Sharif Digital Repository

This paper discusses the effects of substrate motions on the performance of microgyroscopes modeled as ring structures. Using the Extended Hamiltonian Principle, the equations of motion of a ring micro-gyroscope are derived, and the natural frequency equation and response characteristics are extracted in closed form for the case where the substrate undergoes normal rotation. The Galerkin approximation is then used to arrive at the ordinary differential equations of motion for the ring. In these equations, the effects of angular, centripetal and Coriolis accelerations are all apparent. The response of the system to different inputs is studied and the system sensitivity to variation in input... 

Rings are widely used in mechanical equipment, and their fitness may undergo some damage under severe vibration. In these structures, functionally graded rings can be used to optimize the resistance, energy consumption, and fitness. Due to their complexity, the finite-element analysis may be implemented using special elements. Enhancement of accuracy and minimization of time consumption play an important role in the analysis of these rings. In this study, a new cylindrical superelement for the FGM rings is designed and implemented to facilitate the vibration analysis of the rings. The power-law distribution is used for the modeling of the FGM rings in the thickness direction. Natural... 

Keratoconus (KC) is a progressive corneal disease. It is mostly characterized by corneal thinning and irregular steepening as well as formation of cone-shaped regions on the cornea. Amongst treatment modalities, implantation of Intrastromal Corneal Ring Segments (ICRSs) could decrease corneal surface irregularities. However, controversy exists over the selection of proper ICRSs. Here, the aim was to compare the effects of two sets of ICRSs. The ICRSs were identical both in cross-section and total arc length. The results showed that under normal physiological range of intraocular pressures (IOPs), the two selected ICRSs caused slightly different deformations in the same in-silico keratoconic... 

It is well-known that helicopters descending fast may enter the so-called Vortex Ring State (VRS), a region in the velocity space where the blade's lift differs significantly from regular regions and high amplitude fluctuations are often present. These fluctuations may lead to instability and, therefore, this region is avoided, typically by increasing the horizontal speed. This paper researches this phenomenon in the context of small-scale quadcopters. The region corresponding to the VRS is identified by combining first-principles modeling and wind-tunnel experiments. Moreover, we propose that the so-called Windmill-Brake State (WBS) or autorotation region should also be avoided for... 

Lithium perchlorate catalyzed the ring opening of epoxides with amines to provide the corresponding β-aminoalcohols in excellent yields with high regioselectivity. The reaction proceeds rapidly under mild and neutral conditions and worked well with primary, secondary, aliphatic, aromatic, and hindered amines in short times at room temperature, in the absence of solvent. © 2005 NRC Canada