Near-field dynamics of a turbulent round jet with moderate swirl Y. Maciel a, * , L. Facciolo b , C. Duwig c,d , L. Fuchs b,c , P.H. Alfredsson b a Department of Mechanical Engineering, Laval University, Quebec City, Canada G1V 0A6 b Linne ´ Flow Centre, KTH Mechanics, Royal Institute of Technology, SE-100 44 Stockholm, Sweden c Division of Fluid Mechanics, Department Energy Sciences, Lund University Box 118, SE- 22100 Lund, Sweden d Haldor Topsoe A/S, DK-2800 Lyngby, Denmark Received 9 November 2007; received in revised form 1 February 2008; accepted 9 February 2008 Available online 28 March 2008 Abstract The near-field characteristics of a turbulent jet with moderate swirl generated by a fully developed, axially rotating pipe flow are inves- tigated with LDV, time-resolved stereoscopic PIV measurements, as well as with large-eddy simulations. Large-scale vortical structures in either double, triple or even quadruple-helix configuration are found at the pipe exit but rapidly break down or amalgamate after two jet diameters. Further downstream, the swirling jet is dominated by large-scale sweeping motions not present at such a scale and strength in the non-swirling case. Of special interest is the recently discovered counter-rotating core (in the mean) which develops about six jet diameters downstream the jet exit. Data for all six Reynolds stresses is reported at this position and it is argued that the counter-rotation is the result of the transport of angular momentum radially outward by the radial–azimuthal Reynolds shear stress. The mechanisms behind this transport are discussed by qualitative analysis of the time-resolved PIV and LES data and comparisons with the non-swirling case are made. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Swirling jet; Turbulence; LDV; Stereoscopic PIV; Time-resolved PIV; LES 1. Introduction In a turbulent jet, the presence of swirl may strongly affect the mean flow features. At moderate swirl numbers, the turbulence and spreading of the jet increase. At swirl ratios above a critical level so called vortex breakdown occurs which changes the jet behaviour completely. In the following we will limit ourselves to moderate swirl for which vortex breakdown does not occur. A swirling round jet is axisymmetric on average but the swirl breaks the sym- metry, which means for instance that the two Reynolds shear stresses which involve the fluctuating azimuthal com- ponent are non-zero in contrast to the non-swirling case. Crow and Champagne (1971) were among the first to rec- ognize that the large-scale coherent structures found in the near-field of round jets are strongly related to the shear layer instabilities. In non-swirling jets, axisymmetric vortex rings are initially formed close to the nozzle exit but helical distur- bances may appear further downstream. According to the linear stability theory (Michalke, 1984), helical instability waves of azimuthal wavenumber m ¼ 1 (in a modal decom- position of the form exp½iðkx þ mh þ xtÞ) become more unstable than the axisymmetric mode m ¼ 0 when the shear layer is thick. In their low-Reynolds-number experiments, Dimotakis et al. (1983) found large-scale vortical structures that are either nearly axisymmetric, or spiral, or in a transi- tional state between these two configurations. The jet switches back and forth between these different states. At the highest Reynolds numbers (Re  10,000), they found that the dominant mode after one or two jet diameters was the helical one. Other experiments have also revealed helical components appearing close to the nozzle exit at high Reynolds numbers (Ho and Huerre, 1984). From the above discussion, it is expected that the struc- ture of the near-field turbulence should be distinctly 0142-727X/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatfluidflow.2008.02.003 * Corresponding author. Tel.: +1 418 656 7967; fax: +1 418 656 7415. E-mail address: yvan.maciel@gmc.ulaval.ca (Y. Maciel). www.elsevier.com/locate/ijhff Available online at www.sciencedirect.com International Journal of Heat and Fluid Flow 29 (2008) 675–686