Automatic Grid Generation Research Articles

FFVHC-ACE: Fully Automated Cartesian-Grid-Based Solver for Compressible Large-Eddy Simulation

This study presents a fully automated Cartesian-grid-based compressible flow solver, named FrontFlow/Violet Hierarchical Cartesian for Aeronautics based on Compressible-flow Equations (FFVHC-ACE), for large-eddy simulation (LES) and its aeronautical applications. FFVHC-ACE enables high-fidelity LES of high-Reynolds-number flows around complex geometries by adopting three key numerical methods: hierarchical Cartesian grids, wall modeling in LES, and the kinetic-energy and entropy preserving (KEEP) scheme. The hierarchical Cartesian grids allow fully automated grid generation for complex geometries in FFVHC-ACE, and high-fidelity LES of high-Reynolds-number flows around complex geometries is realized by the wall modeling and the KEEP scheme on the non-body-fitted hierarchical Cartesian grids. We apply FFVHC-ACE to wall-modeled LES around high-lift aircraft configurations at wind-tunnel-scale Reynolds number and real-flight Reynolds number , demonstrating the capability of FFVHC-ACE for fully automated grid generation and high-fidelity simulations around complex aircraft configurations. The computed flowfield and aerodynamic forces at the wind-tunnel-scale Reynolds number agree well with the experimental data provided in the past AIAA High Lift Prediction Workshop (Rumsey et al., Journal of Aircraft, Vol. 56, No. 2, 2019, pp. 621–644). Furthermore, the wall-modeled LES at the real-flight Reynolds number shows good agreement of the lift coefficient with flight-test data.

Application of immersed boundary method in turbomachines

Simulating unsteady turbulent flow in turbomachines is still challenging due to the complexity of blade geometry and relative motion between rotor and stator. This study presents an Immersed Boundary Method (IBM) for high-Reynolds turbomachinery internal flows, and shows the advantage of the automatic grid generation techniques and flexible moving boundary treatments. The wall functions are used in the present method to alleviate the wall resolution restriction of turbulence simulation. The Two-Dimensional (2-D) IBM solver, which was previously developed and tested for a low-speed compressor, is further validated for a well-documented Low-Pressure Turbine (LPT) cascade. Both the blade loading and the total pressure losses in the wake are well captured by the present 2-D solver. The complex Three-Dimensional (3-D) effects in turbomachines motivate the further development of an extended 3-D IBM solver by using a curvilinear-coordinate system that facilitates the hub and casing boundary treatment. The good performance of the 3-D solver is demonstrated through comparison with CFX solver solutions for the rotor configuration of Advanced Noise Control Fan (ANCF). Further effects of the grid resolution on capturing the blade wake are discussed. The results indicate that the present 3-D solver is capable of reproducing the evolution of the blade wake with suitable computational grid.

Application of immersed boundary based turbulence wall modeling to the Ahmed body aerodynamics

This paper applies a recently developed immersed boundary-turbulence wall modeling approach to turbulent flows over a generic car geometry, known as the Ahmed body, under massive flow separation within a lattice Boltzmann solver. Although the immersed boundary method combined with hierarchical Cartesian grid offers high flexibility in automatic grid generation around complex geometries, the near-wall solution is significantly deteriorated compared to the body-fitted simulation, especially when coupled to wall models for turbulent flows at high Reynolds number. Enhanced wall treatments have been proposed in the literature and validated for attached flow configurations. In this work, the Ahmed body with a slant surface of angle 35° is considered where the flow separates massively over the slant surface and the vertical base. The large eddy simulation is performed with a Reynolds stress constraint near-wall. The eddy viscosity is computed dynamically by taking into account the actually resolved Reynolds stresses. It approaches the mixing length eddy viscosity in attached boundary layers and returns to the subgrid eddy viscosity in detached boundary layers. An explicit equilibrium wall model has also been proposed to accelerate the calculation. Comparison with the no-slip boundary condition on the separated surfaces shows that the near-wall treatments with the equilibrium wall model operate reasonably well on both attached and detached boundary layers.

