The Phenomenon of Turbulence as Distinct from the Problem of Turbulence

Abstract

The issue is about turbulence as a natural physical phenomenon as related to observations as distinct from the problem of turbulence. The dichotomic distinction between laminar and turbulent flows is problematic in several respects. First, most flows termed “turbulent” are in reality partly turbulent: some portions of the flow as turbulent and some as laminar—the coexistence of the two regimes in one flow is a common feature with continuous transition of laminar into turbulent state via the entrainment process through the boundary between the two. Moreover, the reality is not that simple as the laminar/turbulent dichotomy as, e.g. the behavior of passive objects in flows with small Reynolds number looks as perfectly turbulent reflecting the qualitative difference between the chaotic flow properties in Eulerian and Lagrangian settings. These examples illustrate the enormous difficulties in defining what is both (i) turbulence and (ii) the turbulence problem. As concerns (i) one can only provide a description of major qualitative universal (sic) features of turbulent flows as obtained almost exclusively from observations (rather than by any theoretical deliberations) which form most important part of the “essence” of turbulence. This is because these mostly widely known qualitative features of all turbulent flows are essentially the same, i.e., it is meaningful to speak about qualitative universality of turbulent flows. It has to be stressed that the term “phenomenon of turbulence” as used above is mostly associated with the observational aspects, which in turbulence play far more important role due the unsatisfactory state of “theory”: there seems to exist no such a thing based on first principles. Hence it is vital to put the emphasis on the physical aspects based in the first place on observations.