Introduction

Sir Horace Lamb described the status of our understanding of turbulent flow in 1932 as, “I am an old man now, and when I die and go to Heaven there are two matters on which I hope for enlightenment. One is the quantum electrodynamics and the other is the turbulent motion of fluids. And about the former I am rather optimistic.” For decades it has been known that our understanding of turbulent flow is very incomplete. Despite the enormous research efforts in the last decades, our understanding of turbulence is still poor, but our hopes for understanding it are greater now than they used to be. Perhaps an appropriate quotation might be, “For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.”