Kinetic heating and transition studies at hypersonic speeds

The thesis reports on an experimental and computational study of kinetic heating at hypersonic speeds. Of particular interest is the transition of the laminar boundary layer to a state of turbulent motion. The experiments are performed in a Mach 9 Gun tunnel with a 5 ° semi-angle cone geometry. Twelve hemispherically blunted nose radii are tested at three unit Reynolds numbers. Testing has indicated that as the nose is progressively blunted, the transition region moves downstream. Further amounts of bluntness enhance other instability mechanisms and transition events are witnessed in the near nose regions. There are clearly two transitional regimes, denoted the "small bluntness" and "transition reversal" regime, respectively. This study investigates the structure of the transitional boundary layer in both regimes using thin film heat transfer rate gauges and liquid crystal surface thermography. The heat transfer measurements indicate that the small bluntness transition regime is governed by the rapid formation, growth and merging of turbulent events. Transition occurs over hundreds of boundary layer lengths. The reversal regime transition process is characterised by the birth of turbulent events in the nose and near nose regions. The temporal formation rate of the events is governed by roughness. In a low roughness environment, transition occurs over many model lengths. Increasing the roughness level, increases the spot formation rate, and transition is witnessed immediately downstream of the spherical nose region. The role of roughness is further explored using boundary layer trips. The trip causes a laminar wake which rapidly undergoes transition and forms a turbulent wedge. Event circumferential spreading angles are found for a variety of trip geometries and locations. The heat transfer distribution in the wedge is mapped using the thin film gauges. Computational work is used to perform laminar flow field predictions. Of interest is the entropy layer caused by the presence of the bow shock, and its interaction with the boundary layer. Heat transfer predictions in the transitional region are also performed aided with the experimentally obtained intermittency information.Open Acces