Relaminarization of Fluid Flows
This chapter examines the mechanisms that cause reversion and discusses if there are any similarities or general principles that govern them. In carrying out this examination, a pragmatic definition of reversion is adopted, according to which a flow has relaminarized if its development is understood without recourse to any model for turbulent shear flow. This implies that the turbulent fluctuations need not necessarily have completely vanished in the relaminarized state; but that, if present, their contribution to mean flow dynamics is negligible. Under these circumstances, the flow may be called quasi-laminar; it carries a residual turbulence that is inherited from the previous history of the flow but has been rendered passive. The chapter discusses three reverting flows. In the first, turbulent energy is dissipated through the action of a molecular transport property, such as the viscosity or conductivity, and the governing parameter is typified by the Reynolds number. In the second class, turbulence energy is destroyed or absorbed by work done against an external agency, such as buoyancy forces or flow curvature; the typical parameter is a Richardson number. The third class of reverting flows is exemplified by a turbulent boundary layer subjected to severe acceleration.