Self-Propulsion of Immersed Objects via Natural Convection

Abstract

Natural convection of a fluid due to a heated or cooled boundary has been studied within a myriad of different contexts due to the prevalence of the phenomenon in environmental and engineered systems. It has, however, hitherto gone unrecognized that boundary-induced natural convection can propel immersed objects. We experimentally investigate the motion of a wedge-shaped object, immersed within a two-layer fluid system, due to a heated surface. The wedge resides at the interface between the two fluid layers of different density, and its concomitant motion provides the first demonstration of the phenomenon of propulsion via boundary-induced natural convection. Established theoretical and numerical models are used to rationalize the propulsion speed by virtue of balancing the propulsion force against the appropriate drag force. The phenomenon of natural convection generated by heated and cooled surfaces is ubiquitous. In engineering, for example, the effect is exploited to control transport and reactions in microfluidic devices [1] and in temperature control strategies for nuclear reactors [2] and buildings [3]. In geophysical systems, natural convection due to boundary cooling or heating is prevalent as anabatic and katabatic winds in valleys and over glaciers, respectively [4], and impacts the melting of icebergs [5]. Despite almost a century of research [6], although there have been a few studies of the motion of objects floating on the surface of [7], or immersed within [8], flows driven by natural convection, studies of boundary-induced natural convection have been restricted to systems with fixed boundaries [9]. Recently it has been demonstrated that a related class of boundary-layer flow, driven by molecular diffusion, can propel objects immersed in a fluid [10], albeit very slowly. Boundary-layer flows generated via natural convection are typically orders of magnitude stronger than their diffusive counterpart, and therefore have the potential to generate substantially faster propulsion speeds.To investigate the concept of natural convection driven propulsion, a triangular wedge of length l ¼ 260 mm, slope angle ϕ ¼ 30°and width w ¼ 61 mm was constructed and immersed in a two-layer fluid system, images of which are presented in Fig. 1(a). The dimensions of the wedge were chosen so that is was much narrower than the experimental tank, to limit the impact of sidewalls, and could accommodate an internal power supply and control electronics. A metal heating pad of length l h ¼ 113.5 mm and also 61 mm in width, flush-mounted in one of the sloping surfaces, was used to generate boundary layer convection. The pad was heated by a remotely activated 21 AE 1 W battery within the wedge. To isolate the wedge from surface tension effects that influence motion of an object floating at a free surface, its density was configured such that it was suspended within a two-layer stratification in an experimental tank 740 mm long, 510 mm wide, and 350 mm deep, with the base of the wedge level with the interface between the upper...

Diffusion-Induced Effects in a Stratified Fluid Around a Motionless Wedge

“…The integral force action of convergent flows, absent in the symmetric obstacles (sphere, cylinder or horizontal plate), assumes a finite value on the inclined plate and other bodies, non-symmetrical with respect to the line of gravity. The resulting pressure gradients are large enough and capable of inducing self-motion of the neutral buoyancy free bodies («diffusion fish» in parlance of article [10,11]), which plays an important role in the dynamics of the hydrosphere. Calculation of steady flows on the flat fixed wedge is carried out in [12].…”

The Structure of Induced Diffusion Flows on a Wedge with Curved Edges

“…Между изолиниями одинаковый диапазон изменения величины. Структура изображения согласуется с данными теневой визуализации тече-ний в лабораторном бассейне [11]. Структура поля горизонтальной компоненты градиента возмущения со-лености существенно зависит от знака кривизны боковой поверхности: чем острее вершины у основания клина, тем ярче выражены визуализируемые пучки знакопеременных полос (рис.…”

“…4), его значение зависит от кри-визны боковых сторон. При этом толщина протяженной области дефицита давления на вогнутом клине при Разность давлений создает горизонтальную компоненту интегральной силы x F , толкающую горизонтальный клин в направлении вершины, что со-гласуется с наблюдениями [11,12] Вертикальные компоненты результирующей силы F z , вычисленные от-дельно для верхней и нижней граней, превосходят F x на два порядка. Вслед-ствие антиколлинеарности они компенсируют действие друг друга.…”

“…Возни-кающие градиенты давления достаточно велики и способны вызывать само-движение свободных тел нейтральной плавучести («diffusion fish» по терми-нологии из статьи [11]), играющее важную роль в динамике гидросферы. Расчет установившихся течений на плоском неподвижном клине выполнен в работе [12].…”

Turbulent thermal convection driven by heated inertial particles

“…Recently Frankel et al (2016) explored the two-way coupling between forced homogeneous turbulence and heated inertial particles, demonstrating large effects of buoyant plumes shed by the heated clusters on both settling velocity and turbulent kinetic energy. The work of Mercier et al (2014) also considers the interplay between thermal convection and heat exchange from a solid particle. They show experimentally how natural convection enables to propel a single heated triangular wedge.…”