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Streamwise vorticity in viscous, compressible, steady flow about aircraft is analysed on the basis of the steady Reynolds-averaged Navier-Stokes equations. It is shown, that Streamwise vorticity can develop in the presence of stre...
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Streamwise vorticity in viscous, compressible, steady flow about aircraft is analysed on the basis of the steady Reynolds-averaged Navier-Stokes equations. It is shown, that Streamwise vorticity can develop in the presence of stream-normal vorticity and can be non-negligible only inside e.g. boundary layers, shock layers, viscous (turbulent) wakes, jet exhausts and propeller slipstreams, but also along streamlines leaving such viscous and heat conduction dominated flow regions on their downstream side (e.g. downstream of shock layers). However, the actual development of Streamwise vorticity then still depends on such local flow characteristics as e.g. streamline curvature and streamline convergence or divergence. Two illustrative examples are discussed.
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The structure and dynamics of jet streaks in the extratroprcal upper troposphere are examrned rn the context of a continuously stratified quasi-geostrophic (QG) framework It is hypothesized that jet streaks may result from the sup...
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The structure and dynamics of jet streaks in the extratroprcal upper troposphere are examrned rn the context of a continuously stratified quasi-geostrophic (QG) framework It is hypothesized that jet streaks may result from the superposition of monopolar or dipolar vortices of mesoscale dimensions with the enhanced potential-vorticity gradients constituting the tropopause. Based on this hypothesis, steady-state monopolar and dipolar vortices in a uniform zonal background flow on an f-plane are investigated for their applicability as idealized dynamical representations of jet streaks. The representations of jet streaks satisfy the nonlinear governing equations of the continuously stratified QG framework: the monopolar vortex is specified in terms of axisymmetric distributions of QG potential vorticity in the interior of the domain and perturbation potential temperature on upper (tropopause) and lower (surface) boundaries, whereas the dipolar vortex is adapted from a closed-form analytical solution for the geostrophic stream function. Through the incorporation of vertical structure and divergent circulations, these representations of jet streaks extend those presented previously by the authors using a non-divergent barotropic model. It is shown thatthese vortex representations display characteristic signatures similar to those observed in atmospheric jet streaks. In particular, both the monopole and the dipole exhibit an ageostrophic wind directed towards lower geopotential height in the entrance region of the streak and towards higher geopotential height in the exit region. For the monopole, this ageostrophic wind is entirely rotational and there is no vertical motion. For the dipole, the rotational part of the ageostrophic wind dominates the divergent part; the latter is associated with a four-cell pattern of vertical velocity similar to that described in conceptual models of straight jet streaks. For both the monopole and the dipole, the jet streak is induced by the vortex structure such thatthe wind speed maximum translates at the same speed as the individual vortices; this translation speed is slower than the maximum wind speed in the core of the speed maximum, consistent with observations of jet streaks. It is proposed that the above representations provide a formal theoretical foundation for the conceptual models of jet streaks prevalent in the literature; these conceptual models typically are based on heuristic kinematic or parcel arguments and not on consistent solutions to a physically plausible set of equations. The representations also provide a foundation upon which to explore the unsteady behaviour of jet streaks in terms of the superposition of monopolar and dipolar vortices with non-uniform zonal background flows.
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Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe v...
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Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe vortices around a circular cylinder with a cavity (slot) placed upstream. Through the cavity, no mass flow addition (blowing) or reduction (suction) is applied. With the upstream cavity, adverse pressure gradient is weakened upstream of the cavity whereas it is strengthened downstream of the cavity. Furthermore, a single vortex system is found to form immediately upstream of the cylinder instead of a typical two vortex (primary and secondary vortices) system observed in the absence of the upstream cavity. The strength of the primary vortex is weakened due to the fluid stream engulfed in to the upstream cavity, resulting in diffusion of the mainstream. Consequently, the circulation of the primary vortex is weakened.
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Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe v...
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Horseshoe vortices are formed at the junction of an object immersed in fluid-flow and endwall plate as a result of three-dimensional boundary layer separation. This study presents the variation of the strengths of such horseshoe vortices around a circular cylinder with a cavity (slot) placed upstream. Through the cavity, no mass flow addition (blowing) or reduction (suction) is applied. With the upstream cavity, adverse pressure gradient is weakened upstream of the cavity whereas it is strengthened downstream of the cavity. Furthermore, a single vortex system is found to form immediately upstream of the cylinder instead of a typical two vortex (primary and secondary vortices) system observed in the absence of the upstream cavity. The strength of the primary vortex is weakened due to the fluid stream engulfed in to the upstream cavity, resulting in diffusion of the mainstream. Consequently, the circulation of the primary vortex is weakened.
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We present experimental, numerical, and theoretical studies oi' a vortex front propagating into a region of vortex-free flow of rotating superfluid ~3He-B. We show that the nature of the front changes from laminar through quasicla...
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We present experimental, numerical, and theoretical studies oi' a vortex front propagating into a region of vortex-free flow of rotating superfluid ~3He-B. We show that the nature of the front changes from laminar through quasiclassical turbulent to quantum turbulent with decreasing temperature. Our experiment provides the first direct measurement of the dissipation rate in turbulent vortex dynamics of ~3He-B and demonstrates that the dissipation becomes mutual-friction independent with decreasing temperature, and it is strongly suppressed when the Kelvin-wave cascade on vortex lines is predicted to be involved in the turbulent energy transfer to smaller length scales.
