SCALAR MIXING AT TURBULENT/NON-TURBULENT INTERFACE OF A TURBULENT PLANE JET

Abstract

The interface region that bounds fully developed turbulent shear flow from the vortical non-vortical regions in free shear flows is a long-standing issue in turbulence research. Understanding the local dynamics that take place at such interfacial layer are key to the study of turbulent mixing and entrainment. Dimotakis (2005) highlights the importance of studying the mixing process of a passive scalar in a wide range of engineering applications, e.g. combustion. Recent investigations on the topic focused in the analysis of the properties of both the T/NT interface, particularly in the vorticity structures close to the T/NT interface. da Silva & Taveira (2010) and Reis et al. (2011) investigated these structures showing that the T/NT interface is made of these turbulent structures. Moreover, Reis et al. (2011) and Taveira & da Silva (2013) studied the role of vorticity structures in the enstrophy and kinetic energy transport across the T/NT interface, respectively. The present study aims to explain the dynamics of the turbulent mixing of a passive scalar through the study of scalar gradient and fluctuations transport mechanisms. It is also investigated the topology of the scalar gradient structures, which are responsible for the transport of scalar fluctuations. The present study aims to explain the dynamics of the turbulent mixing of a passive scalar through the study of scalar gradient and fluctuations transport mechanisms. It is also investigated the topology of the scalar gradient structures, which are responsible for the transport of scalar fluctuations. The present work uses data from a single direct numerical simulation of a turbulent plane jet at Reynolds number of Re = 140 and the Schmidt number is 0.7. Structure tracking and conditional statistics are employed allowing to filter out intermittence and a clear perspective of the local dynamics. The instantaneous fields showed that the passive scalar field is mainly arranged in intense scalar gradient sheets that are found along regions of persistent strain, in particular at the T/NT interface. However at the moderate Reynolds number these sheets are not flat as reported in literature, and are seldom arranged parallely. The jump in the mean conditional scalar as width of the order of the Taylor scale, in agreement with Westerweel et al. (2009). Conversely, the jump in the scalar gradient conditional profile suggests the sheet structures to have an average width of the order of the Kolmogorov scale. The topological analysis of the structures shows that on average the intense scalar gradient sheets have a thickness of ≈ 3η and a length of ≈ 2λ . The conditional analysis of the scalar gradient and variance transport revealed that the mixing is taking place particularly close to the T/NT interface, over a region with a thickness of ≈ 12−15η . Introduction In free shear flows, such as jets, sharp and intricate layers separate the irrotational region from the turbulent flow core. These layers establish an interface region the T/NT interface across which the exchanges of mass, energy and momentum, that allow the shear layer to grow and develop naturally, take place. A large effort has been made, in the near past, in order to improve the understanding of the local dynamics that take place at such interfacial region. The experiments of Westerweel et al. (2005) hinted that the main entrainment mechanism is nibbling taking place at the smallest scales, rather than engulfing events driven by large scale of the mean flow (Dimotakis (2005)). The analysis of the geometric properties of both the T/NT interface and its neighbor vorticity structures has shown that the interface is made of these turbulent structures, whose radius is seen to be of the order of the Taylor micro-scale in the case of shear flows (da Silva & Taveira (2010)). Reis et al. (2011) studied, in depth, the role of the intense vorticity structures (IVS), at the T/NT interface, obtaining results that supported the conclusions previously obtained (da Silva & Taveira (2010)). The developments in the methods used to study the local dynamics also allowed a detailed study of the mixing processes taking place between the turbulent and irrotational regions of the flow. For instance, the conditional analysis of the kinetic energy transport in Taveira & da Silva (2013) showed that non-viscous mechanisms play a central role in the entrainment process. Numerous studies showed that chemical reactive flows can be quite