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Licentiate seminar

Spectral-element simulations of separated turbulent internal flows


Defendant Main Advisor Extra Advisor Date
Johan Malm Dan Henningson Philipp Schlatter 2009-12-15

Opponent
Jesper Oppelstrup, KTH, NADA

Evaluation committee

Abstract

The spectral-element method (SEM) offers new possibilities to examine transitional and turbulent flows at high accuracy in complex geometries. Simulations of canonical flow cases such as temporal K-type transition and turbulent channel flow are performed to investigate general resolution requirements and computational efficiency. It is shown that the numerical instability associated with SEM at high Reynolds numbers is cured either by over-integration (dealiasing) or a filter-based stabilization, thus rendering simulations of high Reynolds number flows possible. The generation of high quality turbulent inflow conditions is implemented, tested and finally applied to more complex spatially developing turbulent flows. Examples of such flows investigated in this thesis is the large-eddy simulation (LES) of turbulent separation in a plane asymmetric diffuser, where good agreement with numerical studies of Herbst et al. (2007) is obtained. In particular it is noticed that less grid points can be used to predict the separated flow with similar accuracy, leading to the conclusion that the use of a high-order method is advantageous for flows featuring pressure-induced separation. Another flow case studied is the direct numerical simulation (DNS) of turbulent separation in the truly three-dimensional asymmetric diffuser, experimentally investigated by Cherry et al. (2008). The massively parallel capabilities of the spectral-element method are exploited by running the simulations on up to 32 768 processors. Very good agreement with experimental data is obtained, and it is thus shown that well-resolved simulations of complex turbulent flows are possible at realistic Reynolds numbers with high accuracy even in complicated geometries.
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