An investigation of the near-field flow structure of
coaxial jets with large outer to
inner velocity ratio ru
has been conducted. Since in all cases ru>1,
the outer jet
dominates the near-field flow structure. Two flow regimes are identified
depending on
whether ru is larger or smaller than a
critical value ruc. When
ru<ruc, the
fast annular
jet periodically pinches the central, slow jet near the
end of the inner potential cone.
The pinching frequency corresponds to the outer-jet mode. The length of
the inner
potential cone is strongly dependent on ru
and behaves like A/ru,
where A depends weakly on the initial conditions. When
ru>ruc,
the inner potential cone is truncated
and is followed by an unsteady recirculation bubble with low-frequency
oscillation.The transition from one regime to another is explained by a simple model
whose
ingredients are the turbulent entrainment rate,
governed by the outer-jet mixing layers
and mass conservation. This model satisfactorily predicts the dependence
of the inner
potential cone length on ru and
the critical velocity ratio ruc. The
recirculation bubble
has a wake-type instability. It oscillates at a low frequency and a large
amplitude
compared to the Kelvin–Helmholtz mode. Angular cross-correlations
in the plane
parallel to the jet outlet show moreover that this oscillation displays
an azimuthal
precession such that the rotation time of the phase of the oscillation
equals the
oscillation period. These salient features are discussed in the framework
of the
nonlinear delayed saturation (NLDS) model.
Flow structure and near-field dispersion in arrays of building-like obstacles
