Daniel J. Ruth

I am a Postdoctoral Researcher in the Coletti Group, which is part of the Institute of Fluid Dynamics at ETH Zurich; beforehand I earned my Ph.D. in the Deike Lab at Princeton University.

Broadly, I am interested in multiphase turbulent flows and the experimental and numerical techniques used to study them.

Turbulence under wind-driven waves

The wind-driven shear flow beneath the ocean surface is impacted by wind-generated waves which propagate overhead. I study the turbulent characteristics of these flows in the wind-water-wave flume at ETH Zurich using techniques like particle image velocimetry and laser-induced fluorescence.

The video above shows the velocity field beneath wind-driven waves, which can be decomposed into a depth-dependent mean flow, orbital motions, and turbulence.

Related Publications:

Reconstructing turbulent and orbital motions under wind-driven waves

Daniel J. Ruth, Pim Bullee, Claudio Mucignat, and 1 more author

International Journal of Multiphase Flow, 2026 (In press)

DOI
Three‐Dimensional Measurements of Air Entrainment and Enhanced Bubble Transport During Wave Breaking

Daniel J. Ruth, Baptiste Néel, Martin A. Erinin, and 4 more authors

Geophysical Research Letters, Aug 2022

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Speed and Acceleration of Droplets Generated by Breaking Wind‐Forced Waves

M. A. Erinin, B. Néel, D. J. Ruth, and 4 more authors

Geophysical Research Letters, Jul 2022

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Surface deformations due to turbulence

Turbulence beneath a free surface imprints itself on the surface, causing deformations related to the vorticity (left) and degree of upwelling or downwelling (right).

In this experiment, the velocity beneath the surface is measured with particle image velocimetry and the surface elevation field is measured with background-oriented schlieren. The red and blue on the left shows the surface-normal vorticity and the green and purple on the right show the horizontal divergence of velocity. The surface morphology is represented by the curved lines, which come closer to each other at surface depressions (dimples) and farther apart at bulges in the surface.

Turbulence near free surfaces

A free surface modifies the structure of a turbulent flow: vertical motions are inhibited by the presence of the surface, through which the fluid cannot pass. Further, my recent experimental work has shown that the turbulent cascade of energy is inhibited by the free surface. Approaching the surface, patches of energetic liquid are compressed vertically and extended horizontally, a “pancaking” effect which controls the inter-scale transfer of energy.

On the left in the animation above, homogeneous turbulence is generated in the bulk of a liquid by arrays of randomly-actuated synthetic jets; on the right, wind blown along the water surface in the ETH wind-wave flume induces both surface gravity waves and turbulence in the water. In both setups, particle image velocimetry is used to obtain the vorticity fields (shown in red and blue), and laser-induced fluorescence is used to locate the air-water interface (shown in cyan).

Related Publications:

Structure and energy transfer in homogeneous turbulence below a free surface

Daniel J. Ruth and Filippo Coletti

Journal of Fluid Mechanics, Dec 2024

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We investigate the turbulence below a quasi-flat free surface, focusing on the energy transport in space and across scales. We leverage a large zero-mean-flow tank where homogeneous turbulence is generated by randomly actuated jets. A wide range of Reynolds number is spanned, reaching sufficient scale separation for the emergence of an inertial sub-range. Unlike previous studies, the forcing extends through the source layer, although the surface deformation remains millimetric. Particle image velocimetry along a surface-normal plane resolves from the dissipative to the integral scales. The contributions to turbulent kinetic energy from both vertical and horizontal components of velocity approach the prediction based on rapid distortion theory as the Reynolds number is increased, indicating that discrepancies among previous studies are likely due to differences in the forcing. At odds with the theory, however, the integral scale of the horizontal fluctuations grows as the surface is approached. This is rooted in the profound influence exerted by the surface on the inter-scale energy transfer: along horizontal separations, the direct cascade of energy in horizontal fluctuations is hindered, while an inverse cascade of that in vertical fluctuations is established. This is connected to the structure of upwellings and downwellings. The former, characterized by somewhat larger spatial extent and stronger intensity, are associated with extensional surface-parallel motions. They thus transfer energy to the larger horizontal scales, prevailing over downwellings which favour the compression (and concurrent vertical stretching) of the eddies. Both types of structures extend to depths between the integral scale and the Taylor microscale.

Bubbles in turbulence

In a turbulent flow, bubbles will break apart due to stresses from the surrounding turbulent liquid if they are large enough that surface tension is incapable of preventing severe deformations. Knowledge of the dynamics of the break-ups and the distribution of bubble sizes that are produced is important to modeling bubble-mediated gas transfer, as occurs with breaking waves on the ocean and in industrial bubble column reactors.

Above, a large bubble is suddenly subjected to turbulence. The sizes of the bubbles into which it breaks are identified using high-speed imaging and particle tracking.

Related Publications:

Experimental observations and modelling of sub-Hinze bubble production by turbulent bubble break-up

Daniel J. Ruth, Aditya K. Aiyer, Aliénor Rivière, and 2 more authors

Journal of Fluid Mechanics, Nov 2022

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The effect of nonlinear drag on the rise velocity of bubbles in turbulence

Daniel J. Ruth, Marlone Vernet, Stéphane Perrard, and 1 more author

Journal of Fluid Mechanics, Oct 2021

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Bubble pinch-off in turbulence

Daniel J. Ruth, Wouter Mostert, Stéphane Perrard, and 1 more author

Proceedings of the National Academy of Sciences, Dec 2019

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Capillary driven fragmentation of large gas bubbles in turbulence

Aliénor Rivière, Daniel J. Ruth, Wouter Mostert, and 2 more authors

Physical Review Fluids, Aug 2022

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