429 research outputs found
The westward drift of the lithosphere. A tidal ratchet?
Is the westerly rotation of the lithosphere an ephemeral accidental recent phenomenon or is it a stable
process of Earth’s geodynamics? The reason why the tidal drag has been questioned as the mechanism
determining the lithospheric shift relative to the underlying mantle is the apparent too high viscosity of
the asthenosphere. However, plate boundaries asymmetries are a robust indication of the ‘westerly’
decoupling of the entire Earth’s outer lithospheric shell and new studies support lower viscosities in the
low-velocity layer (LVZ) atop the asthenosphere. Since the solid Earth tide oscillation is longer in one side
relative to the other due to the contemporaneous Moon’s revolution, we demonstrate that a non-linear
rheological behavior is expected in the lithosphere mantle interplay. This may provide a sort of ratchet
favoring lowering of the LVZ viscosity under shear, allowing decoupling in the LVZ and triggering the
westerly motion of the lithosphere relative to the mantle
High speed fluttering skids with elastic suspensions
In a recent project, named SEALAB, a novel marine vehicle has been developed. Its main characteristic is the presence of special skid surfaces surfing over rough water. A suspension system controls the vertical motion of the skid, softening the sequential impacts and vibrations induced by the water, similarly to a wheeled vehicle in off-road trials. The hull-skid-suspension set is modeled by prototypical equations. The system undergoes special regimes when the vessel speed at sea is varied. In particular, for some combinations of the forward speed and sea-state, the skid still maintains the contact with the water. In other navigation conditions the skid indeed jumps out the water with a complete different average transmitted force and vibration characteristics of the hull. This paper presents a theory that outlines these phenomena identifying conditions that lead to the jumping skid condition
On line estimation of rolling resistance for intelligent tires
The analysis of a rolling tire is a complex problem of nonlinear elasticity. Although in the technical literature some tire models have been presented, the phenomena involved in the tire rolling are far to be completely understood. In particular, small knowledge comes even from experimental direct observation of the rolling tire, in terms of dynamic contact patch, instantaneous dissipation due to rubber-road friction and hysteretic behavior of the tire structure, and instantaneous grip. This paper illustrates in details a new powerful technology that the research group has developed in the context of the project OPTYRE. A new wireless optical system based on Fiber Bragg Grating strain sensors permits a direct observation of the inner tire stress when rolling in real conditions on the road. From this information, following a new suitably developed tire model, it is possible to identify the instant area of the contact patch, the grip conditions as well the instant dissipation, which is the object of the present work
Energy exchange between nonlinear oscillators: An entropy foundation
In the field of vibrations of complex structures, energy methods like SEA and a series of mid-frequency methods, represent an important resource for computational analysis. All these methods are based in general on a linear formulation of the elastic problem. However, when nonlinearities are present, for example related to clearance or stiffening of joints, these methods, in principle, cannot be applied. This paper, on the basis of a theory presented recently by one of the authors, proposes a foundation of a new energy method able to deal with nonlinearities when studying the energy exchange between subsystems. The idea relies on the concept of a thermodynamic vibroacoustic temperature, that can be directly defined when introducing the entropy of a vibrating structure. The theory is introduced in general, and examples of calculation of the power flow between nonlinear resonators are presented introducing stiffening and clearences for systems with many degrees of freedom
Prototyping a new car semi-active suspension by variational feedback controller
New suspension systems electronically controlled are presented and mounted on board of a real car. The system consists of variable semi-active magneto-rheological dampers that are controlled through an electronic unit that is designed on the basis of a new optimal theoretical control, named VFC-Variational Feedback Controller. The system has been mounted on board of a BMW Series 1 car, and a set of experimental tests have been conducted in real driving conditions. The VFC reveals, because of its design strategy, to be able to enhance simultaneously both the comfort performance as well as the handling capability of the car. Preliminary comparisons with several industrially control methods adopted in the automotive field, among them skyhook and groundhook, show excellent results
An entropy formulation for the analysis of energy flow between mechanical resonators
Several energy-based methods to approach noise and vibro-acoustic problems are actually under development. These techniques provide a chance of describing the vibro-acoustic behaviour of complex systems by the energies of a limited number of subcomponents. This process is the base of one of the most acknowledged methods in this field, i.e. the statistical energy analysis (SEA). However, SEA invokes only the first law of thermodynamics, i.e. the energy conservation principle. On the contrary, it seems that the formulation of a complete theory of energy transmission among oscillators would claim also for the second principle of thermodynamics. Such direction of investigation, via the entropy concept, is developed in this paper leading to a theoretical energy flow analysis to predict the energy exchanged among complex systems. Some classical SEA results are obtained as a special case of a more general approach. (C) 2002 Elsevier Science Ltd. All rights reserved
Macroscopic description of microscopically strongly inhomogenous systems: A mathematical basis for the synthesis of higher gradients metamaterials
We consider the time evolution of a one dimensional -gradient continuum.
