ALBERT

Description: Controlled, amplitude-modulated excitation of a cylinder at low Reynolds number (Re equals; 136) in the cross-stream direction generates several states of response of the near wake including: a locked-in wake structure, which is periodic at the modulation frequency; a period-doubled wake structure, which is periodic at a frequency half the modulation frequency; and a destabilized structure of the wake, which is periodic at the modulation frequency, but involves substantial phase modulations of the vortex formation relative to the cylinder displacement. The occurrence of each of these states depends upon the dimensionless modulation frequency, as well as the nominal frequency and amplitude of excitation. Transition through states of increasing disorder can be attained by either decreasing the modulation frequency or increasing the amplitude of excitation at a constant value of nominal frequency. These states of response in the near wake are crucial in determining whether the far wake is highly organized or incoherent. Both of these extremes are attainable by proper selection of the parameters of excitation.

Description: The unsteady laminar necklace vortex system formed at the junction of a rectangular bluff body and a flat plate was studied experimentally using hydrogen bubble flow visualization and particle image velocimetry (PIV). The vortex system was found to exhibit unsteady behaviour similar to that described by other investigators for cylinder-flat plate junctures, and is characterized by the periodic formation of necklace vortices upstream of the body that subsequently break away and advect towards the block. Detailed analysis of PIV measurements on the plane of symmetry indicates that the dominant mechanism for vorticity balance in the vortex system is the cross-cancellation of the vorticity of the necklace vortex with vorticity of opposite sign generated by interaction of the necklace vortex with the approach surface to the body. © 1995, Cambridge University Press. All rights reserved.

Description: An impulsively started jet in shallow water gives rise to vortices having a characteristic diameter larger than the water depth. A technique of high-image-density particle image velocimetry allows characterization of the space-time development of the instantaneous flow patterns along planes representing the quasi-two-dimensional and three-dimensional vortex structure. The quasi-two-dimensional patterns exhibit different categories of vortex development and interaction, depending upon the depth of the shallow water layer. Despite these distinctions, the variations of normalized vortex position, diameter, and circulation, as well as peak vorticity within the vortex, are very similar for sufficiently small water depth. These quasi-two-dimensional patterns are, in turn, related to specific forms of three-dimensional flow structure, which is highly ordered. A prevalent feature is a vortex orthogonal to, and just ahead of, the primary, quasi-two-dimensional vortex. Its streamline topology, on a plane parallel to the axis of the quasi-two-dimensional vortex, exhibits a separation bubble with a well-defined separatix at the bottom (bed) surface. Moreover, its vorticity can exceed that of the quasi-two-dimensional pattern by a factor of two. This feature is consistent for all values of water depth. When the depth becomes sufficiently large, however, the three-dimensional vortex pattern involves an array of vorticity concentrations, which extends across the entire depth of the water. On a plane very close to the bottom surface (bed), global instantaneous distributions of velocity and vorticity exhibit large gradients; they are associated with small-scale vorticity concentrations characteristic of rapid transition. The corresponding streamline topology of the averaged flow close to the bed, however, exhibits a stable focus and is a direct indicator of the topology well above the bed.

