Marcello Vanali

Moving microphone arrays to reduce spatial aliasing in the beamforming technique: theoretical background and numerical investigation.

This paper introduces a measurement technique aimed at reducing or possibly eliminating the spatial aliasing problem in the beamforming technique. Beamforming main disadvantages are a poor spatial resolution, at low frequency, and the spatial aliasing problem, at higher frequency, leading to the identification of false sources. The idea is to move the microphone array during the measurement operation. In this paper, the proposed approach is theoretically and numerically investigated by means of simple sound propagation models, proving its efficiency in reducing the spatial aliasing. A number of different array configurations are numerically investigated together with the most important parameters governing this measurement technique. A set of numerical results concerning the case of a planar rotating array is shown, together with a first experimental validation of the method.

Optimization of the current flowing technique aimed at semi-passive multi-modal vibration reduction

The paper deals with the analysis and optimization of a semi-passive multi-modal vibration attenuation technique exploiting piezo-benders actuators. Although there are different methods available in the literature, attention has been focused on the current flowing technique as it has a number of advantages with respect to the other approaches. In this context the control bender is linked to an electrical impedance, designed to maximize the mechanical energy dissipation. Though the method has already been presented in the literature, many points have still to be investigated. The paper gives a deeper analysis of this control technique and then presents an algorithm aimed at optimizing the electrical network linked to the bender.

Vibration Control by Means of Piezoelectric Actuators Shunted with LR Impedances: Performance and Robustness Analysis

This paper deals with passive monomodal vibration control by shunting piezoelectric actuators to electric impedances constituting the series of a resistance and an inductance. Although this kind of vibration attenuation strategy has long been employed, there are still unsolved problems; particularly, this kind of control does suffer from issues relative to robustness because the features of the electric impedance cannot be adapted to changes of the system. This work investigates different algorithms that can be employed to optimise the values of the electric components of the shunt impedance. Some of these algorithms derive from the theory of the tuned mass dampers. First a performance analysis is provided, comparing the attenuation achievable with these algorithms. Then, an analysis and comparison of the same algorithms in terms of robustness are carried out. The approach adopted herein allows identifying the algorithm capable of providing the highest degree of robustness and explains the solutions that can be employed to resolve some of the issues concerning the practical implementation of this control technique. The analytical and numerical results presented in the paper have been validated experimentally by means of a proper test setup.

Stockbridge Type Damper Effectiveness Evaluation, Part I: Comparison between Tests on Span and on the Shaker

This paper deals with an experimental measurement campaign carried out with a modified dynamometric Stockbridge damper clamped to a laboratory test span. The aim is to get the force and the torque exerted between the cable and the damper on a span. This approach obtains the mechanical impedance of the damper under real working conditions, with the possibility to separate the contribution of the torque and of the force out of the global losses, and to compare these results with those obtained on a shaker where only the vertical motion is imposed, with no rotation. More than 100 experimental tests were executed from February 2001 to July 2001, also looking for a metrological validation of the results and for quality assessment of the collected data. Some results are shown, pointing out a comparison between the damper behavior on the span and on dynamic exciter.

Numerical and Experimental Investigation of Stop-Bands in Finite and Infinite Periodic One-Dimensional Structures

Adding periodicity to structures leads to wavemode interaction, which generates pass- and stop-bands. The frequencies at which stop-bands occur are related to the periodic nature of the structure. Thus structural periodicity can be shaped in order to design vibro-acoustic filters for reducing vibration and noise transmission. The aim of this paper is to investigate, numerically and experimentally, stop-bands in periodic one-dimensional structures. Two methods for predicting stop-bands are described: the first method applies to infinite periodic structures using a wave approach; the second method deals with the evaluation of a vibration level difference (VLD) in a finite periodic structure embedded within an infinite one-dimensional waveguide. This VLD is defined to predict the performance in terms of noise and vibration insulation of periodic cells embedded in an otherwise uniform structure. Numerical examples are presented, and results are discussed and validated experimentally. Very good agreement between the numerical and experimental models in terms of stop-bands is shown. In particular, the results show that the stop-bands obtained using a wave approach (applied to a single cell of the structure) predict those obtained from the VLD of the corresponding finite periodic structure.

