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Structure and stability of the finite-area von Kármán street
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View: Figures


Image of FIG. 1.
FIG. 1.

Sketch of the finite-area von Kármán vortex street. The vortex spacing within each row is a, the distance between rows is b, and the stagger is given by d. All distances are measured between vortex centroids.

Image of FIG. 2.
FIG. 2.

Normalized translational velocity for solution families obtained by fixing the street aspect ratio b/a and varying area A/L 2, as computed by Saffman and Schatzman.13,18,20 Large-area solutions with b/a ≲ 0.36 proved costly to resolve in their calculations, and were therefore not computed.

Image of FIG. 3.
FIG. 3.

Map of nondimensional area versus street aspect ratio, as computed by Saffman and Schatzman.13,18 For large b/a, the fold points in Fig. 2 correspond to the black curve, while the limiting states are approximated by the red curve (gray in print) on the right. For small b/a, these results suggested that no turning point was present, and that solutions directly approached a limiting state as A/L 2 increased. The dashed lines indicate expected trends, for regions where solutions were not computed.

Image of FIG. 4.
FIG. 4.

Schematic diagrams, illustrating how turning points are related to changes of stability, following Ref. 24. (a) Velocity-impulse diagram (computed at constant spacing L), and (b) energy-spacing diagram (at constant impulse). Whether the stability boundary corresponds to a loss or gain of stability can be inferred directly from the shape of the diagram.

Image of FIG. 5.
FIG. 5.

Construction of a symmetry-breaking imperfection for the Kármán street. The flow in one periodic strip (of width L*) is made nonsymmetric through a two-step process. The neighborhood of a stagnation point (highlighted by a gray box in (a)) is first altered by introducing a point vortex of strength ΓPV < 0 (marked by the larger bull's eye in (b) and (c)), which changes the local flow topology. This creates two new stagnation points near the original one. One can then introduce a further point vortex at one of these locations (as shown in (c)), thereby breaking all geometric symmetries in the flow.

Image of FIG. 6.
FIG. 6.

Energy-spacing diagram for a Kármań street, for varying L*, and fixed impulse I* = 2−3/2 ≃ 0.3536. Re-computing the steady states after introducing an imperfection, we find that the solution family is broken into two distinct branches (shown by the dashed lines in (c)), revealing an additional loss of stability (marked in by the open circle in (c)).

Image of FIG. 7.
FIG. 7.

Energy-spacing diagram for a Kármán street with impulse I* = 0.1. For these small values of I*, we find a bifurcated branch before the turning point in L*.

Image of FIG. 8.
FIG. 8.

(a) Plot of translational velocity versus spacing for a Kármań street with impulse I* = 5 × 10−5. (b) Bifurcation diagram for I* = 0.

Image of FIG. 9.
FIG. 9.

Possible limiting states that we obtained for the Kármán street, for progressively decreasing impulse: (a) “cat's eyes” (typical of large-impulse vortices), (b) “pie slices”, (c) “teardrops”, and (d) “bowling pins” (which occur for small impulse). Other limiting states of lower symmetry also exist.

Image of FIG. 10.
FIG. 10.

Spacing-impulse solution map, constructed using the results of Saffman and Schatzman.18 (This resembles quite closely the (b/a, A 2/L) map shown earlier in Fig. 3.)

Image of FIG. 11.
FIG. 11.

Spacing-impulse solution map, summarizing the results for the (symmetric) finite-area Kármán street obtained in this paper. Dashed curves denote bifurcations to lower symmetry solutions. Darker regions correspond to values of (I*, L*) for which a greater number of solutions exist. Vertical slices through this map correspond to one-parameter families, as illustrated by the basic curves in the energy-spacing plots of Figs. 6 and 7 (which show cases for I* = 0.1 and 2−3/2 ≃ 0.3536). Horizontal slices through the map correspond to one-parameter families that can be represented in a velocity-impulse diagram. Over 30,000 steady states were calculated to construct this map.

Image of FIG. 12.
FIG. 12.

Examples of possible solution paths (shown by the blue lines) that can be obtained by starting with solutions having I* = 0 and L* < 1.0418, and increasing I* (subsequently, one may try to also vary L*, as shown by the vertical path). In this map, some of these steady vortices overlap with other flows previously shown in this paper. Paths (1) and (2) are described in the text.

Image of FIG. 13.
FIG. 13.

Sketch of the solution structure in the neighborhood of the point (I*, L*) = (0, 1.0418) (marked by an open circle in this and in the previous two figures). The steady states lie on a fold in the (I*, L*, U*) space. The yellow region (light-gray in print) forms part of the map in Fig. 11. With the exception of a few results presented in Fig. 12, the blue region (dark-gray in print) remains essentially unexplored.

Image of FIG. 14.
FIG. 14.

Sketch of the vortex shapes and variable definition for the computation of the “pie slice” vortices.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Structure and stability of the finite-area von Kármán street