MAE Colloquium - Autumn 2016
The Aerospace Engineering Colloquium (AE 598) is a required course that satisfies the professional development component of the Master of Aerospace Engineering (MAE). MAE students are required to complete nine (9) credits of colloquium participation to satisfy the degree requirements. However, all members of the UW community are welcome to attend and participate.
Topics may include current research and advances in aerospace technology as well as other themes relevant to the professional development of aerospace engineers. To earn credit for this course, students must complete a required set of writing assignments.
Mondays, 4:00 - 5:00 pm
Johnson Hall 075
Faculty Coordinator: Tony Waas
No Speaker This Week - MAE Student Orientation
A Renaissance of Fluid-Structure Interactions: From Linear to Highly Nonlinear Applications in Aerospace and Mechanical Engineering
- Dr. Charbel Farhat
Chair, Aeronautics & Astronautics Professor
Mechanical Engineering Director
Army High Performance Computing Research Center
Fluid-structure interactions are among the most important considerations when designing complex engineering systems such as aircraft, spacecraft, turbine blades, Formula 1 cars, and underwater vessels, to name only a few. They are also important for the analysis of aneurysms in large arteries and artificial heart valves. They can significantly affect performance and/or structural integrity. For several decades, attention has focused almost exclusively on the linear subclass of such interactions, or on their linearization for stability and control applications. A well-known example in the first case is the theory of elasto-acoustics that is used today for underwater acoustics and imaging. A most notable example in the second case is the linear theory of aeroelasticity that continues to dominate flutter analysis in the aeronautics industry. However, many fluid-structure interactions are nonlinear, and some are highly nonlinear. In the past, these have been dealt with using tests, or have been simply avoided or ignored. Today, the situation has begun to change, as highly nonlinear fluid-structure interactions have become of the utmost importance for the design of future aircraft, advanced spacecraft, and next-generation underwater systems. This is the case, for example, for the N+3 aircraft concept based on a strut-braced wing, NASA’s new low density supersonic decelerators, and currently envisioned underwater vessels that feature a larger than ever number of implodable volumes. Furthermore, for many reasons ranging from safety to feasibility, there is currently a strong need to predict such highly nonlinear interactions numerically. Unfortunately, because of the aforementioned previous trends, the numerical simulation of highly nonlinear fluid-structure interaction problems is currently at a state where it is fraught with computational challenges. To this effect, this lecture will first overview this renaissance of fluid-structure interactions, then present a unified computational framework for their numerical prediction. It will highlight the recent impact of this framework on the design of micro air vehicles with flexible flapping wings, the prediction of the vertical tail buffeting of fighter jets at high angles of attack, the understanding of parachute inflation dynamics, the aeroelastic tailoring of Formula 1 cars, and the failure analysis of submerged implodable volumes.
To be announced.
No Speaker This Week - State of the Department Address
Gas-Turbine Heat Transfer – Challenges and Opportunities
- Dr. Tom I-P. Shih
School of Aeronautics and Astronautics
Gas turbines are widely used for propulsion and for electrical and mechanical power generation. Though tremendous advances have been made since its invention in the 1930’s with Frank Whittle’s and Hans von Ohain’s patents, there are still huge opportunities for further advances in efficiency, performance, service life, environmental friendliness, and affordability.
One of the most important opportunities that has been exploited for decades and still available today is to improve the thermal efficiency of gas turbines by developing technologies to enable higher gas temperatures at the inlet of the turbine component, which could be as high as the adiabatic flame temperature from the combustion of fuel and air in the gas turbine’s combustor. Though the turbine’s inlet temperatures have steadily increased over the past few decades, they are still far below the maximum possible and hence the opportunity. The challenge is that the current inlet temperatures (up to 2,000 oC) already far exceed, by hundreds of degrees Celsius, the maximum temperature at which even the best turbine materials (e.g., Ni-based super alloys) lose strength and durability. Currently, 15 to 30% of the air entering the gas turbine’s compressor are used to cool turbine material that come in contact with the hot gases – air that could be used to generate thrust or power. Thus, reducing the amount of cooling flow to enable a given turbine’s inlet temperature is another opportunity to improve efficiency.
