Time-resolved tomographic particle image velocimetry experiments show that new hairpin vortices are generated within a fully developed and unperturbed turbulent boundary layer. The measurements are taken at a Reynolds number based on the momentum thickness of 2038, and cover the near-wall region below y⁺ = 140, where y⁺ is the wall-normal distance in wall units. Instantaneous visualizations of the flow reveal near-wall low-speed streaks with associated quasi-streamwise vortices, retrograde inverted arch vortices, hairpin vortices and hairpin packets. The hairpin heads are observed as close to the wall as y⁺ = 30. Examples of hairpin packet evolution reveal the development of new hairpin vortices, which are created upstream and close to the wall in a manner consistent with the auto-generation model (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353-396). The development of the new hairpin appears to be initiated by an approaching sweep event, which perturbs the shear layer associated with the initial packet. The shear layer rolls up, thereby forming the new hairpin head. The head subsequently connects to existing streamwise vortices and develops into a hairpin. The time scale associated with the hairpin auto-generation is 20-30 wall units of time. This demonstrates that hairpins can be created over short distances within a developed turbulent boundary layer, implying that they are not simply remnants of the laminar-to-turbulent transition process far upstream.

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For channel flow, we explore how the interaction of weak eddies produces additional eddies by means of auto-generation. This is done by DNS of two eddies with different initial strengths, initial sizes and initial stream-wise spacing between them. The numerical procedure followed is similar to Zhou et al[1]. The two eddies merge into a single stronger eddy when a larger upstream and a smaller downstream eddy are placed within a certain initial stream-wise separation distance. Subsequently, the resulting stronger eddy is observed to auto-generate new eddies. The non-merging cases with small initial stream wise separation also auto-generate. The auto-generation is characterized by a rapid lift-up of an initial eddy, which blocks the incoming flow and leads to shear- layer roll-up and formation of a new eddy. The same sequence of events is observed in a fully developed turbulent boundary layer[2].@en

The flow in the near-wall region in the turbulent boundary layer has been measured by time-resolved Tomographic-PIV. The resulting instantaneous three-dimensional velocity fields allow visualizing the flow structure in buffer layer or log layer with sufficient spatial resolution of 16×16×16 wall units in a volume spanning 680×170×620 wall units in the streamwise-, wall normal- and spanwise direction respectively. At the measurement location, the thickness of turbulent boundary layer is 71 mm. This corresponds to the Reynolds number based on momentum thickness of 2030. Instantaneous results show the regions of vortical motion detected using the Q-criterion and lowspeed streaks. The distance between low-speed streaks observed in the present experiments is about 100 wall units, which is consistent with earlier experimental studies or numerical studies. In addition, we can visualize vortical motion, such as streamwise vortices, and the autogeneration of a hairpin-like structure. Moreover, the measurement volume extends into the log layer, which enables us to study the effect of the large-scale motions in the log layer passing over the near-wall structures. @en