Vorticity dynamics in breaking waves

Dr. Alessandro Iafrati

Slide at 11:51

Vortex sheets and vortex tubes
¢ Various vortical structures can be identified in turbulent flows. Basically, we may have tubular or filamentary which are the tubes, and structures with nonfilamentary, flat, vorticity distribution, named as sheets (Horiuti & Takagi, 2005).
* In tubes the vorticity predominates the strain rate, whereas in the sheets strain rate and vorticity are comparably large and correlated (Horiuti & Fujisawa, 2008).
¢ Since vortex tubes are mostly responsible for intermittency, they are rather popular and several methods have been developed to identify them, generally based on the invariants of the velocity gradient tensor
* DNS of isotropic turbulence have shown that tubular structures of strong axial vorticity, are responsible for only a negligible part of the energy dissipation (Jiménez & Wray, 1998) most of it being located about the spiraling vortex sheets (Kuwahara, 2005)
* Indeed, vortex sheets, consisting of zones lo locally nearly two-dimensional 4 © (Say shearing motion, provide a dominant contribution to the enstrophy production through vortex stretching and to energy dissipation (Pirozzoli et al., 2010) AS * Tubes and sheet are related: tubes often formed as Kelvin-Helmholtz instability = of the vortex sheet. Tubes may also form by multiple vortex sheets forming a - recirculating flow with a pressure minimum Horiuti & Fujisawa, 2008
SAPIENZA 5% = ISTITUTO DI INGEGNERIA DEL MARE UNIVERSITA DI ROMA © | INSTITUTE OF MARINE ENGINEERING

References

1.
K. Horiuti and Y. Takagi (2005) Identification method for vortex sheet structures in turbulent flows. Physics of Fluids
2.
K. I. Y. O. S. I. HORIUTI and T. A. K. E. H. A. R. U. FUJISAWA (2008) The multi-mode stretched spiral vortex in homogeneous isotropic turbulence. Journal of Fluid Mechanics
3.
J. A. V. I. E. R. JIMÉNEZ and A. A. WRAY (1998) On the characteristics of vortex filaments in isotropic turbulence. Journal of Fluid Mechanics
4.
A. NAKAYAMA and F. KUWAHARA (2005) THREE-DIMENSIONAL NUMERICAL MODELS FOR PERIODICALLY FULLY-DEVELOPED HEAT AND FLUID FLOWS WITHIN POROUS MEDIA.
5.
S. E. R. G. I. O. PIROZZOLI et al. (2010) On the dynamical relevance of coherent vortical structures in turbulent boundary layers. Journal of Fluid Mechanics
6.
K. I. Y. O. S. I. HORIUTI and T. A. K. E. H. A. R. U. FUJISAWA (2008) The multi-mode stretched spiral vortex in homogeneous isotropic turbulence. Journal of Fluid Mechanics

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Summary (AI generated)

During the speech, it was announced that the current method of identifying dissipation in flow was not fully satisfactory. The goal was to understand how to identify the region where the dissipation actually occurred. This had been previously shown in a previous speech. The speaker explained that there were many structural structures present, including tubular or elementary structures which were the tubes, as well as non-filament toroidal or flat participate distributions which were numerous sheets.

In the past, there had been a lot of interest in identifying filamentary structures, but it was noted that the vortex tubes, even though they were responsible for the intermittent flow, were not necessarily the most responsible for energy dissipation. Therefore, there was interest in investigating other structures, such as the vortex sheets, which were zones of locally nearly two-dimensional shear emotion that developed within the flow. These vortex sheets were connected to some extent to the tubes, as the tubes often resulted from the ability of vortex sheets. However, the tubes could also result from the interaction of different vortex sheets that formed a region with a minimum pressure.