Stochastization of Coherent Structures by a Periodic Field

Abstract

Realization of the fact that large-scale orderly structures play an important part in the evolution of turbulent flows (see, for example, [1,2]) gave rise to a number of questions. One of the main ones is when coherent structures (CS) are an intermediate state on the way to developed turbulence and when these CS, developing spontaneously, suppress small-scale turbulence and contribute to relaminarization of the flow. The first step in solving this problem is to study the CS susceptibility to different perturbations which are generally not necessarily small. Such perturbations can appear either due to secondary instability or can be an external forcing including a regular one. It should be natural to use a regular external forcing for stabilization of CS and of the flow as a whole. However, as is apparent from the present paper, external periodic forcing does not always synchronize CS. On the contrary, it can contribute to their stochastization. This is true both for nondissipative and dissipative structures. To illustrate the stochastization of nondissipative structures, nonviscous CS that appear as vortex blobs in two-dimensional free shear flows are considered. It is assumed that in the vortex velocity field the small-scale turbulence is transported by liquid particles. The effect of external regular forcing on dissipative structures is studied using as an example the CS-drops that appear on a viscous capillary water jet discharging in air.