Large Eddy Simulation of transitional flow over a flat plate
The transition of the laminar boundary layer to turbulence in the free stream is of major importance in turbomachinery flows. Transition in the boundary layer has a big impact on flow parameters such as skin friction coefficient and shape factor thereby affecting the design and development process. It is known that the boundary layer transition is affected by properties of the free stream turbulence such as its anisotropy, length scale and its intensity. In the present thesis a Large Eddy Simulation (LES) has been performed of a zero pressure gradient flat plate boundary layer undergoing transition to turbulence under free stream turbulence at a level of 6% In the current work the bypass transition is defined as a superposition of the Blasius layer with freestream continuous isotropic turbulence at an initial intensity level of 6% An effort is made to capture the length and position of boundary layer transition over the flat plate and a subsequent comparison of the skin friction coefficient and shape factor were made with the experimental data. A finite volume method based in house solver in Fortran, namely CALC LES, is used for performing the simulations. The solver of CALC LES is based on a geometric multigrid algorithm to solve the pressure (poisson) equation. Three different subgrid scale (SGS) models are used for performing comparative studies the Smargorinsky Lilly model, the Wall adapting local eddy viscosity (WALE) model and the Scale similarity model. The experimental data for the skin friction coefficient and shape factor were used as a basis to ascertain whether the present simulation manages to capture transition. The present simulations fail to capture the transition appropriately in the boundary layer over the flat plate. The dimensionless coefficients, namely the skin friction coefficient and the shape factor are not in the exact accordance with the experimental data. The mesh independence study showed that the stream wise resolution hardly played any role in the transition. The study of the resolved Reynolds stress components in the boundary layer revealed that the stream wise stress component decreased in magnitude significantly, but it doesn't get distributed into the other components. As a result transition is not observed. Upon further inspection of the turbulent kinetic energy peak showed that probably the stream wise streaks grow, but either they were not big enough to trigger transition or there was no continuous forcing provided by the free stream turbulence to trigger transition. It is not known what aspect of the current LES simulations are at fault but in the light of the general level of agreement obtained, we conclude that,the the simulation qualitatively mimics the real boundary layer even though precise quantitative agreement is not found for all variables.