Vortex-ring-induced stratified mixing

There is tantalizing evidence that some mechanically driven stratified flows tend towards a state of constant mixing efficiency. We provide insight into the energy balance leading to the constant mixing efficiency and isolate the responsible mechanism. The work presented demonstrates an important mixing efficiency regime for periodically forced externally driven stratified flows. Externally forced stratified turbulent mixing is often characterized by the associated eddies within the flow, which are the dominant mixing mechanism (Turner, J. Fluid Mech., vol. 173, 1986, pp. 431–471). Here, we study mixing induced by vortex rings in order to characterize the mixing induced by an individual eddy. By generating a long sequence of independent vortex-ring mixing events in a density-stratified fluid with a sharp interface, we determine the mixing efficiency of each ring. After an initial adjustment phase, we find that the mixing efficiency of each vortex ring is independent of the Richardson number. By studying the mixing mechanism here, we demonstrate consistent features of a volumetrically confined, periodically forced external mixing regime.This work was funded through the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Engineering and Physical Sciences Research Council (EPSRC). The experimental data associated with this study is made available at https://www.repository.cam.ac.uk/handle/1810/249285.This is the author accepted manuscript. The final version is available from Cambridge University Press via http://dx.doi.org/10.1017/jfm.2015.49

Het Bloemendaalse bos; voorstel voor het toekomstig beheer uitgaande van de recreatieve en cultuurhistorische functie in combinatie met een ecologisch bosbeheer

Op verzoek van de gemeente Bloemendaal is een advies geschreven over het beheer van het Bloemendaalse bos, inclusief de laanstructuur daarin. Uitgangspunt bij de aanbevelingen is dat het bos zowel een recreatieve functie als een natuurfunctie heeft. Door middel van ecologisch beheer wordt getracht om natuurlijke processen te benutten, waardoor het beheer zo min mogelijk inzet vergt en waarbij de vitaliteit van de bomen de eventuele verjongingsnoodzaak aangeeft, zowel in de lanen als in de bosgedeelten. Verjonging van de lanen dient bloksgewijs plaats te vinden om opnieuw een goede laanstructuur te creëren, zoals in het oorspronkelijke ontwerp bedoeld is. In de bosgedeelten moet bewust worden omgegaan met open en gesloten gedeelten, zodat geen monotoon bosbeeld ontstaat. Dit risico is aanwezig indien overal tegelijk verjonging gestimuleerd zou worden, bijvoorbeeld door dunningen. De esdoorn zal een grotere rol gaan spelen in het bos. Dit zal een mooi bosbeeld (blijven) opleveren. Lokale bestrijding van esdoorn is slechts nodig indien lichtbehoevende soorten zoals eik te veel verdrongen worden. Vernatting van de bodem doordat de waterwinning in de omgeving stopt, kan gevolgen hebben voor de vitaliteit. Tot nu toe is daarvan in het bos nog niet veel te merken. De vitaliteitsontwikkeling dient goed gevolgd te worden. In een geollustreerde excursiegids zijn de aanbevelingen toegelicht

Vortex-ring-induced stratified mixing: Mixing model

The study of vortex-ring-induced mixing has been significant for understanding stratified turbulent mixing in the absence of a mean flow. Renewed interest in this topic has prompted the development of a one-dimensional model for the evolution of a stratified system in the context of isolated mixing events. This model is compared to numerical simulations and physical experiments of vortex rings interacting with a stratification. Qualitative agreement between the evolution of the density profiles is observed, along with close quantitative agreement of the mixing efficiency. This model highlights the key dynamical features of such isolated mixing events.</jats:p

Three-dimensional visualization of the interaction of a vortex ring with a stratified interface

The study of vortex-ring-induced stratified mixing has long played a key role in understanding externally forced stratified turbulent mixing. While several studies have investigated the dynamical evolution of such a system, this study presents an experimental investigation of the mechanical evolution of these vortex rings, including the stratification-modified three-dimensional instability. The aim of this paper is to understand how vortex rings induce mixing of the density field. We begin with a discussion of the Reynolds and Richardson number dependence of the vortex-ring interaction using two-dimensional particle image velocimetry measurements. Then, through the use of modern imaging techniques, we reconstruct from an experiment the full three-dimensional time-resolved velocity field of a vortex ring interacting with a stratified interface. This work agrees with many of the previous two-dimensional experimental studies, while providing insight into the three-dimensional instabilities of the system. Observations indicate that the three-dimensional instability has a similar wavenumber to that found for the unstratified vortex-ring instability at later times. We determine that the time scale associated with this instability growth has an inverse Richardson number dependence. Thus, the time scale associated with the instability is different from the time scale of interface recovery, possibly explaining the significant drop in mixing efficiency at low Richardson numbers. The structure of the underlying instability is a simple displacement mode of the vorticity field.Support for this work was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and through the Engineering and Physical Sciences Research Council (EPSRC). Additional support has been provided by the EPSRC Mathematical Underpinnings of Stratified Turbulence grant EP/K034529/1

Sense and nonsense DNA

Supramolecular & Biomaterials Chemistr