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The evolution of instabilities in the axisymmetric jet. Part 1. The linear growth of disturbances near the nozzle

Published online by Cambridge University Press:  21 April 2006

J. Cohen  and
I. Wygnanski
J. Cohen
Affiliation:
Aerospace and Mechanical Engineering Department, University of Arizona, Tucson, AZ 85721, USA Present address: Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
I. Wygnanski
Affiliation:
Aerospace and Mechanical Engineering Department, University of Arizona, Tucson, AZ 85721, USA
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Abstract

The modal distributions of coherent structures evolving near the nozzle of a circular jet are considered. The effects produced on the instability modes by transverse curvature, flow divergence, inhomogeneous inflow conditions, and the detailed shape of the mean velocity profile, are investigated both theoretically and experimentally. Linear stability analysis applied to a thin shear layer surrounding a large-diameter jet (i.e. a jet whose diameter is large in comparison with a typical width of the shear layer) indicates that many azimuthal modes are equally unstable. An increase in the relative thickness of the shear layer limits the number of unstable modes, and only one helical mode remains unstable at the end of the potential core. The linear model used as a transfer function is capable of predicting the spectral distribution of the velocity perturbations in a jet. This provides a rational explanation for the stepwise behaviour of the predominant frequency resulting from a continuous increase in the jet velocity.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 176 , March 1987 , pp. 191 - 219
Copyright
© 1987 Cambridge University Press

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