Observation of the instability in the shear layer behind a paraboloidal-nose cylinder

Abstract

Abstract. Flow visualization was used to investigate experimentally the evolution process of the instability in the separated shear layer behind an axisymmetric paraboloidal-nose cylinder at high angles of attack. The appearance of instability is observed as uniformly spaced vortex loops showing wavy folds of a shear layer. The distance between the counter-rotating primary and secondary vortices varies with time and the vortex loops always appear first in the region between the primary and secondary vortex at the time and place where these vortices approach closest together. The second vortex loops appear in the downstream of the first ones delayed by the appearance of the first vortex loops. It is suggested that the short wavelength perturbations of the preceding vortex loops advect on the outer surface of the primary vortex along the fluid path accompanied by the dominating vortex loops in downstream region. IntroductionA flow over a paraboloidal-nose cylinder at incidence provides the fundamental example of the simple body flow field seen in most high angle of attack flows and provides the model flow of three-dimensional separated flows. Three-dimensional flow separation about a slender body at incidence has been the subject of many studies, including topological structures for a body of revolution such as a slender cone [1], a round-nosed body [2], and a prolate spheroid [3,4]. The structures of the symmetric separated flow about a paraboloidal-nose cylinder at incidence with main, primary, and secondary vortices on the lee side has been well established in experiments and is illustrated in figure 1. The feeding shear layers roll up to form a pair of counter-rotating main vortices.It was shown by Riley & Lowson [5] that steady sub-vortical structures appear in the free shear layer over delta wings. They demonstrated that the smoke filaments formed near the leading edge of the wing spiral into the core of the main vortex as they pass downstream. They suggested that the steady sub-vortices are due to a local three-dimensional Kelvin-Helmholtz instability of the free shear layer.In the preset study, we investigate experimentally the instability of the flow behind a paraboloidal-nose cylinder with appearance of vortex loops in the feeding shear layer encircling the main vortices. This instability is unsteady in contrast to the steady sub-vortical structures over delta wings, and little data on this kind of instability were available in previous works. The present measurements were accomplished using flow visualizations together with PIV measurements in a towing system in a settled air chamber.

About the Transition to Turbulence Through Chaotic Distortion of Vortex Shedding

“…Experiments on the flow around other bodies of simple shape (cylinder, disc, plate) record the similar sequence of modes (see review [14]). Figure 8 demonstrates the structure of the shear layer behind a cylinder with axially symmetric paraboloidal nose at high angles of attack [15]. In experiment of [15], the body is dragged through an undisturbed medium in the air chamber.…”

“…Figure 8 demonstrates the structure of the shear layer behind a cylinder with axially symmetric paraboloidal nose at high angles of attack [15]. In experiment of [15], the body is dragged through an undisturbed medium in the air chamber. It ensures a low level of medium fluctuations.…”

“…The following sequence of modes is observed: main recirculating zone (vortex) B, secondary recirculating zone S, primary recirculating zone A, street of adjoining vortex loops C, turbulence D. The secondary recirculating zone S is located between the primary and main ones. The experiment [15] records this sequence of modes in the shear layer while moving downstream along the contour of the streamlined body, accompanied by the growth of local value of Reynolds number Re . This sequence of modes is similar to one observed in the flow behind a bluff body as Re increases.…”

“…This sequence of modes is similar to one observed in the flow behind a bluff body as Re increases. In [15], titanium tetrachloride was employed to produce a dense white smoke, appearing as a result of a chemical reaction with water in the air. The vortex loop structure appears with smoke filaments in the shear layer.…”

“…Fine flow visualization technique of [15] has allowed revealing the evolution of the instability in the shear layer. The experiment of [15] records two coexisting streets of vortex loops. The secondary recirculating zone S gives the origine to preceding vortex loops street.…”