Documents published in Scipedia

• Development lengths for non-newtonian flows in pipes and tubes based on the wall shear stress

  C. Lambride, A. Syrakos, G. Georgiou

  eccomas2022.

  
  

  Abstract

  The development length needed for tube flows to re-adjust from a uniform to the fully-developed velocity profile is usually defined as the length required for the centerline velocity to reach 99% of its fully-developed value. This definition, however, may be quite inaccurate in non-Newtonian flows with almost flat velocity distributions near the centerline, since the velocity far from the axis of symmetry develops more slowly. Shear-thinning and viscoplasticity may cause the flow close to the centerline to evolve faster than that closer to the walls. Thus, alternative definitions of the development length have been proposed for viscoplastic flows. Given that blood exhibits shear thinning, we numerically solve the flow development of power-law fluids in pipes and calculate the development length as a function of the radius, determining the global development length along with the standard centerline estimate. We also consider an alternative definition, based on the evolution of the wall shear stress. Results have been obtained for values of the power-law exponent ranging from 0.

  Abstract
  The development length needed for tube flows to re-adjust from a uniform to the fully-developed velocity profile is usually defined as the length required for the centerline [...]

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• On the effect of Prandtl number to subgrid-scale heat flux models

  F. Trias, D. Santos, J. Hopman, A. Gorobets, A. Oliva

  eccomas2022.

  
  

  Abstract

  Estimations of the grid size and computational cost for direct numerical simulation (DNS) and large-eddy simulation (LES) of Rayleigh-Bénard convection (RBC) are presented in the {Ra, P r} phase space. Computational requirements to reach the so-called asymptotic Kraichnan or ultimate regime of turbulence using DNS are far too expensive. Therefore, we turn to LES to predict the large-scale behavior at very high Ra-numbers. However, a priori alignment studies clearly show why the modelization of the SGS heat flux is the main difficulty that (still) precludes reliable LES of buoyancy-driven flows at (very) high Ra-numbers. This inherent difficulty can be by-passed by carrying out simulations at low-P r numbers where no SGS heat flux activity is expected. This opens the possibility to reach the ultimate regime carrying out LES of RBC at low-P r using meshes of 10101011grid points. Nevertheless, to do so, we firstly need to combine proper numerical techniques for LES (also DNS) with an efficient use of modern hybrid supercomputers.

  Abstract
  Estimations of the grid size and computational cost for direct numerical simulation (DNS) and large-eddy simulation (LES) of Rayleigh-Bénard convection (RBC) are presented [...]

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• Towards proper subgrid-scale model for jet aerodynamics and aeroacoustics

  A. Duben, J. Ruano, F. Trias, A. Gorobets

  eccomas2022.

  
  

  Abstract

  This article presents the investigation of different grey-area mitigation (GAM) techniques towards achieving accurate subsonic turbulent round jet aerodynamics and aeroacoustics results. Combinations of new adapting subgrid length scales with 2D detecting LES models are used as the GAM technique. The numerical simulations are carried out on a set of refining meshes using two different scale-resolving codes: NOISEtte and OpenFOAM. The results show that all the considered techniques provide appropriate accuracy to predict the noise generated and the importance of both the numerical scheme and how subgrid eddy viscosity is modelled.

  Abstract
  This article presents the investigation of different grey-area mitigation (GAM) techniques towards achieving accurate subsonic turbulent round jet aerodynamics and aeroacoustics [...]

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• FEM analysis and monolithic Newton-multigrid solver for thixo-viscoplastic flow problems

  N. Begum, A. Ouazzi, S. Turek

  eccomas2022.

  
  

  Abstract

  In this contribution, we shall be concerned with the question of wellposedness of thixoviscoplastic flow problems in context of FEM approximations.We restrict our analysis to a quasiNewtonian modeling approach with the aim to set foundations for an efficient monolithic Newtonmultigrid solver. We present the wellposedness of viscoplastic subproblems and structure subproblems in parallel/independent fashion showing the possibility for a combined treatment. Then, we use the fixed point theorem for the coupled problem. For the numerical solutions, we choose 4:1 contraction configuration and use monolithic Newton-multigrid solver. We analyse the effect of taking into consideration thixotropic phenomena in viscoplastic material and opening up for more different coupling by inclusions of shear thickening and shear thinning behaviors for plastic viscosity and/or elastic behavior below the critical yield stress limit in more a general thixotropic models.

  Abstract
  In this contribution, we shall be concerned with the question of wellposedness of thixoviscoplastic flow problems in context of FEM approximations.We restrict our analysis [...]

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• Computational micro-magneto-mechanics

  C. Dorn, S. Wulfinghoff

  eccomas2022.

  
  

  Abstract

  We formulate a material model for micro-magneto-mechanics based on the generalized standard material approach. Our model includes exchange, elastic, anisotropy, demagnetizing and Zeeman energy. Furthermore we account for dissipative micro-magnetic behavior by means of a dissipation potential. For the constrained optimization problem w.r.t. magnetization we rely on the exponential map algorithm. We demonstrate our ideas with numerical examples. In particular we apply our model to a thin film composite. With this composite we represent the magneto-mechanical part of a magneto-electric composite sensor (resp. small sensor segment). Our numerical experiments focus on FeCoSiB as the magnetostrictive material. We discuss the coupling effects for the considered thin film composite in detail.

  Abstract
  We formulate a material model for micro-magneto-mechanics based on the generalized standard material approach. Our model includes exchange, elastic, anisotropy, demagnetizing [...]

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• 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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