Documents published in Scipedia

• A multigrid immersed boundary method for the CFD solver Horses3D

  S. Colombo, E. Ferrer, E. Valero

  eccomas2022.

  
  

  Abstract

  This work describes the implementation of the Immersed Boundary Method (IBM) in a high order discontinuous Galerkin framework for the CFD solver Horses3D [1]. High order schemes are very attractive due to their low numerical dissipation, their capability of providing high-accurate solutions and higher efficiency for a given level of accuracy with respect to low order schemes. However, the generation of high order meshes needed by these schemes is still a bottleneck since it requires a large amount of time. IBM tries to tackle the problem by preserving the high order beneficial properties while avoiding the generation of complex meshes.

  Abstract
  This work describes the implementation of the Immersed Boundary Method (IBM) in a high order discontinuous Galerkin framework for the CFD solver Horses3D [1]. High order schemes [...]

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• On highly scalable 2-level-parallel unstructured CFD

  J. Jaegerskuepper, D. Vollmer

  eccomas2022.

  
  

  Abstract

  Exascale HPC systems are just about to become available. Such enormous simulation capabilities from the hardware perspective, however, require software that can effectively make efficient use of this massively parallel hardware. For the domain-decomposition based algorithms generally used for CFD, it has become apparent that running one MPI process, i.e. one domain, per CPU core is no longer apt when there are hundreds of cores available per cluster node. There are just too many processes per node that need to communicate with other remote processes. Message aggregation is thus a key aspect of highly scalable parallel codes. A 2-level parallelization featuring a shared-memory level in addition to the MPI process level seems a promising solution. The CODA (CFD for ONERA, DLR, Airbus) software for high-fidelity compressible-flow simulations of industrial configurations implements such a 2-level domaindecomposition hybrid-parallel approach. The processing of unstructured meshes is particularly challenging with respect to load balancing and non-linear data access. For maximum parallel scalability, CODA's parallelization not only features overlapping communication with computation on the process level, but also a dedicated Single Program (on) Multiple Data (SPMD) programming model for the shared-memory level. Here, the design principles of this parallelization concept are outlined, followed by details concerning its implementation in the Flucs infrastructure (FLIS), which has become part of CODA. Moreover, results of scalability studies with CODA are presented, which impressively demonstrate the extreme scalability realized with this parallelization approach. The studies were performed on two distinct HPC clusters, one based on Intel's Xeon Scalable Processor, the other one based on AMD's EPYC.

  Abstract
  Exascale HPC systems are just about to become available. Such enormous simulation capabilities from the hardware perspective, however, require software that can effectively [...]

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• EXPLICIT AND IMPLICIT LARGE EDDY SIMULATION FOR A NACA0012 USING A HIGH ORDER DISCONTINUOUS GALERKIN SOLVER

  O. Marino, E. Ferrer, E. Valero, J. Errasti

  eccomas2022.

  
  

  Abstract

  We perform Large Eddy Simulation on a 3D NACA0012 airfoil using a high order discontiuous Galerkin spectral element solver at moderate Reynolds numbers, Re = 1.0 · 105. The aim is to compare the results of classic explicit Sub Grid Scale models, e.g. the WALE and VREMAN models, with the ones get by using implicit LES, where no explicit SGS is used, but we use stabilising split-form energy-stable formulations, with two-point Pirozzoli and KennedyGruber fluxes. We compare general qualitative variables such as the Q-criteria as well as local quantitative variables, as boundary layer velocity profiles. We show that both methods achive similar predictions for the integrated global variables, but differences can be appreciated in the local velocity profiles.

  Abstract
  We perform Large Eddy Simulation on a 3D NACA0012 airfoil using a high order discontiuous Galerkin spectral element solver at moderate Reynolds numbers, Re = 1.0 · [...]

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• Free response of a gravitational liquid sheet by means of three-dimensional Volume-of-Fluid simulations

  A. Della Pia, L. Grande, A. Colanera, M. Chiatto, L. Luca

  eccomas2022.

  
  

  Abstract
  The volume-of-fluid (VOF) method is employed to simulate the dynamics of gravitational liquid sheets (curtains) issuing into an initially quiescent gaseous environment.

