Chimera Grid Research Articles

Effects of Protuberances on Surface Loads on the Vega-C Launch Vehicle

The need to meet the demands of the aerospace market in terms of payload capability, mission flexibility, and cost reduction has led the European Space Agency to develop Vega-C as an evolution of the original Vega launcher. The design of a modern launch vehicle like the Vega-C requires accurate evaluation of the thermal and mechanical loads on its surface under conditions as close as possible to real flight conditions. Despite its apparently simple aeroshape, the Vega-C surface is characterized by the presence of antennas, wiring ducts, flanges, and retrorockets. These protuberances interact with the external supersonic stream during the launcher’s flight, giving rise to locally complex flows that can impose stresses on the launcher structure through the generation of severe mechanical and thermal loads. The present work focuses on an in-depth aerothermal analysis of Vega-C protuberances under specific flight conditions. Numerical computations are carried out using the commercial solver CFD++ on multiblock structured Chimera grids to assess which protuberances are subject to the most severe surface loads. Particular emphasis is placed on the role of boundary-layer thickness in the heat flux and pressure peaks that occur on the protuberances.

Применение химерных сеток для явного учета неоднородностей при моделировании распространения упругих волн

The method of chimeric meshes is applied to simulate the propagation of elastic perturbations in media containing porous and fractured inclusions. A model of a linearly elastic isotropic medium is considered, which describes the state of a geological rock. The grid-characteristic method with the third-order accurate Rusanov scheme is used for numerical modeling of the dynamic propagation of elastic disturbances. Special attention is paid to the presence of separate inclusions of pores or fractures, which introduce heterogeneity into the medium and can substantially influence the response of elastic disturbances. The use of the chimera grid method allows for both the position and shape of such inclusions to be described explicitly, taking into account their influence on the propagation of elastic disturbances. As a result of the conducted investigation, a methodology for numerical modeling of the propagation of elastic disturbances in media with porous and fractured inclusions was developed, which can be used to assess the influence of such inclusions on the dynamic response of elastic disturbances. The presented results can be applied in geophysical and seismic research related to modeling the dynamics of various processes in soils and rocks.

Validation of Chimera Grid Method Applied to UMSAPv With Prediction of Carriage Load

본 논문에서는 비정렬 CFD 해석자 UMSAPv에 적용된 중첩 격자 기법의 검증을 위하여 항공기에 장착된 store의 장착하중을 구하였다. 먼저, 잘 알려진 Eglin Wing/Pylon/Finned store 문제에 대해 UMSAPv 의 중첩 격자 기법을 검증하였다. 다음으로 F/A-18C clean 형상에서 대하여 받음각에 따른 아음속 유동해석을 수행하였다. 계산 결과를 정렬 CFD 해석자인 MSAPv을 사용한 선행연구결과와 비교하여 Umsapv의 해석능력을 검증하였다. 마지막으로 F/A-18C/JDAM의 장착하중은 중첩 격자와 단일 격자로 수행하였다. 계산된 결과는 다른 CFD결과, 실험 및 비행시험과 비교하였다.

A simple Chimera grid method with an implicit Dirichlet/Neumann coupling scheme for flows with moving boundaries

A simple Chimera grid method is developed for complex flows with moving boundaries. Based on the interpolation condition, a coupling strategy is proposed where algebraic equations are reconstructed to couple different domains. With respect to other methods, the coupling of different domains can be accomplished with the employment of any discretization scheme and the iterative process or the iterative solver are avoided. The process of reconstruction is simple to be implemented by the node-by-node replacement, which makes little change on the original code. In addition, the reconstructed system matrix keeps symmetric and positive definite, hence the old solver is appliable. The developed method has a clear physical meaning, namely the transmission condition of Dirichlet/Neumann type. The interpolation condition makes the variable continuous across the interior boundaries, which is the Dirichlet condition, while the Neumann condition is imposed by the reconstruction of the system matrix and the right hand side. For the flows with moving boundaries, this method cannot be applied directly as the interpolation condition varies. The corresponding change on reconstruction of the right hand side is conducted to resolve moving boundaries. Finally, the reliability and the accuracy of the present method are validated by several benchmark problems. The simulation results show that the accuracy of the discretization scheme is not affected by the present coupling strategies.

