varied from 33 to 100%. Significant reductions in deposition rates with a decrease in operational load were observed. To uncover the causative mechanisms behind these observations, operational data from the power plant were used to carry out computational fluid dynamic (CFD) simulations of the boiler. After ascertaining that the gas temperatures and velocities at various sections within the boiler were being represented adequately, decoupled simulations of the ash deposition process on the deposit probe were carried out using a finely resolved boundary layer mesh. Fly ash particle size distribution (PSD) and its concentration for the decoupled calculations were determined from stand-alone cyclone barrel simulations. The ash partitioning (mass %) between the fly ash and slag was found to be ~50:50, which was in line with previous field observations, and it did not vary significantly across different cyclone loads. The predicted PSD of the deposit ash was concentrated in the size range 10–30 microns, which was in agreement with cross-sectional images of the deposit obtained from the measurements. At lower loads, sharp variations in the deposition rates were predicted in the gas temperature range 950–1150 K. The particle kinetic energy—particle viscosity-based capture methodology utilized in this study in conjunction with appropriate ash compositions, ash viscosity models and gas temperature estimates can help estimate slagging propensities at different loads reasonably well in these systems.

An hp Weak Galerkin FEM for singularly perturbed problems

We present the analysis for an hp Weak Galerkin-FEM for singularly perturbed reaction-convection-diffusion problems in one-dimension. Under the analyticity of the data assumption, we establish robust exponential convergence, when the error is measured in the energy norm, as the degree p of the approximating polynomials is increased. The Spectral Boundary Layer mesh is used, which is the minimal (layer adapted) mesh for such problems. Numerical examples illustrating the theory are also presented.

An [formula omitted] finite element method for a two-dimensional singularly perturbed boundary value problem with two small parameters

We consider a singularly perturbed boundary value problem, of reaction–convection–diffusion type with two small parameters, and the approximation of its solution by the hp version of the Finite Element Method on the Spectral Boundary Layer mesh. We show that the method converges uniformly, with respect to both singular perturbation parameters, at an exponential rate when the error is measured in the energy norm. Numerical examples illustrating our theoretical findings are also included.

Robust Topological Construction of All-hexahedral Boundary Layer Meshes

We present a robust technique to build a topologically optimal all-hexahedral layer on the boundary of a model with arbitrarily complex ridges and corners. The generated boundary layer mesh strictly respects the geometry of the input surface mesh, and it is optimal in the sense that the hexahedral valences of the boundary edges are as close as possible to their ideal values (local dihedral angle divided by 90°). Starting from a valid watertight surface mesh (all-quad in practice), we build a global optimization integer programming problem to minimize the mismatch between the hexahedral valences of the boundary edges and their ideal values. The formulation of the integer programming problem relies on the duality between boundary hexahedral configurations and triangulations of the disk, which we reframe in terms of integer constraints. The global problem is solved efficiently by performing combinatorial branch-and-bound searches on a series of sub-problems defined in the vicinity of complicated ridges/corners, where the local mesh topology is necessarily irregular because of the inherent constraints in hexahedral meshes. From the integer solution, we build the topology of the all-hexahedral layer, and the mesh geometry is computed by untangling/smoothing. Our approach is fully automated, topologically robust, and fast.

Evaluation of polyhedral mesh performance for large-eddy simulations of flow around an isolated building within an unstable boundary layer

The urban heat island effect has been a noticeable issue in recent years. Much research has gone into understanding the thermal field around buildings to help improve human comfort in urban areas. However, the challenges about accuracy and cost of transient simulations of urban thermal environment still remain. This study aimed to evaluate the performance of polyhedral meshes on wind and thermal fields within an unstable boundary layer using large-eddy simulation. A precursor simulation was conducted to determine the wind and temperature fluctuations for the inflow boundary conditions. Through the grid sensitivity test of hexahedral meshes, the setting of 20 grids along the shortest building edge was recommended. The polyhedral cases were conducted following the grid sensitivity test for hexahedral meshes and shortest building edges were discretized into 20 grids. The accuracy of the predicted results obtained by polyhedral meshes was close to that of hexahedral meshes. Polyhedral cases with uniform boundary layer meshes exhibited 20% smaller relative error for standard deviation of temperature near the ground than the polyhedral case without boundary layer meshes. Moreover, the mesh number of the polyhedral mesh was 50% smaller than that of the hexahedral mesh, and its computational time was nearly 50% of that of the hexahedral mesh. This study expands the guidelines of simulations under non-isothermal condition and lays the foundation for further research of thermal environment in urban street and realistic city models to improve residence comfort.

