Investigations are presented into the stability of vibration suppression employing variable-stiffness actuators. Systems with an arbitrary number of degrees of freedom are considered, and are subject to both parametric excitation and self-excitation. Analytical conditions of stability and instability are derived by applying a singular perturbation technique. These conditions enable a stability classification that naturally leads to the description parametric anti-resonance. The influence of the symmetry property of the parametric excitation matrix on the location of the parametric anti-resonance is discussed. Additionally, the influence of parametric resonance and anti-resonance on the eigenvalues corresponding to the slow motion of a generic system are analysed. A geometric interpretation is presented, enabling deeper insight into the mechanism of vibration suppression, and leading to the interpretation of coupling modes using parametric anti-resonance and amplification of system damping. The basic results obtained can be used for design of a control strategy for variable-stiffness actuators.

The overall efficiency of gas turbines is strongly affected by the performance of the sealing arrangement between the stationary and rotary components. Maintaining an accurate clearance during nominal and transient operation and providing effective damping is of key importance for seal designers. The implementation of brush seals as an alternative to labyrinth seals in a gas turbine engine is outlined in this article. A review of published literature is performed to analyze the leakage performance of brush seals and labyrinth seals. The emphasis lies on the importance of an accurate clearance control and the subsequent reduction in parasitic flows. Based on a discussion of the performance and integrity of brush seals in gas turbine applications their advantages over labyrinth seals are highlighted. Some attention is drawn into common drawbacks of brush seals and how these were improved over time to become more competitive and reliable. Finally, advanced concepts, technological aspects of the design of the brush seals influencing their leakage characteristics and future applications prospective are discussed. It was shown that one of the key advantages of brush seals over conventional labyrinth seals is their positive rotordynamic characteristic. The rotordynamic force coefficients demonstrate very low and generally negative cross-coupled stiffness thereby positively affecting the turbomachine operability in industrial gas turbine applications. The review concluded the need for further work on studying rotordynamic behaviour of brush seals operating at conditions more realistic to real engine operation environment in order to complete the understanding of the seal dynamics in real operation condition.