Numerical investigation of the unsteady distortion for an S-duct intake with mechanical vortex generators

Flow control devices are used within complex intakes to reduce the flow distortion which can adversely impact the stability and performance of embedded engines. There is a need to assess the capability of modern computational methods such as detached eddy based models to compute the unsteady flowfield and to evaluate the potential benefits of flow control devices on the unsteady distortion. This paper investigates the unsteady flowfield for an S-duct using Zonal Detached Eddy Simulations (ZDES) with passive flow control devices modelled with an overlapping Chimera grid method. The ability of ZDES to evaluate the impact of passive flow control devices on the unsteady flow distortion was assessed. The computed unsteady flowfield at the Aerodynamic Interface Plane was compared with experimental data based on total pressure and velocity field measurements. For the baseline configuration, the ZDES model has proven to be able to simulate the unsteady flowfield at the AIP, to provide the time averaged and fluctuating levels of swirl distortion within 1% and 13% respectively of the measurements. The strong impact of the flow control devices on the AIP flowfield was also captured by the ZDES. The overall increase of pressure ratio (PR) at the AIP due to the flow control devices was predicted with less than 1% error. The 65% reduction in swirl distortion fluctuation when the flow control devices are used was predicted within less than 8% error by the ZDES compared with S-PIV measurements. Overall it was determined that the ZDES method is able to simulate the unsteady flow and distortion characteristics for both the baseline reference configuration as well as the case with flow control.

Grid-characteristic method on Chimera meshes using a single background grid with varying elastic properties

Nowadays, traditional hydrocarbon reserves are depleted. The search for and production of oil and gas in hard-to-reach fields require the development of appropriate computing technologies. Therefore, there is a need to develop novel high-precision computational methods that make it possible to more accurately take into account the features of the geological model, and at the same time save computational resources. In this paper, we have proposed a novel modification of the grid-characteristic method on Chimera grids to take into account the complex geometry of the interfaces of geological rocks. This modification differs from those proposed earlier by using a single background grid with varying elastic parameters of the medium and imposes fewer restrictions on the geological model than the previously proposed options. The computational method proposed in the paper is applicable not only for modeling seismic waves in geological media, but also for modeling ultrasonic and sound study, non-destructive testing of various objects, and in medicine. Also, the proposed numerical method can be applied for a more accurate calculation of seismic waves from earthquake hypocenters and man-made sources.

A hybrid recursive regularized lattice Boltzmann model with overset grids for rotating geometries

Simulating rotating geometries in fluid flows for industrial applications remains a challenging task for general fluid solvers and in particular for the lattice Boltzmann method (LBM) due to inherent stability and accuracy problems. This work proposes an original method based on the widely used overset grids (or Chimera grids) while being integrated with a recent and optimized LBM collision operator, the hybrid recursive regularized model (HRR). The overset grids are used to actualize the rotating geometries where both the rotating and fixed meshes exist simultaneously. In the rotating mesh, the fictitious forces generated from its non-inertial rotating reference frame are taken into account by using a second order discrete forcing term. The fixed and rotating grids communicate with each other through the interpolation of the macroscopic variables. Meanwhile, the HRR collision model is selected to enhance the stability and accuracy properties of the LBM simulations by filtering out redundant higher order non-equilibrium tensors. The robustness of the overset HRR algorithm is assessed on different configurations, undergoing mid-to-high Reynolds number flows, and the method successfully demonstrates its robustness while exhibiting the second order accuracy.

