87,579 research outputs found
Structural modifications for squeal noise reduction
No full textBrake squeal is an example of noise caused by vibrations induced by friction forces that can lead to a dynamic instability. The onset of squeal is due to the lock-in phenomenon, i.e. the instability occurring when two modes of the system coalesce and one becomes unstable. The sensitivity of the onset of instabilities with respect to the system parameters is due to the high modal density and to the large uncertainties on the dynamics of the single brake. This makes impossible the design of a 'squeal-free' brake apparatus. This paper suggests an innovative approach to avoid the growth of squeal vibrations, by exploiting the introduction of lumped structural modifications in the disc. The modification rotates with the disc while the disc modes are fixed in space. Depending on the relative position between the concentrated modification and the nodes of the disc modes, the natural frequencies of the disc shift back and forward. This expedient allows to change continuously the lock-in conditions between the system modes that are involved in squeal. The vibrations due to the lock-in between the pair of modes have not enough time to increase because of the repetitive look-out and they can be reduced until they completely disappear
The IMF and star formation history of the stellar clusters in the Vela D cloud
No full textAims.We present the results of a Near-Infrared deep photometric survey of a
sample of six embedded star clusters in the Vela-D molecular cloud,
all associated with luminous ( ) IRAS
sources. The clusters are unlikely to be older than a few 106
yrs, since all are still associated with molecular gas.Methods.We employed the fact that all clusters lie at the same distance and
were observed with the same instrumental setting to derive
their properties in a consistent way, being affected by the same
instrumental and observational biases.
We extracted the clusters' K Luminosity Functions and developed a
simple method to correct them for extinction, based on colour-magnitude
diagrams. The reliability of the method has been tested by
constructing synthetic clusters from theoretical tracks for pre-main
sequence stars and a standard Initial Mass Function.
The clusters' Initial Mass Functions have been derived from the
dereddened
K Luminosity Functions by adopting a set of pre-main sequence
evolutionary tracks and assuming coeval star formation.Results.All clusters are small ( members) and compact (radius
pc); their most massive stars are
intermediate-mass ( ) ones.
The dereddened K Luminosity Functions are likely to arise
from the same
distribution, suggesting that the selected clusters have quite
similar Initial Mass Functions and star formation histories.
The Initial Mass Functions are consistent with those
derived for field stars and clusters. Adding them
together we found that the “global” Initial Mass Function
appears steeper at the high-mass end and
exhibits a drop-off at . In fact,
a standard Initial Mass Function would
predict a star with within one of the clusters,
which is not found. Hence, either high-mass stars need larger
clusters
to be formed, or the Initial Mass Function of the single clusters is
steeper at the
high-mass end because of the physical conditions in the parental gas
Synthesis of thiourea-tethered C-glycosyl amino acids via isothiocyanate-amine coupling
No full textA new class of C-glycosyl amino acids displaying a thiourea segment as a linker has been designed and
synthesized by addition of peracetylated glycosylmethyl isothiocyanates to an amine-functionalized
amino acid (N-Fmoc-beta-amino-L-alanine). Three pairs of compounds with alpha- and beta-galacto, alpha- and
beta-gluco, and alpha- and beta-manno configuration have been prepared with yields ranging between 70 and
75%. The orthogonal set of protective groups (O-acetyl in the carbohydrate moiety and N-Fmoc in the
amino acid residue) makes these compounds suitable substrates for the co-translational modification of
natural peptides. The couplings of model hydroxy-free and perbenzylated glycosylmethyl
isothiocyanates with the above N-Fmoc-beta-amino-L-alanine and the N-Boc-protected analogue have
been carried out as well, thus broadening the scope of the coupling reaction. Nevertheless, there are
limitations of this isothiocyanate–amine coupling in complex systems, and these are briefly discussed
Notes sur la préhistoire en Romagna
No full textSono presentati i risultati delle ricerche preistoriche e protostoriche del territorio romagnol
Young stellar clusters in the Vela D molecular cloud
No full textIt is now well established by means of direct and indirect observations that most, if not all, stars are formed in groups rather than in isolation (Clarke, Bonnell & Hillenbrand 2000). An important result that strongly constrains theories of massive star and stellar cluster formation is that the stellar density of young stellar clusters seems to depend on the mass of the most massive star in the cluster. Low-mass stars are usually found to form in loose groups with typical densities of a few stars per cubic parsec (Gomez et al. 1993), while high-mass stars are found within dense stellar clusters of up to 104 stars per cubic parsec (e.g. the Orion Nebula Cluster, Hillenbrand & Hartmann 1998). To explain this different behaviour, it has been proposed that massive stars may form with a process that is drastically different from the standard accretion picture, e.g. by coalescence of lower mass seeds in a dense cluster environment. The transition between these two modes of formation should occur in the intermediate-mass regime, namely 2 <= M/M0 <= 15
Cross-Subject EEG Channel Selection for the Detection of Predisposition to Alcoholism
No full textElectroEncephaloGram (EEG) is a powerful technology for the early detection, among others, of alcoholism. However, multiple electrodes placed on the scalp to record brain signals may introduce noisy and redundant information, hinder performance and increase computational times in the task of automated decoding of
EEG signals. In this work we propose a novel end-to-end Representation Learning-based algorithm to select the most relevant EEG channels to perform detection of predisposition to alcoholism, in a subject-agnostic way. Indeed, EEG signals are characterized by strong subject-specific variance potentially affecting the generalizability
of the selection. Results are promising, especially compared to the very limited literature on cross-subject EEG channel selection
Squeal propensity characterization of brake lining materials through friction noise measurements
No full textDisc brake systems are a technology widely adopted within the automotive and rail industry, especially when high performance is needed. The interaction between the disc and the pads is responsible for friction-induced vibrations, leading often to squeal noise emission. Squeal vibrations are generated by the onset of an unstable mode, which is triggered by an external excitation. Local phenomena occurring at the contact interface, and resulting in friction noise, can be responsible of the dynamic excitation triggering the squeal instability. This work proposes a new approach for characterizing friction lining materials, by measuring the friction noise coming from the contact between different pad materials and a disc rotor, in order to quantify and compare the attitude of materials to trigger squeal. