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is known that small differences between nominally identical turbomachine blades, known as mistuning, can lead to significant variation in their vibration response levels. A commonly used term in mistuning studies called the "amplification factor" is clearly defined in this paper, and the high .sensitivity of high-cycle-fatigue-related fatigue life to the level of vibration response levels is presented. Computer simulations are run to study the di.stribution of the amplification factor in three situations, namely, (i) bladed disks with damping mistuning, (ii) EO excitation of bladed disk modes in the veering region, and (Hi) apparently tuned bladed disks. In addition to running simulations, the upper bound of the adjusted amplification factor in damping-mistuned bladed disks is derived theoreticaily. BackgroundBlade vibration has been a major problem in gas turbines since the early days of their development [1]. One of the contributing factors is that small differences between nominally identical blades, known as mistuning, can lead to a huge variation of blade vibration responses. This leads to some blades failing much earlier than their predicted lives due to vibration-induced high-cycle fatigue (HCF). This issue, called the blade mistuning problem, has been investigated in numerous analyses [2], The first half of the present paper attempts to refine a commonly used norm of measuring vibration response increase due to mistuning, and to relate the increased vibration response levels to the shortened fatigue lives of blades.The dependence of the variation of vibration responses on design parameters and the permissible level of mistuning in a typical scenario has been investigated [3] by (i) assuming the level of damping is equal in every blade, (ii) characterizing blade responses by the response of one blade-alone mode shape, and (iii) using the blade-alone frequency to measure the degree of mistune. In this paper, these assumptions are removed one at a time to study (i) bladed disks with blades having unequal levels of damping (known as damping-mistuned bladed disks), (ii) excitation of bladed disks in the veering region, and (iii) apparently tuned bladed disks.

Broadband noise with profound low-frequency profile is prevalent and difficult to be controlled mechanically. This study demonstrates effective broadband sound absorption by reducing the mechanical reactance of a loudspeaker using a shunt circuit through electro-mechanical coupling, which induces reactance with different signs from that of loudspeaker. An RLC shunt circuit is connected to the moving coil to provide an electrically induced mechanical impedance which counters the cavity stiffness at low frequencies and reduces the system inertia above the resonance frequency. A sound absorption coefficient well above 0.5 is demonstrated across frequencies between 150 and 1200 Hz. The performance of the proposed device is superior to existing passive absorbers of the same depth (60 mm), which has lower frequency limits of around 300 Hz. A passive noise absorber is further proposed by paralleling a micro-perforated panel with shunted loudspeaker which shows potentials in absorbing band-limit impulse noise.

Integral bladed disks (also known as blisks) are more widely used in modern aeroengine compressor designs because of the potential weight savings, but there are challenges in controlling the extreme vibration response levels in mistuned blisks, which are blisks with blades slightly different from each other. As blisks lack the uncertainty and variability of friction properties related to joints, the maximum vibration response level of a blisk test piece in operation can be predicted prior to installation. A previously proposed response-level prediction procedure for mistuned blisks is outlined, and its robustness is studied. A method of improving the results, given noisy experimental data, is proposed. Some of the issues discussed are validated by using experimental data.

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