A new model for the structure of the DACs and SACs regions in the Oe and Be stellar atmospheres

In this paper we present a new mathematical model for the density regions where a specific spectral line and its SACs/DACs are created in the Oe and Be stellar atmospheres. In the calculations of final spectral line function we consider that the main reasons of the line broadening are the rotation of the density regions creating the spectral line and its DACs/SACs, as well as the random motions of the ions. This line function is able to reproduce the spectral feature and it enables us to calculate some important physical parameters, such as the rotational, the radial and the random velocities, the Full Width at Half Maximum, the Gaussian deviation, the optical depth, the column density and the absorbed or emitted energy. Additionally, we can calculate the percentage of the contribution of the rotational velocity and the ions' random motions of the DACs/SACs regions to the line broadening. Finally, we present two tests and three short applications of the proposed model.Comment: 9 pages, 5 figures, accepted for publication in PAS

Prediction and analysis of impact of thermal barrier coating oxidation on gas turbine creep life

Thermal barrier coatings (TBCs) have been widely used in the power generation industry to protect turbine blades from damage in hostile operating environment. This allows either a high turbine entry temperature (TET) to be employed or a low percentage of cooling air to be used, both of which will improve the performance and efficiency of gas turbine engines. However, with continuous increases in TET aimed at improving the performance and efficiency of gas turbines, TBCs have become more susceptible to oxidation. Such oxidation has been largely responsible for the premature failure of most TBCs. Nevertheless, existing creep life prediction models that give adequate considerations to the effects of TBC oxidation on creep life are rare. The implication is that the creep life of gas turbines may be estimated more accurately if TBC oxidation is considered. In this paper, a performance-based integrated creep life model has been introduced with the capability of assessing the impact of TBC oxidation on the creep life and performance of gas turbines. The model comprises of a thermal, stress, oxidation, performance, and life estimation models. High pressure turbine (HPT) blades are selected as the life limiting component of gas turbines. Therefore, the integrated model was employed to investigate the effect of several operating conditions on the HPT blades of a model gas turbine engine using a creep factor (CF) approach. The results show that different operating conditions can significantly affect the oxidation rates of TBCs which in turn affect the creep life of HPT blades. For instance, TBC oxidation can speed up the overall life usage of a gas turbine engine from 4.22% to 6.35% within a one-year operation. It is the objective of this research that the developed method may assist gas turbine users in selecting the best mission profile that will minimize maintenance and operating costs while giving the best engine availability

Closed-form Approximations and Series Representations for Partially Saturated Ocean Acoustic Processes

An approximate, closed‐form expression for the value of the integral encountered in the calculation of the probability density function (PDF) of the envelope of a partially saturated ocean acoustic process is obtained. Furthermore, an expression of this PDF as a series of modified Bessel functions is presented. The results may also be directly applied to the evaluation of the PDF encountered in the structural reliability analysis of rotating machinery components. Numerical applications show that the closed‐form expression is always within 1%2% of the exact result. The required computational effort is substantially lower than that required by direct numerical integration.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86232/1/Perakis14.pd

Passive Control of Bridges

In the design of bridges with large spans, the significant values of the moments at the main deck-structure require very heavy members, either in the case of a beam or a truss deck-structure. In order to minimize the influence of bending moments, several applications of passive control of displacements using cable nets are herein proposed. The base for all the proposed systems is the cables supported beam nets with additional prestressing control on support cables to optimize the structural behavior of the system. The passive control design problem leads to an optimal control problem for structures governed by variational inequalities. In this presentation several bridge systems are proposed and studied as applications of this method

The complex structure of the Mg II {\lambda\lambda} 2795.523, 2802.698 {\AA} regions of 64 Be stars

Here is studied the presence of absorption components shifted to the violet or the red side of the main spectral line (satellite, or discrete absorption components, i.e. SACs or DACs), in Mg II resonance lines' regions in Be stars and their kinematical characteristics. Namely our objective is to check if exists a common physical structure for the atmospheric regions creating SACs or DACs of the Mg II resonance lines. In order to do this, a statistical study of the Mg II {\lambda \lambda} 2792.523, 2802.698 {\AA} lines in the spectra of 64 Be stars of all spectral subtypes and luminosity classes is performed. We found that the absorption atmospherical regions where the Mg II resonance lines originated may be formed of several independent density layers of matter which rotate with different velocities. It is attempted also to separate SACs and DACs according to low or high radial velocity. The emission lines were detected only in the earliest and latest spectral subtypes.Comment: 18 pages, 12 figures, accepted for publication in PAS