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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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Quantitative vorticity analyses in naturally deformed rocks are essential for studying the kinematics of flow in shear zones and can be performed using a range of methods, which have been developed over the last two decades. The p...
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Quantitative vorticity analyses in naturally deformed rocks are essential for studying the kinematics of flow in shear zones and can be performed using a range of methods, which have been developed over the last two decades. The purpose of this review is to act as a starting point for the reader who needs a current overview of the existing methods and to indicate in what circumstances these methods can be most suitably applied. The review begins by providing an overview of deformation theory, followed by description of the most promising methods - in terms of assumptions, analytical procedures, and possible sources of uncertainty. Finally, the methods are compared on the basis of their uncertainties and strain memory, and discussed in terms of how they can be used to retrieve information about temporal and spatial variation of flow vorticity in shear zones. This review confirms that, although the existing methods are valuable, they are at an immature stage of development and suffer from limitations and uncertainties leading to interpretational problems, which, at present, can be alleviated by applying as many methods as possible to a given sample. Additional studies are recommended to advance the development of existing and new methods.
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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 dynamics of laminar co-rotating vortex pairs without axial flow have been recently thoroughly studied through theoretical, experimental and numerical studies, which revealed different instabilities contributing to the decay of...
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The dynamics of laminar co-rotating vortex pairs without axial flow have been recently thoroughly studied through theoretical, experimental and numerical studies, which revealed different instabilities contributing to the decay of the vortices. In this paper, the objective is to extend the analysis to the case of co-rotating vortices with axial flow at low Reynolds numbers. A high-order incompressible Navier-Stokes flow solver is used. The momentum equations are spatially discretized on a staggered mesh by finite differences and all derivatives are evaluated with 10th order compact finite difference schemes with RK-4 temporal discretization. The initial condition is a linear superposition of two co-rotating circular Batchelor vortices with q = 1. It is found that there is an initial evolution that resembles the evolution that single q = 1 vortices go through. Azimuthal disturbances grow and result in the appearance of large-scale helical sheets of vorticity. With the development of these instability waves, the axial velocity deficit is weakened. The redistribution of both angular and axial momentum between the core and the surroundings drives the vortex core to a more stable configuration, with a higher q value. After these processes, the evolution is somewhat similar to a pair of co-rotating Lamb-Oseen vortices. A three-dimensional instability develops, with a large band of unstable modes, with the most amplified mode corresponding scaling with the vortex initial separation distance.
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Mean and rms profiles of the wall-normal gradient of the streamwise velocity were obtained in a boundary layer using a recently proposed laser velocity gradient technique (LVG). An analysis was also carried out to estimate data ra...
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Mean and rms profiles of the wall-normal gradient of the streamwise velocity were obtained in a boundary layer using a recently proposed laser velocity gradient technique (LVG). An analysis was also carried out to estimate data rates and to determine potential sources of measurement error. The results of this study demonstrate the viability of this novel measurement technique in shear flows. [References: 7]
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The present experimental work is concerned with "twin jets in crossflow" (TJICF) concentrating upon the vorticity distribution and turbulent vorticity transport associated with the dominant vortical structure of the TJICF. The con...
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The present experimental work is concerned with "twin jets in crossflow" (TJICF) concentrating upon the vorticity distribution and turbulent vorticity transport associated with the dominant vortical structure of the TJICF. The configuration of the TJICF consists of a pair of identical jet nozzles and jet-flow conditions at the nozzle exits. The twin jets are issuing normally into a crossflow. The mean-flow velocity vector and associated turbulence statistics of the TJICF are determined using the standard crossed hot-wire anemometry technique. In the present contribution two geometrically symmetric TJICF arrangements, namely tandem and side-by-side arrangements (both with a nozzle centre-to-centre separation of 5D). are examined, focusing upon the dominant vortical structure rather similar to that of the ell-known contrarotating vortex pair of the single JICF. The formation and decay of this dominant cortical structure is closely associated with the turbulent vorticity transport. The two TJICF arrangements under consideration have their own specific vorticity transport features which are. firstly, visualized and interpreted in terms of turbulent vorticity fluxes. Secondly, the vorticity transport analysis shows the vortex-strength decay (i.e. circulation decay) as described by the corresponding integral decay-rate formula. The similarities and differences between the vorticity (distribution and transport) features associated with the two different TJICF arrangements. tandem and side-by-side, and the single-jet case are examined and discussed. The TJICF flow phenomenon represents an interaction of two single JICF and the gross qualitative features of the vortex formation process of the resulting (mean-flow) dominant vortical structure are described. (C) 2003 Elsevier Science Inc. All rights reserved. [References: 24]
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This study is aimed to investigate numerically and experimentally the interaction of a solitary wave with a bottom-mounted vertical thin plate with focuses on the wave induced fluid kinematics and vortex flow patterns. A streamfun...
