AN EXPERIMENTAL STUDY OF HAIRPIN-TYPE VORTICES AS A POTENTIAL FLOW STRUCTURE OF TURBULENT BOUNDARY LAYERS (HEMISPHERES)

It has been shown in numerous studies that the dominant indentifiable structure in near-wall region of turbulent boundary layers in the low-speed streak. These structures are of particular importance since they are the apparent site of the near-wall "bursting" activity which is the primary source of turbulence production in the boundary layer. A water channel study has been done to examine the suggestion that horseshoe or hairpin vortices exist in the near-wall region of turbulent boundary layers and play a key role in the formation, persistence and breakdown of the low-speed streaks.;The flow patterns created by the presence of the hairpin vortices have been documented using flow visualization and hot-film anemometry techniques and cross compared with the patterns observed in the near-wall of a fully turbulent boundary layer. The degree of comparison is quite striking, indicating essential identity between the flow patterns generated by the synthetically generated hairpin vortices and many of the predominant flow patterns observed in the near-wall of a turbulent boundary layer (e.g. streaks, bursts, pockets, ejections, etc.). Velocity measurement were made at various downstream locations of the hemisphere and synthetic low-speed streak wake using a hot-film anemometry data acquisition system. Results indicate that strong inflectional profiles just downstream of the hairpin vortex generation region evolve into fuller profiles, with downstream distance, which eventually begin to resemble that of a turbulent boundary layer.;Based on this study, a comprehensive model of the flow process in the near-wall of a turbulent boundary layer is hypothesized.;The hairpin vortices were synthetically generated by the interaction of both a hemisphere protuberance and a low-speed fluid region (created by fluid injection techniques) with a developing laminar boundary layer. Under proper conditions, hairpin vortices shed extremely periodically which allows detailed examination of their behavior. Shedding characteristics of the hemispheres and roll-up characteristics of the synthetic streaks were determined using hot-film anemometry techniques. The stability characteristics of the synthetic streaks were compared with the natural stability characteristics of different velocity profiles. The results indicate that three-dimensionality of the flow around the synthetic streak results in the amplification of the disturbances which have smaller wavelengths compared to two dimensional flows.