Effects of surface features and vibration of flat plate on boundary layer transition

Bhatia, Dinesh Devraj (2016) Effects of surface features and vibration of flat plate on boundary layer transition. (PhD thesis), Kingston University, .


Transitional flow of fluids has been a subject of great interest in the field of aerodynamics. Reduction of drag through maintaining laminar flow and delaying the onset of boundary layer transition has become one of the key areas of research in this field. Laminar flow control has been achieved through the use of active and passive devises. However, a number of these solutions are difficult to implement because the mechanisms of the controlling techniques are not fully understood. This research focuses on comparing the aerodynamic performance of a number of novel solutions to obtain passive laminar flow control i.e. Natural Laminar flow control. Experimental work was undertaken on a flat plate with different leading edges varying from aspect ratio AR1 to AR12. At the same time, flat plate transition simulations were conducted for aspect ratio AR1 to AR20. The effects of these leading edges were recorded and compared with the results obtained through CFD. ANSYS FLUENT was the chosen solver and the CFD model was validated against a number of existing test cases from literature as well as our own cases. The simulations were performed using the [gamma]-Re[theta] model. The effects of changes in turbulence intensity and the velocity of incoming flow were also recorded to fully understand the transitional phenomenon of the flow. The simulations showed that AR20 is 4% better than the default AR12 leading edge configuration for the same velocity and turbulence intensity of the incoming flow. The use of wavy surfaces to delay transition has been undertaken in this research endeavour. The surface of the plate had a number of different wave configurations consisting of 32, 64,128 and 256 waves along a 2.4 meters long plate. For each of these wave configurations, the amplitude of the wave was changed and the influence of the amplitude change was studied. The presence of a wavy surface with the amplitude lesser than the laminar sub boundary layer resulted in delaying transition. In some cases, the presence of an extremely wavy surface i.e. 256 waves proved to be the best configuration and was roughly 4.1% better than the flat plate configuration. However, as the velocity increases the impact of a higher wavenumber plate starts to diminish. When the velocity is maintained 30 m/s and 50 m/s respectively, the 256 wave configuration has the best aerodynamic performance. When the velocity is increased to 80 m/s, the trend reverses and a 32 wave plate tends to have the best performance. Forward Facing steps find applications in aircraft wing repairs through the use of patches. The presence of a Forward Facing Step (FFS) to delay transition has also been discussed. Steps were placed at crucial locations within the laminar boundary layer on the flat plate in the research. Step heights were determined by using a constant-boundary thickness to step height ratio (o h). The results shown that the position of the step about 100 mm from the transition point with a o h ratio of 12 helped delay transition by roughly 3.2%. The effects of a plate moving in the direction perpendicular to the flow was studied experimentally and quantified. The influence of the motion on boundary layer transition was to bring forward the transition onset location. The aerodynamic performance was found to be worse when the flat plate is subjected to this motion as compared to a stationary flat plate.

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