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This study is aimed to investigate numerically and experimentally the interaction of a solitary wave with a bottom-mounted vertical thin plate with focuses on the wave induced fluid kinematics and vortex flow patterns. A streamfunction-vorticity free-surface (SVFS) based fully nonlinear viscous wave model solved in a transient boundary-fitted coordinate system with locally overlaid grids is applied to study this wave-plate interaction problem. Qualitative comparisons between numerically generated flow patterns around the plate and the experimental observations, including images from both the particle-tracking and the laser-dye visualization methods, are presented. The shear-layer velocity profiles are compared quantitatively with the experimental measurements. The detailed flow characteristics and formed vortices including the effect of the shape of the plate top on the vortex formation and development are numerically investigated. For a thin vertical plate, the flow characteristics that are affected by the dimensionless parameters of incident-wave height and plate height are also investigated. The numerical visualizations illustrated by the virtual transport of particle-tracing, streamlines, and equi-vorticity lines are useful to understand the kinematic behaviors of the induced vortical motions. Furthermore, the pressure gradient, shear stress, and maximum wave force are examined to detail the hydrodynamic impacts on the submerged plate.
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The mechanism of three-dimensional rotational flow in a curved duct caused by distorted inflow is investigated numerically. The steady, laminar flow of viscous, incompressible fluid in a 90-deg. curved duct with a square cross sec...
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The mechanism of three-dimensional rotational flow in a curved duct caused by distorted inflow is investigated numerically. The steady, laminar flow of viscous, incompressible fluid in a 90-deg. curved duct with a square cross section is simulated by the pseudo-compressibility method. A uniform velocity gradient is assumed at the entrance of the computational domain.
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Vortex shedding behind a tapered circular cylinder with taper ratio 75 placed normal to the inflow has been studied. The Reynolds numbers based on the uniform inflow velocity and the diameter of the cylinder at the wide and narrow...
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Vortex shedding behind a tapered circular cylinder with taper ratio 75 placed normal to the inflow has been studied. The Reynolds numbers based on the uniform inflow velocity and the diameter of the cylinder at the wide and narrow ends were 300 and 102, respectively. In the present direct numerical simulation study it was observed that even with a very long time sampling a discrete cellular shedding pattern prevails. This is in contrast to what Parnaudeau [J. Turbulence 8, 13 (2007)] speculated in their tapered cylinder study, where they suggested that with a longer time sampling a diffused cellular pattern might appear. In the present investigation it was found that streamwise vorticity increases as vortex dislocation occurs, an effect also reported by Piccirillo and Van Atta [J. Fluid Mech. 246, 163 (1993)] in their experimental study. Flow visualizations revealed that both modes A and B secondary flow structures coexist along the span of the present tapered cylinder. The wavelength of mode B is in surprisingly good agreement with the experimental value found by Williamson [J. Fluid Mech. 328, 345 (1996)] for uniform circular cylinders. The present numerical calculation revealed a spanwise secondary motion, both in the front stagnation zone and also in the wake of the cylinder. In the front stagnation zone, the secondary flow was driven by a spanwise pressure gradient. The secondary flow pattern in the wake was found to be rather complex. This complex behavior of the secondary motion was attributed to the intrinsic secondary instabilities induced by the transition process itself. This is in contrast to what Parnaudeau [J. Turbulence 8, 13 (2007)] speculated in their tapered cylinder study, where they attributed this to the oblique and cellular vortex shedding. In spite of this secondary flow in the base region, the local formation length in the present tapered cylinder study is in surprisingly good agreement with the results of uniform circular cylinders.
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The structure of initially isotropic homogeneous turbulence interacting with a columnar vortex (with circulation Gamma and radius sigma), idealized both as a solid cylinder and a hollow core model is analysed using the inhomogeneo...
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The structure of initially isotropic homogeneous turbulence interacting with a columnar vortex (with circulation Gamma and radius sigma), idealized both as a solid cylinder and a hollow core model is analysed using the inhomogeneous form of linear rapid distortion theory (RDT), for flows where the r.m.s. turbulence velocity u(0) is small compared with Gamma/sigma. The turbulent eddies with scale L are distorted by the mean velocity gradient and also, over a distance L from the surface of the vortex, by their direct impingement onto it, whether it is solid or hollow. The distortion of the azimuthal component of turbulent vorticity by the differential rotation in the mean flow around the columnar vortex causes the mean-square radial velocity away from the cylinder to increase as (Gamma t/2 pi r(2))(2)(L-x/r)u(0)(2), when (r - sigma) > L-x, but on the surface of the vortices ((r - sigma) < L-x) where (u(r)(2)) is reduced, (u(z)(2)) increases to the same order, while the other components do not grow. Statistically, while the vorticity field remains asymmetric, the velocity field of small-scale eddies near the vortex core rapidly becomes axisymmetric, within a period of two or three revolutions of the columnar vortex. Calculation of the distortion of small-scale initially random velocity fields shows how the turbulent eddies, as they are wrapped around the columnar vortex, become like vortex rings, with similar properties to those computed by Melander & Hussain (1993) using a fully nonlinear direct numerical simulation. A mechanism is proposed for how interactions between the external turbulence and the columnar vortex can lead to non-axisymmetric vortex waves being excited on the vortex and damped fluctuations in its interior. If the columnar vortex is not significantly distorted by these linear effects, estimates are made of how nonlinear effects lead to the formation of axisymmetric turbulent vortices which move as result of their image vorticity (in addition to the self-induction velocity) at a velocity of order u(0)t Gamma/sigma(2) parallel to the vortex. Even when the circulation (gamma) of the turbulent vortices is a small fraction of Gamma, they can excite self-destructive displacements through resonance on a time scale sigma/u(0). [References: 31]
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