Simulations of Turbulent Spots and Wedges over Textured Surfaces

  • Turbulent spot visualized by iso-surface of swirling strength colored by free stream velocity. Swirling strength Flow is from bottom left to top right. Notice the overall arrowhead shape as well as the intricate forest of hairpin (hooks and legs) coherent structure. Even though turbulence is chaotic, there is still order within it.
    Turbulent spot visualized by iso-surface of swirling strength colored by free stream velocity. Swirling strength Flow is from bottom left to top right. Notice the overall arrowhead shape as well as the intricate forest of hairpin (hooks and legs) coherent structure. Even though turbulence is chaotic, there is still order within it.
  • Figure 1 - Laminar to turbulent transition. Source - Viscous Fluid Flow by F. White.
    Figure 1 - Laminar to turbulent transition. Source - Viscous Fluid Flow by F. White.
  • Turbulent wedge visualized by iso-surface of streamwise velocity colored by distance from the bottom wall. Low speed streaks (streaks that juts away from the wall) can be observed. Low speed streaks is a probable link in the cycle of turbulence self-regeneration. If low-speed streaks can be controlled, then maybe we can control spreading of turbulence.
    Turbulent wedge visualized by iso-surface of streamwise velocity colored by distance from the bottom wall. Low speed streaks (streaks that juts away from the wall) can be observed. Low speed streaks is a probable link in the cycle of turbulence self-regeneration. If low-speed streaks can be controlled, then maybe we can control spreading of turbulence.
  • Turbulent spot moving over straight riblets  (left) and turned riblets (right). Turbulent spots at three time snapshots are superimposed on one picture.  The turned riblets inspired asymmetric development of turbulent spots.  This is evidence that realistic and relatively small surface textures can modify turbulent spot behavior on a macroscopic level.
    Turbulent spot moving over straight riblets (left) and turned riblets (right). Turbulent spots at three time snapshots are superimposed on one picture. The turned riblets inspired asymmetric development of turbulent spots. This is evidence that realistic and relatively small surface textures can modify turbulent spot behavior on a macroscopic level.

Turbulent spots are arrowhead shaped pockets of turbulent that form in the late stages of laminar to turbulent transition process (red circle in the schematic below). These spots increase in size as they travel downstream and form fully turbulent flow as they merge together. My research is looking at the formation and growth mechanisms of turbulent spot as well as interactions of millimeter scale surface textures with spots. If laminar to turbulent transition can be delayed using surface textures, then drag could be reduced.

The simulations are done using a channel flow spectral DNS code modified with immersed boundary to allow for boundary layer simulations. Rex ranges from about 524,000 to 675,000.

 

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