Vortex structures under dimples and scars in turbulent free-surface flows

Turbulence beneath a free surface leaves characteristic long-lived signatures on the surface, such as upwelling 'boils', near-circular 'dimples' and elongated 'scars', easily identifiable by eye, e.g., in riverine flows. In this paper, we use Direct Numerical Simulation...

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Bibliographic Details
Published in:arXiv.org 2024-09
Main Authors: Aarnes, Jørgen R, Babiker, Omer, Anqing Xuan, Shen, Lian, Ellingsen, Simen Å
Format: Article
Language:English
Subjects:
Alignment
Dimpling
Direct numerical simulation
Elongated structure
Free surfaces
Horizontal orientation
Inclination angle
Scars
Signatures
Tubes
Turbulence
Turbulent flow
Vertical orientation
Vortices
Vorticity
Weber number
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Summary:Turbulence beneath a free surface leaves characteristic long-lived signatures on the surface, such as upwelling 'boils', near-circular 'dimples' and elongated 'scars', easily identifiable by eye, e.g., in riverine flows. In this paper, we use Direct Numerical Simulations to explore the connection between these surface signatures and the underlying vortical structures. We investigate dimples, known to be imprints of surface-attached vortices, and scars, which have yet to be extensively studied, by analysing the conditional probabilities that a point beneath a signature is within a vortex core as well as the inclination angles of sub-signature vorticity. The analysis shows that the likelihood of vortex presence beneath a dimple decreases from the surface down through the viscous and blockage layers in a near-Gaussian manner, influenced by the dimple's size and the bulk turbulence. When expressed as a function of depth over the Taylor microscale \(\lambda_T\), this probability is independent of Reynolds and Weber number. Conversely, the probability of finding a vortex beneath a scar increases sharply from the surface to a peak at the edge of the viscous layer, at a depth of approximately \(\lambda_T/4\). Distributions of vortical orientation also show a clear pattern: a strong preference for vertical alignment below dimples and an equally strong preference for horizontal alignment below scars. Our findings suggest that scars can be defined as imprints of horizontal vortices approximately a quarter of the Taylor microscale beneath the surface, analogous to how dimples can be defined as imprints of surface-attached vertical vortex tubes.
ISSN:2331-8422