The absolute or convective character of inviscid instabilities in parallel shear flows can be determined by examining the branch-point singularities of the dispersion relation for complex frequencies and wavenumbers. According to a criterion developed in the study of plasma instabilities, a flow is convectively unstable when the branch-point singularities are in the lower half complex-frequency plane. These concepts are applied to a family of free shear layers with varying velocity ratio $R = \Delta U/2\overline{U}$, where ΔU is the velocity difference between the two streams and $\overline{U}$ their average velocity. It is demonstrated that spatially growing waves can only be observed if the mixing layer is convectively unstable, i.e. when the velocity ratio is smaller than Rt = 1.315. When the velocity ratio is larger than Rt, the instability develops temporally. Finally, the implications of these concepts are discussed also for wakes and hot jets.
The goal of this study is to characterize the various breakdown
states taking place
in a swirling water jet as the swirl ratio S and
Reynolds number Re are varied. A
pressure-driven water jet discharges into a large tank, swirl being imparted
by means
of a motor which sets into rotation a honeycomb within a settling chamber.
The
experiments are conducted for two distinct jet diameters by varying the
swirl ratio
S while maintaining the Reynolds number
Re fixed in the range 300<Re<1200.
Breakdown is observed to occur when S reaches a well defined threshold
Sc≈1.3–1.4
which is independent of Re and nozzle diameter used. This critical
value is found to
be in good agreement with a simple criterion derived in the same spirit
as the first
stage of Escudier & Keller's (1983) theory. Four distinct forms
of vortex breakdown
are identified: the well documented bubble state, a new cone configuration
in which
the vortex takes the form of an open conical sheet, and two associated
asymmetric
bubble and asymmetric cone states, which are only observed at large Reynolds
numbers. The two latter configurations differ from the former by the precession
of
the stagnation point around the jet axis in a co-rotating direction with
respect to the
upstream vortex flow. The two flow configurations, bubble or cone, are
observed to
coexist above the threshold Sc at the same
values of the Reynolds number Re and
swirl parameter S. The selection of breakdown state is extremely
sensitive to small
temperature inhomogeneities present in the apparatus.
When S reaches Sc, breakdown
gradually sets in, a stagnation point appearing in the downstream turbulent
region
of the flow and slowly moving upstream until it reaches an equilibrium
location.
In an intermediate range of Reynolds numbers, the breakdown threshold displays
hysteresis lying in the ability of the breakdown state to remain
stable for S<Sc
once it has taken place. Below the onset of breakdown, i.e.
when 0<S<Sc, the
swirling jet is highly asymmetric and takes the shape of a steady helix.
By contrast
above breakdown onset, cross-section visualizations indicate that the cone
and the
bubble are axisymmetric. The cone is observed to undergo slow oscillations
induced
by secondary recirculating motions that are independent of confinement
effects.
The global linear stability of incompressible, two-dimensional shear flows is investigated under the assumptions that far-field pressure feedback between distant points in the flow field is negligible and that the basic flow is only weakly non-parallel, i.e. that its streamwise development is slow on the scale of a typical instability wavelength. This implies the general study of the temporal evolution of global modes, which are time-harmonic solutions of the linear disturbance equations, subject to homogeneous boundary conditions in all space directions. Flow domains of both doubly infinite and semi-infinite streamwise extent are considered and complete solutions are obtained within the framework of asymptotically matched WKBJ approximations. In both cases the global eigenfrequency is given, to leading order in the WKBJ parameter, by the absolute frequency ω0(Xt) at the dominant turning pointXtof the WKBJ approximation, while its quantization is provided by the connection of solutions acrossXt. Within the context of the present analysis, global modes can therefore only become time-amplified or self-excited if the basic flow contains a region of absolute instability.
The possible existence of global modes or self‐excited linear resonances in spatially developing systems is explored within the framework of the WKBJ approximation. It is shown that the existence and properties of the dominant global mode may be deduced from the variations of the local absolute frequency ω0 with distance X. The main results are summarized in two theorems: (1) A system with no region of absolute instability does not sustain temporally growing global modes with an O(1) growth rate. (2) If the singularity X, closest to the real X‐axis of the complex function ω0(X) is a saddle point, the most unstable global mode has, to leading order in the WKBJ approximation, a complex frequency ω0(Xs). Thus, it will be temporally growing only if ω0(Xs) is positive.
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