ALBERT

Description: Vortex formation in the near wake of shallow flow past a vertical cylinder can be substantially delayed by base bleed through a very narrow slot. The structure of the wake associated with this delay changes dramatically with the dimensionless momentum coefficient of the slot bleed. At very low values, where substantial vortex delay is attainable, the bleed flow is barely detectable. For progressively larger values, various forms of jets issue from the slot, and they undergo ordered, large-amplitude undulations, not necessarily synchronized with the formation of the large-scale vortices. When the cylinder is subjected to appropriate rotational perturbations, in the presence of small-magnitude base bleed, it is possible to transform the delayed vortex formation to a form characteristic of the naturally occurring vortices and, furthermore, to induce a large change of the phase, or timing, of the initially formed vortex, relative to the cylinder motion. These features of the near-wake structure are assessed via a technique of high-image-density particle image velocimetry, which provides whole-field patterns of vorticity, Reynolds stress, amplitude distributions of spectral peaks, and streamline topology at and above the bed, for both the delayed and recovered states of the wake. Among the findings is that even small bleed can substantially alter the patterns of streamline topology and Reynolds stress at the bed, which has important consequences for the bed loading. These alterations of th e near-wake structure occur in conjunction with modifications of the shallow approach flow, which is incident upon the upstream face of the cylinder. The topology at the bed, which is altered in accord with attenuation of the well-defined vorticity concentration of the horseshoe (standoff) vortex, shows distinctive patterns involving new arrangements of critical points. © 2005 Cambridge University Press.

Description: The unstable structure of the near wake of a vertical cylinder, in a fully developed laminar free-surface layer, is characterized in relation to the unsteadiness of the horseshoe (necklace) vortex system about the upstream surface of the cylinder. A cinema technique of high-image-density particle image velocimetry allows space-time imaging of the critical regions of the flow and thereby wholefield representations of patterns of the flow structure, in conjunction with spectra and cross-spectra at a large number of points over the flow domain. The unsteadiness of the near wake was examined over a range of wake stability parameter S = cfD/hw, in which cf is the bed friction coefficient, D is the cylinder diameter, and hw is water depth; this range of S was selected such that the classical Káarmán mode of vortex formation remained completely suppressed. Within this range, increase of the Reynolds number, based on depth hw of the shallow layer and D of the cylinder, yielded the onset and development of an instability mode that takes the form of a varicose, as opposed to a sinuous, pattern of vortices. It is related to the unsteadiness of the horseshoe (necklace) vortex system on the upstream side of the cylinder. The process of vortex formation in the near wake is interpreted in terms of multiple, coexisting layers of vorticity due to both the horseshoe vortices and the vorticity layer associated with separation from the cylinder. Furthermore, it is demonstrated that when the near wake is stable at sufficiently low values of the Reynolds number, based on depth hw and cylinder diameter D, application of external perturbations via small-amplitude rotational oscillations of the cylinder, at the most unstable frequency of the separating shear layers, can lead to destabilization of the near wake in a sinuous mode of small-scale vortical structures. Moreover, this type of rotational perturbation of the cylinder, applied at the expected frequency of large-scale Kármán vortex formation, can also yield destabilization of the near wake in this mode. These types of perturbations lead to substantial alterations of the patterns of vorticity and streamline topology, as well as Reynolds stresses and entrainment velocities of the separating shear layers, along the bed, relative to patterns above the bed. © 2005 Cambridge University Press.

Description: The wake states from a circular cylinder undergoing controlled sinusoidal oscillation transverse to the free stream are examined. As the frequency of oscillation passes through the natural Kármán frequency there is a transition between two distinctly different wake states: the low- and high-frequency states. The transition corresponds to a change in the structure of the near wake and is also characterized by a jump in the phase and amplitude of both the total and vortex lift. Over the range of flow and oscillation parameters studied the wake states exhibit a number of universal features. The phases of the vortex lift and drag forces have characteristic values for the low- and high-frequency states, which appear to be directly related to the phase of vortex shedding. A split force concept is employed, whereby instantaneous force traces and images allow discrimination between the actual loading and the physics, and their conventional time-averaged representations. The wake states for the forced oscillations show some remarkable similarities to the response branches of elastically mounted cylinders. The equivalence between forced and self-excited oscillations is addressed in detail using concepts of energy transfer. © 2005 Cambridge University Press.

Description: Interaction of a deep-water wave with a cylinder gives rise to ordered patterns of the flow structure, which are quantitatively characterized using a technique of high-image-density particle image velocimetry. When the cylinder is stationary, the patterns of instantaneous flow structure take on increasingly complex forms for increasing Keulegan-Carpenter number KC. These patterns involve stacking of small-scale vorticity concentrations, as well as large-scale vortex shedding. The time-averaged consequence of these patterns involves, at sufficiently high KC, an array of vorticity concentrations about the cylinder. When the lightly damped cylinder is allowed to undergo bidirectional oscillations, the trajectories can be classified according to ranges of KC. At low values of KC, the trajectory is elliptical, and further increases of KC allow, first of all, both elliptical and in-line trajectories as possibilities, followed by predominantly in-line and figure-of-eight oscillations at the largest value of KC. Representations of the quantitative flow structure, in relation to the instantaneous cylinder position on its oscillation trajectory, show basic classes of patterns. When the trajectory is elliptical, layers of vorticity rotate about the cylinder surface, in accordance with rotation of the relative velocity vector of the wave motion with respect to the oscillating cylinder. Simultaneously, the patterns of streamline topology take the form of large-scale bubbles, which also rotate about the cylinder. When the cylinder trajectory is predominantly in-line with the wave motion, generic classes of vortex formation and shedding can be identified; they include sweeping of previously shed vorticity concentrations past the cylinder to the opposite side. Certain of these patterns are directly analogous to those from the stationary cylinder. © 2007 Cambridge University Press.

Description: Shear flow along a slotted plate can give rise to highly coherent, small-scale oscillations within each slot; the dimensionless frequency is an order of magnitude lower than the frequency of the classical Kelvin-Helmholtz instability. Within a given slot, the unsteady development of a swirl pattern, as well as the flow into and out of the slot, is compatible with the evolution of a large-scale structure, which shows increased agglomeration of vorticity along the surface of the plate. Moreover, the streamwise phase shift of the unsteady flow rates through the slots is in accord with the phase shift of the velocity fluctuation of the downstream propagating disturbance along either side of the plate. This phase shift is associated with development of the large-scale structure, and corresponds to a dimensionless frequency that scales on the plate length, as verified by experiments over ranges of length and inflow velocity. The value of this dimensionless frequency is close to that in the absence of the slotted plate i.e. the Kelvin-Helmholtz instability of the free shear layer. © 2005 Cambridge University Press.