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  • 1.
    Brynjell-Rahkola, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Barman, Emelie
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. Swedish Defence Research Agency, FOI.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    On the stability of flat plate boundary layers subject to localized suction2015Report (Other academic)
    Abstract [en]

    The stability of the Blasius boundary layer subject to localized suction is revisited. Using tools of global stability analysis, the leading direct and adjoint eigenpairs are determined, and novel insight into the sensitivity and receptivity of the flow is obtained. The problem is addressed through high-order spectral element simulations, which enables the inclusion of a suction pipe into the domain. Due to this, a detailed investigation of the connection between the pipe flow and the boundary layer flow is possible. For all cases investigated, the former always turns out to transition for a lower Reynolds number and suction rate than the latter, and the transition scenario is found to be due to a global instability originating inside a separation bubble at the pipe inlet. Identification of such regions, provides information that is valuable in further development of algorithms for laminar flow control.

  • 2.
    Noorani, Azad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Informal introduction to program structure of spectral interpolation in nek5000Manuscript (preprint) (Other academic)
    Abstract [en]

    The algorithm of the interpolation implementation in the spectral element codenek5000is documented informally. The original code is written by James Lottes at Argonne National Laboratories. The various steps of the operations are generally described and visualised for a typical deformed mesh. The corresponding routines and their argument lists for each stage of the interpolation are also explained. The memory structure of the implementation is briefly discussed. Finally, the code overview of the routines is presented.

  • 3.
    Offermans, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Marin, Oana
    Fischer, Paul
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Towards adaptive mesh refinement for the spectral element solver Nek50002017Report (Refereed)
    Abstract [en]

    Hypre, a library for linear algebra, is used to replace a Matlab code for performing the setup step of an Algebraic Multigrid Method (AMG). The AMG method is used to compute part of the preconditioner in Nek5000, a code for Computational Fluid Dynamics based on the spectral element method. However, the solution of the AMG problem is not performed via Hypre but by Nek5000’s internal solver. The new AMG setup is shown to be faster by at least one order of magnitude, while it does not significantly impact the efficiency of the AMG solver, as is shown from its application to relevant test cases.

  • 4.
    Offermans, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Marin, Oana
    Argonne National Laboratory.
    Fischer, Paul
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Towards Adaptive Mesh Refinement for the Spectral Element Solver Nek50002019In: Direct and Large-Eddy Simulation XI, Springer, 2019, 25, p. 9-15Chapter in book (Refereed)
    Abstract [en]

    Hypre, a library for linear algebra, is used to replace a Matlab code for performing the setup step of an Algebraic Multigrid Method (AMG). The AMG method is used to compute part of the preconditioner in Nek5000, a code for Computational Fluid Dynamics based on the spectral element method. However, the solution of the AMG problem is not performed via Hypre but by Nek5000’s internal solver. The new AMG setup is shown to be faster by at least one order of magnitude, while it does not significantly impact the efficiency of the AMG solver, as is shown from its application to relevant test cases.

  • 5.
    Offermans, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Marin, Oana
    Argonne National Laboratory.
    Merzari, Elia
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Performance of preconditioners for large-scale simulations using Nek50002019Conference paper (Refereed)
    Abstract [en]

    BoomerAMG, the algebraic multigrid solver from the hypre library, is used to solve a coarse grid problem which is part of the preconditioning strategy for thepressure equation arising from the numerical resolution of the Navier–Stokes equations. A set of optimal parameters for the setup phase is determined and used for selected strong scaling tests on two different supercomputers, namely Mira and Hazel Hen, on up to 131, 072 compute cores. The results are compared to an existing algebraic multigrid solver, designed specifically for the coarse gridproblem at hand. It is shown that the BoomerAMG solver is fast and scalable, and that performance depends on the computer architecture. The test cases considered are the turbulent flow past a NACA4412 airfoil and the turbulent flow inside wire-tapped pin bundles.

