ALBERT

Description: This paper contains temporally and spatially resolved flow visualization and DPIV measurements characterizing the frequency - amplitude response and three-dimensional vortex structure of a circular cylinder mounted like an inverted pendulum. Two circular cylinders were examined in this investigation. Both were 2.54 cm in diameter and ∼140 cm long with low mass ratios, m* = 0.65 and 1.90, and mass - damping ratios, m*ζ = 0.038 and 0.103, respectively. Frequency - amplitude response analysis was done with the lighter cylinder while detailed wake structure visualization and measurements were done using the slightly higher-mass-ratio cylinder. Experiments were conducted over the Reynolds number range 1900≤ Re ≤ 6800 corresponding to a reduced velocity range of 3.7 ≤ U* ≤ 9.6. Detailed examination of the upper branch of the synchronization regime permitted, for the first time, the identification of short-time deviations in cylinder oscillation and vortex-shedding frequencies that give rise to beating behaviour. That is, while long-time averages of cylinder oscillation and vortex-shedding frequencies are identical, i.e. synchronized, it is shown that there is a slight mismatch between these frequencies over much shorter periods when the cylinder exhibits quasi-periodic beating. Data are also presented which show that vortex strength is also modulated from one cylinder oscillation to the next. Physical arguments are presented to explain both the origins of beating as well as why the quasi-periodicity of the beating envelopes becomes irregular; it is believed that this result may be generalized to a broader class of fluid - structure interactions. In addition, observations of strong vertical flows associated with the Kármán vortices developing 2-3 diameters downstream of the cylinder are presented. It is hypothesized that these three-dimensionalities result from both the inverted pendulum motion as well as free-surface effects. © 2008 Cambridge University Press.

Description: The development of a turbulent streamwise vortex core in the wake of a half delta wing has been examined using high-resolution DPIV. The objective of this work was to gain understanding of the transport processes at work a short distance downstream of the wing trailing edge as the wake vortex developed. Experiments were conducted in the Rutgers Free Surface Water Tunnel using an in-house DPIV system. A turbulent streamwise vortex was generated by a half delta wing, with 44 cm chord length and 60° sweep angle, mounted at 30° angle of attack. Reynolds number based on chord length was 65 000. Laser sheets oriented perpendicular to the flow direction were positioned 1, 3.5, and 7 chord lengths downstream of the wing trailing edge. Instantaneous vortex centres were identified in order to track vortex meandering as well as for better quantification of turbulence levels in the vortex core. Mean and fluctuating turbulence terms in the mean streamwise vorticity transport equation along with turbulent kinetic energy dissipation and production were evaluated relative to an inertial reference frame as well as relative to a vortex-centred frame. The results of this analysis highlight the importance of this near-wake region to the downstream evolution of the trailing vortices. There is a high degree of dissipation as well as streamwise vorticity convection in the very near wake which decreases rapidly with increasing distance from the trailing edge.

Description: A two-dimensional temporal mixing layer is generated in a stratified tilting tank similar to that used by Thorpe (1968). Extensive flow dynamics visualization is carried out using, for the top and bottom layers, fluids of different densities but of the same index of refraction. The two-dimensional density field is measured with the laser-induced fluorescence technique (LIF). The study examines further the classical problem of the two-dimensional mixing layer and explores the effects of cross-shear on a nominally two-dimensional mixing layer, a situation widespread in complex industrial and natural flows. Cross-shear is another component of shear, in plane with but perpendicular to the main shear of the base flow, generated by tilting the tank around a second axis. In the two-dimensional mixing layer, the pairing process is found not only to govern the growth of the mixing layer as is commonly known, but also to play a critical role in the mixing transition. The flow region between pairing vortices exhibits a complex topography of stretches and folds in the fluid interface, the length of which is measured to grow exponentially in time. But as higher stratification increasingly inhibits the pairing process, the flow topography becomes less complex, with the material interface growing less rapidly (linearly). Also, the total yield of mixed fluid, as calculated from the measurements of the density field, is reduced with higher stratification. The reduced mixing is due in part to the reduction in the fluid entrainment into Kelvin-Helmholtz vortices (both in the overall volume and in the portion of the bottom fluid to the overall volume), the reduced frequency of pairing of those vortices, and the subsequent arrest of turbulence during flow restratification. The stratified mixing layer also exhibits many interesting secondary features which have been previously documented to various degrees - the baroclinic shear-induced instability in the braid region, gravitational convective instability within the cores, vortex tearing, and vortex dislocations of the Kelvin-Helmholtz vortices. The introduction of a critical level of cross-shear to a plane shear layer results in a new type of 'co-rotating' streamwise vortices in the braid region of the primary Kelvin-Helmholtz instability and an appreciable gain in the total yield of mixed fluid. The appearance and dynamics of the secondary streamwise vortices are very similar to those of the primary Kelvin-Helmholtz vortices, both qualitatively (dynamics of roll-up and pairing) and quantitatively (normalized length and time scales). It is also found that if cross-shear is introduced to the shear layer while it is still planar, the resulting flow behaves simply as a normal but oblique two-dimensional mixing layer. The co-rotating streamwise vortices and the corresponding added mixing result only when cross-shear is introduced after the primary shear layer has started to roll up. There is also evidence that even in the absence of 'global' cross-shear, the co-rotating streamwise vortices can develop locally where a high curvature of the density interface baroclinically induces strong local cross-shear.