Dynamics and boundary conditions are key to the universal scaling of drag in wall turbulence

Scaling of surface drag in turbulent flows past solid surfaces is of prime scientific and technological importance. Successful scaling descriptions are available for canonical flow types and Reynolds number ranges that enable predictions in a limited class of flows. However, a truly universal scaling of drag uniformly valid across all types of flows (canonical as well as non-canonical) and covering complete (to date) range of Reynolds numbers has remained elusive so far. Here, we present the universal scaling of drag that comes with remarkable predictive power as a by-product. Theoretical foundation for this universal scaling enables correct choice of length and velocity scales that must be consistent with the flow dynamics; the scales that are customarily used fail to qualify this criterion. The main scaling consists of two aspects: first, a dynamically-consistent definition of dimensionless drag and Reynolds number, and second, accounting for the distinguishing differences in boundary conditions amongst different flows. Several high-quality data sets from a variety of flows in the literature unequivocally confirm the validity and practical utility of the universal scaling of drag.