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Preservation properties of stochastic orders by transformation to Harris family

Abbasi S., Alamatsaz M. H.

Probability and Mathematical Statistics

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2018

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Vol. 38, Fasc. 2

441--458

EN

Stochastic comparisons of lifetime characteristics of reliability systems and their components are of common use in lifetime analysis. In this paper, using Harris family distributions, we compare lifetimes of two series systems with random number of components, with respect to several types of stochastic orders. Our results happen to enfold several previous findings in this connection. We shall also show that several stochastic orders and ageing characteristics, such as IHRA, DHRA, NBU, and NWU, are inherited by transformation to Harris family. Finally, some refinements are made concerning related existing results in the literature.

2

Investigation of inlet distortion effects on axial compressor performance based on streamline curvature method

Abbasi S., Pirnia A., Taghavi-Zenouz R.

Journal of Theoretical and Applied Mechanics

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2018

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Vol. 56 nr 4

1005--1015

EN

In this paper, the effects of inlet flow distortion on performance maps and details of the flow field are investigated using the Streamline Curvature Method. The results are presented for both design and off-design conditions and compared with experimental data, which shows good agreement. The effects of inlet flow distortion are investigated by inlet total pressure variation in three different cases in the way that the average total pressure remains constant and equal to the design value. The results show that a relative increase in the total pressure at tip causes an increase in the pressure ratio and efficiency as well as a better performance in the choking region. Alternatively, a relative increase in the total pressure in hub causes opposite behavior and a better performance at the surging region.

3

Multiple spike stall cells in low speed axial compressor rotor blade row

Taghavi-Zenouz R, Abbasi S.

Journal of Theoretical and Applied Mechanics

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2015

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Vol. 53 nr 1

47--57

EN

Inception and development of multiple stall cells of short length scales are numerically investigated in an axial compressor rotor blade row. The method of investigation is based on time accurate three-dimensional full annulus simulations. Time dependent flow structure results revealed that there are two criteria responsible for inception of a special kind of stall, introduced as spike stall in the literature. These criteria are defined as leading edge spillage and trailing edge backflow, which occur at specific mass flow rates near to stall conditions. The numerical results revealed that once the spike stall cells appear, they cover roughly two blade passages in the circumferential direction and cover about 25% of the blade height. By further revolution of the blade row, the number of cells tends to increase. For the present case study, the number of stall cells increased to three after 8.5 rotor revolutions from the moment of the initial spike stall occurrence. Even at this moment, both of the above mentioned criteria for the spike stall inception have been observed within the blades passages. These events caused the inlet relative flow angle to the blade rows, and therefore the flow incidence angle and consequent blockage to the main flow, to increase. The tip leakage flow frequency spectrum has been studied through surveying instantaneous static pressure signals imposed on pressure side of the blades and also on the casing walls. These latter results showed that any further revolving of the rotor blade row, exceeding 8.5 revolutions, causes the spike stall to disturb the flow structure significantly.

4

A semi-analytical solution on static analysis of circular plate exposed to non-uniform axisymmetric transverse loading resting on Winkler elastic foundation

Abbasi S., Farhatnia F., Jazi S. R.

Archives of Civil and Mechanical Engineering

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2014

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Vol. 14, no. 3

476--488

EN

This paper is concerned with static analysis of functionally graded (FG) circular plates resting on Winkler elastic foundation. The material properties vary across the thickness direction so the power-law distribution is used to describe the constituent components. The differential transforms method (DTM) is utilized to solve the governing differential equations of bending of the thin circular plate under various boundary conditions. By employing this solution method, governing differential equations are transformed into recurrence relations and boundary/regularity conditions are changed into algebraic equations. In this study, the plate is subjected to uniform/non-uniform transverse load in two cases of boundary conditions (clamped and simply-supported). Some numerical examples are presented to show the influence of functionally graded variation, different elastic foundation modulus, and variation of the symmetrical transverse loads on the stress and displacement fields. Based on the results, the obtained out-plane displacement coincide with the available solution for a homogenous circular plate. It can be concluded that the applied method provides accurate results and it is easily used for static analysis of circular plates on an elastic foundation.