Hydrodynamic Stability and Magnetic Reconnection in Disks and Stars

The purpose of this grant is to study parametric instability. The simplest example of parametric instability is a harmonic oscillator with a periodic modulation of the spring constant. If the modulation frequency is close to twice the natural frequency of the oscillator, the amplitude of oscillation tends to grow exponentially. The growth rate is proportional to the strength of the modulation, but it also depends upon the closeness to resonance of the two frequencies, and upon natural damping rate or "Q" of the oscillator. Parametric instabilities are very common in physics. A familiar example is a jogger's ponytail--normally a very strongly damped pendulum, it can be destabilized by the variation in effective gravity during the jogger's stride. Observation confirms that the period of the pendulum is half that of the jogger's vertical motion. In astrophysics, parametric instability may occur by external tidal forcing, or by interaction among eigenmodes. In the latter case, an energetic eigenmode may destabilize modes of half its frequency, provided some weak nonlinearity exists to couple them. Under a previous Astrophysical Theory grant (NAGW-2419), the PI discovered a parametric instability of tidally forced disks such as the accretion disks in cataclysmic variables and X ray binaries [2]. The destabilized modes are tightly-wound, incompressible, three-dimensional waves analogous to g-modes and r-modes in stars. Later work has confirmed our analysis [4]. It was hoped that these modes might provide a source of turbulence and angular momentum transport in accretion disks. However, a follow-up investigation of this instability by local numerical simulations, although confirming the analytically estimated growth rates, found negligible angular momentum flux [3]. Other work, partly supported by the ATP, now strongly indicates that the transport mechanism in such disks is magnetohydrodynamic turbulence [6]. Nevertheless, the parametric mechanism may truncate the outer edges of disks in close binaries [2], and it may be important in disks of very low ionization such as protostellar disks, or even cataclysmic-variable disks in quiescence where the MHD mechanism may be ineffective [5]. All analyses up to 1996 were done in a local approximation where the orbital frequency, shear rate, and tidal field were treated as constants. The locally computed growth rate turns out to depend strongly on radius, and it was unclear how to average these local rates to obtain the correct global rate. This is a critical issue for accretion disks in close binaries, because the local growth rate is comparable to the orbital frequency towards the outer edge of the disk but decreases rapidly inwards. Paper #1 examined this issue in a simplified global model where the destabilizing terms vary with position. We found that the global growth rate is essentially equal to the maximum local rate, provided that the latter is smoothed over a radial range equal to the distance that the destabilized wave propagates at its group speed in one growth time. Thus, in an accretion disk, waves would grow rapidly in the outer parts but would propagate both inwards and outwards at a maximum group speed of order the disk thickness divided by the orbital period

Contaminated Confessions Revisited

A second wave of false confessions is cresting. In the first twenty-one years of post-conviction DNA testing, 250 innocent people were exonerated, forty of which had falsely confessed. Those false confessions attracted sustained public attention from judges, law enforcement, policymakers, and the media. Those exonerations not only showed that false confessions can happen, but did more by shedding light on the problem of confession contamination, in which details of the crime are disclosed to suspects during the interrogation process. As a result, false confessions can appear deceptively rich, detailed, and accurate. In just the last five years, there has been a new surge in false confessions — a set of twenty-six more false confessions among DNA exonerations. All but two of these most recent confessions included crime scene details corroborated by crime scene information. Illustrating the power of contaminated false confessions, in nine of the cases, defendants were convicted despite DNA tests that excluded them at the time. As a result, this second wave of false confessions should cause even more alarm than the first. In the vast majority of cases there is no evidence to test using DNA. Unless a scientific framework is adopted to regulate interrogations, including by requiring recording of entire interrogations, overhauling interrogation methods, providing for judicial review of reliability at trial, and informing jurors with expert testimony, the insidious problems of confession contamination will persist

Barnes Hospital Bulletin

https://digitalcommons.wustl.edu/bjc_barnes_bulletin/1174/thumbnail.jp

Team Joseph\u27s Bike Trailer

This final design report shows the results of this senior project’s design process of creating a custom bike trailer for Team Joseph. As done in the Scope of Work, Preliminary Design report, and Critical Design report, current products, relevant technologies, and American Society for Testing and Materials (ASTM) standards are fully researched and benchmarked to aid in the design selection process. Customer requirements are looked at and developed into engineering specifications. A detailed design was created for CDR to show to Team Joseph, and manufacturing and testing plans were laid out. This final design report adds the final design revisions made to the trailer, details from the manufacturing process, and testing results

Easterner, Vol. 22, No. 3, October 13, 1971

This issue includes articles about Associated Students amendments and referendums, a series of lectures given in a hot air balloon, a debate over alcohol on campus, the search for a Black Studies director, the establishment of the Office of Institutional Research, the Listening Lab in the library that contains audio visual learning aids, and the football and cross country season.https://dc.ewu.edu/student_newspapers/1426/thumbnail.jp

Grand Valley Forum, volume 009, number 23, January 28, 1985

Grand Valley Forum is Grand Valley State\u27s faculty and staff newsletter, published from 1976 to the present

Vista: November 08, 1968

https://digital.sandiego.edu/vista/1361/thumbnail.jp