Author Archives: Gregor Gassner

Snapshot: Dam break against a flexible structure with Smoothed Particle Hydrodynamics

To simulate elastic structures, we use a total Lagrangian SPH formulation [1].
We simulate a dam break against a flexible structure based on the setup given by [1].
For the fluid dynamics, a weakly-compressible Lagrangian SPH formulation with an artificial viscosity by Monaghan [2] and boundary force particles to model the tank [3] was used.
This animation shows the simulation in 0.1x real time with 8.7k fluid particles and 455 particles to model the flexible structure.

References:
[1] O’Connor, J., Rogers, B. D. (2021). “A fluid–structure interaction model for free-surface flows and flexible structures using smoothed particle hydrodynamics on a GPU.” In: Journal of Fluids and Structures, 104. https://doi.org/10.1016/J.JFLUIDSTRUCTS.2021.103312
[2] Monaghan, J. J. (1989). “On the Problem of Penetration in Particle Methods”. In: Journal of Computational Physics, 82(1), 1–15. https://doi.org/10.1016/0021-9991(89)90032-6
[3] Monaghan, J. J., & Kajtar, J. B. (2009). “SPH particle boundary forces for arbitrary boundaries”. In: Computer Physics Communications, 180(10), 1811–1820. https://doi.org/10.1016/j.cpc.2009.05.008

Snapshot: Dam break simulation with Smoothed Particle Hydrodynamics

To study the Smoothed Particle Hydrodynamics method, we simulate a dam break based on the setup given by [1].
We use a weakly-compressible Lagrangian SPH formulation with an artificial viscosity by Monaghan [2] and boundary force particles to model the tank [3].

This animation shows the simulation in 0.5x real time with 125k fluid particles and 7k boundary particles.

References:
[1] Marrone, S., Antuono, M., Colagrossi, A., Colicchio, G., le Touzé, D., & Graziani, G. (2011). “δ-SPH model for simulating violent impact flows”. In: Computer Methods in Applied Mechanics and Engineering, 200(13–16), 1526–1542. https://doi.org/10.1016/J.CMA.2010.12.016
[2] Monaghan, J. J. (1989). “On the Problem of Penetration in Particle Methods”. In: Journal of Computational Physics, 82(1), 1–15. https://doi.org/10.1016/0021-9991(89)90032-6
[3] Monaghan, J. J., & Kajtar, J. B. (2009). “SPH particle boundary forces for arbitrary boundaries”. In: Computer Physics Communications, 180(10), 1811–1820. https://doi.org/10.1016/j.cpc.2009.05.008

Talk: Bernhard Müller, Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, Norway [04.02.2020, 11am]

On Tuesday, 4 February 2020 at 11:00 Bernhard Müller will talk on the topic “Immersed Boundary Method for the Compressible Navier-Stokes Equations Using High Order Summation-By-Parts Operators” based on a joint work with M. Ehsan Khalili and Knut Emil Ringstad, NTNU, and Martin Larsson, Sportradar AS, Trondheim.

Location: Gyrhofstraße 8a (building 158a), Room 1.105 (1st floor), 50931 Cologne

Abstract: An efficient and versatile immersed boundary method (IBM) for simulating compressible viscous flows with complex and moving convex boundaries has been developed. The compressible Navier-Stokes equations are discretized by globally fourth order summation-by-parts (SBP) difference operators with in-built stability properties and the classical fourth order explicit Runge-Kutta method. The proposed Cartesian grid based IBM builds on the ghost point approach in which the solid wall boundary conditions are applied as sharp interface conditions. The interpolation of the flow variables at image points and the solid wall boundary conditions are used to determine the flow variables at three layers of ghost points within the solid body in order to introduce the presence of the body interface in the flow computation and to maintain the overall high order of accuracy of the flow solver. Two different reconstruction procedures, bilinear interpolation and weighted least squares method, are implemented to obtain the values at the ghost points. A robust high order immersed boundary method is achieved by using a hybrid approach in which the first layer of ghost points is treated by using a third order polynomial combined with the weighted least squares method and the second and third layers of ghost points are treated by using bilinear interpolation to find the values at the image points of the corresponding ghost points. After demonstrating the accuracy of the present IBM for low Mach number flow around a circular cylinder, it is applied to simulate flow in the cross-section of the upper airways of a specific obstructive sleep apnea patient. The IBM solver has been further verified and validated for moving boundaries by applying it to a transversely oscillating cylinder in freestream flow and an in-line oscillating cylinder in an initially quiescent fluid. Sound waves generated by the in-line oscillation of the cylinder exhibit both quadrupole and monopole types. The present IBM is also verified for fluid-structure interaction of an elastically mounted circular cylinder in freestream flow at Reynolds number 200, and the rate of energy transferred between the fluid and the structure is investigated.

