The turbulent structure of the jet in cross-flow
In this thesis the structure of the jet in cross flow in the far field was investigated experimentally using time-resolved, multi-scale and statistically independent Stereoscopic Particle Image Velocimetry measurements to reveal the mean and instantaneous three-dimensional (3D) structures. All of the measurements were performed in the Counter-rotating Vortex Pair (CVP) plane for a high velocity ratio and jet Reynolds number. Statistical measurements at various downstream locations and velocity ratios are presented. Probability density functions of the streamwise vorticity field showed that each CVP core is instantaneously made of a number of small vortex tubes rather than a single vortex core. The characteristic ‘kidney’ shape was illustrated in the rms velocity profiles and the Reynolds stress profiles exhibited a high level of organisation which showed an evolving shape with downstream distance and persisted well into the far field. Two point spatial correlations pointed to a common structure for all conditions whose mean shape generates the ‘kidney’ shape, as well as evidence of wake structures. Time-resolved measurements were carried out in a moving and stationary frame of reference, converted to 3D measurements via the use of Taylor’s hypothesis. The origin of the ‘kidney’ shape and large degree of spatial order in the far field was found to be a result of an organised ‘train’ of consecutive hairpin, roller and wake structures. Together, these structures provide a physical explanation that reconciles the statistical and instantaneous structure of the CVP.