Doctoral defense
Direct and Large-eddy Simulations of Turbulent Boundary Layers with Heat Transfer
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| Defendant |
Main Advisor |
Extra Advisor |
Date |
| Qiang Li |
Philipp Schlatter |
Dan Henningson |
2011-10-10 |
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| Opponent |
| Gary Coleman, Engineering Sciences, University of Southampton
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| Evaluation committee |
Gunilla Efraimsson, KTH Aeronautics and Vehicle Engineering
Sinisa Krajnovic, Chalmers tekniska högskolan
Johan Revstedt, LTH
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AbstractA new parallelisation of the existing fully spectral research code has been implemented and validated, and is used to perform simulations on massively parallel computer architectures with O(1000) cores. Using the parallelised code, direct numerical simulations (DNS) and large-eddy simulations (LES) of a spatially developing turbulent boundary layer with and without passive scalars over a flat plate under zero-pressure gradient (ZPG) have been carried out. The Navier-Stokes equations are solved employing a spectral method with up to 7 billion grid points. The Reynolds numbers obtained are the highest for a turbulent boundary layer obtained to date for the various setups considered. An extensive number of turbulence statistics for both flow and scalar fields are computed and compared to the well-established experimental/numerical database. In general, good agreement for all considered quantities with existing literature results, both experimentally and numerically is found. Premultiplied spanwise and temporal spectra are also used to identify the large-scale motions in the outer part of the boundary layer. The similarities shared by the streamwise velocity and the scalar with $Pr=0.71$ indicate that they might be generated by the same mechanism. The effects from the different Prandtl numbers and wall boundary conditions are also discussed in detail. Furthermore, the effects of the free-stream turbulence (FST) on the heat transfer on the wall are examined. This problem is of great interest in industrial applications as such boundary layers are rarely developing below clean ambient free streams. The momentum and heat transfer on the wall is compared with those obtained with a clean free stream and augmentations of both momentum and heat transfer in the turbulent region are found. In addition, the boundary layer structures are studied and a change of the structures in the outer region are found due to the presence of the free-stream turbulence. By examining the one-dimensional spanwise spectrum, it is speculated that the increase of the momentum and heat transfer are associated with the large-scale motions in the outer layer. In addition, rare events occuring in the viscous sublayer, i.e. isolated regions of flow reversal and high intermittent values of the wall-normal velocity fluctuations leading to high flatness values, are studied, and an attempt to quantify their occurance and origin is given. In a similar spirit, a recent DNS database has been postprocessed in an effort to educe the dominant vortical structures found in the near-wall region. It is found that hairpin vortices are reminiscent of the transition process and tend to disappear in the fully turbulent region. Finally, statistics of a turbulent boundary layer have also been studied with the concept of SED (structural ensemble dynamics), and comparisons to channel flows are made. Furthermore, closure models are proposed for both mean velocity profile and also energy budget terms.
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