An Automatic Isotropic Triangular Grid Generation Technique Based on an Artificial Neural Network and an Advancing Front Method

To improve the efficiency and automation of the traditional advancing front method (AFM) of unstructured grid generation, a novel isotropic triangular generation technique is developed based on an artificial neural network (ANN). First, some existing high-quality triangular grids are used as data sources, and then an automatic extraction method of training dataset is proposed. Second, the dataset is input into the ANN to train the network by the back-propagation (BP) algorithm, and then some typical patterns are identified through iterative learning. Third, after inputting the initial discretized fronts, the grid generator starts from the shortest front, and the adjacent front information is collected as the input of the neural network to choose the most proper pattern and predict the coordinates of the new point until the grid covers the whole computational domain. Finally, the initial grid is smoothed to further improve the grid quality. Some typical two-dimensional (2D) geometries are tested to validate the capability of the ANN-based advancing front triangle generator. The experimental results demonstrate that the efficiency of the proposed ANN-based triangular grid generator is about 30 percent higher than that of the traditional AFM, and grid quality has also been improved significantly.

Aerodynamic Prediction of Aircraft High Lift Configuration Using Hierarchical Cartesian Grid and Immersed Boundary Method

In this study, the steady RANS simulation around CRM-HL is conducted to investigate the prediction capability of the hierarchical Cartesian grid flow solver to predict the aerodynamic performance of an aircraft high lift configuration. To calculate the turbulent boundary layer, the immersed boundary method and the wall function are used. Three types of grids are used, and their grid numbers are about 100M, 200M, and 400M. The calculated aerodynamic coefficients agreed well with the wind tunnel test results in the low angle of attack region. On the other hand, the calculated aerodynamic coefficients showed a discrepancy in the aerodynamics coefficients to the wind tunnel test at high angle of attack. In addition, grid convergence could not be confirmed for CD and CM at high angle of attack. However, the RANS method used in this paper is not a model which can capture physical phenomena related to separation. Hence, reasonable results for RANS were obtained semi-automatically through automatic grid generation.

Voxel-based simulation of flow and temperature in the human nasal cavity

The nasal airway is an extremely complex structure, therefore grid generation for numerical prediction of airflow in the nasal cavity is time-consuming. This paper describes the development of a voxel-based model with a Cartesian structured grid, which is characterized by robust and automatic grid generation, and the simulation of the airflow and air-conditioning in an individual human nasal airway. Computed tomography images of a healthy adult nose were used to reconstruct a virtual three-dimensional model of the nasal airway. Simulations of quiet restful inspiratory flow were then performed using a Neumann boundary condition for the energy equation to adequately resolve the flow and heat transfer. General agreements of airflow patterns, which were a high-speed jet posterior to the nasal valve and recirculating flow that occupied the anterior part of the upper cavity, and temperature distributions of the airflow and septum wall were confirmed by comparing in-vivo measurements with numerical simulation results.

Large-Scale Finite Difference Grid Generation and Visualization Method Based on Ray Casting Algorithm

Finite difference method (FDM) is one of the most popular method for explosion simulation. Large-scale, accuracy and automatic grid generation is the essential for numerical simulation. Two kinds of grid generation method are wildly used: ray casting method and slicing method. In this paper, the ray casting method is optimized and some special cases of algorithm are analyzed, which improved the efficiency and stability for large-scale grid generation. A facet searching method is proposed to improve the performance of ray casting algorithm. An efficient data storage method is proposed for storing and visualizing grid data, which makes it possible to display trillions of grids. The advantages of modified ray casting algorithm are presented through performance comparison between ray casting and slicing method in different situation. Finally, examples of grid generation and explosion simulation are presented. The accuracy and practicability of algorithm is proved.

Computational optimization of the combustion system of a heavy duty direct injection diesel engine operating with dimethyl-ether

A genetic algorithm optimization methodology is applied to the design of the combustion system of a heavy-duty diesel engine fueled with dimethyl ether (DME). The optimization includes the key combustion system related hardware, bowl geometry and injection nozzle design, together with the most relevant air management and injection settings. The GA was linked to the KIVA computational fluid dynamics code and an automated grid generation tool to perform a single-objective optimization. The optimization target focused on maximizing efficiency, while keeping NOx emissions, peak pressure and maximum pressure rise rate under the baseline engine levels. This research work not only provides the optimum combustion system definition, but also the cause-effect relation between the inputs and outputs under investigation, identifying the most relevant parameters controlling the performance of a DME fueled engine. Piston bowl geometry is found to primarily influence heat transfer and combustion efficiency due to its impact on the surface area and fuel distribution, respectively. Mixing is most affected by the injection system parameters. Finally, the optimum DME engine configuration provides 6.9% absolute net indicated efficiency improvement over the baseline engine fueled with DME. This study confirms the potential of DME as a promising fuel for the future generation of compression ignition engines and demonstrates the need to co-optimize the fuel and combustion system.

