Documents published in Scipedia

• Unsteady Aerodynamic Sensitivity Analysis with FEniCS

  C. Ruiz, M. Chávez-Modena, A. Martínez-Cava

  eccomas2022.

  
  

  Abstract

  Sensitivity analysis is considered a fundamental tool in aerospace engineering, allowing to evaluate the impact of parameters variations, and optimize the aerodynamic and structural design. There has been much effort in the development of both theoretical and numerical frameworks for sensitivity calculations through adjoint solutions. Regarding the implementation of these analyses into agile and versatile numerical tools, the use of scalable and transferable libraries has become of particular importance. In this work, FEniCS is used to calculate the sensitivity of aerodynamic observables in different flow condition. A comparison with different theoretical and benchmark cases is used to validate the methodology before applying it to further and more complex configurations, expanding the scope of the analyses to unsteady flows.

  Abstract
  Sensitivity analysis is considered a fundamental tool in aerospace engineering, allowing to evaluate the impact of parameters variations, and optimize the aerodynamic and [...]

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• Incorporating effective transmission conditions between fluid and solid domains in transient wave propagation problems using the mortar element method

  A. Imperiale

  eccomas2022.

  
  

  Abstract

  We derive an efficient numerical scheme for transient wave propagation in configurations where a fluid domain and a solid domain are separated by a thin coating material. These type of configurations are numerically challenging for multiple factors: managing fluid ­ solid coupling, enabling non-conform space discretizations, and rendering robust time discrete algorithm w.r.t the thin layer thickness. By combining the mortar element method with effective transmission conditions we are able to address these challenges. We illustrate our approach by proposing relevant 2D numerical illustrations inspired from simple ultrasonic testing experiments.

  Abstract
  We derive an efficient numerical scheme for transient wave propagation in configurations where a fluid domain and a solid domain are separated by a thin coating material. [...]

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• Discretize first, filter next – a new closure model approach

  S. Agdestein, B. Sanderse

  eccomas2022.

  
  

  Abstract

  Simulating multi-scale phenomena such as turbulent fluid flows is typically computationally very expensive. Filtering the smaller scales allows for using coarse discretizations, however, this requires closure models to account for the effects of the unresolved on the resolved scales. The common approach is to filter the continuous equations, but this gives rise to several commutator errors due to nonlinear terms, non-uniform filters, or boundary conditions. We propose a new approach to filtering, where the equations are discretized first and then filtered. For a non-uniform filter applied to the linear convection equation, we show that the discretely filtered convection operator can be inferred using three methods: intrusive (`explicit reconstruction') or non-intrusive operator inference, either via `derivative fitting' or `trajectory fitting' (embedded learning). We show that explicit reconstruction and derivative fitting identify a similar operator and produce small errors, but that trajectory fitting requires significant effort to train to achieve similar performance. However, the explicit reconstruction approach is more prone to instabilities.

  Abstract
  Simulating multi-scale phenomena such as turbulent fluid flows is typically computationally very expensive. Filtering the smaller scales allows for using coarse discretizations, [...]

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• Robust Topology Optimization of Static Systems with Unilateral Frictional Contact

  T. Schmidt, B. Kriegesmann, R. Seifried

  eccomas2022.

  
  

  Abstract

  In this paper a robust topology optimization algorithm for linear elastic structures in unilateral contact is presented. The deformation of the linear elastic structure is constrained by support structures that are modeled with the help of Signorini's contact conditions. The contact conditions in turn are enforced with the augmented Lagrangian approach. Doing so, the robust optimization considers uncertainties at the support such as manufacturing tolerances and its local friction behavior. Due to high numerical costs in robust optimization, the firstorder second-moment approach is used to approximate the mean and variance of the objective. This approximation results in minimal additional costs to approximate the mean, the variance and their gradients. Consequentially, a gradient-based optimization algorithm can be used to minimize a weighted sum of both. The results show that the presented approach indeed improves the robustness with respect to uncertain contact conditions compared to a deterministic optimization.

