Event Details


Aerospace and Mechanical Engineering Seminar

Wed, Mar 21, 2018
3:30 PM - 4:30 PM
Location: SSL 150
Speaker: Peter Hagedorn, Professor, Mechanical Engineering, Technische Universit├Ąt Darmstadt, Germany

Talk Title: New Results on Self-Excitation in Circulatory and Parametrically Excited Systems

Abstract: In mechanical engineering systems, self-excited vibrations are in general unwanted and sometimes dangerous. There are many systems exhibiting self-excited vibrations which up to this day cannot be completely avoided, such as brake squeal, the galloping vibrations of overhead transmission lines, the ground resonance in helicopters and others. Most of these systems have in common that in the linearized equations of motion the self-excitation terms are given by non-conservative, circulatory forces and/or parametric excitation. The presentation will discuss some recent results in linear and nonlinear systems of this type.

Self-excited vibrations have of course been mathematically modelled and studied at least since the times of van der Pol. The van der Pol oscillator is a one degree of freedom system; its linearized equations of motion correspond to an oscillator with negative damping. Sometimes also other self-excited systems present negative damping, which can be made responsible for self-excited vibrations. In all the engineering systems mentioned above however, the self-excitation mechanism is mainly related to the interaction between different degrees of freedom (modes), and the linearized equations of motions contain circulatory terms. This together with parametric resonance is the main excitation mechanism discussed in this paper. Destabilization by 'negative damping' will not be considered. Also stick-slip phenomena are not in the focus of this presentation; they also do not seem to play an important role in all the examples given above.

The systems analyzed in this presentation therefore are characterized by the M, D, G, K, N matrices (mass, damping, gyroscopic, stiffness and circulatory matrices, respectively) which may all be time-dependent. In the unstable case, additional nonlinear terms do of course limit the vibration amplitudes. Different types of bifurcations relevant for these systems have recently been studied in the literature.

In the first part, MDGKN-systems with constant coefficients will be discussed. For a long time it has been well known, that the stability of such systems can be very sensitive to damping, and also to the symmetry properties of the mechanical structure. Recently, several new theorems were proved concerning the effect of damping on the stability and on the self-excited vibrations of the linearized systems. The importance of these results for practical mechanical engineering systems will be discussed. It turns out that the structure of the damping matrix is of utmost importance, and the common assumption, namely representing the damping matrix as a linear combination of the mass and the damping matrices, may give completely misleading results for the problem of instability and the onset of self-excited vibrations.

The second case considered deals with MDGKN-systems with time-periodic coefficients. The stability of these systems can be studied via Floquet theory. A typical property of parametric instability behavior is the existence of combination resonances. However, if parametric excitation in the system is simultaneously present in the K and the N matrices and/or there are excitation terms which are not all in phase, an atypical behavior may occur: The linear system may then for example be unstable for all frequencies of the parametric excitation, and not only in the neighborhood of certain discrete frequencies. Such atypical parametric instability happens even for M, D, G constant and zero mean values for the matrices K(t) and N(t). This was recently observed at the linearized equations of motion for a minimal model of a squealing disk brake. It turns out, that an even much simpler example of such a situation was given about 70 years ago by Lamberto Cesari, but seems to have fallen into oblivion. Until recently it was thought that such out of phase terms in the parametric excitation would not occur in engineering systems. In the presentation it is shown that they may indeed occur for example in the model of a squealing brake and probably in many other mechanical engineering systems, as long as there is slip with friction between solid bodies.

In the unstable case, additional nonlinear terms do of course limit the vibration amplitudes. Different types of bifurcations relevant for these systems are studied using normal form theory, in particular for the 'Cesari equations' with additional nonlinearities.

Host: Department of Aerospace and Mechanical Engineering