To further increase turbine’s inlet temperature and/or reduce coolant flow needed to achieve existing inlet temperatures require a leap beyond existing materials and/or cooling science and engineering. Since it only takes one hot spot where material temperature exceeds the maximum permitted for a turbine vane or blade to fail and cooling efficiency requires the turbine materials to be operating at close their maximum permitted temperatures, achieving the next leap on cooling requires greatly improved understanding on how design and operating parameters affect the detailed fluid-mechanics and conjugate heat-transfer mechanisms in harsh environments and complicated geometries and how that understanding is used in design. This talk provides an overview on turbine cooling, how geometry has been used to create the fluid mechanics that enhance cooling, and addresses challenges and opportunities that could enable a leap forward on cooling.
To be announced.
No Speaker This Week.
Probabilistic Strength, Size Effect and Lifetime of Quasibrittle Structures Based on Interatomic Break Frequency and Finite Weakest-Link Model
- Zdenek P. Bazant
The size effect on structural strength and its statistical distribution is a complex problem for quasibrittle materials because their failure behavior transits from quasi-plastic at small sizes to brittle at large sizes. These are heterogeneous materials with brittle constituents in which the size of inhomogeneinty, or representative volume element (RVE), is not negligible compared to the structure size. Aside from concrete, the archetypical example, they include fiber composites, coarse-grained ceramics, rocks, sea ice, snow slabs, wood, bone, foam, stiff soil, dry snow, masonry, carton, etc., and on the micro- or nano-scale, all brittle materials become quasibritle. Since the break probability is known exactly only for interatomic bonds (being equal to frequency), Kramer’s rule of transition rate theory is applied to nano-crack jumps. Based on proving the rules of multiscale transition of tail probabilities of break to material scale, the probability distribution of strength of one macro-scale representative volume element (RVE) is shown to have a Weibullian tail, calibrated to reach to probability 0.001, the rest being Gaussian. On the structure scale, only Type 1 failure is considered, i.e., the structure fails as soon as the first RVE fails. Hence the weakest-link model applies on the structure scale. But, crucially, the number of links is finite, because of non-negligible RVE. For increasing structure size, the Weibullian portion gradually spreads into the Gaussian core. Only in the infinite size limit the distribution becomes purely Weibull, but, importantly, with a zero threshold. Based on an atomistic derivation of the power law for subcritical macro-crack growth, a similar Gauss-Weibull transition is shown to apply to structure lifetime. The theory is then extended to the size dependence of Paris law and Basquin law for fatigue fracture, to statistics of fatigue lifetime, and to residual strength after a period of preload. The theory is shown to match the existing experimental results on the monotonic strength, residual strength after preload, static and fatigue crack growth rates, and static and fatigue lifetimes, including their distributions and size effects on the distributions. There are three essential consequences: 1) The safety factors must depend on structure size and shape; 2) To predict the pdf of strength, the size effect tests of mean strength suffice; 3) To predict the static and fatigue lifetimes, it suffices to add tests of initial subcritical crack growth rate. An interesting mathematical analogy predicting the lifetime of new nano-scale high-k dielectrics is also pointed out. Finally, an extension to structures failing only after large stable crack growth is outlined and some implications for structural analysis are discussed.
