Velocity statistics for point vortices in the local α-models of turbulence

Abstract

The velocity fluctuations for point vortex models are studied for the α-turbulence equations, which are characterized by a fractional Laplacian relation between active scalar and the streamfunction. In particular, we focus on the local dynamics regime. The local dynamics differ from the well-studied case of 2D turbulence as it allows to consider the true thermodynamic limit. This limit is not defined for 2D turbulence. We show an analytical form of the probability density distribution of the velocity fluctuations for different degrees of locality. The central region of the distribution is not Gaussian, in contrast to the case of 2D turbulence, but can be approximated with a Gaussian function in the small velocity limit. The tails of the distribution exhibit a power law behavior and self similarity in terms of the density variable. Due to the thermodynamic limit, both the Gaussian approximation for the core and the steepness of the tails are independent on the number of point vortices, but depend on the α-model. We also show the connection between the velocity statistics for point vortices uniformly distributed, in the context of the α-model in classical turbulence, with the velocity statistics for point vortices non-uniformly distributed. Since the exponent of the power law depends just on α, we test the power law approximation obtained with the point vortex approximation, by simulating full turbulent fields for different values of α and we compute the correspondent probability density distribution for the absolute value of the velocity field. These results suggest that the local nature of the turbulent fluctuations in the ocean or in the atmosphere might be deduced from the shape of the tails of the probability density functions.

arXiv comment6 figures

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Cited by in corpus (1)

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