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The velocity field and Birkhoff-Rott integral for non-decaying, non-periodic vortex sheets
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The velocity field and Birkhoff-Rott integral for non-decaying, non-periodic vortex sheets

11 Jan 2024
url
https://doi.org/10.48550/arxiv.2401.06055View
Preprint (Author's original)arXiv.org - Non-exclusive license to distribute Open

Abstract

Mathematics - Analysis of PDEs Physics - Fluid Dynamics
The Birkhoff-Rott integral expresses the fluid velocity on a vortex sheet. This integral converges if certain quantities decay at horizontal infinity, but can also be summed over periodic images in the horizontally periodic case. However, non-decaying, non-periodic cases are also of interest, such as the interaction of periodic wavetrains with non-commensurate periods (i.e. spatially quasiperiodic solutions), or non-periodic disturbances to periodic wavetrains. We therefore develop a more general single formula for the Birkhoff-Rott integral, which unifies and extends the cases of decay and periodicity. We verify that under some reasonable conditions this new version of the Birkhoff-Rott integral is the restriction to the vortex sheet of an incompressible, irrotational velocity field, with continuous normal component but with a jump in tangential velocity across the vortex sheet. We give a number of examples of non-decaying, non-periodic sheet positions and sheet strengths for which our assumptions may be verified. While we develop this in the case of two-dimensional fluids, the methodology applies equally well to three-dimensional fluids.

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