Wall-bounded turbulence at high Reynolds number

7 problems

• Does spanwise wall forcing still save net power at very high Reynolds number?

  Open

  Moving a wall sideways back and forth, or in travelling waves, can cut turbulent friction by tens of percent. Whether a net energy saving survives at the very high speeds of ships and airliners is unknown.

• Is the von Karman constant universal across wall-bounded flows?

  Contested

  Close to a smooth wall, the mean speed of a turbulent flow grows with the logarithm of distance from the wall, with a slope set by the von Karman constant. Pipes, channels and boundary layers give slightly different values, and it is unclear whether one number holds for all.

• Does the near-wall peak of streamwise fluctuations grow without bound?

  Contested

  The strength of turbulent fluctuations just above a wall, measured relative to the wall friction, keeps rising as the flow gets faster in all experiments so far. Whether it rises forever or levels off is disputed, and the data cannot yet decide.

• Predicting rough-wall drag from surface topography alone

  Open

  Rough surfaces such as fouled ship hulls or corroded pipes raise turbulent friction, and engineers summarize the effect by one number, an equivalent sand-grain roughness height. Nobody can yet compute that number from a measured map of a surface without testing the surface in a flow.

• What information do Reynolds-averaged models lack to predict smooth-surface separation?

  Open

  Engineering simulations cannot afford to resolve every eddy, so they use approximate models of turbulence. These models still fail to predict reliably where a turbulent flow detaches from a smooth curved surface, which governs stall and drag, and it is not known what information they are missing.

• Universality of the logarithmic law for streamwise turbulence intensity

  Contested

  A picture of wall turbulence as a hierarchy of eddies attached to the wall predicts that the strength of streamwise fluctuations falls with the logarithm of height in the middle of the layer. Whether this law holds with the same slope in pipes, channels, boundary layers and the atmosphere is debated.

• Origin of very-large-scale motions in wall turbulence

  Open

  Turbulent pipe and boundary-layer flows contain very long streaks of fast and slow fluid, ten or more times longer than the pipe radius or layer thickness, carrying much of the turbulent energy. What creates and sustains them is not agreed.