On the large-scale streaks in the logarithmic layer of wall-bounded flows

Laboratory experiments were performed to characterize the distinct features of hairpin-like vortical structures induced via pulsed, wall-normal jets in laminar (LBL), and turbulent boundary layers (TBLs). Hotwire anemometry was used to characterize the disturbed flow at multiple streamwise, transverse, and vertical locations. The dominant spatiotemporal structure of the induced motions was reconstructed using Taylor’s hypothesis and time-averaged, phase-locked data. Results support the concept that the meandering of very-long, low-momentum streaks observed in the logarithmic layer and lower part of wake regions of TBLs results from the upwash of hairpin vortices or packets. Comparing the induced hairpin-like vortices in the LBL and TBL reveals the modulation of the mean shear; using the second-order derivative of the mean streamwise velocity, we show a linkage between the mean shear gradient and appearance of low- and high-momentum streaks in the logarithmic layer. Streamwise velocity profiles with a pitot tube by Smith (Effect of Reynolds number on the structure of turbulent boundary layers, 1994) at ten Reynolds numbers from Reθ = 4601 to 13,189 are also inspected to evaluate such linkage in a broader range of Re. As Reynolds number increased, the vertical regions of the negative second derivative of mean streamwise velocity increased. The minimum value of mean shear gradient decreased, indicating that the mean shear stress gradient increased to form more noticeable low- and high-momentum streaks in the logarithmic layer. Besides, the Reynolds tensor’s principal stresses and principal axes angles associated with the hairpin vortex legs stretching are demonstrated by DNS data (Spalart, P. R. 1988) as Reynolds number increased.