A direct numerical simulation (DNS) was carried out to study twin swirling jets which are issued from two parallel nozzles at a Reynolds number of and three swirl levels of , 1.08, and 1.42, respectively. The basic structures of vortex-vortex interaction and temporal evolution are illustrated. The characteristics of axial variation of turbulent fluctuation velocities, in both the near and far field, in comparison to a single swirling jet, are shown to explore the effects of vortex-vortex interaction on turbulence modifications. Moreover, the second order turbulent fluctuations are also shown, by which the modification of turbulence associated with the coherent or correlated turbulent fluctuation and turbulent kinetic energy transport characteristics are clearly indicated. It is found that the twin swirling flow has a fairly strong localized vortex-vortex interaction between a pair of inversely rotated vortices. The location and strength of interaction depend on swirl level greatly. The modification of vortex takes place by transforming large-scale vortices into complex small ones, whereas the modulation of turbulent kinetic energy is continuously augmented by strong vortex modification. 1. Introduction Swirling flows exist in many engineering applications, such as gas turbine combustors, internal combustion engines, cyclone separators, and industrial burners. It is proven as an effective mixing enhancement approach. Swirling flows are extensively studied in many aspects, such as vortex breakdown [1], instability [2], and recirculation zones [3]. Theoretical and experimental approaches are two important tools to research swirling flow. Most early theories were developed to characterize the conditions of vortex breakdown and predict its location and criterion. Recent theoretical studies tried to describe the whole process and predict the internal structure. Although lots of theories have been proposed, none of them agreed well with all features of breakdown [4]. Therefore, many experiments were conducted to investigate swirling flows in detail. For example, Zohir [5] studied heat transfer characteristics and pressure drop for turbulent airflow with propeller swirl generator. It indicated that inserting the propeller downstream provides considerable improvement of heat transfer rate higher than inserting the propeller upstream. Xie et al. [6] investigated the effect of rotational acceleration on flow and heat transfer in swirl microchannels. They showed complicated flow and heat transfer characteristics at different acceleration directions, and the
References
[1]
M. G. Hall, “Vortex breakdown,” Annual Review of Fluid Mechanics, vol. 4, pp. 195–218, 1972.
[2]
M. Lessen, P. J. Singh, and F. Paillet, “The stability of a trailing line vortex. Part 1. Inviscid theory,” Journal of Fluid Mechanics, vol. 63, no. 4, pp. 753–763, 1974.
[3]
G. de Vahl Davis and G. D. Mallinson, “An evaluation of upwind and central difference approximations by a study of recirculating flow,” Computers and Fluids, vol. 4, no. 1, pp. 29–43, 1976.
[4]
O. Lucca-Negro and T. O'Doherty, “Vortex breakdown: a review,” Progress in Energy and Combustion Science, vol. 27, no. 4, pp. 431–481, 2001.
[5]
A. E. Zohir, “Turbulent heat transfer characteristics and pressure drop in swirling flow at upstream and downstream of an abrupt expansion,” Heat and Mass Transfer, vol. 48, no. 3, pp. 529–539, 2012.
[6]
L. Y. Xie, Y. Q. Xie, J. Z. Yu, H. X. Gao, and Y. M. Xi, “Effects of rotational acceleration on flow and heat transfer in straight and swirl microchannels,” Heat and Mass Transfer, vol. 48, no. 6, pp. 953–964, 2012.
[7]
C. O. Iyogun, M. Birouk, and J. A. Kozinski, “Experimental investigation of the effect of fuel nozzle geometry on the stability of a swirling non-premixed methane flame,” Fuel, vol. 90, no. 4, pp. 1416–1423, 2011.
[8]
A. Adzlan and H. Gotoda, “Experimental investigation of vortex breakdown in a coaxial swirling jet with a density difference,” Chemical Engineering Science, vol. 80, pp. 174–181, 2012.
[9]
I. V. Litvinov, S. I. Shtork, P. A. Kuibin, S. V. Alekseenko, and K. Hanjalic, “Experimental study and analytical reconstruction of precessing vortex in a tangential swirler,” International Journal of Heat and Fluid Flow, vol. 42, pp. 251–264, 2013.
[10]
B. A. Younis, B. Weigand, and A. D. Vogler, “Prediction of momentum and scalar transport in turbulent swirling flows with an objective Reynolds-stress transport closure,” Heat and Mass Transfer, vol. 45, no. 10, pp. 1271–1283, 2009.
