The differential scattering characteristics of closed three-dimensional dielectric objects are theoretically investigated. The scattering problem is solved in a spherical basis by the Extended Boundary Condition Method (EBCM) which results in a system of linear equations for the expansion coefficients of the scattered field in terms of the incident field coefficients. The equations are solved numerically for dielectric spheres, spheroids, and finite cylinders to study the dependence of the differential scattering on the size, shape, and index of refraction of the scattering object. The method developed here appears to be most applicable to objects whose physical size is on the order of the wavelength of the incident radiation.

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We combine three-dimensional (3D) large-eddy simulations (LES) and resolvent analysis to design active separation control techniques on a NACA 0012 airfoil. Spanwiseperiodic flows over the airfoil at a chord-based Reynolds number of 23, 000 and a freestream Mach number of 0.3 are considered at two post-stall angles of attack of 6 • and 9 • . Near the leading edge, localized unsteady thermal actuation is introduced in an open-loop manner with two tunable parameters of actuation frequency and spanwise wavelength. For the most successful control case that achieves full reattachment, we observe a reduction in drag by up to 49% and increase in lift by up to 54%. To provide physicsbased guidance for the effective choice of these control input parameters, we conduct global resolvent analysis on the baseline turbulent mean flows to identify the actuation frequency and wavenumber that provide high energy amplification. The present analysis also considers the use of a temporal filter to limit the time horizon for assessing the energy amplification to extend resolvent analysis to unstable base flows. We incorporate the amplification and response mode from resolvent analysis to provide a metric that quantifies momentum mixing associated with the modal structure. By comparing this metric from resolvent analysis and the LES results of controlled flows, we demonstrate that resolvent analysis can predict the effective range of actuation frequency as well as the global response to the actuation input. Supported by the agreements between the results from resolvent analysis and LES, we believe that this study provides insights for the use of resolvent analysis in guiding future active flow control. 4 Case 6-1A ∆C D = −41%, ∆C L = +5.1% Case 6-2A ∆C D = −43%, ∆C L = +1.2% Case 6-4A ∆C D = −36%, ∆C L = +11% 6 Case 6-1B ∆C D = −40%, ∆C L = +7.8% Case 6-2B ∆C D = −44%, ∆C L = −2.2% Case 6-4B ∆C D = −41%, ∆C L = +0.3% 12 Case 6-1C ∆C D = −38%, ∆C L = +2.8% Case 6-2C ∆C D = −37%, ∆C L = +0.3% Case 6-4C ∆C D = −30%, ∆C L = −2.4% 15 Case 6-1D ∆C D = −2.2%, ∆C L = +0.4% Case 6-2D ∆C D = −2.8%, ∆C L = −0.4% Case 6-4D ∆C D = +2.8%, ∆C L = −4.1% 2 Case 9-0A ∆C D = −35%, ∆C L = +39% Case 9-1A ∆C D = −19%, ∆C L = +28% Case 9-2A ∆C D = −17%, ∆C L = +29% 5.5 Case 9-0B ∆C D = −35%, ∆C L = +16% Case 9-1B ∆C D = −38%, ∆C L = +47% Case 9-2B ∆C D = −37%, ∆C L = +53% 12 Case 9-0C ∆C D = −43%, ∆C L = +28% Case 9-1C ∆C D = −46%, ∆C L = +41% Case 9-2C ∆C D = −49%, ∆C L = +37% 16 Case 9-0D ∆C D = −1.7%, ∆C L = −3.5% Case 9-1D ∆C D = +1.9%, ∆C L = −3.0% Case 9-2D ∆C D = +0.7%, ∆C L = −7.4%

The problem of electromagnetic wave propagation along a dielectric cylinder of elliptical cross section is considered. Two infinite determinants representing the characteristic equations for the two types of hybrid waves (the HEmne, and the HEmn0 waves) are derived. It is found that there exists two dominant waves which possess zero cutoff frequencies. The characteristic roots of these two dominant waves are computed for various values of eccentricity and relative dielectric constant. Theoretical propagation constants for the dominant waves are verified by experiments.

The submillimeter wave or terahertz (THz) band (1 mm-100 microm) is one of the last unexplored frontiers in the electromagnetic spectrum. A major stumbling block hampering instrument deployment in this frequency regime is the lack of a low-loss guiding structure equivalent to the optical fiber that is so prevalent at the visible wavelengths. The presence of strong inherent vibrational absorption bands in solids and the high skin-depth losses of conductors make the traditional microstripline circuits, conventional dielectric lines, or metallic waveguides, which are common at microwave frequencies, much too lossy to be used in the THz bands. Even the modern surface plasmon polariton waveguides are much too lossy for long-distance transmission in the THz bands. We describe a concept for overcoming this drawback and describe a new family of ultra-low-loss ribbon-based guide structures and matching components for propagating single-mode THz signals. For straight runs this ribbon-based waveguide can provide an attenuation constant that is more than 100 times less than that of a conventional dielectric or metallic waveguide. Problems dealing with efficient coupling of power into and out of the ribbon guide, achieving low-loss bends and branches, and forming THz circuit elements are discussed in detail. One notes that active circuit elements can be integrated directly onto the ribbon structure (when it is made with semiconductor material) and that the absence of metallic structures in the ribbon guide provides the possibility of high-power carrying capability. It thus appears that this ribbon-based dielectric waveguide and associated components can be used as fundamental building blocks for a new generation of ultra-high-speed electronic integrated circuits or THz interconnects.