arXiv subjects

Ali Semati

Publications and source records attributed to Ali Semati.

3 recordsLinked to original sources

arXiv2026

Sub-surface turbulence and free-surface features

Many turbulent flows encountered in nature -- seas, oceans and rivers -- are bounded by a deformable free surface. A question that remained to be fully explored is to what extent the underlying turbulent flow field can be revealed solely by observing the surface deformations. In this study, we attempt to correlate free-surface topological deformations with the underlying turbulent flow field. We report an experimental investigation of the free surface in the wake of a surface-piercing cylinder and turbulence created by an active grid in an open-channel flow. We are able to study instantaneous events of surface indentations and their related sub-surface coherent structures, as well as statistical properties of velocity and surface motion. We observe weak cross-correlation between the vorticity field and the surface when considering the global surface elevation field. Slightly stronger correlations emerge when conditioning the surface on specific regions, even in the case of three-dimensional homogeneous isotropic turbulence.

physics.flu-dyn↗

arXiv2025

Simultaneous free-surface profilometry and subsurface velocimetry with fringe projection and PIV

This work presents a novel combination of two well-established techniques: fringe projection profilometry (FPP) and particle image velocimetry (PIV). Despite seemingly conflicting requirements---FPP requires an opaque surface to project onto, while PIV requires a transparent fluid---both requirements are met by adding low concentrations (4--25 mg/L) of fluorescein dye to the water. This dye strongly absorbs the blue light projected onto the surface for FPP while remaining nearly transparent to the green PIV laser, achieving simultaneous opacity and transparency depending on wavelength. A set of three optical filters suppresses fluorescence-induced noise in the PIV images and specular reflections in the profilometry images, enabling clean simultaneous acquisition. Validated against point laser-induced fluorescence measurements of the surface, the method achieves a mean absolute error in surface elevation of 18 $\mu$m at a dye concentration of 12 mg/L, above which further increases in concentration yield little improvement. PIV correlation values remain robust up to 20 mg/L. The technique resolves surface features differing in amplitude by two orders of magnitude and is demonstrated on flow past a cylinder interacting with surface waves and on droplet impacts on a quiescent surface.

physics.flu-dyn↗

arXiv2025

Experimental investigation relating free-surface features to sub-surface turbulence

Turbulent flows beneath a free surface play a central role in the Earth system, yet their coupling to observable surface features remains incompletely understood. Recent studies using Direct Numerical Simulations (DNS) have reported strong correlation between observable surface features and surface divergence as well as velocity statistics directly beneath, but were limited to Reynolds numbers ($Re$) far below those typical of natural flows, and do not carry the inherent challenges of measurement and flow fidelity that real flows present. We present a laboratory study in which free-surface topology and sub-surface turbulent velocity are measured simultaneously in a jet-stirred tank, extending these numerical results to the physical domain. Using a novel combination of particle-image velocimetry (PIV) and free-surface profilometry, we access $Re$ up to two orders of magnitude higher than in the DNS. A computer vision method developed for identifying turbulent imprints on the free surface is successfully applied to experimental data, enabling direct comparison with the DNS. The correlation between time series of mean-square surface divergence and surface features is found to persist as strongly at higher Reynolds numbers, despite the increased disparity of turbulent scales. Beyond the thin viscous layer, all surface-to-bulk correlations scale with the integral length scale across both experimental and numerical cases. The normalized cross-correlation between mean-square horizontal velocity divergence and surface area covered by structures decreases linearly with depth and remains significant even two integral scales beneath the surface, unlike point-to-point correlations which decay fast, illustrating how correlations are near-instantaneous but spatially non-local. These results demonstrate that visible surface features provide considerable... [truncated due to arXiv length constraint]

physics.flu-dyn↗