Description: A vertical cylinder is located in a free-surface wave, and a two-camera version of high-image-density particle image velocimetry is employed to characterize the spanwise modes of the flow structure in terms of instantaneous velocity and vorticity. These modes are classified according to organized patterns of velocity in the near wake, and are further interpreted in terms of distinctive arrangements of streamwise vorticity concentrations. At low Keulegan-Carpenter number, which corresponds to small wave height, locally two-dimensional vortices having small scale and circulation tend to form as a symmetrical pair and remain attached, or in close proximity, to the surface of the cylinder. Along the span of the cylinder, the near wake shows either a sinuous S or a unidirectional U mode. The spanwise wavelength λ of the S modes, relative to the cylinder diameter D, lies in the range 1 ≲ λ/D ≲ 4.5. These values of λ/D represent the spacing between extrema of patterns of crossflow velocity, as well as between clusters of streamwise vorticity of like sign. As the free surface is approached, the value of λ/D scales with the ratio of the minor to major axes of the elliptical particle trajectory of the wave. At moderate values of the Keulegan-Carpenter number, locally two-dimensional vortices having large scale and circulation are shed from the cylinder in an asymmetric arrangement. The corresponding spanwise mode represents the phase variation of this shedding along the span of the cylinder. These sinuous S modes involve large-scale distortions of patterns of both crossflow velocity and streamwise vorticity, which have wavelengths in the range 10 ≲ λ/D ≳ 110, in contrast to the spacing between individual concentrations of vorticity, which is 1.5D to 4D. Remarkably, it is possible to attain a unidirectional U mode, whereby the phase of the locally two-dimensional vortex shedding is preserved along the entire extent of the cylinder. Signatures of the moments due to the transverse and in-line forces on the cylinder were acquired simultaneously with the patterns of instantaneous velocity and vorticity. Severe modulations of the moment due to the transverse force are associated with spontaneous transformations between basic forms of the sinuous S and unidirectional U modes. The overall form of the signature of the moment due to the in-line force is, however, not generally affected by the spontaneous transformation between modes, but distortion of its peaks is evident.

Description: Oscillations of impinging flows, which date back to the jet-edge phenomenon (Sondhaus 1854), have been observed for a wide variety of impingement configurations. However, alteration of the structure of the shear layer due to insertion of an impingement edge (or surface) and the mechanics of impingement of vortical structures upon an edge have remained largely uninvestigated. In this study, the impingement of a shear layer upon a cavity edge (or corner) is examined in detail. Water is used as a working fluid and laser anemometry and hydrogen bubble flow visualization are used to characterize the flow dynamics. Reynolds numbers (based on momentum thickness at separation) of 106 and 324 are employed. Without the edge, the shear layer produces the same sort of non-stationary (variable) velocity autocorrelations observed by Dimotakis & Brown (1976). When the edge is inserted, the organization of the flow is dramatically enhanced as evidenced by a decrease in variability of autocorrelations and appearance of well-defined peaks in the corresponding spectra. This enhanced organization is not locally confined to the region of the edge but extends along the entire length of the shear layer, thereby reinforcing the concept of disturbance feedback. Comparison of spectra with and without insertion of the edge reveals a remarkable similarity to those of a non-impinging shear layer with and without application of sound at a discrete frequency (Browand 1966; Miksad 1972); with enhanced organization at the fundamental frequency, simultaneous enhancement occurs also at the sub- and higher-harmonics. Visualization of the vortical structures in the vicinity of the impingement edge shows that an impinging structure may experience one of three possible events: complete clipping, whereby the structure is swept down into the cavity; partial clipping, which results in severing of the vortex; or escape, involving deformation of the vortex while it is swept (intact) downstream past the edge. In general, no one of these events persisted continuously over a long period, but tended to occur alternately, meaning that “jitter” of an impinging structure occurs. Plots of paths of these structures versus time showed that the convective speed of the vortex was locally influenced a distance of about four momentum thicknesses upstream of impingement, which is less than the estimated diameter of an impinging vortical structure. Furthermore, this upstream influence of the edge is also evident in the distributions of transverse velocity. Laser measurements indicate that the presence of the edge substantially increases the local value of transverse velocity fluctuation in the region immediately upstream of the edge. © 1979, Cambridge University Press

