Direct numerical simulation of the near ®eld dynamics of a rectangular reactive plume X. Jiang * , K.H. Luo Department of Engineering, Queen Mary and West®eld College, University of London, Mile End Road, London E1 4NS, UK Received 15 October 2000; accepted 21 June 2001 Abstract Spatial direct numerical simulation DNS) is used to study the near ®eld dynamics of a buoyant diusion ¯ame established on a rectangular nozzle with an aspect ratio of 2:1. Combustion is represented by a one-step ®nite-rate Arrhenius chemistry. Without applying external perturbations at the in¯ow boundary, large vortical structures develop naturally in the ¯ow ®eld, which interact with the ¯ame and temporally create localized holes within the reaction zone in which no chemical reactions take place. The in- teraction between density gradients and gravity plays a major role in the vorticity generation of the buoyant plume. At the downstream of the reactive plume, a more disorganized ¯ow regime characterized by small scales has been observed, following the breakdown of the large vortical structures due to three-dimensional 3D) vortex interactions. Analysis of energy spectra shows that the spatially developing reactive plume has a tendency of transition to turbulence under the eects of combustion-induced buoyancy. The buoyancy eects are found to be very important to the formation, development, interaction, and breakdown of vortices in reactive plumes. In contrast with the relaminarization eects of chemical exothermicity via viscous damping and volumetric ex- pansion on non-buoyant jet diusion ¯ames, the tendency towards transition to turbulence in reactive plumes is greatly enhanced by the buoyancy eects. Ó 2001 Elsevier Science Inc. All rights reserved. Keywords: DNS; Buoyancy; Transition; Combustion; Non-circular jets 1. Introduction As an ecient technique of passive ¯ow control, jets in non- circular con®gurations, such as elliptic, rectangular, square and triangular geometries, are encountered in various engi- neering applications such as combustors, cooling of energy conversion devices, and exhaust of aerospace vehicles. Non- circular jets have attracted an extensive research interest in recent years, both experimentally e.g. Ho and Gutmark, 1987; Hertzberg and Ho, 1995; Zhang, 2000) and numerically e.g. Miller and Madnia, 1995; Grinstein and Kailasanath, 1995; 1996; Grinstein and DeVore, 1996; Wilson and Demuren, 1998). Recently, the topic has been reviewed by Gutmark and Grinstein 1999). In a broad range of practical applications, buoyancy due to density inhomogeneity under the in¯uence of gravity plays a major role in the ¯ow development of non-circular jets. The density inhomogeneity can result from inhomogeneities in temperature, dierences in concentration of chemical species, changes in material phase, and many other eects in the ¯ow ®eld. Buoyancy eects are especially important to low-speed combustion applications, such as ®res. However, buoyancy eects have not been taken into consideration in the existing numerical studies of non-circular jets Miller and Madnia, 1995; Grinstein and Kailasanath, 1995, 1996; Grinstein and DeVore, 1996; Wilson and Demuren, 1998). Free jets and plumes in open-boundary domains with buoyancy eects usually consist of a strongly buoyant near ®eld and a turbulent, weakly buoyant far ®eld. In the near ®eld, the most prominent feature of buoyant jets and plumes is that the large vortical structures dominate the ¯ow ®eld of both reacting and non-reacting ¯ows e.g. Cetegen and Ahmed, 1993; Katta et al., 1994; Lingens et al., 1996; Cetegen et al., 1998; Maxworthy, 1999; Jiang and Luo, 2000a). The mechanism leading to the formation of these vortical structures is believed to be an absolute ¯ow insta- bility Lingens et al., 1996; Maxworthy, 1999; Jiang and Luo, 2000c), which is dierent from that of non-buoyant jets. It was denti®ed Jiang and Luo, 2000c) that the evo- lution of vortices in buoyant jets and plumes does not rely on the spatial ampli®cation of external perturbations applied at the ¯ow boundary but on the interactions between the density gradients and gravity. The near ®eld dynamics of buoyant jets and plumes, such as the formation and transport of vortices and laminar to turbulent ¯ow transition, still has not been fully understood. In general, theoretical analysis is not able to deal with such complicated ¯ow patterns. Experimental measurements of International Journal of Heat and Fluid Flow 22 2001) 633±642 www.elsevier.com/locate/ijh * Corresponding author. Fax: +44-20-8983-3052. E-mail address: x.jiang@qmw.ac.uk X. Jiang). 0142-727X/01/$ - see front matter Ó 2001 Elsevier Science Inc. All rights reserved. PII:S0142-727X01)00123-0