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 ( criterion).
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.
The plot below shows h-convergence results of our DG code Strukti for a compressible turbulent round-jet, using polynomials with degree N=5. Increasing the total DOF per equation from 21 mill. up to 170 mill. DOF shows that the range of scales of the DG solution still changes and thus shows that the approximation is not converged yet…
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!
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 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.
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…