NEW DEVELOPMENTS AND CLASSICAL THEORIES OF TURBULENCE

Abstract

In this paper we review certain classical and modern concepts pertinent for the theory of developed turbulent flows. We begin by introducing basic facts concerning the properties of the Navier-Stokes equation with the emphasis on invariant properties of the vorticity field. Then we discuss classical semiempirical approaches to developed turbulence which for a long time have constituted a basis for engineering solutions of turbulent flows problems. We do it for two examples, homogeneous isotropic turbulence and flat channel turbulent flow. Next we discuss the insufficiency of classical semi-empirical approaches. We show that intermittency is an intrinsic feature of all turbulent flows and hence it should be accounted for in any reasonable theoretical approach to turbulence. We argue that intermittency in physical space is in one to one correspondence with certain phase coherence of turbulence in an appropriate dual space, e.g. Fourier space for the case of homogeneous isotropic turbulence. In the same time the phase coherence has its origin in invariant topological properties of vortex lines in inviscid flows, modified by the presence of small molecular viscosity. This viewpoint is expounded again using the examples of homogeneous isotropic turbulence and channel flow turbulence. Finally we briefly discuss the significance of phase coherence and intermittency in turbulence for the fundamental engineering challenge of turbulence control.