Experimental observation of hairpin auto-generation events in a turbulent boundary layer

Abstract

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.

Transitional–turbulent spots and turbulent–turbulent spots in boundary layers

“…The three aforementioned more recent PIV experiments (6,48,49) were performed in turbulent ZPGSFPBLs at 2,000 < Re θ < 3,000, also overlapping with our DNS Reynolds number range. Tennekes and Lumley (50) argued that, in the inertial sublayer of high Reynolds number wall-bounded flows, the Reynolds shear stress asymptotes to ρu 2 τ , and the mean velocity gradient is approximately u τ =κy.…”

Resolving vorticity and dissipation in a turbulent boundary layer by tomographic PTV and VIC+

“…A full description of the experiment is given by Jodai and Elsinga (2016). Relevant details related to the boundary layer properties and measurement setup are repeated here for completeness.…”

“…The data ensemble along both directions is considered for the statistical evaluation of the turbulent properties. For further details of the experimental setup, the reader is referred to Jodai and Elsinga (2016).…”

“…Tomographic PIV processing was performed by Jodai and Elsinga (2016) and salient aspects are recalled in this section. The intensity volumes were reconstructed using the MART algorithm (Elsinga et al 2006) and the average intensity profile of the reconstruction in wall-normal direction as calculated from the reconstructed objects is reported in Fig.…”

“…In the present manuscript, the VIC+ technique is applied to an actual experiment in a turbulent boundary layer, in order to assess the accuracy of the velocity gradients, vorticity and dissipation. For the present analysis, the existing experimental data of Jodai and Elsinga (2016) are used. These authors have performed time-resolved tomographic PIV measurements to obtain a detailed characterization of the turbulent motions in a turbulent boundary layer.…”