Experimental studies of vortex disconnection and connection at a free surface

Gharib, Morteza; Weigand, Alexander

doi:10.1017/s0022112096007641

Cited by 57 publications

(65 citation statements)

References 11 publications

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“…Since unsteady surface motion is clearly in evidence in the experiment, it is a likely source of vorticity. A detailed mathematical treatment of this and other mechanisms has been presented by Wu (1993), see also Gharib et al (1992) and Rood (1994). It is interesting to note that techniques such as the one used here for the velocity field measurement can, in principle, be used to test vorticity creation mechanisms, though the temporal resolution of the data (15 Hz) is insufficient in our case.…”

Section: Mechanisms Of Vorticity Generationmentioning

confidence: 94%

The flow field downstream of a hydraulic jump

1995

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A control-volume analysis of a hydraulic jump is used to obtain the mean vorticity downstream of the jump as a function of the Froude number. To do this it is necessary to include the conservation of angular momentum. The mean vorticity increases from zero as the cube of Froude number minus one, and, in dimensionless form, approaches a constant at large Froude number. Digital particle imaging velocimetry was applied to travelling hydraulic jumps giving centre-plane velocity field images at a frequency of 15 Hz over a Froude number range of 2-6. The mean vorticity determined from these images confirms the control-volume prediction to within the accuracy of the experiment. The flow field measurements show that a strong shear layer is formed at the toe of the wave, and extends almost horizontally downstream, separating from the free surface at the toe. Various vorticity generation mechanisms are discussed. IntroductionAt sufficiently high Froude number, the flow downstream of a hydraulic jump is so obviously rotational that it can be seen with the naked eye. The mechanism of vorticity generation is, however, still controversial. There is, of course, a source of vorticity at the solid boundary below the bottom fluid. The sign of the vorticity generated there depends on whether this boundary is stationary relative to the jump or relative to the upstream lower fluid. This vorticity stays within the boundary layer, however, and is not the concern here. In a recent publication, Yeh (1991) discusses mechanisms of vorticity generation in a steady-flow hydraulic jump. Three contributions are identified: viscous shear at the interface between the two fluids, the baroclinic torque brought about by the static pressure gradient in the upper fluid, and the baroclinic torque brought about by the dynamic pressure gradient associated with a suitable velocity field in the upper fluid. The last of these is shown to dominate the other two and is proportional to the density ratio. It requires the vertical component of the pressure gradient in the upper fluid to be of opposite sign to that corresponding to the static gradient, so that a significant velocity field with prescribed features has to be present in the upper fluid. Since this velocity field will depend on whether the far-field upper fluid is at rest relative to the wave or relative to the upstream lower fluid, the vorticity generation would be different in these two cases.When the density ratio is very large, such as for an air/water interface, where it is approximately 1000, such a dependence on the motion of the tenuous upper fluid seems too sensitive. One might carry the argument to the extreme case of a t Permanent address:

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Section: Mechanisms Of Vorticity Generationmentioning

confidence: 94%

The flow field downstream of a hydraulic jump

1995

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“…commonly used in order to study vortex (dis)connection, both in experiments, for instance by Bernal & Kwon (1989), Song et al (1992), Gharib & Weigand (1996) and Weigand (1996), and in numerical simulations by Zhang et al (1999) among others. As a vortex ring approaches a free surface, it too tends to break up into smaller vortex tubes that end at the surface.…”

Section: Free-surface Deformationsmentioning

confidence: 99%

Experiments on free-surface turbulence

Savelsberg

Water

2009

J. Fluid Mech.

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We study the free surface of a turbulent flow, in particular the relation between the statistical properties of the wrinkled surface and those of the velocity field beneath it. Channel flow turbulence is generated using an active grid. Through a judicial choice of the stirring protocol the anisotropy of the subsurface turbulence can be controlled. The largest Taylor Reynolds number obtained is Reλ = 258. We characterize the homogeneity and isotropy of the flow and discuss Taylor's frozen turbulence hypothesis, which applies to the subsurface turbulence but not to the surface. The surface gradient field is measured using a novel laser-scanning device. Simultaneously, the velocity field in planes just below the surface is measured using particle image velocimetry (PIV). Several intuitively appealing relations between the surface gradient field and functionals of the subsurface velocity field are tested. For an irregular flow shed off a vertical cylinder, we find that surface indentations are strongly correlated with both vortical and strain events in the velocity field. For fully developed turbulence this correlation is dramatically reduced. This is because the large eddies of the subsurface turbulent flow excite random capillary–gravity waves that travel in all directions across the surface. Therefore, the turbulent surface has dynamics of its own. Nonetheless, it does inherit both the integral scale, which determines the predominant wavelength of the capillary–gravity surface waves, and the (an)isotropy from the subsurface turbulence. The kinematical aspects of the surface–turbulence connection are illustrated by a simple model in which the surface is described in terms of waves originating from Gaussian wave sources that are randomly sprinkled on the moving surface.

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“…where an equation presented by Gharib & Weigand (1996) is used along with one from Edwards, Brenner & Wasan (1991) to derive the last term, and s is the surface component. Here the dilitational (κ s ) and shear (µ s ) viscosities of the surfactant film are included to account for their rheological effects at the free surface.…”

Section: Theorymentioning

confidence: 99%

“…2.2. Vorticity and vorticity flux Using the curvilinear coordinate system defined in figure 2, the shear stress condition at the free surface, and the equation given by Gharib & Weigand (1996), the surfaceparallel vorticity (out of the page, or z-direction) is defined as…”

Section: Theorymentioning

confidence: 99%

“…They observed opposite-sign vorticity being generated at the free surface in the contaminated case, as opposed to a rebounding of the pair in the clean case. Gharib & Weigand (1996) looked at the connection of an inclined vortex ring to a free surface. In the clean case they observed the formation of two separate half rings, while in the contaminated case only one half ring was formed as the connection process was altered by the surface contamination.…”

Section: Introductionmentioning

confidence: 99%

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Experimental study of the wake behind a surface-piercing cylinder for a clean and contaminated free surface

Lang

Gharib

2000

J. Fluid Mech.

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This experimental investigation into the nature of free-surface flows was to study the effects of surfactants on the wake of a surface-piercing cylinder. A better understanding of the process of vorticity generation and conversion at a free surface due to the absence or presence of surfactants has been gained. Surfactants, or surface contaminants, have the tendency to reduce the surface tension proportionally to the respective concentration at the free surface. Thus when surfactant concentration varies across a free surface, surface tension gradients occur and this results in shear stresses, thus altering the boundary condition at the free surface. A low Reynolds number wake behind a surface-piercing cylinder was chosen as the field of study, using digital particle image velocimetry (DPIV) to map the velocity and vorticity field for three orthogonal cross-sections of the flow. Reynolds numbers ranged from 350 to 460 and the Froude number was kept below 1.0. In addition, a new technique was used to simultaneously map the free surface deformation. Shadowgraph imaging of the free surface was also used to gain a better understanding of the flow. It was found that, depending on the surface condition, the connection of the shedding vortex filaments in the wake of the cylinder was greatly altered with the propensity for surface tension gradients to redirect the vorticity near the free surface to that of the surface-parallel component. This result has an impact on the understanding of turbulent flows in the vicinity of a free surface with varying surface conditions.

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Experimental studies of vortex disconnection and connection at a free surface

Cited by 57 publications

References 11 publications

The flow field downstream of a hydraulic jump

The flow field downstream of a hydraulic jump

Experiments on free-surface turbulence

Experimental study of the wake behind a surface-piercing cylinder for a clean and contaminated free surface

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