  Abstract
  In this paper a robust topology optimization algorithm for linear elastic structures in unilateral contact is presented. The deformation of the linear elastic structure is [...]

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• A fluid-structure interaction study of hemodynamics in arterial bypass-graft anastomoses

  G. Bletsos, L. Radtke, A. Düster, T. Rung

  eccomas2022.

  
  

  Abstract

  This paper reports on numerical experiments on arterial bypass-graft anastomoses. Bypass-grafts are oftentimes used in surgical procedures to divert blood around narrowed or occluded parts of an artery. The diverted blood flow is crucial to the success of the operation as it may lead to undesirable peculiarities that can result to a renewed occlusion in the distal connection of the graft. However, an a priori prediction of detrimental hemodynamic aspects due to undesirable flow properties is difficult to perform in vitro or in vivo conditions. To this end, this work targets to enhance our understanding of harming mechanisms through in silico experiments using computational fluid dynamics (CFD) and fluid-structure interaction (FSI) simulations. The latter are realized through a partitioned coupled approach which is verified for a 2D benchmark case against literature-reported results. Finally, we present numerical results on grafts with different cuff sizes. Wall shear stress (WSS), oscillatory shear index (OSI) and hemolysis are monitored and compared in the context of either rigid or elastic walls and cuff sizes. Special interest is given to the prediction of hemolysis induction which is often not considered in such studies. We show that wall elasticity is the key parameter in terms of WSS prediction while cuff size mainly affects the estimation of OSI.

  Abstract
  This paper reports on numerical experiments on arterial bypass-graft anastomoses. Bypass-grafts are oftentimes used in surgical procedures to divert blood around narrowed [...]

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• Low Mach preconditioned non-reflecting boundary conditions for the harmonic balance solver

  P. Sivel, C. Frey

  eccomas2022.

  
  

  Abstract

  In computational fluid dynamics (CFD), unsteady computations are cost-intensive. The Harmonic Balance (HB) method [7] represents a cost-efficient alternative. Here, for timeperiodic flows, the governing equations are recasted in the Fourier domain. In the low Mach regime, the compressible governing equations are stiff. Therefore, densitybased solvers converge slowly. Low Mach preconditioning equalizes the eigenvalues of the system of equations, to improve the condition number and remove the stiffness of the system [14]. In this paper, low Mach preconditioning is applied to the HB method, with emphasis on the non-reflecting boundary conditions (NRBCs). These boundary conditions have a crucial impact on the flow inside the truncated computational domains used in CFD. Improper boundary conditions reflect waves exiting the computational domain and deteriorate the quality of the solution. However, NRBCs [6] avoid spurious reflections. We explain that to precondition the NRBC its formulation in terms of characteristics has to be adapted. An academic wave propagation test case is computed for different wave configurations to validate the preconditioned boundary conditions. The use of non-preconditioned NRBCs in a preconditioned computation leads to instabilities and reflections at the boundaries of the domain. A consistent setup with preconditioned NRBCs improves the stability and leads to good non-reflecting properties for all presented wave configurations.

  Abstract
  In computational fluid dynamics (CFD), unsteady computations are cost-intensive. The Harmonic Balance (HB) method [7] represents a cost-efficient alternative. Here, for timeperiodic [...]

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• High fidelity fluid-structure interaction simulation of a multi-megawatt airborne wind energy reference system in cross-wind flight

  N. Pynaert, J. Wauters, G. Crevecoeur, J. Degroote

  eccomas2022.

  
  

  Abstract

  Airborne wind energy (AWE) is an emerging technology for the conversion of wind energy into electricity by flying crosswind patterns with a tethered aircraft connected to a generator either on board or on the ground. Having a proper understanding of the unsteady interaction of the air with the flexible and dynamic system during operation is key to developing viable AWE systems. The research goal is to simulate the time-varying fluid-structure interaction (FSI) of an AWE system in a crosswind flight maneuver using high fidelity simulation tools. In this work a framework is presented that serves as a proof of concept to perform high fidelity simulations of airborne wind energy systems. This is done using a partitioned and explicit approach in the open-source coupling tool CoCoNuT. An existing finite element method (FEM) model of the wing structure is coupled with a newly developed computational fluid dynamics (CFD) model of the wing aerodynamics including rigid body motion. It has been found that the mesh deformation is quite sensitive to dynamic mesh parameters. On the other hand, the overset/Chimera technique has been proven to be a robust approach to simulate the motion of an AWE system in CFD simulations.

