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Sixth International Symposium on Turbulence and Shear Flow Phenomena
June, 22-24, 2009 , Seoul National University, Seoul, Korea

DOI: 10.1615/TSFP6

NUMERICAL SIMULATION OF STRONGLY SWIRLING TURBULENT FLOWS THROUGH AN ABRUPT EXPANSION

pages 779-784
DOI: 10.1615/TSFP6.1250
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ABSTRACT

Turbulent swirling incompressible flow through an abrupt axisymmetric expansion is investigated numerically using detached-eddy simulation at Reynolds numbers = 3.0 ×104 and 1.0 × 105. The effects of swirl intensity on the coherent dynamics of the flow are systematically studied by carrying out numerical simulations over a range of swirl numbers from 0.17 to 1.23. Comparison of the computed solutions with the experimental measurements of Dellenback et al. [AIAA Journal. Vol 26, pp. 669-681] shows that the numerical simulations resolve both the axial and swirl mean velocity and turbulence intensity profiles with very good accuracy. Our simulations show that critical prerequisite for accurate predictions of the flow downstream of the expansion is the specification of inlet conditions at a plane sufficiently far upstream of the expansion in order to avoid the spurious suppression of the low-frequency, large-scale precessing of the vortex core, which occurs in simulations with short inlet segment. Further improvements in the accuracy of the computed solutions can be obtained by moderate mesh refinement. Coherent structure visualizations with the q-criterion and Lagrangian particle tracking are used to elucidate the rich dynamics of the flow as a function of the swirl number with emphasis on the onset of the bubble-type vortex breakdown just downstream of the expansion, strong on-axis recirculation along the pipe core, and large scale instabilities along the shear layers and the pipe wall.

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