Technique for imaging ablation-products transported in high-speed boundary layers by using naphthalene planar laser-induced fluorescence

textA new technique is developed that uses planar laser-induced fluorescence (PLIF) imaging of sublimated naphthalene to image the transport of ablation products in a hypersonic boundary layer. The primary motivation for this work is to understand scalar transport in hypersonic boundary layers and to develop a database for validation of computational models. The naphthalene is molded into a rectangular insert that is mounted flush with the floor of a Mach 5 wind tunnel. The distribution of naphthalene in the boundary layer is imaged by using PLIF, where the laser excitation is at 266 nm and the fluorescence is collected in the range of 320 to 380 nm. To investigate the use of naphthalene PLIF as a quantitative diagnostic technique, a series of experiments is conducted to determine the linearity of the fluorescence signal with laser fluence, as well as the temperature and pressure dependencies of the signal. The naphthalene fluorescence at 297 K is determined to be linear for laser fluence that is less than about 200 J/m². The temperature dependence of the naphthalene fluorescence signal is found at atmospheric pressure over the temperature range of 297K to 525K. A monotonic increase in the fluorescence is observed with increasing temperature. Naphthalene fluorescence lifetime measurements were also made in pure-air and nitrogen environments at 300 K over the range 1 kPa to 40 kPa. The results in air show the expected Stern-Volmer behavior with decreasing lifetimes at increasing pressure, whereas nitrogen exhibits the opposite trend. Preliminary PLIF images of the sublimated naphthalene are acquired in a Mach 5 turbulent boundary layer. Relatively low signal-to-noise-ratio images were obtained at a stagnation temperature of 345 K, but much higher quality images were obtained at a stagnation temperature of 380 K. The initial results indicate that PLIF of sublimating naphthalene may be an effective tool for studying scalar transport in hypersonic flows.Aerospace Engineering and Engineering Mechanic

Performance and efficiency of an industrial direct diode source with an extremely low BPP in laser cutting of Fe-based and reflective alloys

The performance and efficiency of a 2kW industrial direct diode laser source with an extremely low BPP are investigated when carbon and stainless steels as well as aluminium and brass sheetsare laser cut. The results confirm the industrial feasibility and robustness of the direct diode laser source as tool for laser processes. In particular in the oxidation laser cutting of iron-based alloys the low BPP together with relative larger transport fiber diameter allows quality and cutting speed equivalentto the active fiber and disk laser sources in a very large range of thickness (up to 15 mm). When higher power densities are required, because inert laser fusion cutting of structural steel is carried out or because high reflective alloys need to be cut, the low BPP and the shorter wavelength are favorable figures and produce comparable performances with the mentioned laser sources

Laser Cutting of Copper and Brass Alloys by High Brilliance Diode Source with an Extremely Low BPP

Copper and copper-based alloys are difficult materials to laser cut due to their high thermal conductivity, high reflectivity and tendency to lose zinc on the zone adjacent to the cut. The study explores the potential of laser cutting process of copper and brass sheets with a TeraDiode TeraBlade direct diode source whose high brilliance is given by the very low BPP and by the small fiber diameter (100 micron). Results show that it is possible to obtain good quality cuts operating with nitrogen and oxygen assist gas in the case of brass and copper alloys respectively. Moreover larger process stability, higher productivity and easier-to-cut conditions are obtained cutting brass alloys than pure copper

Risky sexual behavior and substance use among adolescents: A meta-analysis

This study presents the results of a meta-analysis of the association between substance use and risky sexual behavior among adolescents. 87 studies fit the inclusion criteria, containing a total of 104 independent effect sizes that incorporated more than 120,000 participants. The overall effect size for the relationship between substance use and risky sexual behavior was in the small to moderate range (r = .22, CI = .18, .26). Further analyses indicated that the effect sizes did not substantially vary across the type of substance use, but did substantially vary across the type of risky sexual behavior being assessed. Specifically, mean effect sizes were smallest for studies examining unprotected sex (r = .15, CI = .10, .20), followed by studies examining number of sexual partners (r = .25, CI = .21, .30), those examining composite measures of risky sexual behavior (r = .38, CI = .27, .48), and those examining sex with an intravenous drug user (r = .53, CI = .45, .60). Furthermore, our results revealed that the relationship between drug use and risky sexual behavior is moderated by several variables, including sex, ethnicity, sexuality, age, sample type, and level of measurement. Implications and future directions are discussed