If Community College Students Are So Poor Why Do Only 16.9% Of Them Receive Pell Grants?

In this paper the authors attempt to address the discrepancy between the perception of income levels for community college students, and the seemingly low percentage of those students who receive Pell grants. The authors try to solve this paradox using data, published and unpublished, from the U. S. Department of Labor

Magnetic bearings for a high-performance optical disk buffer

An optical disk buffer concept can provide gigabit-per-second data rates and terabit capacity through the use of arrays of solid state lasers applied to a stack of erasable/reusable optical disks. The RCA optical disk buffer has evoked interest by NASA for space applications. The porous graphite air bearings in the rotary spindle as well as those used in the linear translation of the read/write head would be replaced by magnetic bearings or mechanical (ball or roller) bearings. Based upon past experience, roller or ball bearings for the translation stages are not feasible. Unsatisfactory, although limited experience exists with ball bearing spindles also. Magnetic bearings, however, appear ideally suited for both applications. The use of magnetic bearings is advantageous in the optical disk buffer because of the absence of physical contact between the rotating and stationary members. This frictionless operation leads to extended life and reduced drag. The manufacturing tolerances that are required to fabricate magnetic bearings would also be relaxed from those required for precision ball and gas bearings. Since magnetic bearings require no lubricant, they are inherently compatible with a space (vacuum) environment. Magnetic bearings also allow the dynamics of the rotor/bearing system to be altered through the use of active control. This provides the potential for reduced vibration, extended regions of stable operation, and more precise control of position

Daring to Lead 2006

Presents findings from a survey that examines why nonprofit executives leave their jobs. Offers recommendations for strengthening and supporting current leadership, creating more diversity, and developing the next generation of nonprofit leaders

Why RKKY exchange integrals are inappropriate to describe ferromagnetism in diluted magnetic semiconductors

We calculate Curie temperatures and study the stability of ferromagnetism in diluted magnetic materials, taking as a model for the exchange between magnetic impurities a damped Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and a shor t range term representing the effects of superexchange. To properly include effects of spin and thermal fluctuations as well as geometric disorder, we solve the effective Heisenberg Hamiltonian by means of a recently developed semi-analytical approach. This approach, ``self-consistent local Random Phase Approximation (SC-L RPA)'', is explained. We show that previous mean-field treatments, which have been widely used in the literature, largely overestimate both the Curie temperatures and the stability of ferromagnetism as a function of carrier density. The discr epancy when compared to the current approach was that effects of frustration in RKKY oscillations had been strongly underestimated by such simple mea n-field theories. We argue that the use, as is frequent, of a weakly-disordered RKKY exchange to model ferromagnetism in diluted III-V systems is inconsistent with the observation of ferromagnetism over a wide region of itinerant carrier densities. This may be puzzling when compared to the apparent success of calculations based on {\it ab-initio} estimates of the coupling; we propose a resolution to this issue by taking RKKY-like interactions between resonant states close to the Fermi level.Comment: Accepted for publication in Physical Review B. 22 pages, 7 figure

Simulating the aerodynamic performance and wake dynamics of a vertical-axis wind turbine

The accurate prediction of the aerodynamics and performance of vertical-axis wind turbines is essential if their design is to be improved but poses a signifi cant challenge to numerical simulation tools. The cyclic motion of the blades induces large variations in the angle of attack of the blades that can manifest as dynamic stall. In addition, predicting the interaction between the blades and the wake developed by the rotor requires a high-fi delity representation of the vortical structures within the fl ow fi eld in which the turbine operates. The aerodynamic performance and wake dynamics of a Darrieus-type vertical-axis wind turbine consisting of two straight blades is simulated using Brown’s Vorticity Transport Model. The predicted variation with azimuth of the normal and tangential force on the turbine blades compares well with experimental measurements. The interaction between the blades and the vortices that are shed and trailed in previous revolutions of the turbine is shown to have a signifi cant effect on the distribution of aerodynamic loading on the blades. Furthermore, it is suggested that the disagreement between experimental and numerical data that has been presented in previous studies arises because the blade–vortex interactions on the rotor were not modelled with sufficient fidelity

Biology of some British psocoptera, with particular reference to species frequenting foliage

Imperial Users onl

Magnetic bearings for a spaceflight optical disk recorder

The development and testing of a magnetic bearing system for the translator of the read/write head in a magneto-optic disk drive are discussed. The asymmetrical three-pole actuators with permanent magnet bias support the optical head, and its tracking and focusing servos, through their radial excursion above the disk. The specifications for the magnetic bearing are presented, along with the configuration of the magnetic hardware. Development of a five degree of freedom collision model is examined which allowed assessment of the system response during large scale transients. Experimental findings and the results of performance testing are presented, including the roll-off of current-to-force due to eddy current loss in the magnetic materials

Early Entry in the NBA Draft: The Influence of Unraveling, Human Capital and Option Value

In an influential article, “Unraveling in Matching Markets,” Li and Rosen (1998) note the first seven picks, and 17 among 29 first round selections of the 1997 NBA draft, were not college seniors. In 2004, the first pick in the NBA draft was a high school senior, and 25 of the first 29 picks were not college seniors. Li and Rosen (1998) suggest early entry is a form of unraveling in a labor market as firms attempt to secure the most promising player. We suggest recent NBA contract provisions implemented to slow the early entry of talented players have instead provided additional incentives to both players and firms for early entry into the NBA. In particular, the lowering of the fixed wage contract and lengthening of rookie contracts have given firms limited monopsonistic power and the ability to capture economic rents. We explore two competing models that predict why teams choose a talented player sooner under the new rookie contract system. The first model is the traditional human capital model, and the second is the Lazear (1995) option value model. To test why unraveling occurs, we use a panel study of all NBA players for 12 years from 1989 through 2002. The data include individual player performance statistics on a season-to-season basis, salary, and draft number.