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• A level-set model for two-phase flow with variable surface tension: Thermocapillary and surfactants

  N. Balcazar-Arciniega, J. Rigola, A. Oliva

  eccomas2022.

  
  

  Abstract

  An unstructured conservative level-set method for two-phase flow with variable surface tension is introduced. Surface tension is a function of temperature or surfactant concentration on the interface. Consequently, the called Marangoni stresses induced by temperature gradients or surfactant concentration gradients on the interface lead to a coupling of momentum transport equation with thermal energy transport equation or interface surfactant transport equation. The finite-volume method discretizes transport equations on 3D collocated unstructured meshes. The unstructured conservative level-set method is employed for interface capturing, whereas the multiple marker approach avoids the numerical coalescence of fluid particles. The fractional-step projection method solves the pressure-velocity coupling. Unstructured flux-limiters are proposed to discretize the convective term of transport equations. A central difference scheme discretizes diffusive terms. Gradients are evaluated by the weighted least-squares method. Verifications and validations are reported.

  Abstract
  An unstructured conservative level-set method for two-phase flow with variable surface tension is introduced. Surface tension is a function of temperature or surfactant concentration [...]

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• Comparison of different entropy stabilization techniques for discontinuous Galerkin spectral element methods

  J. Markert, G. Gassner

  eccomas2022.

  
  

  Abstract

  We review and compare two techniques to get entropy stability for nodal Discontinuous Galerkin Spectral Element Methods (DG) in compressible flows. One technique is based on entropy split-forms, e.g., [8, 15, 5] and one is based on a direct algebraic correction [1]. We have implemented the Flux Differencing methodology for both, Legendre-Gauss-Lobatto (Lobatto) and Legendre-Gauss (Gauss) based spectral element basis functions. While the Lobatto operators belong to the class of diagonal norm summation-by-parts (SBP) operators, the Gauss operators belong to the generalized class of SBP operators, where it is not necessary that the boundary nodes are included. To reach entropy stability, respectively guaranteed entropy dissipation, a key ingredient is an entropy conserving numerical flux function. With this ingredient, only the volume integral term of the DG method has to be modified accordingly. We have also implemented an alternative technique, which is in general applicable for a wide range of discretizations. Abgrall [R. Abgrall, A general framework to construct schemes satisfying additional conservation relations. Application to entropy conservative and entropy dissipative schemes. Journal of Computational Physics, vol 372, 2018] introduced an algebraic correction term that retains conservation of the primary quantities and is furthermore constructed such, that an entropy (in-) equality can be shown. The second technique is at first sight a simpler alternative to the split-form based approach. Hence, questions regarding its advantages and disadvantages naturally come up.

  Abstract
  We review and compare two techniques to get entropy stability for nodal Discontinuous Galerkin Spectral Element Methods (DG) in compressible flows. One technique is based [...]

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• A Machine Learning based Expert System for Optimizing CFD Solver Parameters

  L. El Zaatari, T. Leicht, S. Langer, P. Bekemeyer, S. Görtz

  eccomas2022.

  
  

  Abstract

  Computational Fluid Dynamics is a viable tool in the field of aerodynamics enabling to reduce time, effort and budget required for experimental testing. Although powerful and established for various years, it remains a complex tool calling for experienced users to ensure consistent high-quality results. This complexity primarily stems from the underlying model, namely the Navier-Stokes equations typically combined with a set of equations resolving the effects of turbulence. Additionally, to obtain accurate high-fidelity result appropriate meshes are required. As a consequence, a substantial number of parameters needs to be selected carefully and the quality of a result often highly depends on individual knowledge and experience of a user. Hence, a strong desire exists to reduce the number of input parameters without causing a loss of accuracy and efficiency. Such reduction of parameters might be viewed as a prerequisite to CFD as a tool in process chains for multidisciplinary applications where typically no user interaction is possible. In this article we propose a machine-learned Expert System for CFD to provide guidance for users in selecting optimal or at least near optimal parameter combinations. The proposed Expert System is divided into two macro steps, the surrogate model and a genetic algorithm to determine from the surrogate model the parameters. Numerical examples are presented to demonstrate the approach.

