# Licentiate seminar

## Simulations of turbulent boundary layers with heat transfer

 Defendant Main Advisor Extra Advisor Date Qiang Li Dan Henningson Philipp Schlatter 2009-10-22 Opponent Ulrich Rist, Stuttgart University, Germany Evaluation committee

### Abstract

A new parallelisation of the existing fully spectral research code has been implemented and validated, and used to perform simulations on massively parallel computer architectures with $\mathcal O(1000)$ 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 600 M grid points. The Reynolds numbers obtained are the highest for a turbulent boundary layer obtained to date with such a numerical setup. 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 agreements are 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. The momentum and heat transfer on the wall are 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.