On the physical nature of the turbulent/turbulent interface

Abstract

The existence of a turbulent/turbulent interface (TTI) has recently been verified in the far wake of a circular cylinder exposed to free-stream turbulence (Kankanwadi & Buxton, J. Fluid Mech., vol. 905, 2020, p. A35). This study aims to understand the physics within the TTI. The wake boundary, approximately 40 diameters downstream of a circular cylinder subjected to grid-generated turbulence, was investigated through simultaneous cinematographic, stereoscopic particle image velocimetry and planar laser induced fluorescence experiments. With no grid placed upstream of the cylinder, the behaviour of the resultant interface, our closest approximation to a turbulent/non-turbulent interface, exactly matched what is observed in existing literature. When background turbulence is present, viscous action is no longer the only method by which enstrophy is transported to the background fluid, unlike for turbulent/non-turbulent interfaces. The presence of rotational fluid on both sides of the TTI allows the vorticity stretching term of the enstrophy budget equation to be the dominant actor in this process. The role of viscosity within a TTI is greatly diminished as the vorticity stretching term takes over responsibilities for enstrophy production. The turbulent strain rate normal to the TTI was found to be enhanced in the interfacial region. Decomposing the vorticity stretching term into components aligned with the three principal strain-rate directions, it was found that the term most aligned with the interface-normal direction contributed to the largest share of enstrophy production. This indicates that better ‘organised’ vorticity on the wake side of the interface yields the enstrophy amplification leading to the previously discovered enstrophy jump across the TTI by Kankanwadi & Buxton (J. Fluid Mech., vol. 905, 2020, p. A35).

On the scalar turbulent/turbulent interface of axisymmetric jets

“…Hence, the structure of the VSL may not be fully recovered using a high- passive scalar threshold in the quiescent ambient. When, however, background turbulence is present, Kankanwadi & Buxton (2022) concluded that viscosity plays a negligible role within the TTI and that the VSL may not exist at all for cases of intense external turbulence. Here, we are interested in the geometry of the scalar TNTI and TTI outlines, as well as their fractal property, which exists in the inertial range.…”

“…However, the interface between a turbulent ambient and a turbulent flow, that is, the turbulent/turbulent interface (TTI), has scarcely been explored, despite its existence in numerous industrial and environmental applications. The presence of the TTI, across which the vorticity and scalar adjust between the surrounding ambient and the turbulent flow akin to the TNTI layer, was only recently shown for a cylindrical wake in a grid-generated turbulent ambient (Kankanwadi & Buxton 2020, 2022). The outer boundary of the TTI or the TNTI layer, denoted as ‘outline’ hereafter, is usually detected by applying a low-magnitude threshold to a flow-dependent scalar field (e.g.…”

Spatial evolution of the turbulent/turbulent interface geometry in a cylinder wake

“…2014). Kankanwadi & Buxton (2022) also demonstrated that the vorticity in the vicinity of the TTI is ‘organised’ in such a way on the wake side of the TTI that it exploits the enhanced strain rates in the interface-normal direction, previously reported for TNTIs (e.g. Cimarelli et al.…”

The influence of freestream turbulence on the near-field growth of a turbulent cylinder wake: turbulent entrainment and wake meandering

“…a transport of fluid from the wake to the background, as background turbulence intensity was increased. Kankanwadi and Buxton (2022) subsequently showed that the physics of the TTI were fundamentally different to those of the TNTI since the inertial vorticity stretching term ω i s ij ω j , where ω i is the vorticity vector and s ij is the fluctuating rate-of-strain tensor, dominates viscous diffusion. ω i s ij ω j , like all velocity-gradient quantities, is an intermittent quantity, perhaps explaining the importance of intermittency in determining the entrainment rate between two adjacent streams of turbulence.…”