Abstract
Direct Numerical Simulation of turbulent flows is a computationally demanding problem that requires efficient parallel algorithms. We investigate the applicability of the time-parallel Parareal algorithm to an instructional case study related to the simulation of the decay of homogeneous isotropic turbulence in three dimensions. We combine a Parareal variant based on explicit time integrators and spatial coarsening with the space-parallel Hybrid Navier–Stokes solver. We analyse the performance of this space–time parallel solver with respect to speedup and quality of the solution. The results are compared with reference data obtained with a classical explicit integration, using an error analysis which relies on the energetic content of the solution. We show that a single Parareal iteration is able to reproduce with high fidelity the main statistical quantities characterizing the turbulent flow field.
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Notes
We define the parallel efficiency as the ratio of the parallel speedup over the number of processes involved.
Since there is no mean flow in this configuration, we cannot use the classical definition of the Reynolds number. We rather use \(Re_\lambda \)(see e.g., [11, 44]), which characterizes the ratio of turbulent eddy viscosity, over the fluid viscosity. This Reynolds number, at least one order of magnitude lower than the classical one, has therefore no relevance regarding laminar to turbulent transition, but instead informs about the range of length scales present in an already turbulent flow (see e.g., [31] for a complete description).
For isotropic turbulence \(\lambda \) is conveniently expressed as \(\lambda = u^{\prime }\sqrt{15 \langle \mu \rangle /(\langle \rho \rangle \epsilon )}\) (see, e.g., [31, Sec. 6.3, p.199]).
available at https://gitlab.com/tlunet/parallel-in-time.
This value of the theoretical ratio of coarse-to-fine execution is explained by the coarsening in space by a factor of 2 in each direction and by the choice of the coarse time step detailed in Sect. 3.4.
We refer the reader to only Sec. 4.3 for numerical experiments using two iterations (\({\widehat{K}}=2\)) of Parareal with spatial coarsening.
In Hybrid the computation of the right-hand side of the Navier–Stokes equations involves 39 gradient evaluations (3 for (14), 18 for (15), 6 for (16), 9 for (18) and 3 for (19), respectively). Since the RK4 time integration method requires 4 evaluations of right-hand sides, the total amount of gradient evaluations for one time step of the coarse solver is then 156. Finally, 60 stencil evaluations per coarse point are required for the interpolation of the 5 different fields.
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Acknowledgements
The authors thank the two reviewers for their comments. The authors would like to acknowledge CALMIP for the dotation of computing hours on the EOS computer. This work was granted access to the HPC resources of CALMIP under allocation P1425 and P17005. Thibaut Lunet would like to thank Daniel Ruprecht, Michael Minion, Debasmita Samaddar, Jacob B. Schroder, Martin J. Gander, Robert Speck, Andreas Schmidt, Felix Kwok, Beth Wingate and Raymond J. Spiteri for the interesting discussions at the 5th Workshop on Parallel-in-time integration at the Banff International Research Station (BIRS), and the Jülich Supercomputing Center for granting a Student Travel Award for attending this conference.
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This research is partly funded by “Région Occitanie/ Pyrénées-Méditerranée” as part of the “Développement de nouvelles stratégies pour le calcul massivement parallèle à l’échelle exa en mécanique des fluides numérique (DENSE)” project.
A. Computation of the energy spectrum
A. Computation of the energy spectrum
We briefly describe the computation of the energy spectrum. More details can be found in the literature (see, e.g., [20, Ap. B]). We denote by \(\hat{\varvec{u}}(\kappa _x,\kappa _y,\kappa _z)\) the Discrete Fourier Transform (DFT) of \(\varvec{u}(x,y,z)\), using 1 (\(1/N_L^3\)) to normalize the direct (inverse) DFT, respectively. We define \(\kappa _x\), \(\kappa _y\) and \(\kappa _z\) the discrete one-dimensional wavenumbers as:
and the associated vector norm:
The 3D Fourier space is discretized into three-dimensional shells of thickness \(d\kappa \) (usually \(\frac{2\pi d\kappa }{L}=1\)). To deduce the energy corresponding to the wavenumber \(\kappa _i\), a bin count is then performed using an integer number of shells:
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Lunet, T., Bodart, J., Gratton, S. et al. Time-parallel simulation of the decay of homogeneous turbulence using Parareal with spatial coarsening. Comput. Visual Sci. 19, 31–44 (2018). https://doi.org/10.1007/s00791-018-0295-0
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DOI: https://doi.org/10.1007/s00791-018-0295-0