We report the design, simulation and experimental demonstration of low loss subwavelength grating waveguide (SWG) bends. With trapezoidal shape silicon pillars, the average insertion loss of a 5μm SWG waveguide bend is reduced drastically from 5.43dB to 1.10dB per 90°bend for quasi-TE polarization.
The plume and acoustic field produced by a cluster of two and four rocket nozzles is visualized by way of retroreflective shadowgraphy. Both steady state and transient operations of the nozzles (start-up and shut-down) were conducted in the fully-anechoic chamber and open jet facility of The University of Texas at Austin. The laboratory scale rocket nozzles comprise thrust-optimized parabolic (TOP) contours, which during start-up, experience free shock separated flow, restricted shock separated flow, and an “end-effects regime” prior to flowing full. Shadowgraphy images are first compared with several RANS simulations during steady operations. A proper orthogonal decomposition (POD) of various regions in the shadowgraphy images is then performed to elucidate the prominent features residing in the supersonic annular flow region, the acoustic near field and the interaction zone that resides between the nozzle plumes. Synchronized surveys of the acoustic loads produced in close vicinity to the rocket clusters are compared to the low-order shadowgraphy images in order to identify the various mechanisms within the near-field that are responsible for generating sound.
Several years of research at The University of Texas at Austin concerning the sound field produced by large area-ratio rocket nozzles is presented [Baars et al., AIAA J. 50(1), (2012); Baars and Tinney, Exp. Fluids, 54 (1468), (2013); Donald et al., AIAA J. 52(7), (2013)]. The focus of these studies is on developing an in-depth understanding of the various acoustic mechanisms that form during start-up of rocket engines and how they may be rendered less efficient in the generation of sound. The test articles comprise geometrically scaled replicas of large area ratio nozzles and are tested in a fully anechoic chamber under various operating conditions. A framework for scaling laboratory-scale nozzles is presented by combining established methods with new methodologies [Mayes, NASA TN D-21 (1959); Gust, NASA TN-D-1999 (1964); Eldred, NASA SP-8072 (1972); Sutherland AIAA Paper 1993–4383 (1993); Varnier, AIAA J. 39:10 (2001); James et al. Proc. Acoust. Soc. Amer. 18(3aNS), (2012)]. In particular, both hot and cold flow tests are reported which comprise single, three and four nozzle clusters. An effort to correct for geometric scaling is also presented.
e The effects of static annealing on recovery, recrystallization and grain growth in a magnesium AZ31B-H24 alloy sheet are investigated at 50°C to 450°C. Full recrystallization is observed after annealing at 250°C or higher temperatures. Recrystallized grain size increases with temperature through normal grain growth. Room-temperature hardness drops abruptly following recrystallization and then decreases with increasing grain size. Predictive relationships are proposed for recrystallized grain size as a function of temperature and time and for hardness as a function of recrystallized grain size.The effects of recrystallization and grain growth on plastic flow and anisotropy will also be discussed.