An Efficient of Chimera Grid Scheme with Spline Interpolation in FBTS Inversion Technique for Extremely Dense Breast Cancer Detection

Microwave imaging system is classified as non-invasive, simple to perform and inexpensive compared to MRI and X-Ray machine. Therefore, the novel idea of this research work is to develop a Chimera Grid Scheme (CGS) incorporate with the microwave inverse scattering technique in a low cost, non-ionising and safe short-range. The CGS with spline interpolation in Forward Backward Time-Stepping (FBTS) inversion technique can determined an accurate result especially for the biological anomalies like breast tumours at an early curable stage due to the high electrical properties contrast between malignant cells and normal cells. The findings showed that the proposed method successfully detected and reconstructed the breast structure in relative permittivity profiles. The quantitative information of reconstructed images, such as location, shape, size and internal composition also can be obtained. Furthermore, the normalised functional error for proposed method was also lower than the FDTD method in FBTS. At 150th iteration, the difference of normalised functional error between these two methods was 1.74 × 10−5. The result shows that the CGS method in FBTS inversion technique would reconstruct breast composition more precisely.

Efficient Methodology with Grid Configurations for HSI Noise Prediction in Hover

Complex and unsteady flow patterns occur around rotating rotor blades owing to the generation of shock waves at the blade tip and interference between the blade and tip wake when the blade rotates at high speeds, thereby resulting in specific noise characteristics. Furthermore, these noise characteristics cause noise pollution in private areas and degrade detectability of helicopters in military applications. Therefore, it is important to accurately analyze noise characteristics around rotor blades. In this study, a novel computational methodology with a chimera wake grid was proposed to efficiently and accurately predict high-speed impulsive (HSI) noise due to the shock wave at the blade tip. The proposed method enables accurate analysis of a complex flow region by overlapping a wake grid in the vicinity of shock waves. A 1/7-scaled UH-1H rotor blade was used to compare and verify the accuracy of HSI noise prediction and computational efficiency. The chimera grid method was applied to the present simulation for considering the blade motion and moving effects. A permeable surface for wrapping the surface of the physical blade was constructed to include quadrupole noise source generated from the control volume, thickness noise source, and loading noise source. Furthermore, the permeable Ffowcs Williams and Hawkings (FW–H) equations were used with the Kirchhoff approach to realize efficient far-field noise prediction. The proposed HSI noise analysis via chimera wake grid indicated that the strength of the shock wave is more precisely predicted, thereby resulting in improved prediction of the HSI noise. The proposed chimera methodology allows efficient noise prediction using a much coarser background grid system.

Accounting for curved boundaries in rocks by using curvilinear and Chimera grids

Reconstruction of rock boundaries plays an important role in seismic exploration of minerals. Seismic data processing requires multiple solutions to the corresponding direct problem. Therefore, it is of interest to develop numerical methods for solving direct problems of seismic exploration, taking into account curvilinear boundaries of rocks and surface topography. In this paper, we propose two approaches to accounting for curvilinear boundaries: using curved meshes and using Chimeric meshes. We have used the grid-characteristic numerical method for the solution of boundary value problem of elastic wave equation. To compare the results obtained, we used the method of studying wave phenomena Wave Logica. The analysis of the costs of computing resources was carried out.

Large eddy simulation analysis of the turbulent flow in an optically accessible internal combustion engine using the overset mesh technique

Computational fluid dynamics has become a fundamental tool for the design and development of internal combustion engines. The meshing strategy plays a central role in the computational efficiency, in the management of the moving components of the engine and in the accuracy of results. The overset mesh approach, usually referred to also as chimera grid or composite grid, was rarely applied to the simulation of internal combustion engines, mainly because of the difficulty in adapting the technique to the specific complexities of internal combustion engine flows. The article demonstrates the feasibility and the effectiveness of the overset mesh technique application to internal combustion engines, thanks to a purposely designed meshing approach. In particular, the technique is used to analyze the cycle-to-cycle variability of internal combustion engine flows using large eddy simulation. Fifty large eddy simulation cycles are performed on the well-known TCC-III engine in motored condition. Results are analyzed in terms of tumble center trajectory and using proper orthogonal decomposition to objectively characterize the spatial and temporal evolution of turbulent flow field in internal combustion engines. In particular, an original decomposition method previously applied by the authors to the TCC-III measured flow fields is here extended to computational fluid dynamics results.