Sensitivity of Rotorcraft Hover Predictions to Mesh Resolution in Strand Grid Framework

The strand grid approach is a flow solution method where a prismatic like grid using “strands” is grown to a short distance from the body surface to capture the viscous boundary layer and the rest of the domain is covered using an adaptive Cartesian grid. The approach offers a potential for nearly automatic grid generation and adaptation requiring very few input parameters while, at same time, offering the ability to obtain fast and efficient flow solution. This paper explores the sensitivity of rotor hover predictions to various meshing parameters in the strand grid framework. Surface mesh resolution has minimal impact on the predictions, whereas both the outer extent of the strand mesh and the Cartesian mesh resolution have significant effect on rotor performance and wake predictions. A strand mesh extending out to half a chord distance away and a Cartesian mesh spacing of less than two times the outer boundary spacing of the strand mesh are recommended for most rotor blade simulations.

Grid generation on free-form surface using guide line advancing and surface flattening method

Automatic grid generation on a curved surface is important to more efficient design of free-form structures. However, it is neither a convenient nor an obvious task for engineers to create a discrete grid structure on a complex surface that meets the architectural requirements. Besides, research on the rapid grid generation methodology for free form structural design is still limited. In order to achieve better grid distribution of rods on free-form surface, a grid generation methodology which combines surface flattening technique with guide line method is put forward. The parametric domain of the free-form surface was firstly divided into a number of parts and a discrete free-form surface was accordingly formed by mapping the generated dividing points onto the curved surface. The free-form surface was then flattened based on the principle of identical area. Accordingly, the flattened rectangular lattices were fitted into the 2D surface where grids were formed by using the guide line method. Finally, the 2D grids were mapped onto the 3D surface, and grids were therefore generated on the given surface. A grid shape and rod length quality index was proposed to evaluate the shape of grid cells and rod lengths. The results show that the grid shape quality index and the deviation of rod length of the grid structure are reduced by up to 47% and 34% respectively by using the guide line method with surface flattening when compared to the method without surface flattening.

Development and validation of a multi-strand solver for complex aerodynamic flows

Abstract The strand grid approach is a flow solution method where a prismatic-like grid using “strands” is grown to a short distance from the body surface to capture the viscous boundary layer and the rest of the domain is covered using an adaptive Cartesian grid. The approach offers several advantages in terms of nearly automatic grid generation and adaptation, ability to implement fast and efficient flow solvers that use structured data in both the strand and Cartesian grids, and the development of efficient and highly scalable domain connectivity algorithm. An improvement to this approach is the multi-strand strategy, where multiple strands are allowed from each surface vertex to enhance grid resolution near sharp corners. This paper introduces a fully parallel and highly efficient flow solver called mStrand that is developed from ground-up to operate on multi-strand meshes. The strand solver is integrated to HPCMP CREATETM-AV Helios framework to simulate complex aerodynamic flows. Detailed validation of the solver is shown on problems with varying degrees of complexity and comparison with experimental data. A performance study shows that the strand solver is nearly as efficient as a structured grid solver.

HPCMP CREATE-SH Integrated Hydrodynamic Design Environment

The HPCMP CREATE-SH Integrated Hydrodynamic Design Environment (IHDE) is a workbench-like desktop application that integrates a suite of hull form design and analysis tools allowing a user to execute round-trip evaluations of hydrodynamic performance, including visualization, in a simplified and timely manner. Ship hull designers are able to assess ship performance in areas of resistance, seakeeping, hydrodynamic loads, and operability for several different mission types. The development plan calls for additional capabilities related to maneuvering performance and multi-objective optimization. The IHDE also includes an analysis tool validation engine, which provides the user with validation information by leveraging historical model test data for comparisons. The advantages of the IHDE include automated analysis preparation, automated grid generation, and integrated visualization. The IHDE has been used to support several Navy design studies and this paper describes the capabilities of IHDE and shows examples of typical workflow processes and sample analysis results.

