Dynamics of structures in transition to turbulence

  • Wesfreid, José Eduardo (pmmh)
  • Semin, Benoit (pmmh)
  • Liu, Tao (pmmh)
  • Godoy-Diana, Ramiro (pmmh)

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In the transition regime in wall-bounded shear flows, active turbulence is characterized by coherent structures described by streamwise vortices (rolls) and modulations of the streamwise velocity (streaks). We investigate experimentally, in a plane Couette-Poiseuille channel (see figure 1A), the dynamics of these structures and the detailed interplay of their components. We study the decay of turbulence using a 'quench' protocol, i.e. an abrupt decrease of the Reynolds number $Re$ from a fully turbulent state to a laminar regime and we show that the rolls decay faster than the streaks (Liu et al. 2021). Additionnaly, we investigate the waviness of streaks using vortex generators to induce unstable wavy streaks. We apply a spatial filter to separate the straight part and the wavy part of the streamwise velocity. The normal vorticity from this latter component (wavy vorticity), is used as a quantitative measurement of the waviness of the streaks Spanwise velocity (see (see figures 1B and 1C) is correlated to the increase of the waviness of the streaks, as expected from non-linear self-sustaining models (Waleffe 1997) . For streaks of low waviness, this value is smaller and related to the amplitude of the streak, as expected for linear lift-up.