A parallel implicit hole-cutting method based on background mesh for unstructured Chimera grid

As an efficient simulation method for complex configurations and moving boundary problems in computational fluid dynamics, Chimera or overset grid techniques have been widely used in many aspects of aeronautics and astronautics. However, there are still some bottlenecks for the current hole-cutting method to handle large scale mesh, such as memory storage limitation and parallel efficiency. In this paper, a parallel implicit hole-cutting method based on unstructured background mesh is presented. The method is based on the parallel searching of donor cells for all grid nodes. In order to reduce the memory consumption of the searching procedure for the large-scale grids, a global-to-local (GTL) searching strategy as well as the background grid approach is developed. To improve the connectivity of overset domains, a parallel front advancing method is adopted to automatically distinguish the active regions. Finally, the efficiency and effectiveness of the present Chimera grid method are validated by some test cases and applications, including multi-store separation from a fighter and a missile pitching-up maneuvering with rudder deflection under a control command. The numerical results demonstrate the potential for steady and unsteady CFD simulations for complex geometries.

A parallel algorithm for chimera grid with implicit hole cutting method

The chimera grid methods have been widely used in the simulation of flow over complex configurations and unsteady moving boundary process. Lee and Baeder presented the implicit hole cutting (IHC) method, which improves the practicability and robustness of chimera grid method. But the excessive time consumption of this method restricts the scalability of parallelism. In this article, based on the parallel implementation of IHC method with structured multi-block grid, the factors which restrict the performance and efficiency are analyzed. Cartesian auxiliary grid is introduced to reduce the communication and computing cost. Finally, test cases are presented to demonstrate the effectiveness of this algorithm, and the calculation and data communication are reduced on the premise of maintaining accuracy.

Numerical investigation on the assistant restarting method of variable geometry for high Mach number inlet

To compromise the compression efficiency and the starting properties, the inner contraction ratio (ICR) of a general high Mach number inlet is usually designed in the range of dual solution area. When going into an unstarted status, the high Mach inlet needs an assistant method to restart. This work explores a variable geometry method to restart the inlet. The rotating cowl is adopted to a typical Mach 4 cruising inlet, and the unsteady computation method with a dynamic Chimera grid technique is applied to simulate the rotating process of the inlet cowl. The change characteristics of the restarted performance at different rotating angle amplitude of the inlet cowl are investigated systematically. The numerical results reveal that the unstarted status of this typical inlet induced by the effect of high backpressure failed to restart if the inlet cowl rotating angle amplitude is under a small critical value, which is called lower critical angle. The inlet could restart if the cowl rotating angle amplitude is a little larger than the lower critical angle, and the flow may rapidly go to a steady condition after the inlet cowl returns to the design position. However, the performance of the restarted inlet is still worse than the design condition, because of the existence of an stable separation bubble on its shoulder, even if the inlet cowl stops rotating. The separation bubble becomes shrunk with an increasing the cowl rotating angle amplitude. When the inlet cowl rotating angle amplitude reaches a large critical value which is called upper critical angle, the separation bubble disappears, and all the separation is swallowed by the mean flow. Therefore the design performance of the inlet can be recovered, which means that the flow mass capture coefficient, total pressure recovery coefficient, drag and the outlet Mach number go back to the design level. It also shows that within the range of the lower and upper critical angles, the larger the rotating angle amplitude is, the more rapidly the separation bubble reaches stable state.

Enhancement of Navier–Stokes solver based on an improved volume-of-fluid method for complex interfacial-flow simulations

In this study, an extended validation of an improved volume-of-fluid (VOF)-based method for 3D highly nonlinear, complex, and breaking free-surface problems in engineering is presented. The VOF interface-tracking scheme is implemented with a Navier–Stokes solver in a curvilinear coordinate system. The numerical procedure is advanced by implementing the scheme on moving Chimera (overset) grids for application in complex engineering problems. The overset grids are used to facilitate the flow simulations of complex geometries and arbitrary motions of objects and improve computational productivity by taking advantage of their flexibility. To solve the six degrees of freedom (6DOF) motions of a rigid body simultaneously, the nonlinear 6DOF motion equations in a vector form system are strongly combined with a flow solver. A collection of problems is carefully selected such that a large density ratio and complex cases under a wide variety of Froude numbers are presented with the aim of demonstrating the capabilities of the new enhanced method on a complicated moving overset grid system. The method is validated for a wide range of parameters. The results of different free-surface problems are presented. The numerical results are compared with numerical alternatives and experimental measurements, and accurate approximations and good agreement are obtained for complex flows.