Development and Application of an Incompressible Strand Solver

Computational fluid dynamics (CFD) is an ever-increasing component of the naval design and analysis process. Most current high-fidelity CFD methods used by the US Navy require body-fitted volume grids, which are time-consuming and depend on seasoned engineers with significant grid generation experience. Automated grid generation using a strand gridding approach is a relatively new concept that could transform the role of higher fidelity CFD within the Navy's design and analysis process. The HPCMP CREATE-SH team seeks to expand the compressible strand solver under development as part of the HPCMP CREATE-AV effort to incompressible flow problems for applications of interest to the Navy. The completed incompressible strand solver would allow quicker turnaround time for experienced CFD engineers while allowing more naval architects to use higher fidelity CFD as a part of the overall design process. The incompressible strand solver is a new development of the HPCMP CREATE-SH Hydro team, and this work will demonstrate the solver on validation cases of naval interest.

A novel approach for automatic unstructured grid generation for dynamic simulation of heterogeneous reservoirs

Precise studying of the static and dynamic characteristics of the reservoir plays a key role in the accurate evaluation of the reservoir performance. In this paper, a novel technique has been presented for generating a dynamic mesh in which static and dynamic parameters of the reservoir were combined with the capability of updating. Static parameters including permeability distribution and the location of active wells were combined with fluid patterns in the reservoir. An exact element size map was generated by comparing all mentioned effective parameters and making the element size indicator by selecting the maximum value. This element size map was applied for unstructured mesh generation. This intelligent mesh generator was employed in multi-phase flow simulation. Results showed that the generated grid is capable of predicting the results of breakthrough time more accurately in comparison to uniform grid. Moreover, it is faster and computationally less expensive in comparison to the fine model. [Received: August 15, 2014; Accepted: March 2, 2015]

A novel approach for automatic grid generation for multi-phase flow simulators: A robust computational framework for mass transfer at bubble point pressure

Abstract Coarsening from fine geological model to dynamic coarse scale is an essential task in reservoir simulation. In this paper a new approach is introduced to create a coarse dynamic model based on effective parameters of fluid flow in porous media. In this technique, the effects of fine scale permeability map, key flow paths (streamlines) and well location are considered to build a coarse dynamic model from a fine geological one. An exact element size map is generated by comparing all the mentioned effective parameters and making the element size indicator by selecting the maximum value. This element size map (background grid) is applied to build an unstructured mesh for discretization of the reservoir. Afterward, this intelligent mesh generator was employed in three-phase flow simulation. Moreover, a novel computational framework is developed for the evaluation of bubble point pressure to prevent divergence of the solution when the reservoir conditions change. To evaluate the performance of the developed model, the fluid flow rate obtained by this model is compared to that obtained by the uniform grid model. It is found that the proposed method provides more accurate results for fluid flow rate compared to the uniform grid model. Moreover, it is faster and computationally less expensive than the fine model. This model can be applied in reservoir simulators and provides more accurate and reliable results in less CPU time compared to the traditional mesh generation techniques. In addition, the proposed model solves the problem of divergence of the solution at the bubble point pressure.

OPTIMIZATION OF THE BLADE PROFILE AND COOLING STRUCTURE IN A GAS TURBINE STAGE CONSIDERING BOTH THE AERODYNAMICS AND HEAT TRANSFER

The need to design high performance of a cooled gas turbine is considered with emphasis made on coupled aerodynamic and heat transfer optimization of the vane, blade, and single stage cooled gas turbine by using a multiobjective optimization method. The aerodynamic profile is designed to have three sections and the cooling structure to consist of a serpentine passage, with a tail transverse channel and trailing edge slots. The optimization platform is built up in an in-house code using a cooling structure parametric method based on MATLAB, as well as automatic grid generation methods, a blade profile parametric program in FORTRAN, the soft ware ISIGHT and ANSYS-CFX. The optimization platform evaluates the aerodynamic effects through the aerodynamic efficiency and presents the cooling effect by the high-temperature coefficient. The pressure drop is described by a pressure drop function. The multiobjective optimization method is accomplished by optimizing the inlet flow angle, installation angle, and the post-corner angle of the vane and blade profiles, while the position of partition is the optimized variable of the cooling structure. The results show that there exists an optimum case in aerodynamic efficiency, high-temperature coefficient, and pressure drop in a Pareto-optimal front. (Less)

