Luigi Carassale

Wind modes for structural dynamics: a continuous approach

Load on structural systems is often represented by a multi-dimensional and/or multi-variate random process. The cross-correlation often existing between loading components acting in different points of the structure introduces conceptual and computational difficulties in many practical problems. It is the case, for example, of the projection of the external load on the vibration modes in the modal analysis of linear systems or of the simulation of multi-correlated time series for a Monte Carlo-based analysis of non-linear structures. The use of the proper orthogonal decomposition (POD) introduces some formal simplifications in the solution of the aforementioned problems, but requires the evaluation of the eigenquantities of some statistical representations of the loading process. The knowledge of such quantities in analytic form yields computational advantages and enables important physical interpretations. In the present paper, an analytic expression of POD is developed for a class of processes, which includes models usually adopted to represent the atmospheric turbulence. Examples of linear analysis of a wind-excited slender structure and of simulation of turbulence fields are presented.

Modeling Nonlinear Systems by Volterra Series

The Volterra-series expansion is widely employed to represent the input-output relationship of nonlinear dynamical systems. This representation is based on the Volterra frequency-response functions (VFRFs), which can either be estimated from observed data or through a nonlinear governing equation, when the Volterra series is used to approximate an analytical model. In the latter case, the VFRFs are usually evaluated by the so-called harmonic probing method. This operation is quite straightforward for simple systems but may reach a level of such complexity, especially when dealing with high-order nonlinear systems or calculating high-order VFRFs, that it may loose its attractiveness. An alternative technique for the evaluation of VFRFs is presented here with the goal of simplifying and possibly automating the evaluation process. This scheme is based on first representing the given system by an assemblage of simple operators for which VFRFs are readily available, and subsequently constructing VFRFs of the t...

Human-Induced Vibrations on Two Lively Footbridges in Milan

This article describes the results of extensive full-scale experiments carried out on two lively footbridges recently built in Milan, Italy. The objective of the study was to assess the dynamic characteristics of the bridges and the expected vibration level induced by pedestrians. A preliminary vibration analysis of the bridges revealed their potential sensitivity to human-induced vibrations in the vertical direction. Two sets of experiments were carried out, one before and one after the installation of tuned mass dampers. Furthermore, bridge vibrations during a marathon event were recorded and analyzed. Experimental and analytical results were compared for several loading scenarios.

Nonlinear Aerodynamic and Aeroelastic Analysis of Bridges: Frequency Domain Approach

A frequency domain approach for nonlinear bridge aerodynamics and aeroelasticity, based on the Volterra series expansion, is introduced in this paper. The Volterra frequency-response functions and the associated linear equations are formulated utilizing a topological assemblage scheme and are identified utilizing an existing full-time-domain nonlinear bridge aerodynamics and aeroelasticity analysis framework. A two-dimensional sectional model of a long-span bridge is used to illustrate this approach. The results show a good comparison between the time-domain simulation and the proposed frequency-domain model. The availability of Volterra frequency-response functions enables gaining a qualitative insight into nonlinear bridge aerodynamics and aeroelasticity.

Estimation of Damping for Turbine Blades in Non-Stationary Working Conditions

Turbine blades are critical components in thermal power plants and their design process usually includes experimental tests in order to tune or confirm numerical analyses. These tests are generally carried out on full-scale rotors having some blades instrumented with strain gauges and usually involve a run-up and/or a run-down phase. The quantification of damping in these conditions is rather complicated, since the finite sweep velocity produces a distortion of the vibration amplitude in contrast to the Frequency-Response Function that would be expected for an infinitely slow crossing of the resonance. In this work, we show through a numerical simulation that the usual identification procedures lead to a systematic overestimation of damping due both to the finite sweep velocity, as well as to the variation of the blade natural frequency with the rotation speed. An identification procedure based on the time-frequency analysis is proposed and validated through numerical simulations.Copyright

Modal Identification of Dynamically Coupled Bladed Disks in Run-Up Tests

The spin test is a standard industrial practice employed for the qualification of rotor blades and disks. The expected results are the modal properties of blades and assemblages at different rotation velocities. If a significant dynamic coupling among the blades exists, global vibration modes appear, reflecting into a set of closely spaced natural frequencies for each mode family. In case of perfectly-tuned bladed disks, the circumferential structure of the mode shapes is known and can be exploited during the identification process so that traditional single-dof models may be applied. On the contrary, the mode irregularities produced by mistuning prevents the use of single-dof models requiring the development of more sophisticated approaches. In this work, we propose a multi-dof identification technique organized as follow: 1) the FRF of the bladed disk in the neighborhood of a resonance crossing is identified by the wavelet transform of the measured response; 2) the modal parameters of the system are estimated using a mixed stochastic-deterministic subspace algorithm formulated in the frequency domain. The procedure is validated using a realistic numerical simulation.Copyright

Damping Estimation for Turbine Blades Under Non-stationary Rotation Speed

Turbine blades are critical components in thermal power plants and their design process usually includes experimental tests in order to tune or confirm numerical analyses. These tests are generally carried out on full-scale rotors having some blades instrumented with strain gauges and usually involve a run-up and/or a run-down phase. The quantification of damping in these conditions is rather complicated, since the finite sweep velocity produces a distortion of the vibration amplitude with respect to the Frequency-Response Function that would be expected for an infinitely slow crossing of the resonance. In this work, we demonstrate through a numerical simulation that the usual identification procedures procedure lead to a systematic overestimation of damping due both to the finite sweep velocity, as well as to the variation of the blade natural frequency with the rotation speed. An identification procedure based on the time-frequency analysis is proposed and validated through numerical simulations.