Expansion wave diffraction around sharp convex corners

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering. Johannesburg, December 2017Much work has been previously invested towards the study of shock wave phenomena. However, little or no literature can be found involving studies of expansion waves undergoing similar effects. The study of expansion wave diffraction was rst considered by Mohamed and Skews [1]. The investigation focussed on studying expansion wave diffraction around a 90 corner by using both experimental and computational methods. While a good agreement between the results was found there were several features which were apparent in the experimental imaging but could not be resolved by a numerical model. This included the large-scale turbulent structures in the separation bubble, shear layer instability and vortex shedding as well as a large wake region which was noted downstream of the bubble. In the current work, expansion wave di raction is examined using Large Eddy Simulation in an attempt to provide a better description of the ow eld. To make the analysis more feasible the Embedded LES hybrid technique was employed. Several subgrid-scale LES models were tested and the Wall Modelled LES technique showed the most promising results. The LES solution showed much improvement to the RANS solutions from the previous work. Turbulence in the separation bubble was evident and shear-layer instability and vortex shedding was observed. Due to the very low velocity uctuations the LES model did not resolve the wake region although, in certain cases, evidence of a wake region beginning to form could be seen. Using these results a further analysis into the structure of the bubble through the depth of the ow was conducted. As an extension of previous work the current study also examined expansion wave diffraction around other convex corners. The study involved an experimental as well as RANS and LES computational methods. The wedge angles investigated were 45 , 15 and 5 . The effects of changing the initial diaphragm pressure ratio and the diaphragm distance from the di raction corner were analysed. The results showed a high resemblance to what was found previously for the 90 case. However, signifi cant differences were noted in the shape of the bubble due to the inclination angle of the ow along vertical wall and the much higher velocity gradients of the ow at the diffraction corner and surrounding the bubble.MT 201