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Helical instabilities of slowly divergent jets

Published online by Cambridge University Press:  19 April 2006

Peter Plaschko
Peter Plaschko
Affiliation:
Hermann Föttinger Institut für Thermo- u. Fluiddynamik, Technische Universität Berlin, West Germany
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Abstract

The inviscid spatial growth of spiral modes of circular, slowly diverging jets is analysed. A multiple-scales expansion is used to develop a linear stability study for non-axisymmetric disturbances of arbitrary helicity. The numerical evaluation is restricted to axisymmetric modes and to the first two helical modes. It is shown that in the case of comparatively high values of the Strouhal number the modes exhibit a very rapid growth and reach their maximal amplification after a short distance, the axisymmetric instabilities being excited more strongly than their spiral counterparts. Contrary to this, the modes grow comparatively slowly in the case of smaller values of the Strouhal number and exhibit their maximal amplification further downstream. In the latter case the first spiral mode is more unstable than the axisymmetric one. A comparison with experiments seems to support these results.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 92 , Issue 2 , 28 May 1979 , pp. 209 - 215
Copyright
© 1979 Cambridge University Press

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