Flow around a sphere

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A snapshot of a recent computation with the unstructured DGSEM code Flexi of the flow around a sphere at RE=1000 and Ma=0.3. The computation was run on the new Cray XE6 “Hermit” at HLRS on 4096 cores. The picture below shows the laminar separation, the vortex street and the transition in the wake (Latex formula criterion).

High vs. low order DGSEM for Taylor-Green Vortex

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Here’s an interesting plot of the dissipation rate of kinetic energy of the Taylor-Green vortex problem computed with our Discontinuous Galerkin Spectral Element Method workhorse code “Strukti”. The Reynolds number of the problem is 800, Mach number is 0.1 and all computations were done with a Lax-Friedrichs formulation for the inviscid flux and BR1 for the viscous fluxes. All simulations were done on 64 Nehalem cores, the times shown in the plots are the total (overall) wall times of each computation.

Taylor Green Vortex at Re = 800

“N” in the figures above denotes the degree of the polynomial approximation inside each grid cell, the DNS solution from Brachet (384 DOF per direction, pseudo-spectral code) is shown as square symbols for comparison.
Two related issues are obvious from the plot above:
1) While both formulations (O(2) and O(16)) converge to the DNS as expected, the O(2) solution requires significantly more degrees of freedom and prohibitively more wall time to reach an acceptable agreement with the DNS data.
2) Conversely, for the same number of DOF (64³, clear underresolution for DNS), the O(16) scheme produces an acceptable result close to the DNS, while the O(2) solution is too dissipative to capture the physics of the flow and to allow transition to turbulence.

Visit by Dr. Christophe Bogey

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On November 9th, Dr. Christophe Bogey from the École Centrale Lyon visited our research group and presented his work on “Large-Eddy Simulation based on relaxation filtering: quality assessment and application to jet”. We had a full day of very fruitful and interesting discussions and would like to thank Dr. Bogey for his visit!

Note on Compressibility Effects for Taylor Green Vortex Analysis

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The classical Taylor Green vortex problem is defined for an incompressible flow field. If the flow is computed with a compressible code at an essentially “incompressible” low Mach number (Ma=0.1 in this case), compressibility effects are deemed to be negligible for the resulting flow field. However, diagnostic quantities that depend on the spatial derivatives of the velocity field and/or the Latex formula condition can show a significantly different behavior for their incompressible or compressible formulation, depending on the resolution of the problem.

The plot below compares the compressible and incompressible dissipation rate for the Taylor Green vortex problem. The difference between both formulations (the pressure term and the divergence condition) for this LES-type resolution can be very significant. It should be noted that this difference decreases as one approaches a DNS resolution.
So if an essentially incompressible problem is solved by a compressible method, care should be taken to include compressibility effects in the analysis, even for low Mach numbers.

Launch party!

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After years of discussion, planing, shouting and tears we finally did it and launched our new group research page!
This blog section is intended to provide a platform for discussion and scientific exchange. Feel free to comment…