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In this review, we describe turbulent drag reduction in a variety of flowsusing a universal framework of energy flux. In a turbulent flow with dilutepolymers and magnetic field, the kinetic energy injected at large scalescascades to the velocity field at intermediate scales, as well as to thepolymers and magnetic field at all scales. Consequently, the kinetic energyflux, $ \Pi_u(k) $, is suppressed in comparison to the pure hydrodynamicturbulence. We argue that the suppression of $\Pi_u(k)$ is an important factorin the reduction of the inertial force $\langle {\bf u \cdot \nabla u} \rangle$and \textit{turbulent drag}. This feature of turbulent drag reduction isobserved in polymeric, magnetohydrodynamic, quasi-static magnetohydrodynamic,and stably-stratified turbulence, and in dynamos. In addition, it is shown thatturbulent drag reduction in thermal convection is due to the smooth thermalplates, similar to the turbulent drag reduction over bluff bodies. In all theseflows, turbulent drag reduction often leads to a strong large-scale velocity inthe flow.

Turbulent Drag Reduction in Magnetohydrodynamic Turbulence and Dynamo from Energy Flux Perspectives