Showing papers by "Philip Geoffrey Saffman published in 1992"

The structure of intense vorticity in homogeneous isotropic turbulence

Abstract: The structure of the intense vorticity regions is studied in numerically simulated homogeneous, isotropic, equilibrium turbulent flow fields at four different Reynolds numbers in the range Re(sub lambda) = 36-171. In accordance with previous investigators, this vorticity is found to be organized in coherent, cylindrical or ribbon-like, vortices ('worms'). A statistical study suggests that they are just especially intense features of the background, O(omega'), vorticity. Their radii scale with the Kolmogorov microscale and their lengths with the integral scale of the flow. An interesting observation is that the Reynolds number based on the circulation of the intense vortices, gamma/nu, increases monotonically with Re(sub lambda), raising the question of the stability of the structures in the limit of Re(sub lambda) approaching infinity. One and two-dimensional statistics of vorticity and strain are presented; they are non-gaussian, and the behavior of their tails depends strongly on the Reynolds number. There is no evidence of convergence to a limiting distribution in our range of Re(sub lambda), even though the energy spectra and the energy dissipation rate show good asymptotic properties in the higher Reynolds number cases. Evidence is presented to show that worms are natural features of the flow and that they do not depend on the particular forcing scheme.

A Simple Model for the Effect of Water Shear on the Generation of Waves by Wind

Abstract: The stability of the air-water interface with piecewise linear velocity profiles in the air and the water is analysed to study the effect of shear in the water on the generation of waves by wind. This simple formulation reduces the eigenvalue problem to the solution of a quartic equation, which facilitates the exploration of the dependence on the various physical parameters. The results indicate that the presence of water shear tends to enhance the Kelvin-Helmholtz-type instability and to lessen the Miles-type mechanism. In addition, the presence of water shear leads to a selection rule by which the growth of certain short wave components is suppressed. This may be relevant to high-frequency radar imaging of the ocean.