Dynamic Behavior of the Palazzo Lombardia Tower: Comparison of Numerical Models and Experimental Results

AbstractThe Palazzo Lombardia Tower, located at the heart of the city of Milan, Italy, recently was completed. At 161.30 m high, the new building is currently the tallest in Italy and one of the most prominent features of the city skyline for many years to come. Given the strategic importance and the strong impact of the new building, it was decided to perform a series of dynamic excitation tests, with special emphasis on the Tower. Thus, it was possible to estimate its basic modal features and then compare them to the numerical simulation predictions provided by the same finite-element (FE) numerical analysis used for design. This offered the chance to start an important and interesting model-updating procedure, taking advantage of both the actual material properties measured during the construction phase and dynamic measurements, the latter having added significant insight to the structural behavior. The mechanical engineering department of the Polytechnic University of Milan was in charge of the design...

The Behaviour of Mistuned Piezoelectric Shunt Systems and Its Estimation

This paper addresses monoharmonic vibration attenuation using piezoelectric transducers shunted with electric impedances consisting of a resistance and an inductance in series. This type of vibration attenuation has several advantages but suffers from problems related to possible mistuning. In fact, when either the mechanical system to be controlled or the shunt electric impedance undergoes a change in their dynamical features, the attenuation performance decreases significantly. This paper describes the influence of biases in the electric impedance parameters on the attenuation provided by the shunt and proposes an approximated model for a rapid prediction of the vibration damping performance in mistuned situations. The analytical and numerical results achieved within the paper are validated using experimental tests on two different test structures.

Effects of People Occupancy on the Modal Properties of a Stadium Grandstand

It is well known that of people standing or sitting on a structure can change the dynamic behavior of the structure itself. Particularly, when a significant number of people are occupying a structure, high variations of non-dimensional damping ratios and natural frequencies are often experienced. The extent of these changes depends not only on the number of persons, but also on the properties of the empty structure and on people position and postures.

Quantification of Damping Effect of Humans on Lightly Damped Staircases

There is experimental evidence that the people interacting with structures are not only an active load but also affect the structural properties. Particularly, considerable damping ratio value changes are often experienced. This fact assumes a relevant importance in assessing the structure serviceability against vibrations due to pedestrian induced loads. There is therefore ground to find methods capable of estimating the effect induced by people interacting with a structure, in terms of both changes of modal parameters and of loading effect. This work aims at presenting a model, partly based on modal approach, able to describe people effect on a structure. Each person is modelled as a two degrees of freedom spring-mass-damper system and is introduced locally on the structure. Several tests on a lightly damped staircase were then carried out to validate the model.

Geometry effects on the vibro-acoustic behavior of railway resilient wheels

Noise generated by railway vehicles is nowadays one of the most important concerns in the railway engineering community and sometimes becomes one of the key points in the discussions preceding the design of a new line. Many researchers have contributed to achieve important improvements in the reduction of railway wheel noise during recent years. One of the solutions effectively able to reduce rolling noise is the adoption of resilient wheels, nowadays widely used on city trams and sometimes on high-speed trains. Some studies have shown the important role played by the wheel rubber blocks in reducing rolling noise and a strict link between their stiffness and the emitted noise has been demonstrated as well. On the other hand the resilient wheel effectiveness in mitigating squeal noise has not yet been deeply investigated. Moreover the effects of factors, different from the rubber block features, have not been completely stated. The aim of the present paper is to analyze which are the main parameters affecting the resilient wheel vibro-acoustic behavior and the consequences on squeal noise emission. This study considers not only the rubber block features but also other parameters, such as wheel geometry. The research approach is both experimental and numerical. The analysis has been carried out up to 5 kHz. Two different resilient wheels have been tested performing modal analyses and getting sound pressure levels at the same time. The results have been compared and commented. It has been found that the wheel geometry plays an important role in determining the wheel vibro-acoustic behavior, although rubber blocks remain the most influent elements. Particularly the wheel width value has significant effects on the tread dynamics and, as the main consequence, on the wheel sound radiation.