Publications by Type: Conference Paper

2017
J. Selvakumar and Bakolas, E., “A pursuit-evasion game in the orbital plane,” in 27th AASAIAA Space Flight Mechanics Meeting, San Antonio, TX, 2017.
D. Pylorof, Bettadpur, S., and Bakolas, E., “On the Robust Attitude Regulation for Earth Observation Spacecraft Under Hybrid Actuation,” in 27th AASAIAA Space Flight Mechanics Meeting, San Antonio, TX, 2017.
2016
Z. Wang, Xu, X., Fan, D., Wang, Y., and Chen, R., “Low-loss Subwavelength Grating Waveguide Bends Based on Index Engineering,” in CLEO: Science and Innovations 2016, San Jose, California, United States, 2016. Publisher's VersionAbstract
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.
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D. Pylorof, Bakolas, E., and Russel, R., “A Nonlinear Controller for Low Thrust Stabilization of Spacecraft on CRTBP Orbits,” in 26th AASAIAA Space Flight Mechanics Meeting, Napa, CA, 2016.
2014
A. Canchero, Tinney, C. E., Murray, N. E., and Ruf, J. H., “

Low-dimensional acoustic structures in the near-field of clustered rocket nozzles

,” in 168th Meeting of the Acoustical Society of America, 2014, vol. 136:2167. Publisher's VersionAbstract
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.
G. Mack, Tinney, C. E., and Ruf, J., “

Scaling metrics for predicting rocket noise

,” in 168th Meeting of the Acoustical Society of America, 2014, vol. 136:2168. Publisher's VersionAbstract
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.
A. R. Antoniswamy, Carter, J. T., Louis G. Hector, J., and Taleff, E. M., “Static Recrystallization and Grain Growth in AZ31B-H24 Magnesium Alloy Sheet,” in Proceedings of the 2014 TMS Annual Meeting, San Diego, CA,, 2014. Publisher's VersionAbstract
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.
C. E. Tinney and Baars, W. J., “

Where are the nonlinearities in jet noise?