Born and educated in Prague (Ph.D. 1963), Bažant joined Northwestern in 1969, where he has been W.P. Murphy Professor since 1990 and simultaneously McCormick Institute Professor since 2002, and Director of Center for Geomaterials (1981-87). He was inducted to NAS, NAE, Am. Acad. of Arts & Sci., Royal Soc. London; to the academies of Italy, Austria, Spain, Czech Rep. and Lombardy; to Academia Europaea and Eur. Acad. of Sci. & Arts. Honorary Member of: ASCE, ASME, ACI, RILEM. Received: Austrian Cross of Honor for Science and Art I. Class; 7 honorary doctorates (Prague, Karlsruhe, Colorado, Milan, Lyon, Vienna, Ohio State); ASME Timoshenko, Nadai and Warner Medals; ASCE von Karman, Newmark, Biot, Mindlin and Croes Medals and Lifetime Achievement Award; SES Prager Medal; RILEM L’Hermite Medal; Exner Medal (Austria); Torroja Medal (Madrid); ˇSol´ın and Bažant, Sr., Medals (Prague), etc. He authored six books: Scaling of Structural Strength, Inelastic Analysis, Fracture and Size Effect, Stability of Structures, Concrete at High Temperatures, and Concrete Creep. H-index: 112, citations: 53,000 (on Google, Sept.2016, incl. self-cit.), i10 index: 546. In 2015, ASCE established ZP Bažant Medal for Failure and Damage Prevention. He is one of the original top 100 ISI Highly Cited Scientists in Engrg. (www.ISIhighlycited.com).
To be announced.
- Noel Clemens
Aerospace Engineering and Engineering Mechanics
University of Texas
Shock wave / boundary layer interactions are an important feature of high-speed flow that occur in supersonic aircraft inlets, aircraft control surfaces, missile base flows, nozzles, and rotating machinery. These interactions are often associated with severe boundary layer separation, which is highly unsteady, and which exhibits high fluctuating pressure and heat loads. The unsteady motions are characterized by a wide range of frequencies, including low-frequency motions that are about two orders of magnitude lower than the integral-scale fluctuations in the upstream boundary layer. It is these low-frequency motions that are of most interest because they have been the most difficult to explain and model. Despite significant work over the past few decades, the source of the low-frequency motions remains a topic of intense debate. Some argue that the low-frequency unsteadiness is primarily driven by disturbances in the upstream boundary layer, whereas others argue that it is driven by an intrinsic instability of the separated flow. In this seminar I will discuss the experimental research that we have conducted on this topic over the past 20 years, including our recent work using 50 kHz particle image velocimetry to obtain time-resolved information of the separated flow unsteadiness. I will also propose a point-of-view that seems to reconcile the seemingly contradictory mechanisms that have been proposed in the literature.
Dr. Noel Clemens holds the Bob R. Dorsey Professorship in the Department of Aerospace Engineering and Engineering Mechanics at The University of Texas at Austin and serves as department chair. He received a B.S. in Mechanical Engineering from the University of Massachusetts/Amherst in 1985, and M.S. and Ph.D. degrees in Mechanical Engineering from Stanford University in 1986 and 1991, respectively. From 1991 to 1993 he was a post-doctoral fellow at the Combustion Research Facility at Sandia National Laboratories in Livermore, CA. Dr. Clemens began as an Assistant Professor at UT in 1993 and was promoted to full professor in 2005. His areas of research include turbulent mixing, combustion, laser diagnostics, shock wave/boundary layer interactions, inlet unstart and high-speed flow control. He received the Presidential Faculty Fellow Award in 1995, the College’s Faculty Excellence Award in 1997, the award for “Outstanding Teaching by an Assistant Professor” in 1998, the ASE/EM Department Teaching Award in 2000, and the Lockheed Martin Award for Excellence in Engineering Teaching in 2011. He is a Fellow of the American Physical Society and he served as Editor-in-Chief of Experiments in Fluids from 2009 to 2012.
To be announced.
No Speaker This Week.
Role of Mechanical Non-equilibrium on a Hypersonic Turbulent Boundary Layer
- Rodney Bowersox
To be announced.
Performance Enhancement of Vertical Tails with Sweeping Jet Actuators
- Mory Gharib
To be announced.