Robust Scheme for Control of Skin Friction and Heat Transfer in Turbulent Boundary Layers via a New Instability Mechanism

摘要

Using direct numerical simulations of turbulent channel flow, we present new insight into the generation of streamwise vortices near the wall, and an associated drag reduction strategy. Growth of x-dependent spanwise velocity disturbances w(x) is shown to occur via two mechanisms: (i) linear transient growth, which dominates early-time evolution, and (ii) linear normal- mode instability, dominant asymptotically at late time (for frozen base flow streaks). Approximately 25% of streaks extracted from near-wall turbulence are shown to be strong enough for linear instability (above a critical vortex line Lift angle). However, due to viscous annihilation of streak normal vorticity COy, normal mode growth ceases after a factor of two energy growth. In contrast, the linear transient disturbance produces a 20-fold amplification, due to its rapid, early-time growth before significant viscous streak decay. Thus, linear transient growth of w(x) is revealed as a new, apparently dominant, generation mechanism of x-dependent turbulent energy near the wall.