  • 6.
    Offermans, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Marin, Oana
    Argonne National Laboratory.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Adaptive mesh refinement for steady flows in Nek5000Manuscript (preprint) (Other academic)
    Abstract [en]

    Adaptive mesh refinement is performed in the framework of the spectral element method augmented by approaches to error estimation and control. The h-refinement technique is used for adapting the mesh, where selected grid elements are split by a quadtree (2D) or octree (3D) structure. Continuity between parent–child elements is enforced by high-order interpolation of the solution across the common faces. Parallel mesh partitioning and grid management respectively, are taken care of by the external libraries ParMETIS and p4est. Two methods are considered for estimating and controling the error of the solution. The first error estimate is local and based on the spectral properties of the solutionon each element. This method gives a local measure of the L2-norm of the solution over the entire computational domain. The second error estimate uses the dual-weighted residuals method — it is based on and takes into account both the local properties of the solution and the global dependence of the error in the solution via an adjoint problem. The objective of this second approach is to optimize the computation of a given functional of physical interest. The simulations are performed by using the code Nek5000 and three steady-state test cases are studied: a two-dimensional lid-driven cavity at Re = 7, 500, a two-dimensional flow past a cylinder at Re = 40, and a three-dimensional lid-driven cavity at Re = 2, 000 with a moving lid tilted by an angle of 30 degrees. The efficiency of both error estimators is compared in terms of refinement patterns and accuracy on the functional of interest. In the case of the adjoint error estimators, the trend on the error of the functional is shown to be correctly represented up to a multiplicative constant.

  • 7.
    Offermans, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Marin, Oana
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Adjoint error estimators and adaptive mesh refinement in Nek50002017Report (Other academic)
    Abstract [en]

    The development of adaptive mesh refinement capabilities in the field of computational fluid dynamics is an essential tool for enabling the simulation of larger and more complex physical problems. In this report, we describe recent developments that have been made to enable adaptive mesh refinement in Nek5000 and we validate the method on simple, two-dimensional, steady test cases.We start by describing the modifications brought to Nek5000 that enable the presence of hanging nodes in the mesh. Thanks to this new feature, we can use the h-refinement technique for mesh adaptation, where selected elements are split via quadtree (2D) or octree (3D) structures. Then, two methods are considered to estimate and control the error. The first method is local and based on the spectral properties of the solution on each element. The second method is goal-oriented and takes into account both the local properties of the solution and the global dependence of the error in the solution via the resolution of an adjoint problem. Finally, the use of automatic mesh refinement is demonstrated in Nek5000 on two test cases: the lid-driven cavity at Re = 7, 500 and the flow past a cylinder at Re = 40. Both error estimation methods are compared andare shown to efficiently reduce the number of degrees of freedom required to reach a given tolerance on the solution compared to conforming refinement. Moreover, the gains in terms of mesh generation, accuracy and computational cost are discussed by analysing the convergence of some functional of interest and the evolution of the mesh as refinement proceeds.

  • 8.
    Offermans, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Unsteady adjoint error estimators and adaptive mesh refinement in Nek50002019Report (Other academic)
    Abstract [en]

    Unsteady adjoint error estimators based on the dual-weighted residuals method are implemented for the spectral element method in Nek5000. The time-integration of the adjoint problem is performed based on the nonlinear direction solution recomputed via the revolve algorithm, which uses an optimal check-pointing strategy. Adaptive mesh refinement is performed on the flow inside a constricted periodic channel, the so-called periodic hill case, at four different Reynolds numbers, Re = 700, 1400, 2800 and 5600. This case is fully turbulent at all regimes, with significant flow separation, requires curved meshes, but yet has a number of accurate reference solutions in the literature. The chosen method to adapt the mesh is h-refinement, where selected elements are split by an oct-tree structure in three dimensions. The objective function for the adjoint estimators is the integral of the friction forces along the flat bottom wall between the hills, for which the location of the reattachment becomes crucial. The refinement process is compared between the adjoint error estimators and classical straightforward a posteriori spectral error indicators based on the local approximation properties of the solution.The turbulent simulations using mesh adaptation are stable, free of spurious numerical noise and accurate, as shown by comparing the statistical profiles of relevant flow quantities with reference data. The comparison between the error estimators shows that the adjoint error estimators tend to refine the mesh only around localized regions in the computational domain while leaving other areas under-resolved. However, only the locally refined regions are shown to have a significant impact on the value of the objective function and thus on the location of the reattachment point. Conversely, the spectral error indicatorstend to homogenize the error on the solution over the whole domain but have a lesser direct influence on the location of the reattachment point.

