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Direct numerical simulation of spatially developing, three-dimensional swirling jets. (English) Zbl 1083.76543

Summary: Vortex breakdown of nominally axisymmetric, swirling incompressible jets and wakes issuing into a semi-infinite domain is studied by means of direct numerical simulations, as well as by local and global linear stability analyses. A two-parameter low entrainment velocity profile, for which the steady axisymmetric breakdown is well studied [W. J. Grabowski and S. A. Berger, J. Fluid Mech. 75, 525-544 (1976; Zbl 0336.76007)], is selected to discuss the role of the applied swirl in the existence and mode selection of vortex breakdown. As the swirl parameter is increased, bubble, helical and double-helical breakdown modes are observed for the moderate Reynolds number applied. It is shown that a local transition from supercritical to subcritical flow, as defined by T. B. Benjamin [J. Fluid Mech. 14, 593-629 (1962)], accurately predicts the swirl parameter yielding breakdown. Thus the basic form of breakdown is axisymmetric. A transition to helical breakdown modes is shown to be caused by a sufficiently large pocket of absolute instability in the wake of the bubble, giving rise to a self-excited global mode. Preliminary axisymmetric results of a global linear instability analysis agree favourably with the direct numerical simulation and thus encourage extension of the global analysis to helical modes.

MSC:

76F65 Direct numerical and large eddy simulation of turbulence
76D25 Wakes and jets
76E99 Hydrodynamic stability

Citations:

Zbl 0336.76007
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References:

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