Turbulent coherent structures, from taxonomy to engineering

Abstract This review synthesises contemporary understanding and engineering applications of turbulent coherent structures (TCS) in boundary layer flows and evaluates a spectrum of active and passive control techniques. We begin by classifying seven TCS “species”–from wall streaks and quasi-streamwise vortices to hairpin vortices, vortex packets, super-streaks, temperature ramps, and thermal roll-cells–detailing their characteristic scales, formation cycles, and mutual interactions. Two self-sustaining cycles, confined, respectively, to the near-wall (inner) and outer regions, are presented alongside mechanisms of interscale modulation, including amplitude and frequency modulation and generation excitation. Building on this physical framework, we review control methods that target specific TCS, covering compliant coatings, riblets, blowing/suction, spanwise wall oscillations, polymer additives, plasma actuators, and thermal actuation. For each, we discuss the governing mechanism, practical implementation, predictive correlations, and performance criteria, highlighting common signatures of effective inner-cycle disruption (e.g., reduced Reynolds stress, thicker viscous layers, extended hibernation periods), and outer layer actuation strategies suited to high–Reynolds number flows. We conclude by comparing net energy savings–typically limited to about 10% and outlining two promising research directions: hybrid control schemes and theory-guided optimisation. Finally, we emphasise that successful drag reduction hinges on a deep, cycle-level understanding of TCS dynamics and their response to control inputs.