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We study a twisted vortex bundle where quantized vortices form helices circling around the axis of the bundle in a "force-free" configuration. Such a state is created by injecting vortices into a rotating vortex-free superfluid. U...
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We study a twisted vortex bundle where quantized vortices form helices circling around the axis of the bundle in a "force-free" configuration. Such a state is created by injecting vortices into a rotating vortex-free superfluid. Using continuum theory we determine the structure and the relaxation of the twisted state. This is confirmed by numerical calculations. We also present experimental evidence of the twisted vortex state in superfluid ~3He-B.
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With significant centrifugal effects and Dean vortices, the spiral microchannels are being widely used in microfluidic systems, especially for high-efficiency mixing. However, the spirals are with various shapes and their radii co...
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With significant centrifugal effects and Dean vortices, the spiral microchannels are being widely used in microfluidic systems, especially for high-efficiency mixing. However, the spirals are with various shapes and their radii continuously vary along the spiral, which causes the flow fields and mixing process complicated. This research focused on the flow behaviors and mixing performance of spiral micromixers with various spiral structures. Five classical spirals of Archimedean, Logarithmic, Hyperbolic, Golden and Fibonacci spirals, and two structures of centrosymmetric spiral structure and single spiral structure were investigated. Numerical simulation and experimental validation were conducted with Re from 0.1 to 100. The results showed that the Archimedean spiral presented the best mixing performance among the five spirals, since its radius of curvature kept a small value in the whole flow path which brought about stronger Dean vortices. Moreover, it was found that Dean vortices was inapplicable to characterize chaotic advection in the centrosymmetric spiral structure due to vortices degradation. Vortices degradation resulted from the counter-rotating Dean vortices, which suppressed the chaotic advection and the mixing process. The results also revealed that vortices varied periodically along the flow path in single spiral structure. Increasing the Reynolds number accelerated the periodical variation of vortices, and therefore mixing developed more rapidly. Moreover, a novel parameter named vortices advection intensity (Vai) was proposed to characterize chaotic advection. The calculated results of Vai agreed well with the results of mixing performance under the condition of chaotic advection dominated mixing. The parameter Vai could reliably characterize chaotic advection.
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Two-dimensional vorticity flow fields created in the wake of a plunging breaker were investigated for regular turbulent flow at a Reynolds number of 30 000. Velocity flow fields obtained from an earlier study that had employed dig...
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Two-dimensional vorticity flow fields created in the wake of a plunging breaker were investigated for regular turbulent flow at a Reynolds number of 30 000. Velocity flow fields obtained from an earlier study that had employed digital particle image velocimetry, were analysed to determine vorticity shedding patterns and the interactions between the vorticity filaments as flow progressed. Central difference approximations were applied to the velocity fields to determine vorticity at each point in the field. Most of the strong instantaneous vorticity observed in the flow field was in the form of filaments. A hierarchy of filaments of different lengths were observed, with the longest being as long as the height of the wave used. During the early phases of the flow, instantaneous vorticity tended to organise into thin filaments of counter-rotating pairs. Eventually, the co-rotating vorticity filaments coalesced and ultimately merged in the turbulent flow as flow progressed, while counter-rotating vorticity filaments were cancelled by viscous dissipation. The results suggested that filaments travel more slowly than the wave velocity and drifted towards the bed as they became elongated, and the number of filaments remaining in the flow were observed to decrease as flow progressed. Whereas phase-resolved instantaneous vorticity results showed pairs of counter-rotating vorticity filaments near the crest, the phase-averaged vorticity description of flow fields showed a dominant primary positive vorticity filament around the shear boundary layer.
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The dynamics of n vortices in the self-dual Chern-Simons-Higgs system defined on the infinite plane is investigated. In adiabatic approximation, the vortex dynamics is determined by considering a rigid motion of a vortex configura...
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The dynamics of n vortices in the self-dual Chern-Simons-Higgs system defined on the infinite plane is investigated. In adiabatic approximation, the vortex dynamics is determined by considering a rigid motion of a vortex configuration and a motion around a fixed center of mass. A motion of two vortices is studied in detail. [References: 19]
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An experimental measurement of all three components of the velocity and vorticity vectors, as well as the temperature and its gradient, and potential vorticity, has been described using four acoustic anemometers. Anemometers were ...
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An experimental measurement of all three components of the velocity and vorticity vectors, as well as the temperature and its gradient, and potential vorticity, has been described using four acoustic anemometers. Anemometers were placed at vertices of a tetrahedron, the horizontal base of which was a rectangular triangle with equal legs, and the upper point was exactly above the top of the right angle. The distance from the surface to the tetrahedron its base was 5.5 m, and the lengths of legs and a vertical edge were 5 m. A covariance-correlation matrix for turbulent variations in all measured values has been calculated. In the daytime horizontal and vertical components of the helicity are of the order of-0.03 and +0.01 m s(-2), respectively. The nighttime signs remain unchanged, but the absolute values are several times smaller. The cospectra and spectral correlation coefficients have been calculated for all helicity components. The time variations in the components of "instantaneous" helicity and potential vorticity are demonstrated.
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