Evidence of non-universality of von Kármán’s k

A notable universal feature of wall-bounded turbulent flows is the universal logarithmic law of the wall deduced by Theodore von Kármán. This law of the wall describes how the time-averaged streamwise velocity changes with the distance from the wall. Despite the law of the wall having a universal von Kármán constant k = 0.41 that governs the slope of the log-law velocity profile, as is commonly known over a period of about 80 years, in fluvial streams there are number of in-stances on the non-universality of k. To be specific, it behaves as a variable in flows with low relative submergence, or where there is bed-load and/or suspended-load sediment transport. This article focuses on the aspect of non-universality of k by inviting various open questions relating to future research directions.