Hp-FEM for reaction–diffusion equations. II: robust exponential convergence for multiple length scales in corner domains

Abstract In bounded, polygonal domains $\varOmega \subset {{\mathbb {R}}}^2$ with Lipschitz boundary $\partial \varOmega $ consisting of a finite number of Jordan curves admitting analytic parametrizations, we analyze $hp$-FEM discretizations of linear, second-order, singularly perturbed reaction–diffusion equations on so-called geometric boundary layer meshes. We prove, under suitable analyticity assumptions on the data, that these $hp$-FEM afford exponential convergence in the natural ‘energy’ norm of the problem, as long as the geometric boundary layer mesh can resolve the smallest length scale present in the problem. Numerical experiments confirm the robust exponential convergence of the proposed $hp$-FEM.

Boundary layer mesh resolution in flow computation with the Space–Time Variational Multiscale method and isogeometric discretization

We present an extensive study on boundary layer mesh resolution in flow computation with the Space–Time Variational Multiscale (ST-VMS) method and isogeometric discretization. The study is in the context of 2D flow past a circular cylinder, at Reynolds number ranging from [Formula: see text] to [Formula: see text]. It was motivated by the need to have in tire aerodynamics a better understanding of the mesh resolution requirements near the tire surface. The focus in the study is on the normal-direction element length for the first layer of elements near the cylinder, with that length varying by a refinement factor ranging from 2 to 40. The evaluation is based mostly on the velocity profile near the cylinder. As the element length for the first layer is varied, the element lengths for the other layers of the disk-shaped inner mesh are adjusted, with no increase in the number of elements for the refinement factors 2, 3, and 4, and with modest increases only in the radial direction for refinement factors beyond that. The computations are performed with quadratic NURBS basis functions in space and linear basis functions in time. The expressions for the stabilization parameters used in the ST-VMS and for the related local lengths scales are those targeting isogeometric discretization, introduced in recent years. The mesh resolution study is based mostly on the strong enforcement of the Dirichlet boundary conditions on the cylinder, but also includes some computations with the weakly-enforced conditions. We expect that the data generated and observations made will be helpful in setting proper near-surface mesh resolution in VMS-based computations with isogeometric discretization, not only for cylindrical shapes but also for comparable geometries. We furthermore expect that although the data generated and observations made are based on computations with nonmoving meshes, they will also be applicable to computations with moving meshes where the mesh around the solid surface rotates with the surface in the framework of the ST Slip Interface method.

Effects of Boundary Layer and Local Volumetric Cells Refinements on Compressor Direct Noise Computation

<div class="section abstract"><div class="htmlview paragraph">The use of turbochargers with downsized internal combustion engines improves road vehicles’ energy efficiency but introduces additional sound sources of strong acoustic annoyance on the turbocharger’s compressor side. In the present study, direct noise computations (DNC) are carried out on a passenger vehicle turbocharger compressor. The work focuses on assessing the influence of grid parameters on the acoustic predictions, to further advance the maturity of the acoustic modelling of such machines with complex three-dimensional features. The effect of the boundary layer mesh structure, and of the spatial resolution of the mesh, on the simulated acoustic signatures is investigated on detached eddy simulations (DES). Refinements in the core mesh are applied in areas of major acoustic production, to generate cells with sizes proportional to the local Taylor microscale values. Such an educated guess allows for quality enhancement with a smaller increase in computational costs as compared to more general overall refinements. The reflection-free simulation results are validated against experiments. The experimental data were post-processed with methods from the two-port theory to represent pure acoustic source power density for the acoustic modes, cleaned from test-domain-specific reflections. A detailed comparison between experiments and numerical simulations is carried out. As a result of this study, the most critical parameters for the numerical prediction of turbocharger noise are presented. The results can, furthermore, be used to improve the understanding of grid construction when predicting noise signature for compressor flows.</div></div>