다중 투과면을 이용한 UH-1H 로터 블레이드의 제자리 비행 시 원방 소음 해석 연구

In this study, aero-acoustics in far-field is computed for the UH-1H rotor blade in hovering flight. A Chimera grid method is applied in this simulation to consider the blade motion and moving effects. Multi-permeable surfaces surrounding a blade are constructed to include the flow noise generated from the shock wave and the tip vortices. A computation is performed with three-dimensional Euler’s equations. An acoustic analogy code using the Kirchhoff approach to Ffowcs Williams and Hawkings equations is applied to efficient the noise prediction caused by volume noise sources in far-field. The high speed impulsive noise on the rotor blade is predicted and validated according to the permeable surface locations. It is confirmed that the noise source caused by the shock waves generated on the blade surface is a dominant factor in the far-field noise prediction.

Specific features of the Chimera calculation methodology implemented for unstructured grids

The main steps of implementing the calculation technique on Chimera grids are considered. The attention is focused on the specifics of implementing the algorithm on grids that are composed of arbitrary polyhedrons. Methods of creating intersections of countable regions of arbitrary shape are proposed. The operability and efficiency of the technique are illustrated by examples with the available experimental data.

A Higher-Order Chimera Method for Finite Volume Schemes

In this work a higher-order accurate finite volume method for the resolution of the Euler/Navier–Stokes equations using Chimera grid techniques is presented. The formulation is based on the use of Moving Least Squares approximations in order to obtain higher-order accurate reconstruction and connectivity between the overlapped grids. The accuracy and performance of the proposed methodology is demonstrated by solving different benchmark problems.

Navier–Stokes solver for water entry bodies with moving Chimera grid method in 6DOF motions

A preconditioned Navier–Stokes (NS) method is developed for multiphase flow with application to the water entry problem of moving bodies. The method employs a dual time-preconditioned technique with multiblock and parallel computing to improve the computational productivity. Novel, generalized pseudo-time terms are added into the multiphase NS equations, taking advantage of two previous suggestions reported in the literature. The NS solver is formulated in the moving curvilinear coordinate system for multiphase flow, including water, vapor, and air fluids. In order to handle the motion of an object in three-dimensional (3D) water entry problems, a six degrees-of-freedom (6DOF) rigid body motion model and a moving Chimera grid scheme are dynamically integrated into the NS solver. The Chimera domain decomposition scheme uses an overlapping, embedded, and moving grid approach to facilitate the flow simulation of arbitrary translationally and rotationally complex geometries among various computational blocks. The domain communication module is fully parallel and is ideally suited for handling moving body problems. To demonstrate the capacities of the method, several sets of example computations are performed and presented in this paper.

On the Spurious Mode Generation Induced by Spectral-Like Optimized Interpolation Schemes Used in Computational Acoustics

AbstractThe present work constitutes a fraction of a more extensive study that is devoted to numerical methods in acoustics. More precisely, we address here the interpolation process, which is more and more frequently used in Computational Acoustics–whether it is for enabling multi-stage hybrid calculations, or for easing the proper handling of complex configurations via advanced techniques such as Chimera grids or Immersed Boundary Conditions. In that regard, we focus on high-order interpolation schemes, so as to analyze their intrinsic features and to assess their effective accuracy. Taking advantage of specific insights that had been previously achieved by the present authors regarding standard high-order interpolation schemes (of centered nature), we here focus on their so-calledspectral-like optimizedcounterparts (of both centered and noncentered nature). The latter spectral-like optimized schemes are analyzed thoroughly thanks to dedicated theoretical developments, which allow highlighting better what their strengths and weaknesses are. Among others, the various ways such interpolation schemes can degrade acoustic signals they are applied to are carefully investigated from a theoretical point-of-view. Besides that, specific criteria that could help in optimizing interpolation schemes better are provided, along with generic rules about how to minimize the signal degradation induced by existing interpolation schemes, in practice.