Large Scale Dynamics of a Jet in a Counter Flow

Abstract

Numerous studies of turbulent jets have contributed to a better understanding of the flow physics with applications in vehicle engineering, power generation or chemical industry. Different configurations (e.g. single jet in quiescent environment, jet in cross-flow) have receive much attention whereas only few studies focused on the case of a jet issuing into a uniform counter-flow (JiCF). Among important studies, Bernero and Fiedler [1] evidenced some large scale flapping like motion but restricted their analysis to plane data. Tsunoda and Saruta [2] highlighted the bimodal shape of the scalar pdf as a consequence of the large scale fluctuation. The purpose of the present work is to forther increase the understanding of the JiCF with the focus on identifying the large scale coherent structures using 3D LES data. In the LES framework, we consider the filtered incompressible continuity and momentum equations [3]. The subgrid scale (SGS) term is modeled using a classical eddy viscosity type model based on the Filtered Structure Function Model by Ducros et al. [4]. To solve the filtered governing equations on Cartesian grids a high-order finite difference code is used.The spatial discretization is done using a fourth order centered scheme. However, for the convective terms a small portion of hyper-viscosity is added to the fourth order scheme, for stability reasons. A second order implicit finite difference scheme is used for time discretization. Multi-grid iterations are used to solve the implicit parts of the system. These advantages make the present approach suitable for LES of turbulent flows, in particular jets, e.g. Maciel et al. [5].A commonly followed post-processing procedure consists of computing mean and RMS fields of the velocity variables. However, information related to large scale coherent motions is lost. We seek to supplement the mean and RMS fields by employing Proper Orthogonal Decomposition (POD) and we focus on the most energetic eddies/modes. Consequently, one seeks to project the turbulent flow field on a vector base that maximizes the turbulent kinetic ⋆ also at Haldor Topsoe A/S, DK-2800 Lyngby