| Summary: | 博士 === 國立成功大學 === 航空太空工程學系 === 83 === The low level time harmonic excitation of the piezoelectric
actuators have been successfully employed to study the
development of helical modes. When forcing m=1, the flow
becomes eccentric, and the flow moves radially outward or
inward as the crest of pressure wave passes the shear layer.
When forcing m=2, the major and minor axes of the forced jet
are continuously interchanged during each cycle of the
modulating wave train. In the forcing m=1 case, the streamwise
vortex pairs apparently grow up in the braid region, and
usually one of those is larger, which appears as the streamwise
vortex tube. In the forcing m=2 case, there are two major
streamwise vortices in the transverse cross section. The
breaking of rotation symmetry may ultimately lead to the
production of intensive (large radial outward velocity) side
jets. Then at the tip of the radial ejection, it creates a
pattern which is similar to the mushroom type of vortex, and
the vortex entrains the external fluid. The jet flame is
stabilized at locations of roll up or pairing of vortices in
the hysteresis region, where the large scale coherent
structures prevail. Usually, both sides of the flame base of
the lifted flame are not located at the same positions, which
is shifted by a wave-length of coherent vortex. The hairpins
(mushroom type) in the longitudinal section are the streamwise
vortices in the transverse section. Due to the entrainment
increase, the streamwise vortices are flammable. If the strain
rate of streamwise vortices is not large enough to extinct the
flame, the streamwise vortices will serve to provide a path
between the two adjacent vortices for the flame base to
propagate to the next up-coming vortex.
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