Event Details

Nov13Wed

AME Seminar

Wed, Nov 13, 2019
3:30 PM - 4:30 PM
Location: 102
Speaker: Paolo Luzzatto-Fegiz,

Talk Title: Two Problems in Wall-Bounded Flow: Fluid Energy Extraction in Wind Farms, and Surfactant Effects in Superhydrophobic Drag Reduction

Abstract: In this talk, we consider two fluid problems directly linked to decarbonization efforts. In the first part, we investigate fundamental limits to the performance of large wind farms. Since wind turbines are often deployed in arrays of hundreds of units, wake interactions can lead to drastic losses in power output. Remarkably, while the theoretical Betz maximum has long been established for the output of a single turbine, no corresponding theory appears to exist for a generic, large-scale energy extraction system. We develop a model for an array of energy-extracting devices of arbitrary design and layout, first focusing on the fully-developed regime, which is relevant for large wind farms. We validate our model against data from field measurements, experiments and simulations. By defining a suitable ideal limit, we establish an upper bound on the performance of a large wind farm. This is an order of magnitude larger than the output of existing arrays, thus supporting the notion that large performance improvements may be possible.

In the second part of this talk, we examine flow past superhydrophobic surfaces (SHS). These coatings have long promised large drag reductions; however, experiments have provided inconsistent results, with many textures yielding little or no benefit. By performing surfactant-laden simulations and unsteadily-driven experiments, we demonstrate that surfactant-induced Marangoni stresses can be to blame. We find that extremely low surfactant concentrations, unavoidable in practice, can drastically increase drag, at least in laminar flows. To obtain accurate drag predictions on SHS, one must therefore solve the mass, momentum, bulk surfactant and interfacial surfactant conservation equations, which is not feasible in most applications. To address this issue, we propose a theory that captures how the near-surface dynamics depend on the seven dimensionless groups for surfactant. We validate our theory extensively in 2D, and describe progress toward 3D and turbulent models. Our theory significantly improves predictions relative to a surfactant-free one, which can otherwise overestimate drag reduction by several orders of magnitude.

Here are links to papers/resources that form the basis for this talk:

https://doi.org/10.1103/PhysRevFluids.3.093802
https://doi.org/10.1073/pnas.1702469114
https://doi.org/10.1103/PhysRevFluids.3.100507
https://doi.org/10.1103/APS.DFD.2017.GFM.V0098
https://arxiv.org/abs/1904.01194

Biography: Paolo Luzzatto-Fegiz graduated with a BEng in Aerospace Engineering from the University of Southampton, where he received the Royal Aeronautical Society Prize for highest first-class degree and the Graham Prize for best experimental project in the School of Engineering Sciences. After a summer working with the ATLAS Magnet Team at CERN, he completed an MSc in Applied Mathematics at Imperial College, and an MS and PhD in Aerospace Engineering at Cornell University. His doctoral work received the Acrivos Award of the American Physical Society for outstanding dissertation in Fluid Dynamics at a U.S. university. He was awarded a Devonshire Postdoctoral Scholarship from the Woods Hole Oceanographic Institution, as well as a Junior Research Fellowship from Churchill College, Cambridge. He is currently an Assistant Professor in Mechanical Engineering at UCSB, where he has received the Northrop Grumman Teaching Award and a Gallery of Fluid Motion Award from APS-DFD. He co-invented a salinity sensor for oceanography that has been adopted by 20 institutions, and led the first microgravity experiment from NSF CBET, which successfully returned in January 2019 from the International Space Station.

Host: AME Department

More Info: https://ame.usc.edu/seminars/