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Show These models use "gradient diffusion" assumptions for modeling turbulent flux (momentum, species) terms, thus neglecting the contribution of large scale vortical structures on the entrainment, turbulent transport and mixing processes in turbulent flows. Though the evidence is far from comprehensive, numerical studies [18,20,21] indicate that for these conventional modeling approaches, jet spread is underpredicted, entrainment is underpredicted and velocity decay is underpredicted in the buoyancy dominated, plume regime. These trends are not seen in the current study and are consistent with the lack of accounting for entrainment via large scale structures expected in the plume regime of buoyant, turbulent jet flames. Recently developed Large Eddy Simulations (LES), now available in FLUENT V5, accounts for turbulent transport via large scale coherent structures. In LES, The governing transport equations are spatially averaged, with a subgrid scale closure model provided to describe turbulence fluctuations in the inertial range. The advantage of L E S is that large scale vortical structures, which tend to be geometry dependent, are modeled exactly while the subgrid scale fluctuations, which have been shown to be far more universal in nature, are modeled. Though the resolution requirements for LES are much less restrictive than D N S , LES is still far more computationally expensive than RANS approaches. It is only due to the tremendous strides seen in computer hardware performance and the availability of parallel processing capabilities that L E S has begun to be considered a possibility for engineering applications. Future efforts will include examining the benefits of LES for flows with significant turbulent transport made via large scale eddies, like those found in the buoyancy dominated regime of turbulent jet flames. Acknowledgments The assistance of Sandipan Das in the creation of the mesh for these simulations is gratefully acknowledged. References 1 Investigations of round vertical turbulent buoyant jets (1988) P.N. Papanicolaou & E.J. List, Jou. Fl. Mech., 195, 341-391. 2 Some measurements in the self-preserving jet (1969) I. Wygnanski & H. Fiedler, Jou. Fl. Mech., 38, 577-612. 3 Similarity solutions for turbulent jets and plumes (1981) C.S. Yih, Journal of the Engineering Mechanics Division, ASCE, 107, 455-478. 4 An experimental study of a turbulent axisymmetric jet issuing into a coflowing airstream (1975) S. Biringen, VKI Tech., Note 110. 5 Effects of heat release on turbulent shear flows. Part I: A general equivalence principle for nonbuoyant flows and its application to turbulent jet flames. (1998) K.M. Tacina & W.J.A. Dahm, Laboratory for turbulence and combustion, Univ. of Michigan, Report # 036407-3 6 Entrainment, momentum flux and temperature in vertical free turbulent diffusion flames (1978) H.A. Becker & S. Yamazaki, Comb. Flame, 33, 123-149. 7 Characterization of turbulent H2/N2/air jet diffusion flames by single-pulse spontaneous raman scattering (1996) W. Meier, S. Prucker, M.-H. Cao & W . Strieker, Combust. Sci. Technol. 118, 293. Also web site: www.th-darmstadt.de/fb/ekt/flamebase.html. 8 FLUENT V5 User's Guide, Volume 2 (1998) Fluent, Inc. 9 Lectures in mathematical models of turbulence (1972) B.E. Launder and D.B. Spalding, Academic Press, London, England. 10 A new k-e eddy-viscosity model for high Reynolds number turbulent flows-model development and validation (1995) T.-H. Shih, W.W. Liou, A. Shabbir & J. Zhu, Computers Fluids 24(3), 227-238. 11 Fundamentals of turbulence for turbulence modeling and simulation (1987) W.C. Reynolds, Lecture notes for Von Karman Institute, A G A R D Report # 755. 12 Computations of Complex Turbulent flows using the commercial code FLUENT (1997) S.-E. Kim, D. Choudhury & B. Patel, |