Mixed convection–radiation on stagnation-point flow of nanofluids over a stretching/shrinking sheet in a porous medium with heat generation and viscous dissipation

Abstract This paper is concerned with the effects of thermal radiation of mixed convection boundary layer flow towards a stagnation-point flow over a stretching/shrinking embedded in porous medium in nanofluids with heat generation and viscous dissipation. Dual solutions are obtained for shrinking sheet case. The basic equations are solved numerically by using Runge–Kutta–Fehlberg method with shooting technique. This method is more suitable to the present boundary value problem as it has advantages like automatic grid generation and convergence test in order to get very accurate results compared to Runge–Kutta-fourth order method which lacks these features. Results are compared with the previously published results and excellent agreement has been obtained. It is found that suction parameter decreases the velocity and temperature profiles for stretching sheet, whereas reverse trend is obtained for shrinking sheet. It is also found that the boundary layer thickness for second solution is higher than first solution for the three types of nanofluids for shrinking sheet. It is important to note that addition of nanoparticles into the base fluid produced an increase in the skin-friction.

A Comprehensive Investigation of Preswirled Flow Through Rotating Radial Holes

This paper reports on the flow (centrifugal = radially outward, centripetal = radially inward) through rotating radial orifices with and without preswirl in the flow approaching the orifice in the outer annulus. The aerodynamical behavior of flow through radial rotating holes is different from the one through axial and stationary holes due to the presence of centrifugal and Coriolis forces. To investigate the flow phenomena and the discharge coefficient of these orifices in detail, an existing test rig containing two independently rotating shafts (corotating and counter rotating) was used. To simulate conditions of real gas turbines, where the flow is often influenced by upstream components, various preswirl angles were used in the test rig. Measurements of the flow discharge coefficient in both flow directions through the orifices (centripetal and centrifugal), with and without preswirl generation in the outer annulus, are presented at various flow conditions (pressure ratios across orifices, Mach numbers of approaching flow) and for different geometric parameters (length to diameter ratios, sharp/rounded inlet edges). Flow effects that occur with preswirled flow are clarified. A comparison of the experimental data, for both flow directions, shows a similar behavior of the discharge coefficients with increasing shaft speeds. To supplement the experimental data and to better understand the experimental findings, numerical simulations were performed, which show a good agreement with the experimental results. Furthermore, an optimization model with complete automatic grid generation, computational fluid dynamics (CFD) simulation, and postprocessing, was built to enable large parametric studies, e.g., grid independence of the solutions.

An automatic grid generation approach over free-form surface for architectural design

An essential step for the realization of free-form surface structures is to create an efficient structural gird that satisfies not only the architectural aesthetics, but also the structural performance. Employing the main stress trajectories as the representation of force flows on a free-form surface, an automatic grid generation approach is proposed for the architectural design. The algorithm automatically plots the main stress trajectories on a 3D free-form surface, and adopts a modified advancing front meshing technique to generate the structural grid. Based on the proposed algorithm, an automatic grid generator named “St-Surmesh” is developed for the practical architectural design of free-form surface structure. The surface geometry of one of the Sun Valleys in Expo Axis for the Expo Shanghai 2010 is selected as a numerical example for validating the proposed approach. Comparative studies are performed to demonstrate how different structural grids affect the design of a free-form surface structure.

Massively parallel grid generation on HPC systems

The automatic grid generation on high performance computers is a challenging task under the restriction of computational power and memory availability. The increasing demand for high grid resolutions to simulate complex flow configurations necessitates parallel grid generation on multicore machines with distributed memory. In this study, a new robust algorithm to automatically generate hierarchical Cartesian meshes on distributed multicore HPC systems with multiple levels of refinement is presented. The number of cells is only restricted by the number of available cores and memory. The algorithm efficiently realizes a computational domain decompositioning for an arbitrary number of cores based on a Hilbert curve. The grids are efficiently stored and accessed via a high capacity parallel I/O method. The efficiency of the approach is demonstrated by considering human nasal cavity and internal combustion engine flow problems.