A robust equal-peak method for uncertain mechanical systems

The linear vibration absorber is a widely-used vibration mitigation device. However, when the absorber is tuned according to Den Hartog's equal-peak method, the resulting narrow bandwidth may decrease its effectiveness, especially when the host structure is uncertain or in the presence of environmental variability. In this paper, a new tuning strategy of the linear absorber, based on the concept of robust equal peaks, is introduced for mitigating a specific resonance of an uncertain mechanical system. Both analytical and numerical investigations are carried out to demonstrate the robustness of the proposed absorber. For 20% uncertainty in the stiffness of the host system, the performance improvement brought by the robust equal-peak method amounts to more than 30% with respect to Den Hartog's tuning rule. © 2017 Elsevier Lt

Frictional characteristics of geosynthetic-geosynthetic and geosynthetic-soil interfaces determined by the inclined plane apparatus

The problem of managing the unneeded and unusable materials is a serious challenge for modern societies. The progress of civilization is accompanied by an increase of the human population. This results in a constant increase of waste production. An average EU citizen generates over 500 kg of municipal waste per year, of which less than one third is recycled. The rest is either disposed of in the landfills or incinerated. Every landfill needs to be properly protected by an impermeable barrier – a liner. Its use is necessary to prevent soil and ground water contamination. The protection barrier needs to be implemented both at the bottom and at the top of a landfill. Various geosynthetic materials are utilized for a liner design. Waste disposed of in a landfill creates high embankments with steep slopes. In these conditions, the problem of liner stability arises. The tests, the results of which are presented in this paper, were conducted to analyze the behaviour of several different combinations of geosynthetic materials used for liner design and to calculate the friction angle of the interface between them. A set of combinations, crucial for the stability of a landfill capping system was tested: geomembrane–geospacer, geospacer–geotextile and geotextile–soil. All of the tests were conducted at the LTHE Laboratory at Joseph Fourier University in Grenoble, France

Dynamic vibration absorbers for ground resonance control in hinged rotor helicopters

International audienceThis paper provides a numerical analysis on the influence of dynamic vibration absorbers (DVA) on the ground resonance phenomenon in an undamped articulated rotor helicopter. A torsional lead/lag absorber to be installed on the blade lag hinges is proposed. A simplified linearized four bladed helicopter model taking into account only the planar motion of the fuselage and the lead/lag motion of the blades is considered for the simulations. The instability zones for this model are known beforehand. The use of Coleman variables allows an eigenvalue analysis to be performed in the full system (helicopter + DVAs) within a typical range of rotor speed. By verifying the influence of each DVA parameter, namely, the inertia ratio, the natural frequency and the damping coefficient, on the stabilization, an optimality condition can be met

Post critical analysis of helicopter ground resonance

International audienceIn helicopter with hinged blades, an unstable dynamic phenomenon known as ground resonance may occur during take-off and landing phase leading to the possible destruction of the aircraft. The prediction of the stability boundary has been addressed for various helicopter configuration and by using different methods. However, in order to study the behavior of the system when the rotor speed is in the range of instability, an analysis of the post-critical behavior is needed. To do this, the behavior of a helicopter with isotropic blades and fuselage properties is investigated. The nonlinear terms due to large blades rotation are kept up to cubic order and a coordinate array transformation is used to simplify the equation of motion. The resulting nonlinear system is analyzed using the method of multiple scales and the normal form of the bifurcation is obtained. The results are compared and validated with numerical simulations

The Effects of Fuselage Stiffness Asymmetry in the Post-critical Dynamic Behavior of Helicopters

International audienceThe present paper contributes to a better understanding about the ground resonance phenomenon in helicopters. A detailed study is presented on the post-critical dynamic analysis of the aircraft behavior at the unstable region. In addition to periodic solutions, the dynamic system may exhibits quasi-periodic and chaotic response depending on the fuselage asymmetry level

The Effects of Fuselage Stiffness Asymmetry in the Post-critical Dynamic Behavior of Helicopters

International audienceThe present paper contributes to a better understanding about the ground resonance phenomenon in helicopters. A detailed study is presented on the post-critical dynamic analysis of the aircraft behavior at the unstable region. In addition to periodic solutions, the dynamic system may exhibits quasi-periodic and chaotic response depending on the fuselage asymmetry level

Post critical analysis of helicopter ground resonance

International audienceIn helicopter with hinged blades, an unstable dynamic phenomenon known as ground resonance may occur during take-off and landing phase leading to the possible destruction of the aircraft. The prediction of the stability boundary has been addressed for various helicopter configuration and by using different methods. However, in order to study the behavior of the system when the rotor speed is in the range of instability, an analysis of the post-critical behavior is needed. To do this, the behavior of a helicopter with isotropic blades and fuselage properties is investigated. The nonlinear terms due to large blades rotation are kept up to cubic order and a coordinate array transformation is used to simplify the equation of motion. The resulting nonlinear system is analyzed using the method of multiple scales and the normal form of the bifurcation is obtained. The results are compared and validated with numerical simulations

Post critical analysis of helicopter ground resonance

International audienceIn helicopter with hinged blades, an unstable dynamic phenomenon known as ground resonance may occur during take-off and landing phase leading to the possible destruction of the aircraft. The prediction of the stability boundary has been addressed for various helicopter configuration and by using different methods. However, in order to study the behavior of the system when the rotor speed is in the range of instability, an analysis of the post-critical behavior is needed. To do this, the behavior of a helicopter with isotropic blades and fuselage properties is investigated. The nonlinear terms due to large blades rotation are kept up to cubic order and a coordinate array transformation is used to simplify the equation of motion. The resulting nonlinear system is analyzed using the method of multiple scales and the normal form of the bifurcation is obtained. The results are compared and validated with numerical simulations