Description: Oscillations of a cavity shear layer, involving a downstream-travelling wave and associated vortex formation, its impingement upon the cavity corner, and upstream influence of this vortex-corner interaction are the subject of this experimental investigation. Spectral analysis of the downstream-travelling wave reveals low-frequency components having substantial amplitudes relative to that of the fundamental (instability) frequency component; using bicoherence analysis it is shown that the lowest-frequency component can interact with the fundamental either to reinforce itself or to produce an additional (weaker) low-frequency component. In both cases, all frequency components exhibit an overall phase difference of almost 2k (k = 1,2,.) between separation and impingement. Furthermore, the low-frequency and fundamental components have approximately the same amplitude growth rates and phase speeds; this suggests that the instability wave is amplitude-modulated at the low frequency, as confirmed by the form of instantaneous velocity traces. At the downstream corner of the cavity, successive vortices, arising from the amplified instability wave, undergo organized variations in (transverse) impingement location, producing a low-frequency component(s) of corner pressure. The spectral content and instantaneous trace of this impingement pressure are consistent with those of velocity fluctuations near the (upstream) shear-layer separation edge, giving evidence of the strong upstream influence of the corner region. © 1982, Cambridge University Press. All rights reserved.

Description: This investigation addresses the unsteady wake from a blunt trailing edge subjected to controlled perturbations. The relationship between the structure of the near wake, the surface loading on the edge, and the motion of the edge is characterized by flow visualization in conjunction with velocity and pressure measurements. The response of the near wake can be classified into two general categories: a modulated wake, characterized by ordered variations in the near-wake flow structure over a number of cycles of oscillation of the trailing edge; and a phase-locked wake, whereby the near-wake structure does not change from cycle to cycle of the edge oscillation. For the modulated wake, there are large, repetitive excursions of the near-wake vortex pattern in the streamwise direction due to coexistence of the self-excited global instability of the wake and the applied excitation. These excursions can have an amplitude two orders of magnitude larger than the amplitude of the edge motion. The duration of these excursions, in relation to the cyclic motion of the trailing edge, is deterministic. For the phase-locked wake, small changes of the edge oscillation frequency produce large changes in the phase shift of the initially formed vortex from the edge. These phase shifts are due to changes in the times required for vortex formation and departure from the near wake. The corresponding mechanisms are interpreted in terms of the crucial topological features of the near wake and a phase clock concept. © 1993, Cambridge University Press

Description: A cylinder is subjected to frequency-modulated (FM) excitation and the structure of its wake is characterized in terms of the modulation frequency and the frequency deviation. It is possible to destabilize or restabilize the degree of organization of the vortical structures in the near wake and thereby substantially manipulate the spectral content, relative to the case of purely sinusoidal excitation. These processes of destabilization and restabilization are attainable by varying the frequency deviation while holding the modulation frequency constant or vice versa. A phase-locked periodicity of the near-wake response is attainable at the period of the modulation frequency, as well as at double its period. This phase-locked periodicity, or lack of it, is related to the degree of organization of the wake. The structure of the far wake is strongly dependent upon the nature of the near wake modification. Either coherent or destabilized wake structure can be induced in the far wake, at a given value of nominal excitation frequency, by employing appropriate FM excitation. © 1994, Cambridge University Press

Description: The stages of evolution of a quasi-steady breaker from the onset of a capillary pattern to a fully evolved breaking wave are characterized using high-image-density particle image velocimetry, which provides instantaneous representations of the free surface and the patterns of vorticity beneath it. The initial stage, which sets in at a low value of Froude number, involves a capillary pattern along each trough—crest surface of a quasi-stationary wave. The successive crests of the capillary pattern exhibit increasing scale and culminate in a single largest-scale crest of the free surface. Immediately upstream of the large-scale crest, the capillary pattern shows counterclockwise concentrations of vorticity at its troughs and regions of clockwise vorticity beneath its crests. The onset of the final, largest-scale crest exhibits two basic forms: one involving no flow separation; and the other exhibiting a small-scale separated mixing layer. At an intermediate value of Froude number, a breaker occurs and the capillary pattern is replaced by large-scale distortions of the free surface. The onset of separation, which involves flow deceleration along a region of the free surface having a large radius of curvature, leads to formation of a long mixing layer, which has substantial levels of vorticity. Downstream of this breaker, the long-wavelength wave pattern is suppressed. At the largest value of Froude number, the onset of flow separation rapidly occurs in conjunction with an abrupt change in slope of the surface, giving rise to vorticity concentrations in the mixing layer. © 1995, Cambridge University Press. All rights reserved.

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.