  • 9.
    Peplinski, Adam
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Fischer, P. F.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Parallel performance of h-type Adaptive Mesh Refinement for Nek50002016In: ACM International Conference Proceeding Series, Association for Computing Machinery , 2016Conference paper (Refereed)
    Abstract [en]

    We discuss parallel performance of h-type Adaptive Mesh Refinement (AMR) developed for the high-order spectral element solver Nek5000 within CRESTA project. AMR is a desired feature of the future simulation software, as it gives possibility to increase the accuracy of numerical simulations at minimal computational cost by resolving particular region of the domain. At the same time it increases complexity of the communication pattern and introduces load imbalance, that can have negative effect on the code scalability. In this work we concentrate on the parallel performance of different tools required by AMR and the resulting algorithm limitations. Our implementation is based on available libraries for parallel mesh management (p4est) and partitioning (ParMetis) that provide necessary information for grid refinement/coarsening and redistribution performed within nonconforming version of Nek5000. For simplicity we consider advection-diffusion problem instead of the full Navies-Stokes equations and study both strong and weak scalability for the convected-cone problem. It is a synthetic test case allowing to test AMR with frequent dynamic mesh adjustments.

  • 10.
    Peplinski, Adam
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Offermans, Nicolas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Marin, Oana
    Argonne National Laboratory.
    Fischer, Paul
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Non-conforming elements in Nek5000: Pressure preconditioning and parallel performance2019Conference paper (Refereed)
    Abstract [en]

    Adaptive mesh refinement (AMR) is an important component of modern numerical solvers, as it allows to control the computational error during the simulation, increasing the reliability of the numerical modelling and giving the possibility to study a broad range of different phenomena even without knowing the physics a priori. In this work we present selected aspects of the implementation and parallel performance of a new h−type AMR framework developed for the high-order CFD solver Nek5000; the development was done within the ExaFLOW EU project. We utilise in this case the natural domain decomposition inherent to the spectral element method (SEM), which constitutes the main source of parallelism and provides meshing flexibility that can be exploited in AMR. We use standard libraries for parallel mesh management (p4est) and partitioning (ParMetis) and focus on developing efficient preconditioners for the pressure problem solved on non-conforming meshes. Two different approaches are considered: an additive overlapping Schwarz and a hybrid Schwarz-multigrid method.The strong scaling is shown on the example of the simulation of the turbulent flow around a NACA4412 wing section at Rec = 200, 000.

  • 11.
    Peplinski, Adam
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fischer, P. F.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability tools for the spectral-element code Nek5000: Application to Jet-in-Crossflow2014In: Lecture Notes in Computational Science and Engineering, ISSN 1439-7358, Vol. 95, p. 349-359Article in journal (Refereed)
    Abstract [en]

    We demonstrate the use of advanced linear stability tools developed for the spectral-element code Nek5000 to investigate the dynamics of nonlinear flows in moderately complex geometries. The aim of stability calculations is to identify the driving mechanism as well as the region most sensitive to the instability: the wavemaker.We concentrate on global linear stability analysis, which considers the linearised Navier–Stokes equations and searches for growing small disturbances, i.e. so-called linear global modes. In the structural sensitivity analysis these modes are associated to the eigenmodes of the direct and adjoint linearised Navier–Stokes operators, and the wavemaker is defined as the overlap of the strongest direct and adjoint eigenmodes. The large eigenvalue problems are solved usingmatrix-freemethods adopting the time-stepping Arnoldi approach.We present here our implementation in Nek5000 with the ARPACK library on a number of test cases.