,” in 168th Meeting of the Acoustical Society of America, Indianapolis, 2014, vol. 136:2101. Publisher's VersionAbstract
For some time now it has been theorized that spatially evolving instability waves in the irrotational near-field of jet flows couple both linearly and nonlinearly to generate far-field sound [Sandham and Salgado, Philos. Trans. R. Soc. Am. 366 (2008); Suponitsky, J. Fluid Mech. 658 (2010)]. An exhaustive effort at The University of Texas of Austin was initiated in 2008 to better understand this phenomenon, which included the development of a unique analysis technique for quantifying their coherence [Baars et al., AIAA Paper 2010–1292 (2010); Baars and Tinney, Phys. Fluids 26, 055112 (2014)]. Simulated data have shown this technique to be effective, albeit, insurmountable failures arise when exercised on real laboratory measurements. The question that we seek to address is how might jet flows manifest nonlinearities? Both subsonic and supersonic jet flows are considered with simulated and measured data sets encompassing near-field and far-field pressure signals. The focus then turns to considering nonlinearities in the form of cumulative distortions, and the conditions required for them to be realized in a laboratory scale facility [Baars, et al., J. Fluid Mech. 749 (2014)].
2013
T. J. Watt, Yasuda, S., Ichitani, K., Takata, K., Carpenter, A., Jodlowski, J., and Taleff, E. M., “The Effect of Magnesium Content on Microstructure Evolution during Hot Deformation of Aluminum Alloys,” in Proceedings of the 2013 TMS Annual Meeting, San Antonio, TX, 2013, pp. 499–503. Publisher's Version
A. J. Carpenter, Carter, J. T., Louis G. Hector, J., and Taleff, E. M., “Gas-pressure Bulge Forming of Mg AZ31Sheet 450°C,” in Proceedings of the 2013 TMS Annual Meeting, San Antonio, TX, 2013, pp. 139–144. Publisher's Version
M. A. Morovat, Engelhardt, M. D., Helwig, T. A., and Taleff, E. M., “Influence of Creep on the Stability of Steel Columns Subjected to Fire,” in Proceedings, Annual Stability Conference, St. Louis, MO, 2013. Publisher's Version
R. Rojo, Tinney, C. E., and Baars, W. J., “Near-field/far-field study of the end-effects regime produced by large area ratio nozzles,” in 166th Meeting of the Acoustical Society of America, San Fransisco, CA, 2013, vol. 134:5, Pt 2.
H. Daigle and Dugan, B., “Unlocking the Secrets of Subsalt Sedimentary Environments,” in Chikyu+10 International Workshop, Tokyo, Japan, 2013.
2012
J. Lee, Morovat, A., Engelhardt, M., and Taleff, E., “Creep Behavior of ASTM A992 Steel at Elevated Temperatures,” in Proceedings, 7th International Conference on Structures in Fire (SiF), Zurich, Switzerland, 2012. Publisher's Version
P. A. Sherek, Carpenter, A. J., Louis G. Hector, J., Krajewski, P. E., Carter, J. T., Lasceski, J., and Taleff, E. M., “The Effects of Strain and Stress Sate in Hot Forming of Mg AZ31 Sheet,” in Magnesium Technology 2012, TMS Annual Meeting, Orlando, FL, 2012. Publisher's VersionAbstract
Wrought magnesium alloys, such as AZ31 sheet, are of considerable interest for light-weighting of vehicle structural components. The poor room-temperature ductility of AZ31 sheet has been a hindrance to forming the complex part shapes necessary for practical applications. However, the outstanding formability of AZ31 sheet at elevated temperature provides an opportunity to overcome that problem. Complex demonstration components have already been produced at 450°C using gas-pressure forming. Accurate simulations of such hot, gas-pressure forming will be required for the design and optimization exercises necessary if this technology is to be implemented commercially. We report on experiments and simulations used to construct the accurate material constitutive models necessary for finite-element-method simulations. In particular, the effects of strain and stress state on plastic deformation of AZ31 sheet at 450°C are considered in material constitutive model development. Material models are validated against data from simple forming experiments.
A. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “High-Temperature Creep Buckling Phenomenon of Steel Columns Subjected to Fire,” in Proceedings, 7th International Conference on Structures in Fire (SiF), Zurich, Switzerland, 2012. Publisher's Version
M. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “Investigation of Time-Dependent Buckling of Steel Columns Exposed to Fire Temperatures,” 2012, pp. 2095–2106. Publisher's VersionAbstract
One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of applied load. The phenomenon of time-dependent buckling can have a significant impact on the safety of steel columns subjected to fire. This phenomenon has received relatively little research attention, and is not currently explicitly considered in code-based design formulas for columns at elevated temperatures, such as those in the Eurocode 3 or those in the AISC Specification. This paper presents some results of on-going research, which aims at developing analytical, computational and experimental predictions of the phenomenon of creep buckling in steel columns subjected to fire. Analytical solutions using the concept of time-dependent tangent modulus are developed to model time-dependent buckling behavior of steel columns at elevated temperatures. Results from computational creep buckling studies using Abaqus are also presented, and compared with analytical predictions. Material creep data on ASTM A992 steel is also presented in the paper and compared to existing creep models for structural steel at high temperatures. Both analytical and computational methods utilize material creep models for structural steel developed by Harmathy, by Fields and Fields, and by the authors. Predictions from this study are also compared against those from Eurocode 3 and the AISC Specification. Results of this work show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.
J. T. Lee, Carpenter, A. J., Jodlowski, J. P., and Taleff, E. M., “Predicting Hot Deformation of AA5182 Sheet,” in In Proceedings of the 13th International Conference on Aluminum Alloys (ICAA-13), Pittsburgh, PA, 2012. Publisher's Version
2011
C. N. Dolder, Villanueva, M. A., Haberman, M. R., and Tinney, C. E., “Application of boundary layer suction for reducing hydrophone sensing noise,” in 162nd Meeting of the Acoustical Society of America, San Diego, CA, 2011, vol. 130:4, Pt 2.
M. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “Creep Buckling of Steel Columns Subjected to Fire,” in In Proceedings of the 35th International Symposium on Bridge and Structural Engineering, London, England, 2011. Publisher's VersionAbstract
One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of applied load. Although material creep and consequently the phenomenon of creep buckling can significantly impact the safety of steel columns subjected to fire, they have received relatively little research attention, and are not currently explicitly considered in code-based design formula for columns at elevated temperatures, such as those in the Eurocode 3 or in the AISC Specification. This paper will propose a preliminary methodology to study the phenomenon of creep buckling in steel columns subjected to fire. Preliminary analytical solutions are presented, and compared with computational predictions for creep buckling. The analytical and computational results clearly indicate that accurate knowledge of material creep is essential in studying creep buckling phenomenon at elevated temperatures. In addition, the results show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.

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