Intensity of vortices: from soap bubbles to hurricanes

T. Meuel, Y. L. Xiong, P. Fischer, C. H. Bruneau, M. Bessafi, H. Kellay
2013 Scientific Reports  
By using a half soap bubble heated from below, we obtain large isolated single vortices whose properties as well as their intensity are measured under different conditions. By studying the effects of rotation of the bubble on the vortex properties, we found that rotation favors vortices near the pole. Rotation also inhibits long life time vortices. The velocity and vorticity profiles of the vortices obtained are well described by a Gaussian vortex. Besides, the intensity of these vortices can
more » ... followed over long time spans revealing periods of intensification accompanied by trochoidal motion of the vortex center, features which are reminiscent of the behavior of tropical cyclones. An analysis of this intensification period suggests a simple relation valid for both the vortices observed here and for tropical cyclones. V ortices are prominent features of fluid flows and span length scales ranging from an insect's length 1 to planetary sizes 2 . Understanding the displacement of vortices, their structure, and their long time dynamics is crucial for different aspects be they at the level of small engines, turbulent flows, or planetary atmospheres 3 . Vortices may be found in turbulent flows, where they appear at all scales, in the wake of a bluff body, where they come in pairs, or in atmospheric flows whether on earth or on other planets 3,4 . In the latter case, these single vortices may be giant and very long lived such as the famous great red spot on Jupiter 2 or the great dark spot in Neptune 5 . Or they may be short lived and capable of traveling long distances such as tropical cyclones (TC) on earth 6,7 . The exact structure of planetary vortices and notably that of the great red spot is still debated but some of their features are known. The two great spots are elliptical for example 5, 8, 9 and tropical cyclones can be reasonably modeled with a modified version of an ideal vortex known as the Rankine vortex 10 or even more refined models introduced recently 11 . Nonetheless and when it comes to predicting their trajectories (as for TCs on earth), the evolution of their intensity, or their lifetime, vortices still pose a number of problems mainly because the interactions with the environment are complex 7,12,13 . For example, the intensity of tropical cyclones and its temporal evolution is an issue which has been tackled over the past few decades with some success as thermodynamical models as well as simple coupled ocean atmosphere models seem to capture some of the essential features 12,14 . Its prediction remains an arduous task as there is need to consider several factors such as the interaction with the sea and the environment, the structure of the TC itself, as well as the role of rotation on the dynamics of such vortices 13,15 . It is therefore highly desirable to devise experimental systems to study the generic properties as well as the effects of such single vortices [16] [17] [18] [19] [20] [21] . It goes without saying that such experimental systems may bring insight into the vastly complex problem of Tropical cyclone intensity or the stability of giant planetary vortices, however, care has to be taken in using results from such systems considering the large differences in the mechanisms at play between the model systems and their natural counterparts. Here we use a half bubble heated from below on which large isolated vortices are observed. We first characterize the general properties of these vortices (their displacement and location on the bubble surface, their life time) with and without rotation of the bubble to gauge the effects of rotation on such a situation. Second, we characterize these vortices by measuring their velocity and vorticity profiles. In a third stage, and based on the preceding velocity measurements, we analyze the long time behavior of the rotation rate of these vortices. The measurements show that the system at hand exhibits a number of intriguing features such as intensification of vortices and trochoidal motion which are common in natural vortices such as tropical cyclones. For the sake of making such qualitative observations more quantitative, the measurements of intensification periods are then compared to the long time variation of the intensity of tropical cyclones. The rationale behind this comparison is very simple. Since the vortices here and Tropical cyclones are large vortices with dimensions much larger than the thickness of the bubble membrane or of the atmosphere respectively, both may be OPEN SUBJECT AREAS: ATMOSPHERIC DYNAMICS FLUID DYNAMICS SURFACES, INTERFACES AND THIN FILMS CLIMATE AND EARTH SYSTEM MODELLING
doi:10.1038/srep03455 pmid:24336410 pmcid:PMC3861805 fatcat:ahvulnyyvzdmhld6rte4ih6tja