Small-scale aspects of flows in proximity of the turbulent/nonturbulent interface

Abstract

The work reported below is a first of its kind study of the properties of turbulent flow without strong mean shear in a Newtonian fluid in proximity of the turbulent/non-turbulent interface, with emphasis on the small scale aspects. The main tools used are a three-dimensional particle tracking system (3D-PTV) allowing to measure and follow in a Lagrangian manner the field of velocity derivatives and direct numerical simulations (DNS). The comparison of flow properties in the turbulent (A), intermediate (B) and non-turbulent (C) regions in the proximity of the interface allows for direct observation of the key physical processes underlying the entrainment phenomenon. The differences between small scale strain and enstrophy are striking and point to the definite scenario of turbulent entrainment via the viscous forces originating in strain.

Characteristics of the turbulent/non-turbulent interface of a non-isothermal jet

“…nibbling). Similar observations were done for the progressing turbulent/non-turbulent interface in a tank with an oscillating grid [10,11], and in the simulation of a time-evolving planar jet [12]. Close examination of the laser-induced fluorescence (LIF) images reveals possible nibbling events, where previously detrained fluid (in which the vorticity has dissipated) is re-entrained, as shown in figure 1.…”

Interfaces and inhomogeneous turbulence

“…These quantities have a probability distribution with a high flatness factor (normalized on the whole flow region) within, but not outside, the layer. These interfaces at n 1 and n 2 (defined in terms of a coordinate normal to the interface) are more sharply defined on the upstream face (n = n 1 ) where the layers are generally moving relative to the large-scale flow (as with boundary interfaces [27]). The thickness of the layers is approximately 4-5l.…”

Interfaces and internal layers in a turbulent boundary layer

“…Previous research has recognized the significance of these interfaces on the overall dynamics of the turbulent flow. [1][2][3][4][5][6][7][8][9] Knowledge on physical mechanisms occurring at these interfaces is especially important as a number of processes take place at or in close vicinity of the interface, e.g., it governs the overall growth of the turbulent flow region as entrainment of non-turbulent fluid into the turbulent flow occurs across this interface. Previous research on the turbulent/non-turbulent (T/NT) interface has focused on the definition of the interface and on the physical mechanism of entrainment at the interface.…”