12,710 research outputs found

    Three-dimensional instability in flow over a backward-facing step

    Get PDF
    Results are reported from a three-dimensional computational stability analysis of flow over a backward-facing step with an expansion ratio (outlet to inlet height) of 2 at Reynolds numbers between 450 and 1050. The analysis shows that the first absolute linear instability of the steady two-dimensional flow is a steady three-dimensional bifurcation at a critical Reynolds number of 748. The critical eigenmode is localized to the primary separation bubble and has a flat roll structure with a spanwise wavelength of 6.9 step heights. The system is further shown to be absolutely stable to two-dimensional perturbations up to a Reynolds number of 1500. Stability spectra and visualizations of the global modes of the system are presented for representative Reynolds numbers

    "It's Not Too Aggressive": Key Features of Social Branding Anti-Tobacco Interventions for High-Risk Young Adults.

    Get PDF
    Purpose. Peer crowd-targeted campaigns are a novel approach to engage high-risk young adults in tobacco use prevention and cessation. We elicited the perspectives of young adult key informants to understand how and why two social branding interventions were effective: (1) "COMMUNE," designed for "Hipsters" as a movement of artists and musicians against Big Tobacco, and (2) "HAVOC," designed for "Partiers" as an exclusive, smoke-free clubbing experience. Design. Qualitative study (27 semistructured qualitative phone interviews). Setting. Intervention events held in bars in multiple U.S. cities. Participants: Twenty-seven key informants involved in COMMUNE or HAVOC as organizers (e.g., musicians, event coordinators) or event attendees. Measures. We conducted semistructured, in-depth interviews. Participants described intervention events and features that worked or did not work well. Analysis. We used an inductive-deductive approach to thematically code interview transcripts, integrating concepts from intervention design literature and emergent themes. Results: Participants emphasized the importance of fun, interactive, social environments that encouraged a sense of belonging. Anti-tobacco messaging was subtle and nonjudgmental and resonated with their interests, values, and aesthetics. Young adults who represented the intervention were admired and influential among peers, and intervention promotional materials encouraged brand recognition and social status. Conclusion. Anti-tobacco interventions for high-risk young adults should encourage fun experiences; resonate with their interests, values, and aesthetics; and use subtle, nonjudgmental messaging

    Inflation meets neutrinos

    Full text link
    Constraints on inflationary models typically assume only the standard models of cosmology and particle physics. By extending the neutrino sector to include a new interaction with a light scalar mediator (mϕm_{\phi}\simMeV), it is possible to relax these constraints, in particular via opening up regions of the parameter space of the spectral index nsn_s. These new interactions can be probed at IceCube via interactions of astrophysical neutrinos with the Cosmic Neutrino Background for nearly all of the relevant parameter space.Comment: 9 pages, 4 figures, comments welcom

    Spectroscopy of Close Companions to QSOs and the Ages of Interaction-Induced Starbursts

    Get PDF
    We present low-resolution absorption-line spectra of three candidate close ( < 3 arcsec) companions to the low redshift QSOs 3CR 323.1, PG 1700+518, and PKS 2135-147. The spectra were obtained with LRIS on the Keck telescopes and with the Faint Object Spectrograph on the University of Hawaii 2.2 m telescope. For 3CR 323.1 and PG 1700+518, we measure relative velocities that are consistent with an association between the QSOs and their companion galaxies. The spectral features of the companion galaxy to 3CR 323.1 indicate a stellar population of intermediate age (approx. 2.3 Gyr). In contrast, the spectrum of the companion object to PG 1700+518 shows strong Balmer absorption lines from a relatively young stellar population, along with the Mg Ib absorption feature and the 4000 A break from an older population. By modeling the two stellar components of this spectrum, it is possible to estimate the time that has elapsed since the end of the most recent major starburst event: we obtain approx. 0.1 Gyr. This event may have coincided with an interaction that triggered the QSO activity. Finally, our spectroscopy shows conclusively that the supposed companion to PKS 2135-147 is actually a projected Galactic G star.Comment: 10 pages, 5 Postscript figures. Latex (AASTEX). To appear in ApJ. Letters, Volume 480 (1997

    The collisional evolution of undifferentiated asteroids and the formation of chondritic meteoroids

    Full text link
    Most meteorites are fragments from recent collisions experienced in the asteroid belt. In such a hyper-velocity collision, the smaller collision partner is destroyed, whereas a crater on the asteroid is formed or it is entirely disrupted, too. The present size distribution of the asteroid belt suggests that an asteroid with 100 km radius is encountered 101410^{14} times during the lifetime of the Solar System by objects larger than 10 cm in radius; the formed craters cover the surface of the asteroid about 100 times. We present a Monte Carlo code that takes into account the statistical bombardment of individual infinitesimally small surface elements, the subsequent compaction of the underlying material, the formation of a crater and a regolith layer. For the entire asteroid, 10,000 individual surface elements are calculated. We compare the ejected material from the calculated craters with the shock stage of meteorites with low petrologic type and find that these most likely stem from smaller parent bodies that do not possess a significant regolith layer. For larger objects, which accrete a regolith layer, a prediction of the thickness depending on the largest visible crater can be made. Additionally, we compare the crater distribution of an object initially 100 km in radius with the shape model of the asteroid (21) Lutetia, assuming it to be initially formed spherical with a radius that is equal to its longest present ellipsoid length. Here, we find the shapes of both objects to show resemblance to each other.Comment: Accepted by Ap

    On finite-size effects in computer simulations using the Ewald potential

    Full text link
    We discuss the origin and relevance for computer simulations of a strong finite-size effect that appears when using the Ewald summation formula. It can be understood as arising from a volume-dependent shift of the potential in a finite, periodic box relative to the infinite volume limit. This shift is due to the fact that the ``zero of energy'' for a periodic system cannot be defined by letting the interacting particles be separated by an infinite distance; the correct definition corresponds to setting its \bbox k=\bbox 0 Fourier mode to zero. The implications of this effect for computer simulations are discussed.Comment: Submitted to Journal of Chemical Physic
    corecore