Boundary-layer transition on a flat plate with a super-elliptic leading edge is visualized using direct numerical simulation (DNS) data. The simulation setup mimics a wind-tunnel experiment of bypass transition, where the initially laminar boundary layer is exposed to free-stream vortical disturbances that cause its breakdown to turbulence [T.A. Zaki, Flow Turbul. Combust., 91, 451-473, (2013)]. The Reynolds number is \(Re = 800\), based on the free-stream velocity \(U_{\infty}\) and half thickness of the plate \(R\). The streamwise extent of the computational domain is \(1050R\), which is sufficiently long to capture the transition region and the development of an equilibrium turbulent boundary layer downstream. Inflow grid turbulence is prescribed from a concurrent DNS of homogeneous isotropic turbulence with intensity \(Tu = 0.027 U_{\infty}\) and integral length scale \(L=1.8R\). Smoke visualization is performed using a passive scalar at Peclet number \(Pe=568\), and vortical structures are identified using the \(\lambda_{2}\)-criteria with a threshold \(\lambda_{2} = -0.003 U_{\infty}^2 / R^{2}\). The visualization captures the interaction of the free-stream turbulence with the boundary layer, which leads to the generation of Klebanoff streaks in the transition region. The high- and low-speed streaks modulate the wall shear stress, and their secondary instability leads to the sporadic inception of localized patches of turbulence. The turbulence spots spread as they are advected downstream, and merge to form the fully turbulent boundary layer. The full spatio-temporal evolution of the flow will be made publicly available as part of the Johns Hopkins Turbulence Database (http://turbulence.pha.jhu.edu).
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