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2012 年 03 月 07 日
Seminar - From Hypersonic flows to Soap film flows; what is next? by Professor Chih-Yung Wen

Date:   7 March 2012 (Wednesday)

Time:   2:00 pm – 3:00 pm

Venue:  EF305, The Hong Kong Polytechnic University

Abstract:
In this talk, the speaker will introduce his researches on compressible flows chronologically. The speaker will start with his first involvement in hypersonic aerothermodynamics and NASP (National Aerospace Program) during his Ph.D Study at Caltech, U.S.A. In this work, the speaker has corrected the previous correlation, found by Hornung (his advisor, 1976, JFM), of dissociative non-equilibrium effects on the flow field in front of blunt bodies considered the dependence of the dimensionless shock stand-off distance on the dimensionless dissociation rate immediately after the normal shock in the simple case of a diatomic gas with only one reaction. The speaker has taken into account an additional parameter of the dimensionless free-stream kinetic energy, and extended Hornung’s correlation to the case of complex gas mixtures with many species and many reactions. The new parameter has a clear physical meaning and leads to a theory for the stand-off distance. Extensive experimental and numerical results of air, nitrogen and carbon dioxide flow over spheres were obtained over a large range of total enthalpy, which substantiate the theory. This correlation is now the rule of thumb for predicting the stand-off distance of hypersonic flows over a sphere in the hypersonic society. The speaker will then continue to focus on a general review of a series of classic hydrodynamic experiments of two-dimensional incompressible and compressible flows conducted with soap films, which he has completed in Taiwan. The thickness of these films ranges commonly from 0.1 to 10 mm, so that the ratio of the characteristic length of the flow structures to the film thickness routinely exceeds 104. Thus soap films constitute the closest physical approximation to the concept of a truly two-dimensional fluid. Therefore, contrary to three-dimensional facilities, effects such as aspect ratios, oblique vortex-shedding and three-dimensional instabilities that may influence the separation process are minimized. For the incompressible part, experiments of two-dimensional turbulence and flows over a circular cylinder, a flat plate and a NACA 0012 wing will be presented. For the compressible part, an experimental investigation of one-dimensional (1-D) moving shock waves in vertical soap films will be demonstrated. The shock waves were generated by bursting the films with a perforating spark. Images of propagating shock waves and small disturbances were recorded using a fast line scan CCD camera. An aureole and a “shock wave” preceding the rim of the expanding hole were clearly observed. These images are similar to the x-t diagrams in gas-dynamics and give the velocities of shock and sound waves. The moving shock waves cause jumps in thickness. The variations of the induced Mach number and the ratio of film thickness across the shock wave are plotted versus the shock Mach number. Both results suggest that soap films are analogous to compressible gases with a specific heat ratio of  . At the end of the talk, the speaker will briefly introduce his recent researches on hypersonic vehicles and scramjet engines at National Cheng Kung University. The program layout will be presented. The numerical tools and ground-test facility built for the program will be introduced, including the Mach 2—Mach 6 shock tunnel, high-speed data acquisition system, optical systems..etc. The preliminary results of hydrogen supersonic combustion will also be presented.

Biosketch:
Dr. Chih-Yung Wen is currently full professor in the Department of Aeronautics and Astronautics, National Cheng-Kung, Taiwan. Dr. Wen received his BSc degree at Department of Mechanical Engineering, National Taiwan University in 1986 and MSc. and Ph.D degrees at the Department of Aeronautics, Caltech in 1989 and 1994, respectively. Dr. Wen worked at Caltech as a research fellow from Feb., 1994 to Jul. 1994, right after he finished his Ph.D degree. He then continued his teaching and research works at Department of Mechanical Engineering, Da-Yeh University, Taiwan. During his 12-year professor career at Da-Yeh University, he has been elected as the Chairman of the department (Aug. 1997~Jul. 2000), promoted to full professor in Feb., 2002, and appointed as the Provost (Aug. 2004~Jul. 2006). In Aug. 2006, Dr. Wen joined Department of Aeronautics and Astronautics, National Cheng Kung University (NCKU). In the last five years, Dr Wen has authored and co-authored 42 refereed journal papers and has been awarded 5 patents; meanwhile, has been Principal Investigator (PI) and Co-PI for more than 18 scientific research projects (total grants are more than NT$164,521,940~US$5,487,500). His current research interests are in the areas of (1) Hypersonic Aerodynamics and Scramjet Engine Design, (2) Aerodynamic Applications of Plasma Actuators in Delta-Winged UAVs and MAVs, (3) Fuel Cell Applications in the Electric Power System of a Micro Spacecraft, (4) Cardiovascular Fluid Mechanics, and (5) Flow Instabilities of Magnetic Fluids and Their Applications in Micro-mixers.  He has received many awards, including Excellent Research Award of National Science Council, Taiwan in 1995, 2000-2011, NCKU Annual Excellent Industrial Cooperation Award in 2010, Excellent Teaching Award of Department of Aeronautics and Astronautics, NCKU in 2010, Best Paper Award of 51th Annual Conference of Aeronautic and Astronautic Society of Republic of China ( AASRC) in 2009, and Yearly Best Paper Award of Journal of Mechanics (SCI) in 2001 and 2010, Best Poster Award, The 12th International Conference on Magnetic Fluids (ICMF12), Aug. 1-5, 2010, Sendai, Japan.