arXiv subjects

Sigurdur T. Thoroddsen

Publications and source records attributed to Sigurdur T. Thoroddsen.

5 recordsLinked to original sources

arXiv2024

Circulation in turbulent flow through a contraction

We study experimentally the statistical properties and evolution of circulation in a turbulent flow passing through a smooth 2-D contraction. The turbulence is generated with an active grids to reach $Re_λ \simeq 220$ at the inlet to the 2.5:1 contraction. We employ time-resolved 3-D Lagrangian Particle Tracking technique with the Shake-The-Box algorithm to obtain volumetric velocity fields which we use to calculate the simultaneous circulation in three perpendicular planes. Forming a circulation vector and studying the PDFs of the relative strength of its components, we can quantify how the mean strain enhances and orients coherent vortical structures with the streamwise direction. This is further studied with streamwise space and time correlations of the circulations over a range of loop sizes. The streamwise component of the circulation, over same-size square loops, shows increased integral length, while the other two components are less affected. The circulation around the compressive direction weakens and reaches prominent negative correlation values, suggesting buckling or sharp reorientation of transverse vortices. The PDFs of circulation transit from non-Gaussian to Gaussian behavior as the loop size is increased from dissipative to large scales.

physics.flu-dyn↗

arXiv2017

Navier slip model of drag reduction by Leidenfrost vapour layers

Recent experiments found that a hot solid sphere that is able to sustain a stable Leidenfrost vapour layer in a liquid exhibits significant drag reduction during free fall. The variation of the drag coefficient with Reynolds number shows substantial deviation from the characteristic drag crisis behavior at high Reynolds numbers. Results obtained with liqiuds of different viscosities show that onset of the drag crisis depends on the viscosity ratio of the vapor to the liquid. The key feature of the vapour layer in facilitating tangential flow and in altering the behaviour of the boundary layer can be captured by the Navier slip model that is defined by a slip length. Here, direct numerical and large eddy simulations of flow past a sphere at moderate to high Reynolds numbers ($10^2 \leq \mbox{Re} \leq 4 \times 10^4$) are employed to quantify comparisons with experimental results in terms of variations of the drag coefficient with Reynolds number and the form of the downstream wake on the sphere. This provides a simple one parameter characterization of the drag reduction phenomenon.

physics.flu-dyn↗

arXiv2016

Vortex ring induced large bubble entrainment during drop impact

For a limited set of impact conditions, a drop impacting onto a pool can entrap an air bubble as large as its own size. The subsequent rise and rupture of this large bubble plays an important role in aerosol formation and gas transport at the air-sea interface. The large bubble is formed when the impact crater closes up near the pool surface and is known to occur only for drops which are prolate at impact. Herein we use experiments and numerical simulations to show that a concentrated vortex ring, produced in the neck between the drop and pool, controls the crater deformations and pinch-off. However, it is not the strongest vortex rings which are responsible for the large bubbles, as they interact too strongly with the pool surface and self-destruct. Rather, it is somewhat weaker vortices which can deform the deeper craters, which manage to pinch off the large bubbles. These observations also explain why the strongest and most penetrating vortex rings emerging from drop impacts, are not produced by oblate drops but by more prolate drop shapes, as had been observed in previous experiments.

physics.flu-dyn↗

arXiv2012

Bubble Rings Entrapment

We show how micro-bubble rings are entrapped under a drop impacting onto a pool surface. This fluid dynamics video is submitted to the APS DFD Gallery of Fluid Motion 2012, part of the 65th Annual Meeting of the American Physical Society's Division of Fluid Dynamics (18-20 November, San Diego, CA, USA).

physics.flu-dyn↗

arXiv2012

Bubble Chandeliers

When a drop impacts at very low velocity onto a pool surface it is cushioned by a thin layer of air, which can be stretched into a hemispheric shape. We use ultra-high-speed video imaging to show how this thin air-layer ruptures. The number and locations of these ruptures determines the morphology of the resulting myriad of micro-bubbles. This fluid dynamics video is submitted to the APS DFD Gallery of Fluid Motion 2012, part of the 65th Annual Meeting of the American Physical Society's Division of Fluid Dynamics (18-20 November, San Diego, CA, USA).

physics.flu-dyn↗