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Published Articles

The Volume 7, No 1, March 2002

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The Influence of Rotor Detuning on Rotor-Stator Interaction Generated Acoustic Response

Scott Sawyer, John M. Feiereisen and Sanford Fleeter


The stator row of a turbomachine is excited by the upstream rotor and responds at multiples of rotor blade pass frequency, resulting in a discrete blade pass frequency tone. Rotor detuning, i.e., unequal circumferential rotor blade spacing, decreases this fundamental tone by breaking the fundamental periodicity of blade passing. However, the stator row responds at additional multiples of rotor shaft pass frequency. This paper is directed at investigating the influence of rotor detuning on discrete frequency noise generation. Complete data were acquired including acoustic mode and stator vane unsteady surface pressure data. Microphones in a circumferential array upstream of the rotor were sampled simultaneously over one rotor revolution, and an ensemble-averaged data set acquired. A dual transform was then utilised to determine the sound pressure amplitude as a function of frequency and spatial mode. The stator acoustic response was also calculated from unsteady stator surface pressure measurements, and the response spectrum was compared with the microphone array data. Rotor detuning is shown to have a significant effect on discrete frequency noise, with an overall reduction in the total sound intensity level for a range of rotor speeds.

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Moderately Large Vibrations of Imperfect Elastic-Plastic Composite Beams with Thick Layers

Christoph Adam


Flexural vibrations of elastic-plastic composite beams with three thick layers are analysed. Geometric non-linear effects arising from longitudinally immovable supports are taken into account. The layers are perfectly bonded and their arbitrary thickness and material properties are symmetrically distributed about the central beam axis. Layerwise continuous and linear in-plane displacement fields are implemented, and as such, model both the global and the local elastic-plastic response of laminate beams. By means of proper definition of an effective cross-sectional rotation, the complex problem reduces to the simpler case of a homogenised shear-deformable beam on fixed supports with effective stiffness and proper boundary conditions. Resultants of plastic strains are determined and the response of the linear elastic and homogenised beam due to the action of these resultants is determined. Geometric non-linear terms of the differential equation of motion are treated as lateral forces acting on the linear elastic and homogenised shear-deformable beam. Since the response of the homogenised beam is considered to be linear within a given time step, efficient solution methods of the linear theory of flexural vibra- tions become applicable.

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The Effect of a Pressurised Mid-Span Gas Seal on Fluid-Induced Rotor Instability

John J. Yu, Paul Goldman, Donald E. Bently and John S. Jacob


Fluid-induced instability of a rotor-bearing system occurs frequently in rotating machinery. This involves two types of instability, i.e., fluid whirl and whip. Whirl is a predominately rigid-body mode and tracks at around one-half shaft running speed. Its threshold speed can be arbitrarily raised by using externally-pressurised fluid bearings, which allow for adjustable pressure for high stiffness. Whip is mainly a rotor-bending mode and locks into the rotor natural frequency. It appears when increases in bearing stiffness no longer influence the instability threshold. As demonstrated both analytically and experimentally in this paper, however, use of an externally- pressurised gas seal in the mid-span of the rotor can successfully eliminate whip. Its threshold speed can be greatly raised beyond running speed by adding the mid-span seal, to nearby the antinode of the rotor-bending mode. An experimental rig was operated at speeds up to 10,000 rpm, far above the rotor natural frequency of less than 2000 rpm without the seal. The gas supply pressure could be adjusted to yield the stiffness required for eliminating whip, which would otherwise appear even with an externally pressurised fluid bearing at the shaft end. This approach encourages utilisation of existing process gas as the supporting medium to eliminate fluid- induced instability.

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A Symbolic Manipulation Approach for Modelling and Performance Analysis of Flexible Manipulator Systems

Z. Mohamed and M.O. Tokhi


This paper presents a symbolic manipulation approach for modelling and performance analysis of a flexible ma- nipulator system using finite element methods. A constrained planar single-link flexible manipulator is consid- ered. A symbolic algorithm characterising the dynamic behaviour of the system is developed using a symbolic language. Using this approach, the system transfer functions are obtained in symbolic forms. Analyses and in- vestigations in terms of system stability, time response to an input command and vibration frequencies are pre- sented. Numeric and experimental results are presented for validation and assessment of the symbolic model.

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Wavelet Analysis of Longitudinal Impact Responses

Peter Eberhard, Bin Hu and Zhuo Li


The dynamics of a mechanical system during impact and after impact has very different characteristics since im- pact responses are transient and have very high frequency components. For the numerical simulation of impact responses, the discretisation of elastic bodies plays an important role in modal analysis and in the finite element method. In this paper, we use wavelet analysis as a tool to investigate the influence of the discretisation on im- pact responses. As an example, a longitudinal impact system of a rigid body striking an elastic rod is used. We discuss how well the impact response of this continuous impact system is approximated using a discrete system with finite degrees of freedom based on the modal approach from a wavelet analysis point of view. Numerical simulation results are compared with experimental results. Furthermore, in mechanical systems weak collisions may appear. They are difficult to detect experimentally since motion measurements are often available only par- tially and measured signals are always disturbed by measurement noise. It is shown that wavelet analysis is a powerful tool to detect weak collisions even from noisy signals.

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Pressure Fluctuations on the Impeller Blades of a Centrifugal Turbomachine: a Comparative Analysis Between Air and Water Tests

A. Talha, J.P. Barrand and G. Caignaert


In this paper a comparison between pressure fluctuations measurements in the time domain is proposed. These measurements were carried out on two centrifugal turbomachines, a fan with air and a pump with water. In order to gather a better understanding of their internal acoustics, time records of the pressure fluctuations were made on the blades in the outlet of the impellers and near the volute tongue. These showed that the flow is very un- steady at the impeller outlet and in the tongue region. An analysis of the pressure fluctuations and their propaga- tion showed that the main noise sources are caused by the blade-tongue interactions and depend on the fluid type. In the case of the pump, the hydraulic nature of the noise source is dominant. Using a one-dimensional propagation model, it is possible to explain some differences in the behaviour of the turbomachines which is re- lated in particular to the different sound speeds.

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Theoretical and Experimental Studies of Tubular Valve Dynamics

Minvydas Ragulskis and Arvydas Palevicius


Construction of a novel vibratory valve and its design optimisation is presented in this paper. The principle of the system?s operation is based on the effect of the dynamic positioning of a steel ball in a vibrating tube. A theoretical analysis of the stability of this non-linear system together with an experimental study of the operating valve forms the basis of this study. Laser holographic interferometry was used for the identification and optimi- sation of the regimes of operation of the system.

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