Propulsion through Diffusion

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).0854230 Peacock, Thomas The PIs have discovered a remarkable new propulsion mechanisms that operates in density-stratified environments. The underlying mechanism is a little-studied phenomenon, diffusion-driven flow, first identified within the context of salt transport in rock fissures and ocean boundary-mixing; although its origins date back to Prandtl's study of thermal winds. Diffusion-driven flow spontaneously arises due to an interaction between diffusion and gravity on sloping surfaces, which are the rule rather than the exception for floating, neutrally-buoyant objects. If a floating object is asymmetric, diffusion-driven flow produces an unbalanced force that propels it. The PIs have demonstrated this concept in salt-stratified water, using a carefully designed triangular wedge. At first sight, the effect is highly counterintuitive, requiring no moving parts and therefore seemingly generating propulsion from nowhere; when in fact, the kinetic energy of motion is drawn from the underlying molecular diffusion process. Many fundamental questions remain to be answered, including: How does the propulsion speed scale with the governing parameters? Does suppression of vertical fluid motion by stratification dictate that confinement is always an influence? And is this effect exploited to transport particles and organisms in natural settings, such as lakes and oceans? This study will develop understanding of a new physical effect in fluid dynamics: propulsion through diffusion. The planned studies center on a carefully coordinated program of laboratory experiments, designed to investigate the interdependency of the dimensionless groups that govern the propulsion speed as affected by, three-dimensionality, shape, stratification, diffusivity, viscosity and inertia.. The planned Particle Image Velocimetry (PIV) is difficult for such small-scale, low-speed flows. This data will reveal the underlying flow structure, to help develop analytical and numerical models. Undergraduate research students will be involved in the experiments through the MIT UROP program and through a flow-visualization research project coordinated with Dr. Jim Bales at the MIT Edgerton Lab. Visualizations will be posted to repositories of fluid mechanics, and benchmark data for testing numerical simulations of stratified flow will be posted on the PI's website.