title: Swept boundary layer transition
Creators: name: Yuile, Adam
Publisher: name: University of Liverpool Repository
Year: 2014

description: Boundary layer transition has been investigated for incompressible three-dimensional mean flows on a flat plate with a 60° swept leading edge for a nominally zero, a positive, and a negative pressure gradient for three freestream turbulence intensities using a low speed blower tunnel with a 1.22 x 0.61 m working section at the University of Liverpool. The freestream turbulence intensities were generated using grids upstream of the leading edge, producing turbulence levels of approximately 0.2 %, 1.25 % and 3.25 %. For each of these nine (3 x 3) test cases detailed boundary layer traverses were obtained at ten streamwise measurement stations, at a fixed spanwise location, using single-wire constant temperature hot-wire anemometry techniques and digital signal processing. The location for the onset and end of transition was obtained for each case, in terms of distance from the leading edge and local momentum thickness Reynolds number. These results are compared with the 2-D unswept empirical transition correlations of Abu-Ghannam and Shaw (1980) and the differences in the results between the two flows are highlighted. It was found that transition starts and ends earlier than for similar unswept flows, complementing the transition observations of Gray (1952) for swept wings. Further to this the receptivity of the swept boundary layers to freestream turbulence (in the bypass transition regime) was determined by comparing near wall and local freestream spectra, for the pre-transitional boundary layers. These experimental results were compared with numerical predictions from a fourth order accurate computational fluid dynamics method which considered a multitude of perturbation waveforms. This numerical approach was also able to identify the waveform frequency and orientation combinations which drive receptivity in swept boundary layer transition and indicate the manner in which receptivity scales with momentum thickness Reynolds number. It was found that the most receptive waveforms correspond to the streamwise streaks which are frequently observed in flow visualisations and direct numerical simulation studies of pre-transitional boundary layers. Additionally it was also found that the numerical receptivities to freestream turbulence were highest for the positive pressure gradient and, in contrast, lowest for the negative pressure gradient – a similar finding to that in 2-D boundary layers. Transition was seen to commence prior to the advent of the intended non-zero pressure gradients in the experiments and thus direct comparisons are not strictly available. The results obtained, and synthesis undertaken for this thesis, contribute towards an improved understanding of the transition process, particularly with respect to receptivity, in regard to flat plates with swept leading edges in various pressure gradients and highlight the differences between swept and unswept flows. Furthermore, additional avenues have been identified for future work on more complicated topologies where potential problems have also been highlighted.