Effect of Cavitation Formation on Velocity and Vorticity

In this study, flow in a rectangular shape nozzle has been simulated. Fully hexahedral mesh elements are utilized with having boundary layer mesh in order to avoid excessive mesh destiny in the orifice area. It was found that velocity magnitude, axial velocity and vorticity magnitude increases intensely at the beginning of the orifice area which makes it possible for the cavitation phenomena to occur rapidly. The cavitation that occurs inside diesel fuel injector nozzles has a significant impact on atomization, and modelling this phenomenon will be useful in future injector development. The Schnerr cavitation model was chosen among three distinct cavitation models available in Ansys Fluent v21 because it is capable of properly forecasting choke conditions in cavitating flow. The two-phase flow inside a nozzle is assumed to be a homogenous combination of vapor, liquid, and noncondensable gas in this model. The pressure and velocity profiles derived from the simulations were compared to planar throttle flow experimental findings.

Study of typical water header flow structure by large eddy simulation and RANS

Water header is the most common structure in the design of flow system for energy and power system. The complex flow structure could result in some problems when CFD simulation is applied in the whole system analysis. The rapid change in velocity distribution of the flow field leads to difficulties to create suitable boundary-layer mesh, and the complex flow structure will also make residuals hard to reach convergence criteria. Large eddy simulation is promising to promote these studies, it is more accurate than RANS method and can capture many non-steady-state characteristics those RANS method cannot obtain. In this study a typical water header flow structure is investigated by RANS and large eddy simulation methods. By comparing the detailed flow structures in the results of two methods, the deficiency of RANS method was found. The results of large eddy simulation can be used to guide the establishment of meshes and the application of time-averaged turbulence models to improve efficiency in engineering. The asymmetric Reynolds stresses may induce asymmetric flow field in symmetric geometry.

Mixed hp finite element method for singularly perturbed fourth order boundary value problems with two small parameters

AbstractWe consider fourth order singularly perturbed boundary value problems with two small parameters, and the approximation of their solution by the version of the finite element method on the spectral boundary layer mesh from Melenk et al. We use a mixed formulation requiring only basis functions in two‐dimensional smooth domains. Under the assumption of analytic data, we show that the method converges uniformly, with respect to both singular perturbation parameters, at an exponential rate when the error is measured in the energy norm. Our theoretical findings are illustrated through numerical examples, including results using a stronger (balanced) norm.

Comparison of hexahedral, tetrahedral and polyhedral cells for reproducing the wind field around an isolated building by LES

The wind field around a 1:1:2 isolated building was predicted by large-eddy simulation for evaluating the pedestrian-level wind environment. This study focused on the effects of cell types (hexahedral, tetrahedral and polyhedral cells) and boundary layer mesh on the time-averaged and fluctuating wind characteristics. The minimum cell size and stretching ratio were set to be the same among all cases. The case composed of hexahedral cells was found to have the best agreement with the experiment among the three cell types. The accuracies of the polyhedral cases are close to that of the hexahedral case, and better than those of the tetrahedral cases. However, the polyhedral mesh is most economical for the computational resources since the cell numbers of the polyhedral cases are less than half of that of the hexahedral case and about a quarter of those of the tetrahedral cases. It was also found that the boundary layer mesh does not improve the numerical accuracy under any circumstances. For both tetrahedral and polyhedral meshes, the boundary layer mesh can improve the numerical accuracy in the region above the flat ground by reducing the mesh non-orthogonality and skewness. However, the boundary layer mesh was found to worsen the mesh quality in the local region around the sharp corners of the building for both tetrahedral and polyhedral meshes. As a result, the boundary layer mesh did not lead to the expected improvement of numerical accuracy of wind velocity in the sensitive region of the separated flow from the sharp corners.

NNW-GridStar: Interactive structured mesh generation software for aircrafts

The structured meshes are widely utilized in the aircraft industry because of their efficiency in achieving accurate solutions, especially for viscous flows of complex configurations. However, they are usually generated by engineers through enormous and tedious interaction with the graphical user interface of mesh generation software. To address this issue, we implement one piece of interactive structured mesh generation software called NNW-GridStar. First, we present its design details, including the software architecture and data structure. Second, we demonstrate its implementation and emphasize on the accelerated techniques proposed in our software, which consist of automatic boundary-layer mesh generation, rapid block assembly, multiblock stretching, and O-type block stretching. Finally, we perform a deep comparison between the developed NNW-GridStar and another two pieces of well known commercial software, and then demonstrate the application of NNW-GridStar in the productional CFD simulations. Experimental results show that NNW-GridStar has an outstanding generation speed and can produce meshes with high quality.