Our aim is to construct and analyze discrete approximations in terms of
physically realizable mechanical systems, called microscopic because they are
living on a smaller space scale. We validate our construction by proving a
convergence theorem of the microscopic system to the given continuum, as the
scale parameter goes to zero.Comment: 20 page
A multisensing setup for the intelligent tire monitoring
The present paper offers the chance to experimentally measure, for the first time, the internal
tire strain by optical fiber sensors during the tire rolling in real operating conditions. The phenomena
that take place during the tire rolling are in fact far from being completely understood. Despite several
models available in the technical literature, there is not a correspondently large set of experimental
observations. The paper includes the detailed description of the new multi-sensing technology for an
ongoing vehicle measurement, which the research group has developed in the context of the project
OPTYRE. The experimental apparatus is mainly based on the use of optical fibers with embedded
Fiber Bragg Gratings sensors for the acquisition of the circumferential tire strain. Other sensors are
also installed on the tire, such as a phonic wheel, a uniaxial accelerometer, and a dynamic temperature
sensor. The acquired information is used as input variables in dedicated algorithms that allow the
identification of key parameters, such as the dynamic contact patch, instantaneous dissipation and
instantaneous grip. The OPTYRE project brings a contribution into the field of experimental grip
monitoring of wheeled vehicles, with implications both on passive and active safety characteristics of
cars and motorbikes
Car collision avoidance with velocity obstacle approach
The obstacle avoidance maneuver is required for an autonomous vehicle. It is essential to define the system's performance by evaluating the minimum reaction times of the vehicle and analyzing the probability of success of the avoiding operation. This paper presents a collision avoidance algorithm based on the velocity bstacle approach that guarantees collision-free maneuvers. The vehicle is controlled by an optimal feedback control named FLOP, designed to produce the best performance in terms of safety and minimum kinetic collision energy. Dimensionless accident evaluation parameters are proposed to compare different crash scenarios
Ensemble energy average and energy flow relationships for nonstationary vibrating systems
This paper attempts to introduce a new point of view on energy analysis in structural dynamics with particular emphasis to its link with uncertainty and complexity. A linear, elastic system undergoing free vibrations, is considered. The system is subdivided into two subsystems and their respective energies together with the shared energy flow are analysed. First, the ensemble energy average of the two subsystems, assuming uncertain the natural frequencies, is investigated. It is shown how the energy averages follow a simple law when observing the long-term response of the system, obtained by a suitable asymptotic expansion. The second part of the analysis shows how the ensemble energy average of a set of random samples is representative even of the single case if the system is complex enough. The two previous points, combined, produce a result that applies to the energy sharing between two subsystems even independently of uncertainty: for complex systems, a simple energy sharing law is indeed stated. Moreover, in the case of absence of damping, a nonlinear relation between the energy flow and the energy (weighted) difference between the two subsystems is derived; on the other hand, when damping is present, this relationship becomes linear, including two terms: one is proportional to the energy (weighted) difference between the two subsystems, the other being proportional to its time derivative. Therefore, the approach suggests a way for deriving a general approach to energy sharing in vibration with results that, in some cases, are reminiscent of those met in Statistical Energy Analysis. Finally, computational experiments, performed on systems of increasing complexity, validate the theoretical results. (c) 2005 Elsevier Ltd. All rights reserved
- …