Modeling “Morning Glory:” how do we develop simple analytical models for internal bores?
Internal bores, or hydraulic jumps, arise in many atmospheric and oceanographic phenomena. The most well known atmospheric internal bore is the “Morning Glory” cloud off the northwestern coast of Australia, but atmospheric bores occur planet wide and are important because they can worsen already sever weather. In marine environments, internal bores can be caused by the breaking of internal waves or by the interaction of stratified flow with complex topography. It is thought that marine bores play an important role in the global energy cascade and in the transport of sediment.
At its most basic, a model of an internal bore needs to predict its speed of propagation given the bore's size. Historically, analytical models use an integral analysis to relate these two quantities. A control volume is constructed around the front of the bore, and then mass and momentum are conserved across it. For one-layer bores (i.e. bores propagating on the surface of water), these two equations are sufficient to obtain a solution. But, for two-layer internal bores, an additional equation is needed.
In this presentation, I will discuss the history of this third equation, usually an assumption relating the down- and upstream energy fluxes, before developing my own third equation based on observations from two- and three-dimensional direct numerical simulations of internal bores. Finally, I will discuss our most recent work in which we remove the need for a third equation entirely by reframing the conservation of momentum in terms of vorticity.
Zac Borden graduated in 2008 from Franklin W. Olin College of Engineering with a degree in Mechanical Engineering. Now, he is a Ph.D student in his fifth year at UCSB, working in Eckart Meiburg's computational fluid dynamics lab. Zac's research focuses primarily on using the data and insights gathered from highly resolved numerical simulations of large-scale geophysical flows to develop simple analytical models that describe their behavior. But he also spent one year of his studies in Perth, Australia working on the accurate simulation of particle erosion and deposition. After graduating this spring, he will join the ExxonMobil Upstream Research Company as a computational scientist.
Microfabrication of Bio-Inspired Adhesive Systems
Sathya S. Chary
Most geckos can rapidly attach and detach from almost any kind of surface using the weak but universal van der Waals forces. This ability is attributed to the hierarchical structure of their feet (involving macroscale toe pads, arrays of ‘seta’ microfibers, and nanoscale spatula tips), and how these anisotropic structures are moved (articulated) to generate strong adhesion and friction forces on gripping that rapidly relax on releasing. Inspired by the gecko adhesive system, various structured surfaces have been fabricated suitable for robotic applications.
First, vast arrays of both vertical and tilted rectangular PDMS micro-flaps were fabricated using micro-electromechanical systems (MEMS) fabrication techniques. Friction and adhesion force properties were investigated using a custom-built tester with a flat-on-flat test geometry and millimeter-scale contact area. Demonstrating the importance of both asymmetric tilted structures (such as the fibers in geckos) and an optimal articulation mechanism, it was found that the anisotropic structure of the tilted micro-flaps resulted in highly anisotropic adhesion and friction forces when articulated along different directions: high friction and adhesion when sheared along the tilt direction, and low friction and adhesion when sheared against tilt.
Then, the importance of both tilt angle and fiber shape was demonstrated with tilted half-cylinder PDMS microfiber arrays. With suitable articulation, the adhesion force was switched from a maximum of 9.4 kPa for strong attachment to a minimum of zero for easy detachment. Practical applications require adhesives to be highly durable in addition to being anisotropic – this material retained up to 77% of the initial adhesion after 10,000 test cycles.