Snapshot: Postdoctoral researcher Dr. Andrés Rueda-Ramírez joins our research group

Andrés M. Rueda-Ramírez completed his undergraduate studies in Mechanical Engineering at the National University of Colombia (Universidad Nacional de Colombia, UNAL). After graduating, he worked as a research and teaching assistant at the Research Group on biomechanics at UNAL, where he collaborated with a highly interdisciplinary team in the development of a Finite Element software to simulate bone growing processes.

AMRR completed his Ph.D. studies in Aerospace Engineering at the Polytechnic University of Madrid (Universidad Politécnica de Madrid). During his Ph.D., AMRR studied and developed p-adaptation algorithms, implicit time-integration schemes, and multigrid solvers for high-order Discontinuous Galerkin Spectral Element Methods (DGSEM).

AMRR is now a member of the Numerical Simulation Research Group at the University of Cologne, where he joined the development team of the DGSEM code FLUXO. AMRR is currently working on sub-cell shock-capturing schemes and time integrators for the Navier-Stokes and the MHD equations.

Talk: Ruediger Pakmor, Max-Planck-Institute for Astrophysics, Garching

On Friday, 31 January 2020 at 14:00 Ruediger Pakmor will talk on the topic “The numerical schemes behind the moving mesh code Arepo”

Location: Weyertal 86-90, Seminar room 1 (ground floor), 50931 Cologne

Abstract: I will present the schemes behind the moving mesh magnetohydrodynamics code Arepo and discuss in particular the finite volume scheme on a moving mesh and its time integration. I will then show an example of an anisotropic diffusion solver on the unstructured Voronoi mesh used in Arepo and discuss general properties of the Arepo code.

Talk: Cedrick Ansorge, Institut für Geophysik und Meteorologie, Universität zu Köln

On Tuesday, 28 January 2020 at 11.00 Cedrick Ansorge will talk on the topic “Turbulent Ekman flow as virtual lab in geophysical fluid dynamics”

Location:  Gyrhofstraße 8a (Gebäude 158a), Room 1.105 (1st floor), 50931 Cologne

Abstract: The atmospheric boundary layer (ABL) is the lowest part of the atmosphere that is directly linked to the surface through vertical turbulent exchange, typically the lowest 100 to 1000m. There, turbulent mixing is the main vertical transport mechanism for heat, water, momentum and any kind of air constituent. Besides turbulence, the ABL is a multi-physical system comprising also radiative, miro-physical, chemical and other processes on scales from the multi-kilometre range down to the Kolmogorov scale of turbulent motion at the sub-millimetre range in three spatial dimensions. Both the multiphysical complexity and the broad-scale nature are prohibitive for a brute-force approach to numerical modelling of the system. Truncated representations of the ABL are thus inevitable when numerically modeling the ABL.
I will introduce turbulent Ekman flow–the doubly periodic flow over a flat rotating plate—that physically truncates the ABL to its fluid-mechanical core, the Navier–Stokes equations with appropriate boundary conditions. The governing equations are solved by a highly scalable numerical algorithm that is being used on up to 250,000 compute cores to represent the turbulent flow on grids that routinely use 3 × 230 collocation points in space. The reduced physical complexity allows for high-fidelity turbulence simulations where the entire range of turbulent motion is represented directly–without need for turbulence closure. We can thus study the ABL under conditions for which classical approaches to represent turbulence fail–such as partial or complete laminarization and transitional flows.

 

New paper submitted: A Sub-Element Adaptive Shock Capturing Approach for Discontinuous Galerkin Methods

In this paper,a new strategy for a sub-element based shock capturing for discontinuous Galerkin (DG) approximations is presented. The idea is to interpret a DG element as a collection of data and construct a hierarchy of low to high order discretisations on this set of data, including a first order finite volume (FV) scheme up to the full order DG scheme. The different DG discretisations are then blended according to sub-element troubled cell indicators, resulting in a final discretisation that adaptively blends from low to high order within a single DG element. The goal is to retain as much high order accuracy as possible, even in simulations with very strong shocks, as e.g. presented in the Sedov test. The framework retains the locality of the standard DG scheme and is hence well suited for a combination with adaptive mesh refinement (AMR) and parallel computing. The numerical tests demonstrate the sub-element adaptive behaviour of the new shock capturing approach and its high accuracy.