Large-scale coherence in a zero-pressure-gradient turbulent boundary layer is highlighted using data from direct numerical simulation (DNS) at \(Re_\tau \approx 1000\) \(Re \approx 1000\). The visualization features two structures: the large-scale streamwise velocity \(u'\) fluctuations and the turbulent/non-turbulent (T/NT) interface. Firstly, the streamwise velocity fluctuations are identified using the \(u' = \pm 0.1 U_{\infty}\) threshold , \(U_{\infty}\) where is the free-stream speed. High-speed structures are marked red and yellow in the video, and negative ones are blue and cyan. Using appropriate filtering and image processing techniques, the cores are identified and tracked during their spatio-temporal evolution [Lee, Sung & Zaki, J. Fluid Mech. 819, 165-187 (2017)]. In comparison to the original fluctuating field, they are geometrical much less complex. From the population of \(u'\) -structures identified in the time series of flow fields, properties such as the lengths, intensities and lifetimes of the coherent motions can be evaluated. Secondly, the turbulent/non-turbulent (T/NT) interface between the boundary layer and the free stream is identified using the normalized vorticity \(\omega^{*}=0.2\) magnitude . The interface is further low-pass-filtered to highlight its large-scale features and their spatio-temporal evolution. The visualization demonstrates that the undulations of the T/NT interface are spatially correlated with the cores of the large-scale \(u'\)-structure: Positive excursions in the interface height take place above low-speed structures which are formed by ejection events; Conversely, depressions in the interface occur above the high-speed structures that are formed by sweeping motions.
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