A 3C-2D PIV technique was applied to investigate the swirling flow generated by an axial plus tangential type swirl generator. This work is focused on the near-exit region of an isothermal swirling jet to characterize the effect of swirl on the flow field and to identify the large coherent structures both in unconfined and confined conditions for geometrical swirl number, S_{g <\/sub>= 4.6. Effects of the Reynolds number on the flow structure were also studied. The experimental results show significant effects of the confinement on the mean velocity fields and its fluctuations. The size of the recirculation zone was significantly enlarged upon confinement compared to the free swirling jet. Increasing in the Reynolds number further enhanced the recirculation zone. The frequency characteristics have been measured with a capacitive microphone which indicates the presence of periodic oscillation related to the existence of precessing vortex core, PVC. Proper orthogonal decomposition of the jet velocity field was carried out, enabling the identification of coherent structures. The time coefficients of the first two most energetic POD modes were used to reconstruct the phase-averaged velocity field of the oscillatory motion in the swirling flow. The instantaneous minima of negative swirl strength values calculated from the instantaneous velocity field revealed the presence of two helical structures located in the inner and outer shear layers and this structure fade out at an axial location of approximately z\/D = 1.5 for unconfined case and z\/D = 1.2 for confined case. By phase averaging the instantaneous swirling strength maps, the 3D helical vortex structure was reconstructed.<\/p>\r\n","references":"[1]\tK. Gupta, D.G. Lilley, N. Syred, Swirl Flows. Abacus Press, 1984.\r\n[2]\tN. Syred, \u201cA review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems,\u201d Progress in Energy and Combustion. Science, vol. 32(2), pp. 93\u2013161, 2006.\r\n[3]\tP. Billant, J.M. Chomaz, P. Huerre, \u201cExperimental Study of Vortex Breakdown in Swirling Jets,\u201d Journal of Fluid Mechanics, Vol. 376, pp. 183\u2013219, 1998. \r\n[4]\tR. Chanaud, \u201cObservations of oscillatory motion in certain swirling flows,\u201d J Fluid Mech 21:111\u2013127, 1965.\r\n[5]\tO. Lucca-Negro, T. O\u2019Doherty, \u201cVortex breakdown,\u201d A review. Prog. Energy Combust. Sci. 27, 431, 2001.\r\n[6]\tC.E. Cala, E.C.Fernandes, M.V. Heitor, S.I. Shtork, \u201cCoherent structures in unsteady swirling jet flow,\u201d Exp Fluids 40:267\u2013276, 2006.\r\n[7]\tF. Martinelli, F. Cozzi, A. Coghe, \u201cPhase-locked analysis of velocity fluctuations in a turbulent free swirling jet after vortex breakdown,\u201d Exp Fluids. 53:437-449, 2012.\r\n[8]\tN. Syred, J.M. Beer, \u201cCombustion in swirling flows: a review,\u201d Combustion and Flame, 23:143\u2013201, 1974.\r\n[9]\tA.E.E. Khalil, J.M Brooks, A.K. Gupta, \u201cImpact of confinement on flowfield of swirl flow burners,\u201d Fuel 184, 1\u20139. 5, 2016.\t\r\n[10]\tG. Ceglia, S. Discetti, A Ianiro, \u201cThree-dimensional organization of the flow structure in a non-reactive model aero engine lean burn injection system,\u201d Exp ThermFluid Sci 52:164\u2013173, 2014.\r\n[11]\tM. Negri, F. Cozzi, S. Malavasi, \u201cSelf-synchronized phase averaging of PIV measurements in the base region of a rectangular cylinder,\u201d Meccanica 46: 423-435, 2010.