Documents published in Scipedia

• 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 [...]

  • 3
  •  read

• 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 [...]

  • 14
  •  read

• 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 [...]

  • 28
  •  read

• 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 [...]

  • 32
  •  read

• 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, [...]

  • 43
  •  read

• Fluid-structure interaction tool for morphing blades

  G. Abate, J. Riemenschneider

  eccomas2022.

  
  

  Abstract

  The design of aeronautical components commonly involves two highly coupled disciplines: aerodynamics and structural mechanics. The interaction between them becomes even more relevant when morphing aeronautical structures are studied. Considering the importance of morphing technology for the future of the aerospace industry, several tools have already been developed to couple these two disciplines together, but all of them deal with pure twodimensional or three-dimensional aero-structural problems. In some circumstances, the study of aeronautical components requires to couple a 2D computational fluid-dynamics (CFD) analysis with a 3D finite element analysis (FEA). This usually happens in the preliminary design phase of aeronautical engine blades (i.e. compressor blades) where the aerodynamic study of the original 3D geometry is replaced by the analysis of a 2D blade cascade in order to reduce the overall computational cost. However, such an approach requires a specific method to couple the 2D CFD geometry/mesh with the 3D FEA geometry/mesh in order to transfer the aerodynamic loads from the CFD analysis to the structural one. As mentioned before, the existing fluid-structure interaction (FSI) tools cannot be implemented to solve a 2D-3D problem; therefore, a novel 2D3D aero-structure coupling approach needs to be developed. This paper describes step-by-step the 2D-3D aero-structure coupling strategy applied to the performance analysis of a morphing blade cascade with the goal of enhancing its aerodynamic performance. The results show a relevant decrease in the total pressure losses of the morphing cascade thanks to the adapting blade leading-edge. In order to highlight the reliability of the FSI framework, the developed approach is applied to four different blade configurations which differ in size and location of the two morphing devices.

  Abstract
  The design of aeronautical components commonly involves two highly coupled disciplines: aerodynamics and structural mechanics. The interaction between them becomes even more [...]

  • 6
  •  read

• Unstructured high-order solutions of hovering rotors with and without ground effect

  P. A. S. F. Silva, P. Tsoutsanis, A. Antoniadis, K. Jenkins

  eccomas2022.

  
  

  Abstract

  This work describes the computational cost and accuracy of high-order numerical schemes on a simplified concept of multiple reference frame (MRF) technique using mixedelement unstructured grid framework widely tested for aerospace applications. The Reynolds averaged Navier-Stokes equations are approximated with up to fourth spatial order using Spalart Allmaras turbulence model on two types of reconstruction scheme: monotonic upwind scheme for conservation laws (MUSCL) and weighted non-oscillatory (WENO). The calculations were made for both out-of-ground-effect (OGE) and in-ground-effect (IGE) cases and compared with experimental data in terms of pressure distribution, tip-vortex trajectory, vorticity contours and integrated thrust and torque. The predictions were obtained for several ground distances. Our findings suggest that the resolution of the vortex path and wake breakdown were considerably improved with increased scheme order. It is noticeable how low-order scheme struggles to deal with the large amount of diffusion. This numerical nature contributes to the vortex system settle down and achieve this stable ring state structure as seen on a couple radii downstream the rotor. As the wake is transported downwards, it can be clearly seen the interaction primary and secondary structures, which stretch between the tip vortices making an S-shaped path also seen experimentally. The presence of the vortex-ring close to the rotor blade contributes to a larger induced flow and under predictions of thrust coefficient . As we increase the scheme-order, it becomes more evident how the helical system convects down and breakdown toroidal vortex into smaller scales.

  Abstract
  This work describes the computational cost and accuracy of high-order numerical schemes on a simplified concept of multiple reference frame (MRF) technique using mixedelement [...]

  • 30
  •  read

• Large-eddy simulations of turbulent compressible supersonic jet flows using discontinuous Galerkin methods

  D. Abreu, C. Junqueira-Junior, E. Dauricio, J. Azevedo

  eccomas2022.

  
  

  Abstract

  In this work, a discontinuous Galerkin scheme is employed to perform large-eddy simulations of supersonic jet flows. A total of four simulations are performed with different meshes and order of accuracy to identify the resolution requirements to reproduce the physical characteristics from experiments. The number of degrees of freedom from the simulations varies from 50 × 106to 400 × 106. The results indicate that by increasing the resolution of simulation, in general, the results got closer to experimental data. The comparison of velocity distribution in the jet centerline and lipline from the simulation with 400 × 106with experimental shows that important characteristics of the flow are represented. The study investigated a procedure of using lower-order simulations to initialize high-order simulations to reduce the total computational cost of the calculation. This strategy is successful and allows the performance of high-order simulations with only 6% more computational effort than a second-order simulation with the same number of degrees of freedom.

  Abstract
  In this work, a discontinuous Galerkin scheme is employed to perform large-eddy simulations of supersonic jet flows. A total of four simulations are performed with different [...]

  • 9
  •  read

• Accelerating the simulation of elastohydrodynamic lubrication in a line contact using a surrogate model

  N. Delaissé, P. Havaej, D. Fauconnier, J. Degroote

  eccomas2022.

  
  

  Abstract

  Elastohydrodynamically lubricated contacts are an example of a strongly coupled fluid-structure interaction problem. Typically, these problems are solved in a partitioned way and require multiple flow-structure iterations per time step to reach convergence. The manner in which these iterations are performed is determined by the coupling algorithm. In the previous decade, several algorithms have been proposed, most of which are based on a quasi-Newton principle. These methods use an approximate Jacobian, which is constructed during the calculation itself. However, in many cases, a simpler model is available, which provides an approximate solution and Jacobian, and is denoted as surrogate model. For the elastohydrodynamically lubricated contact, this model is the coupled Reynolds-Boussinesq approach, which evaluates significantly faster than the CFD-CSM simulation. The incorporation of a surrogate model in a quasi-Newton method is realized with the IQN-ILSM algorithm. This work is a first step towards employing this coupling method for the elastohydrodynamically lubricated contact and, in this way, combining the speed of the Reynolds-Boussinesq approach with the accuracy and versatility of the CFD-CSM modelling. In the current work, only the surrogate solution will be used as initial solution. The use of the surrogate Jacobian is future work.

  Abstract
  Elastohydrodynamically lubricated contacts are an example of a strongly coupled fluid-structure interaction problem. Typically, these problems are solved in a partitioned [...]

  • 14
  •  read

• An extended discontinuous Galerkin method for high-order shock treatment

  J. Sebastian, F. Kummer

  eccomas2022.

  
  

  Abstract

  In this paper, we are going to present a high-order shock fitting approach based on a cut-cell method. We formulate a suitable Constraint Optimization Problem and develop an algorithm aiming to reconstruct the shock front represented by the zero iso-contour of a Level Set function.

  Abstract
  In this paper, we are going to present a high-order shock fitting approach based on a cut-cell method. We formulate a suitable Constraint Optimization Problem and develop [...]

  • 11
  •  read