  • 12.
    Peplinski, Adam
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Investigations of stability and transition of a jet in crossflow using DNS2015In: 9th International Conference on Direct and Large-Eddy Simulation, 2013, Springer Publishing Company, 2015, p. 207-217Conference paper (Refereed)
  • 13.
    Peplinski, Adam
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Global stability and optimal perturbation for a jet in cross-flow2015In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 49, p. 438-447Article in journal (Refereed)
    Abstract [en]

    We study the stability of a jet in cross-flow at low values of the jet to cross-flow velocity ratio R using direct numerical simulations (DNS) and global linear stability analysis adopting a time-stepper method. For the simplified setup without a meshed pipe in the simulations we compare results of a fully-spectral code SIMSON with a spectral-element code Nek5000. We find the use of periodic domains, even with the fringe method, unsuitable due to the large sensitivity of the eigenvalues and due to the large spatial growth of the corresponding eigenmodes. However, we observe a similar sensitivity to reflection from the outflow boundary in the inflow/outflow configuration, and therefore we use an extended domain where reflections are minimal. We apply in our studies both modal and non-modal linear analyses investigating transient effects and their asymptotic fate, and we find a transient wavepacket to develop almost identically in both the globally stable and unstable cases. The final results of the global stability analysis for our numerical setup show the critical value of R, at which the first bifurcation occurs, to lie in the range between 1.5 and 1.6.

  • 14.
    Peplinski, Adam
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Investigations of stability and transition of a jet in crossflow using DNS2015In: Instability and Control of Massively Separated Flows: Proceedings of the International Conference on Instability and Control of Massively Separated Flows, held in Prato, Italy, from 4-6 September 2013, Kluwer Academic Publishers, 2015, Vol. 107, p. 7-18Conference paper (Refereed)
    Abstract [en]

    We study the stability of a jet in crossflow at low values of the jet-tocrossflow velocity ratio R focusing on direct numerical simulations (DNS) and the global linear stability analysis adopting a time-stepper method. For the simplified setup neglecting a meshed pipe in the simulations, we compare results of the fullyspectral code SIMSON with the spectral-element code Nek5000. We find the calculated critical value R for the first bifurcation to be dependent on the numerical method used. This result is related to a large sensitivity of the eigenvalues and to the large spatial growth of the corresponding eigenmodes, making the use of periodic domains, even with the fringe method, difficult. However, we observe a similar sensitivity to reflection from the outflow boundary in the inflow/outflow configuration as well.We apply in our studies both modal and non-modal analyses investigating transient effects and their asymptotic fate, and we find transient wavepacket that develop almost identically in the stable and unstable cases. Finally, we compare these results with the simulation including the pipe in the computational domain finding the latter one to be much more unstable.

  • 15. Tanarro, Álvaro
    et al.
    Mallor, Fermı́n
    Offermans, Nicolas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Using adaptive mesh refinement to simulate turbulent wings at high Reynolds numbers2019Conference paper (Refereed)
    Abstract [en]

    The implementation of adaptive mesh refinement (AMR) in Nek5000 is used for the first time on the simulation of the flow over wings. This is done by simulating the flow over a NACA4412 profile with 5 degrees angle of attack at chord-based Reynolds number 200,000. The mesh is progressively refined by means of AMR which allows for high resolution near the wall whereas significantly larger elements are used in the far-field. The resultant mesh shows higher resolution than previous conformal meshes, and it allows for larger computational domains,which avoid the use of RANS to determine the boundary condition, all of this with, approximately, 3 times lower total number of grid points. The results ofthe turbulence statistics show a good agreement with the ones obtained with the conformal mesh. Finally, using AMR on wings leads to simulations at higher Reynolds numbers (i.e. Rec = 850, 000) in order to analyse the effect of adverse pressure gradients at high Reynolds numbers.

1 - 15 of 15
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