Influence of non-uniform inflow swirl and hot streak on turbine vane

The turbine vane bears higher and higher temperature of upstream flow with the higher requirements for the engine thrust. The heat resistance of turbine vanes is a severe problem for the lifetime of turbine vanes. Therefore, it is essential to study the flow field and heat transfer of the vane to find better ways to improve the heat resistance of the turbine and increase the thrust of the engine. To improve the efficiency of the turbine vanes, the present paper conducted researches on the heat transfer and the pressure loss of turbine vanes under different radius ratio of swirl and hot streak. The numerical applies CFD 3D RANs solver with the SST turbulent model to get the influence of non-uniform inflow velocity and temperature distribution on turbine vanes. The present numerical model utilizes turbine vane C3X. Considering the viscosity of the fluid causes shear stress on the wall, the paper set the boundary layers mesh in the numerical model. The size of the first boundary layer mesh is 10–6 m. The results show that the hot fluid on the suction side migrates towards the hub, and the hot fluid on the pressure side migrates towards the shroud while there are coupling effect of the swirl and hot streak. The larger the radius ratio of the hot streak and the swirl (RHS) is, the higher the temperature of the local high-temperature regions exist on the suction side of leading edge and the pressure side of the trailing edge is, and the higher the pressure loss is.

ポリヘドラルメッシュを用いた建物周辺気流のLarge-Eddy Simulations: 建物壁面に配置した境界層メッシュは風環境の予測精度の改善に有効か？

ポリヘドラルメッシュを用いた建物周辺気流のLarge-Eddy Simulations: 建物壁面に配置した境界層メッシュは風環境の予測精度の改善に有効か？

Anisotropic boundary layer mesh generation for reliable 3D unsteady RANS simulations

This paper proposes a Computational Fluid Dynamics (CFD) framework with the aim of combining consistency and efficiency for the numerical simulation of high Reynolds number flows encountered in engineering applications for aerodynamics. The novelty of the framework is the combination of a Reynolds-Averaged Navier–Stokes (RANS) model with an anisotropic mesh adaptation strategy handling arbitrary immersed geometries by building the corresponding boundary layer meshes. The numerical algorithm consists of robust and accurate solution of the unsteady incompressible Navier–Stokes equations supplemented with a Spalart–Allmaras turbulence model and boundary layer remeshing relying on a specifically designed metric. The flow solver is formulated as a Variational Multiscale (VMS) finite element method for the momentum balance and the incompressibility constraint, and as an upwind Petrov–Galerkin method for the nonlinear turbulent equation. The boundary layer remeshing strategy is flexible as it allows the adaptation of arbitrary coarse meshes by modifying the size and the orientation of elements along the immersed boundary to ensure a smooth gradation along the curvature of the body's geometry. The solver is capable of handling highly stretched anisotropic elements and is shown to successfully predict both mean and fluctuating drag/lift coefficients. Laminar and turbulent test cases in 2D and 3D are presented to assess the performance of this framework against experimental results relevant to external aerodynamics, including an airship and a flying drone.

Numerical and experimental studies of water disinfection in UV reactors.

Performance of UV reactors for water disinfection is investigated in this paper. Both experimental and numerical studies are performed on base reactor LP24. Enterobacteria phage MS2 is chosen as the challenge microorganism in the experiments. Experiments are conducted to evaluate the effect of different parameters, i.e. flow rate and UV transmission, on the reactor performance. Simulation is carried out based on the commercial software ANSYS FLUENT with user defined functions (UDFs) implemented. The UDF is programmed to calculate UV dose absorbed by different microorganisms along their flow trajectories. The effect with boundary layer mesh and without boundary layer mesh for LP24 is studied. The results show that the inclusion of boundary layer mesh does not have much effect on the reactor performance in terms of reduction equivalent dose (RED). The numerical results agree well with the experimental measurements, hence validating the numerical model. With this achieved, the numerical model is applied to study other scaled reactors: LP12, LP40, LP60 and LP80. Comparisons show that LP40 has the highest RED and log inactivation among all the reactors while LP80 has the lowest RED and log inactivation.