Experimental study of the mutual interactions between waves and tailored turbulence

Smeltzer, Benjamin K.; Rømcke, Olav; Hearst, R. Jason; Ellingsen, Simen Å.

doi:10.1017/jfm.2023.280

Cited by 5 publications

(1 citation statement)

References 39 publications

(79 reference statements)

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“…This suggests the relatively weak role of the viscous effect in the evolution of surface-attached vertical vortices. While the ability of subsurface turbulence to leave imprints on the surface is not governed by Re directly, in practical situations for a given fluid Re, We and Fr are all determined by the turbulent motion and difficult to control independently (turbulence generation with an active grid allows some independent control of turbulence intensity and integral scale; see Smeltzer et al 2023).…”

Section: Dependence On Turbulent Flow Conditionsmentioning

confidence: 99%

Vortex imprints on a free surface as proxy for surface divergence

Babiker,

Bjerkebæk,

Xuan

et al. 2023

J. Fluid Mech.

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In turbulence near a free surface, strong vortices attach to the surface, creating surface imprints visible as nearly circular ‘dimples’. By studying these imprints in direct numerical simulation (DNS) data we make two observations. First, the imprints of surface-attached vortices can be very effectively distinguished from other turbulent surface features using two physical features: they are nearly circular in shape, and persist for a long time compared with other pertinent time scales. Secondly, the instantaneous number of surface dimples from surface-attached vortices in an area, $N(t)$ , is intimately related to its mean-square surface divergence, $\beta ^2(t)$ . We develop a simple and physically transparent computer vision procedure which, using the properties of low eccentricity and longevity, detects and tracks vortices from their surface features only, with sensitivity and accuracy of $90\,\%$ or better. We compare $N(t)$ and $\beta ^2(t)$ , finding a normalised cross-correlation of $0.90$ , with changes in $N$ lagging around $0.8T_\infty$ behind those in $\beta ^2$ ( $T_\infty$ is an integral time scale), confirming the common observation that vortices are spawned by strong upwelling events where $\beta ^2$ is large. These findings suggest that the rate of mass flux across the surface, being closely related to surface divergence, can be estimated remotely in some natural flows using visible free-surface dimples as proxy.

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Section: Dependence On Turbulent Flow Conditionsmentioning

confidence: 99%

Vortex imprints on a free surface as proxy for surface divergence

Babiker,

Bjerkebæk,

Xuan

et al. 2023

J. Fluid Mech.

Self Cite

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The influence of water turbulence on surface deformations and the gas transfer rate across an air–water interface

Bullee,

Weichert,

Nore

et al. 2024

Exp Fluids

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We present experimental results of a study on oxygen transfer rates in a water channel facility with varying turbulence inflow conditions set by an active grid. We compare the change in gas transfer rate with different turbulence characteristics of the flow set by four different water channel and grid configurations. It was found that the change in gas transfer rate correlates best with the turbulence intensity in the vertical direction. The most turbulent cases increased the gas transfer rate by 30% compared to the low turbulence reference case. Between the two most turbulent cases studied here, the streamwise turbulence and largest length scales in the flow change, while the gas transfer rate is relatively unchanged. In contrast, for the two less turbulent cases where the magnitude of the fluctuations normal to the free surface are also smaller, the gas transfer rate is significantly reduced. Since the air–water interface plays an important role in the gas transfer process, special attention is given to the free-surface deformations. Despite taking measures to minimise it, the active grid also leaves a direct imprint on the free surface, and the majority of the waves on the surface originate from the grid itself. Surface deformations were, however, ruled out as a main driver for the increase in gas transfer because the increase in surface area is < 0.25%, which is two orders of magnitude smaller than the measured change in the gas transfer rate.

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