Glass formation, properties, and structure of soda-yttria-silicate glasses

The glass formation region of the soda yttria silicate system was determined. The glasses within this region were measured to have a density of 2.4 to 3.1 g/cu cm, a refractive index of 1.50 to 1.60, a coefficient of thermal expansion of 7 x 10(exp -6)/C, softening temperatures between 500 and 780 C, and Vickers hardness values of 3.7 to 5.8 GPa. Aqueous chemical durability measurements were made on select glass compositions while infrared transmission spectra were used to study the glass structure and its effect on glass properties. A compositional region was identified which exhibited high thermal expansion, high softening temperatures, and good chemical durability

Numerical and Physical Modeling of Turbulent Shear Flows

This dissertation is an evaluation of popular turbulence schemes; both three dimensional and depth-averaged, and also includes an experimental study on shallow near bed jets. The three dimensional and RNG turbulent closure schemes are evaluated for free and bounded shear flows. For free shear flows (circular and plane turbulent jets), the scheme with standard coefficient performs equally well and in some cases better than the renormalized group scheme in predicting growth rate, decay of centerline velocity and longitudinal velocity profiles. For turbulent kinetic energy across the jet, the inner region is better predicted by the RNG scheme. The second case used to evaluate the three dimensional schemes was a submerged hydraulic jump. This flow included a free surface and solid boundary creating larger shearing forces than in a free jet. The results showed the longitudinal velocity profiles and their maximum values, in vertical direction, were estimated better by the RNG scheme. The turbulent kinetic energy was overestimated in both magnitude and elevation of its maximum position in the flow. The elevation of the recirculation region was also over predicted by both schemes; however, its longitudinal extent was predicted well. A two-dimensional, depth-averaged flow model with the depth-averaged parabolic eddy viscosity, mixing length, and turbulent closure schemes was used to simulate flow patterns downstream of lock and dam structures. The mixing length scheme was modified and performed as well as the scheme in predicting the location and size of the recirculation zones, as well as the velocity profiles across the channel. Experimental measurements on shallow near bed jets are performed. For low submergence, the horizontal growth rates have two distinct regions, with the downstream region having a higher growth rate. The longitudinal velocity profiles in the horizontal plane are self-similar. The centerline decay was slower than that of a free jet

The Impact of Multidimensionality on the Detection of Differential Bundle Functioning Using SIBTEST.

In response to public concern over fairness in testing, conducting a differential item functioning (DIF) analysis is now standard practice for many large-scale testing programs (e.g., Scholastic Aptitude Test, intelligence tests, licensing exams). As highlighted by the Standards for Educational and Psychological Testing manual, the legal and ethical need to avoid bias when measuring examinee abilities is essential to fair testing practices (AERA-APA-NCME, 1999). Likewise, the development of statistical and substantive methods of investigating DIF is crucial to the goal of designing fair and valid educational and psychological tests. Douglas, Roussos and Stout (1996) introduced the concept of item bundle DIF and the implications of differential bundle functioning (DBF) for identifying the underlying causes of DIF. Since then, several studies have demonstrated DIF/DBF analyses within the framework of “unintended” multidimensionality (Oshima & Miller, 1992; Russell, 2005). Russell (2005), in particular, examined the effect of secondary traits on DBF/DTF detection. Like Russell, this study created item bundles by including multidimensional items on a simulated test designed in theory to be unidimensional. Simulating reference group members to have a higher mean ability than the focal group on the nuisance secondary dimension, resulted in DIF for each of the multidimensional items, that when examined together produced differential bundle functioning. The purpose of this Monte Carlo simulation study was to assess the Type I error and power performance of SIBTEST (Simultaneous Item Bias Test; Shealy & Stout, 1993a) for DBF analysis under various conditions with simulated data. The variables of interest included sample size and ratios of reference to focal group sample sizes, correlation between primary and secondary dimensions, magnitude of DIF/DBF, and angular item direction. Results showed SIBTEST to be quite powerful in detecting DBF and controlling Type I error for almost all of the simulated conditions. Specifically, power rates were .80 or above for 84% of all conditions and the average Type I error rate was approximately .05. Furthermore, the combined effect of the studied variables on SIBTEST power and Type I error rates provided much needed information to guide further use of SIBTEST for identifying potential sources of differential item/bundle functioning