  Abstract
  Computational Fluid Dynamics is a viable tool in the field of aerodynamics enabling to reduce time, effort and budget required for experimental testing. Although powerful [...]

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• A new projection method for Navier-stokes equations by using Raviart-thomas finite element

  G. Barbi, A. Chierici, A. Cervone, V. Giovacchini, S. Manservisi, L. Sirotti, R. Scardovelli

  eccomas2022.

  
  

  Abstract

  Computational Fluid Dynamics codes usually adopt velocity-pressure splitting to reduce the computational effort in the solution of the Navier-Stokes equations. In standard projection methods, the finite element approximations show difficulties to find a solution with discrete free-divergence velocity field in all space points. In this work, a new velocity-pressure method for Navier-Stokes equations that projects the velocity field inside the discrete free-divergence velocity space is presented. This algorithm computes the velocity field on the discrete free-divergence space by using Raviart-Thomas finite elements. The projection is obtained by the minimization of the distance, over the discrete free-divergence space, between the auxiliary field and the desired Raviart-Thomas interpolation space. The Raviart-Thomas discretization is based on the quadrilateral and hexahedral finite element space and therefore the divergence mimetic computational approach is used to avoid the well-known degradation of the divergence term convergence. The auxiliary velocity field is obtained by solving the velocity-pressure split system used in the classical Chorin­Temam algorithm. The pressure is recovered by the orthogonal space to the projection on the Raviart-Thomas interpolation space. The method is investigated with an explicit and semi-implicit treatment of the pressure terms. The issues on boundary conditions and the errors in the reproducibility of the tangential components are investigated. Several numerical examples are reported to support this new projection method.

  Abstract
  Computational Fluid Dynamics codes usually adopt velocity-pressure splitting to reduce the computational effort in the solution of the Navier-Stokes equations. In standard [...]

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• Algorithmic Differentiation for an effcient CFD solver

  B. Maugars, S. Bourasseau, C. Content, B. Michel, B. Berthoul, J. Nunez Ramirez, I. Salah el Din, P. Raud, L. Hascoët

  eccomas2022.

  
  

  Abstract

  We illustrate the benefits of Algorithmic Differentiation (AD) for the development of aerodynamic flow simulation software. In refining the architecture of the elsA CFD solver, developed jointly by ONERA and Safran, we consider AD as a key technology to cut development costs of some derivatives of interest, namely the tangent, adjoint, and Jacobian. We first recall the mathematical background of CFD applications which involve these derivatives. Then, we briefly present the software architecture of elsA (Cambier et al. [12]) and the design choices which give it its HPC capability while highlighting how these choices strongly constrain the applicability of AD. To meet our efficiency requirements, we select the Source-Transformation approach to AD through the Tapenade tool which is justified by a series of experiments and benchmarks. Finally, we present results on large scale configurations.

  Abstract
  We illustrate the benefits of Algorithmic Differentiation (AD) for the development of aerodynamic flow simulation software. In refining the architecture of the elsA CFD solver, [...]

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• A runtime-based dynamic mesh partitioning approach

  G. Baldan, R. Borrell, J. Jägersküpper

  eccomas2022.

  
  

  Abstract

  Large-scale parallel numerical simulations are fundamental for the understanding of a wide variety of aeronautical problems. Mesh decomposition is applied to make use of parallel hardware. In particular, when using a massively parallel architecture, not only the final quality of the mesh subdivision is relevant. Also the partitioning algorithm itself needs to be robust as well as efficient. A strategy for dynamic mesh partitioning based on runtime measurements is presented. We integrate the 'Geometric Mesh Partitioner' (GeMPa), which is a partitioning library based on Hilbert Space-Filling Curve (HSFC), in the FlowSimulator (FS) software. FS is a platform designed to run multi-disciplinary simulations on massively parallel cluster architectures. The algorithm performance is evaluated on an unstructured mesh representing the ONERA M6 wing. In particular, the load imbalance among processes is evaluated and compared with a well-known graph-based partitioning approach. Finally, we analyze how the number of processes influences the load imbalance.

  Abstract
  Large-scale parallel numerical simulations are fundamental for the understanding of a wide variety of aeronautical problems. Mesh decomposition is applied to make use of parallel [...]

  • 6
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