Journal of Aerospace Mechanics
Year:2008 | Volume:4 | Issue:1 (11)|Papers Count:8

This paper focuses on the development of a complete model of the Timoshenko beam, based on considering the effects of shear deformation. A method based on distributed-lumped modeling approach is proposed to solve the governing equations. Natural frequencies obtained by this method are compared and verified with Chebyshev pseudo spectral method. The effects of shear deformation on natural frequency of pinned and clamped beams are discussed for various diameters. In addition, the effects of lumped mass and its position are investigated for clamped and pinned rotors with different diameters. It is shown that, while the new method leads to highly accurate results, its simplicity and accuracy makes it appropriate for application on industrial systems.

In spite of numerous attractive features, the gas bearings have some disadvantages, primarily due to its relatively low lubricant viscosity. Gas films provide relatively low damping in comparison to the oil films, which makes the instability more severe in gas bearings. The purpose of the present work is to study the effects of mounting angle, as well as the size of the bearing, on the static and dynamic characteristics of three types of non-circular bearings. It is found that the effect of the mounting angle is more significant in two lobe bearings, in comparison with the three- and four-lobe ones. It is also noted that the size of the bearings has significant effect on their behavior.

Generally, constant cross section duct vortex generators are located upstream of the flow, causing increase in the overall diffuser length. Considering the application of these wind tunnel diffusers, this overall length increase lowers the performance of the diffusers. However, with increasing the diverging angle of subsonic diffusers and installing vortex generators, the diffuser length can be decreased. In this paper, experimental analysis of a diffuser with 14 degrees and with divergence installing vortex generator inside a low degree diffuser (instead of constant cross section ducts) is presented. Using this method, the diffuser length was decreased to a half and its performance was increased 20 to 45 percent, depending on Reynolds number. Vortex generators cause decrease in starting power (about 40 percent of consuming energy) too. Also, results show that increasing Reynolds numbers decreases the performance of the vortex generator.

This paper describes an investigation of combustion instability mechanism in premixed gas turbines. First, the preliminary concept of combustion instability is reviewed and then all types of frequency modes of combustion instability are studied. The results show that combustion instability is excited at first longitudinal mode; therefore the first mode or probably the second mode of longitudinal wave is the most important modes. Consequently, in this investigation only first longitudinal mode is considered. This simulation is based on equivalence ratio oscillations, which is an important mechanism for gas turbine combustion instability. The results are validated according to previons analytical and experimental results and show good agreements.

In this paper, a novel multi-dimensional characteristic based upwind scheme for incompressible viscous flows is presented. Unlike conventional characteristic based (CB) schemes, which use one-dimensional characteristic relations, the proposed scheme takes into account the physical multi-dimensional characteristic relations. For the first time, the multi-dimensional characteristic structure of incompressible flows, modified by artificial compressibility, is extracted. The present MCB (multi-dimensional characteristic based) scheme, in conjunction with the finite-volume discretization, is employed to model convective fluxes. This MCB scheme is applied to benchmark incompressible internal and external flows, namely lid driven cavity flow and two-dimensional flow past a circular cylinder. It was found that the proposed scheme presents more accurate results than the conventional CB scheme in both their first and second order counterparts. Using this inherent upwinding technique, no artificial viscosity is required, even at high Reynolds numbers. Another remarkable advantage of MCB scheme lies in its faster convergence rate with respect to CB scheme, which is found to exhibit substantial delays in convergence. Our results are in good agreements when compared to standard benchmark data.

Milling processes are becoming one of the common manufacturing methods in industry. One of the main goals in any manufacturing operation is to increase productivity. In high speed low immersion milling, the ratio of cutting edge engaging time to spindle period is a small value. For this kind of operations, two types of: Instability are proposed: classical Hopf and the Flip bifurcation, which are mainly due to impact effects of entering/exiting of the cutting edge into the cut. In order to increase productivity, it is essential to anticipate stable machining boundaries. In this research, the main goal is to derive and implement the improved TFEA method to include the effects of the helix angle variations for a 2-DOF vibratory system and accurately compute the stability lobe diagram. Results obtained by simulation witnesses that instability due to Flip bifurcation vanish completely from stability lobe diagram with increasing helix angle to 30°. Experimental tests are conducted in order to confirm the result obtained by the improved TFEA method showing good agreements.

This paper deals with experimental results of an axial compressor of a small power plant gas turbine engine. Tests were carried out during the engine operation (along operating line of the compressor). Time averaged axial and radial pressure distributions in each individual stage were measured at different rotational speeds. Acceleration and deceleration phases of the engine were divided into reasonable time intervals of constant rotational speeds. Consequently, data logging was performed during steady operation of the engine. Measured parameters included pressure and temperature distributions and air mass flow rate. Test results were used to calculate axial distribution of load factor along the compressor mean line. Experimental results showed that the span wise total pressure reduces from mean line region towards the hub and casing at each stage. No significant variations in load factor of each stage were observed during acceleration and deceleration phases of the engine. Meanline load factor distribution was increasing from compressor head towards its end within experimental rotational speed range.