Elastic waves are critically used for non-destructive evaluations in industrial and medical fields. Because the wave field is multimodal, involving coupled longitudinal and transverse wave modes, manipulating elastic waves for specific new applications is very challenging. Here, we show that elastic waves can be manipulated unprecedentedly if engineered materials of exotic anisotropy, not found in natural materials, are elaborately used. We present the principle to “design” exotic anisotropy for wave manipulation, actual fabrication through single-phase metamaterials, and their unique applications.

A rapidly expanding research area involves the development of routes to shape programmable three-dimensional (3D) structures with feature sizes in the mesoscopic range (that is, between tens of nanometres and hundreds of micrometres). A goal is to establish methods to control the properties of materials systems and the function of devices, through not only static architectures, but also morphable structures and shape-shifting processes. Soft matter equipped with responsive components can switch between designed shapes, but cannot support the types of dynamic morphing capabilities needed to reproduce continuous shape-shifting processes of interest for many applications. Challenges lie in the establishment of 3D assembly/fabrication techniques compatible with wide classes of materials and 3D geometries, and schemes to program target shapes after fabrication. In this talk, I will introduce a mechanics-guided assembly approach that exploits controlled buckling for constructing complex 3D micro/nanostructures from patterned two-dimensional (2D) micro/nanoscale precursors that can be easily formed using established semiconductor technologies. This approach applies to a very broad set of materials (e.g., semiconductors, polymers, metals, and ceramics) and even their heterogeneous integration, over a wide range of length scales (e.g., from 100 nm to 10 cm). To allow realization of 3D mesostructures that are capable of qualitative shape reconfiguration, we devise a loading-path controlled strategy that relies on elastomer platforms deformed in different time sequences to elastically alter the 3D geometries of supported mesostructures via nonlinear buckling. I will also introduce a recent work on shape programmable soft surface, constructed from a matrix of filamentary metal traces, driven by programmable, distributed electromagnetic forces that follow from the passage of electrical currents in the presence of a static magnetic field. Under the guidance of a mechanics model-based strategy to solve the inverse problem, the surface can morph into a wide range of 3D target shapes and shape-shifting processes. The compatibility of our approaches with the state-of-the-art fabrication/processing techniques, along with the versatile capabilities, allow transformation of diverse existing 2D microsystems into complex configurations, providing unusual design options in the development of novel functional devices.

In this presentation we provide an overview of how knowledge about turbulent boundary layer structure and statistics has had a profound impact on our understanding of wind farm performance. We discuss the asymptotic limit of the fully developed wind turbine array boundary layer in which a double-logarithmic mean velocity distribution emerges for the horizontally averaged velocity. We discuss the question of power density and stress the importance of turbulence in facilitating vertical transport of mean kinetic energy. Of direct relevance to power variability, we show how models of wave-number frequency spectra of turbulent boundary layers can be used to predict frequency spectra of fluctuating power generation of entire wind farms. Focusing on individual wake structures, we then discuss the technique of yawing wind turbines for wake steering, wherein turbine wakes can be redirected to reduce interactions with downstream turbines. The wake deflection is associated with the generation of counter-rotating streamwise vortices of very large size (macro-vortices), whose magnitude can be predicted using classical lifting line theory. Finally, we summarize continuing efforts to couple local wake models with global boundary layer models in new-generation analytical wind farm models, and close by stressing the importance and continued need for reduced analytical models of flow phenomena affecting various aspects of wind energy.

The addition of small amounts of polymers to a Newtonian solvent makes a fluid viscoelastic and can lead to significant viscous drag reduction in high-speed flows. The interaction of viscoelasticity and inertia in a dilute polymer solution results in the emergence of unique inertioelastic instabilities that are still far from being understood. The nonlinear evolution of these instabilities engenders a state of turbulence with significantly different spatiotemporal features compared to Newtonian turbulence, commonly termed elastoinertial turbulence (EIT). We systematically explore EIT by studying the dynamics of low-speed submerged jets of dilute aqueous polymer solutions injected through a nozzle into tanks of quiescent water or polymer solution.