\r\n[12]\tB.W. van Oudheusden, F. Scarano, N.P. van Hinsberg, D.W. Watt, \u201cPhase-resolved characterization of vortex shedding in the near wake of a square-section cylinder at incidence,\u201d Exp Fluids 39: 86-98, 2005.\r\n[13]\tK. Oberleithner, M. Sieber, C.N. Nayeri, C.O. Paschereit, C Petz, H.C. Hege, \u201cThree-dimensional coherent structures in a swirling jet undergoing vortex breakdown: stability analysis and empirical mode construction,\u201d J Fluid Mech 679:383-414, 2011.\r\n[14]\tM. Stohr, R. Sadanandan, W. Meier, \u201cPhase-resolved characterization of vortex-flame interaction in a turbulent swirl flame.\u201d Exp Fluids 51:1153-1167, 2011.\r\n[15]\tH. Chen, D.L. Reuss, V. Sick, \u201cOn the use and interpretation of proper orthogonal decomposition of in-cylinder engine flows,\u201d Meas Sci Technol 23: 085302, 2012.\r\n[16]\tJ.L. Lumley, \u201cThe structure of inhomogeneous turbulence,\u201d In: Yaglom AM, Tatarski VI (eds) Atmospheric turbulence and wave propagation. Nauka, Moscow, 166-178, 1967.\r\n[17]\tF. Cozzi, R. Sharma, A. Coghe, F. Arzuffi, \u201cAn experimental investigation on Isothermal free swirling jet,\u201d XXXVIII Meeting of the Italian Section of the Combustion Institute, 2015, Lecce, Italy.\r\n[18]\tS.M. Soloff, R.J Adrian, Z.C Liu, \u201cDistortion compensation for generalized stereoscopic particle image velocimetry,\u201d Meas. Sci. Technol.8:1441-1454, 1997.\r\n[19]\tN.A. Chigier, A. Chervinsky A, \u201cExperimental investigation of swirling vortex motion in jets,\u201d J Appl Mech 34:443\u2013451, 1967.\r\n[20]\tN. Rajaratnam, Turbulent Jets. Elsevier, Amsterdam, 1976.\r\n[21]\tT.C. Claypole, N. Syred, \u201cThe Effect of Swirl Burner Aerodynamics on NOx Formation,\u201d International Symposium on Combustion, 1981, 18, 81\u201389.\r\n[22]\tG. Berkooz, P. Holmes, J.L. Lumley, \u201cThe proper orthogonal decomposition in the analysis of turbulent flows,\u201d Annu Rev Fluid Mech 25:539-575, 1993.\r\n[23]\tL. Sirovich, \u201cTurbulence and the dynamics of coherent structures,\u201d Quart Appl Math 45: 561-590, 1987.\r\n[24]\tH. Liang, T. Maxworthy, \u201cAn experiment investigation of swirling jets,\u201d J Fluid Mech 525:115\u2013159, 2005.\r\n[25]\tS. Archer, A.K. Gupta, \u201cThe role of confinement on flow dynamics under fuel lean combustion,\u201d In: 2nd international energy conversion engineering conference, 16\u201319 August, Providence, RI. Paper#AIAA-5617, 2004.\r\n[26]\tP. Chong, W. Hongping, W. Jinjun, \u201cPhase identification of quasi-periodic flow measured by particle image velocimetry with a low sampling rate,\u201d Measurement Science and Technology 24:055305, 2013.\r\n[27]\tR. Sharma, F. Cozzi, A. Coghe, \u201cPhase-averaged characterization of turbulent isothermal free swirling jet after vortex breakdown,\u201d Proc.18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics, 2016, Lisbon, Portugal.\r\n[28]\tD.M Markovich, S.S Abdurakipov, L.M Chikishev, \u201cComparative analysis of low- and high swirl confined flames and jets by proper orthogonal and dynamic mode decompositions,\u201d Phys Fluids 26:065109, 2014.\r\n[29]\tG. John, P. Dimitris, G. Manolakis, Digital signal processing (3rd ed.): principles, algorithms, and applications, Prentice-Hall, NJ, USA, 1996.\r\n[30]\tK. E. Meyer, D. Cavar, J. M. Pedersen, \"POD as tool for comparison of PIV and LES data,\u201d 7th International Symposium on Particle Image Velocimetry, 2007, Rome, Italy","publisher":"World Academy of Science, Engineering and Technology","index":"Open Science Index 122, 2017"}}