[11]
S. Roux, G. Lartigue, T. Poinsot, U. Meier, and C. Bérat, “Studies of mean and unsteady flow in a swirled combustor using experiments, acoustic analysis, and large eddy simulations,” Combustion and Flame, vol. 141, no. 1-2, pp. 40–54, 2005.
[12]
S. B. Müller and L. Kleiser, “Large-eddy simulation of vortex breakdown in compressible swirling jet flow,” Computers and Fluids, vol. 37, no. 7, pp. 844–856, 2008.
[13]
Y. Yang and S. K. K？r, “Large-eddy simulations of the non-reactive flow in the Sydney swirl burner,” International Journal of Heat and Fluid Flow, vol. 36, pp. 47–57, 2012.
[14]
S. Patil and D. Tafti, “Wall modeled large eddy simulations of complex high Reynolds number flows with synthetic inlet turbulence,” International Journal of Heat and Fluid Flow, vol. 33, no. 1, pp. 9–21, 2012.
[15]
K. K. J. R. Dinesh, K. W. Jenkins, A. M. Savill, and M. P. Kirkpatrick, “Swirl effects on external intermittency in turbulent jets,” International Journal of Heat and Fluid Flow, vol. 33, no. 1, pp. 193–206, 2012.
[16]
J. Paik and F. Sotiropoulos, “Numerical simulation of strongly swirling turbulent flows through an abrupt expansion,” International Journal of Heat and Fluid Flow, vol. 31, no. 3, pp. 390–400, 2010.
[17]
M. Freitag, M. Klein, M. Gregor et al., “Mixing analysis of a swirling recirculating flow using DNS and experimental data,” International Journal of Heat and Fluid Flow, vol. 27, no. 4, pp. 636–643, 2006.
[18]
M. Freitag and M. Klein, “Direct numerical simulation of a recirculating, swirling flow,” Flow, Turbulence and Combustion, vol. 75, no. 1–4, pp. 51–66, 2005.
[19]
Z. Q. Yin, J. Z. Lin, K. Zhou, and T. L. Chan, “Numerical simulation of the formation of pollutant nanoparticles in the exhaust twin-jet plume of a moving car,” International Journal of Nonlinear Sciences and Numerical Simulation, vol. 8, no. 4, pp. 535–543, 2007.
[20]
V. Kolá？ and E. Savory, “Dominant flow features of twin jets and plumes in crossflow,” Journal of Wind Engineering and Industrial Aerodynamics, vol. 95, no. 9-11, pp. 1199–1215, 2007.
[21]
Q. Li, G. J. Page, and J. J. McGuirk, “Large-eddy simulation of twin impinging jets in cross-flow,” Aeronautical Journal, vol. 111, no. 1117, pp. 195–206, 2007.
[22]
D. Fu and Y. Ma, “A high order accurate difference scheme for complex flow fields,” Journal of Computational Physics, vol. 134, no. 1, pp. 1–15, 1997.
[23]
S. K. Lele, “Compact finite difference schemes with spectral-like resolution,” Journal of Computational Physics, vol. 103, no. 1, pp. 16–42, 1992.
[24]
A. Jameson and W. Schmidt, “Some recent developments in numerical methods for transonic flows,” Computer Methods in Applied Mechanics and Engineering, vol. 51, no. 1–3, pp. 467–493, 1985.
[25]
U. Anantha Krishnaiah, R. Manohar, and J. W. Stephenson, “Fourth-order finite difference methods for three-dimensional general linear elliptic problems with variable coefficients,” Numerical Methods for Partial Differential Equations, vol. 3, no. 3, pp. 229–240, 1987.
[26]
N. Gui, J. Fan, K. Cen, and S. Chen, “A direct numerical simulation study of coherent oscillation effects of swirling flows,” Fuel, vol. 89, no. 12, pp. 3926–3933, 2010.
[27]
N. Gui, J. R. Fan, and S. Chen, “Numerical study of particle-particle collision in swirling jets: a DEM-DNS coupling simulation,” Chemical Engineering Science, vol. 65, no. 10, pp. 3268–3278, 2010.
[28]
I. Orlanski, “A simple boundary condition for unbounded hyperbolic flows,” Journal of Computational Physics, vol. 21, no. 3, pp. 251–269, 1976.
[29]
J. Kim, D. Kim, and H. Choi, “An immersed-boundary finite-volume method for simulations of flow in complex geometries,” Journal of Computational Physics, vol. 171, no. 1, pp. 132–150, 2001.