  Abstract
  Airborne wind energy (AWE) is an emerging technology for the conversion of wind energy into electricity by flying crosswind patterns with a tethered aircraft connected to [...]

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• Novel immersed boundary method for fluid-structure interaction of compressible flow

  F. Kristoffersen, M. Larsson, S. Johnsen, W. Schröder, B. Müller

  eccomas2022.

  
  

  Abstract

  A 3D fluid-structure interaction (FSI) code is under development. The fluid domain (Navier-Stokes) solver will employ a sharp interface ghost node immersed boundary method (IBM) to apply the boundary conditions at fluid-solid interfaces. The Navier-Stokes (N-S) solver has been verified using a classic Poiseuille channel flow. The current version of the immersed boundary method is being tested by solving a heat conduction problem. The order of accuracy of the IBM was shown to be just above second order.

  Abstract
  A 3D fluid-structure interaction (FSI) code is under development. The fluid domain (Navier-Stokes) solver will employ a sharp interface ghost node immersed boundary method [...]

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• A semi-implicit method for thrombus formation in haemodynamic fluid-structure interaction

  R. Schussnig, S. Dreymann, A. Jafarinia, T. Hochrainer, T. Fries

  eccomas2022.

  
  

  Abstract

  Aortic flows with thrombus formation represent a challenging application of fluidstructure interaction (FSI) in biomechanics where blood flow, thrombus, and vessel wall are strongly coupled. Considering patient-specific FSI and thrombus formation on identical time scales remains unfeasible. To resolve this issue, we propose incorporating the dynamics-based thrombus formation model of Menichini et al. [1] into our recently presented semi-implicit, splitstep partitioned FSI scheme for non-Newtonian fluids [2, 3]. Herein, we formulate the basic split-step scheme and present the first promising results, merely coupling the fluid pressure and structure displacement iteratively at each time step.

  Abstract
  Aortic flows with thrombus formation represent a challenging application of fluidstructure interaction (FSI) in biomechanics where blood flow, thrombus, and vessel wall are [...]

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• The iPGDZ+ technique for compressing primal solution time-series in unsteady adjoint - applications & assessment

  A. Margetis, E. Papoutsis-Kiachagias, K. Giannakoglou

  eccomas2022.

  
  

  Abstract

  Gradient-based optimization for large-scale problems governed by unsteady PDEs, in which gradients with respect to the design variables are computed using unsteady adjoint, are characterized by the backward in time integration of the adjoint equations, which require the instantaneous primal/flow fields to be available at each time-step. The most widely used technique to reduce storage requirements, at the expense of a controlled number of recomputations, is binomial check-pointing. Alternatively, one may profit of lossless and lossy compression techniques, such as iPGDZ+, this paper relies upon. iPGDZ+is a hybrid algorithm which consists of (a) an incremental variant of the Proper Generalized Decomposition (iPGD), (b) the ZFP and (c) the Zlib compression algorithms. Two different implementations of iPGDZ+are described: (a) the Compressed Full Storage (CFS ) strategy which stores the whole time-history of the flow solution using iPGDZ+and (b) the Compressed Coarse-grained Check-Pointing (3CP ) technique which combines iPGDZ+with check-pointing. Assessment in aerodynamic shape optimization problems in terms of storage saving, computational cost and representation accuracy are included along with comparisons with binomial check-pointing. The methods presented are implemented within the in-house version of the publicly available adjointOptimisation library of OpenFOAM, for solving the flow and adjoint equations and conducting the optimization.

  Abstract
  Gradient-based optimization for large-scale problems governed by unsteady PDEs, in which gradients with respect to the design variables are computed using unsteady adjoint, [...]

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