Generation of near-wall coherent structures in a turbulent boundary layer

Using direct numerical simulations of turbulent channel flow, we present ne w 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 transie nt growth, which dominates early-time evolution, and (ii) linear normal-mod e instability, dominant asymptotically at late time (for frozen base flow s treaks). Approximately 25% of streaks extracted from near-wall turbulence a re shown to be strong enough for linear instability (above a critical vorte x line lift angle). However, due to viscous annihilation of streak normal v orticity omega(y), normal mode growth ceases after a factor of two energy g rowth. In contrast, the linear transient disturbance produces a 2-fold ampl ification, due to its rapid, early-time growth before significant viscous s treak decay. Thus, linear transient growth of w(x) is revealed as a new, ap parently dominant, generation mechanism of x-dependent turbulent energy nea r the wall. Combined transient growth/instability of lifted, vortex-free lo w-speed streaks (above the instability cutoff of streak strength) is shown to generate new streamwise vortices, which dominate near-wall turbulence ph enomena. This new vortex formation mechanism consists of: (i) streak wavine ss in the horizontal plane caused by w(x) disturbance growth, (ii) generati on of horizontal sheets of streamwise vorticity and induction of positive s tretching partial derivative u/partial derivative x (i.e. positive VISA), i nherent to streak waviness, and finally (iii) vorticity sheet collapse via stretching (rather than roll-up) into streamwise vortices. Significantly, t he 3D features of the (instantaneous) vortices generated by transient/insta bility growth agree well with the coherent structures educed (i.e. ensemble -averaged) from fully turbulent flow, suggesting the prevalence of this mec hanism. Results suggest promising new strategies for drag and heat transfer control, involving large-scale thence more durable) actuators, without req uiring wall sensors or control logic.