Abstract
The role of hydrodynamic instability in the creation of coherent structures in wall-bounded turbulence is reviewed. Because the mean velocity profile for a flat-plate boundary layer or channel flow is highly stable to small wave-like disturbances, the classical Tollmien-Schlichting instability mechanism is not operative for the excitation of the fluctuation field. Instead, this must involve other instability mechanisms and/or nonlinearity. For such stable flows it is found that the transfer of energy from the mean to the fluctuations takes place primarily through algebraic instability [1]. This gives rise to structures of long-lived alternating low and high-speed regions in the near-wall region, i.e., streaks, which give contributions to the Reynolds shear stress proportional to their lifetime. By restraining the spanwise fluctuating velocity component so as both to reduce the spanwise vortex stretching, thereby inhibiting the formation of internal shear layers, and the spanwise asymmetry of the structures, their contribution to the Reynolds stress, and hence the total turbulent stress and the skin frictionx, may be reduced.
Published Version
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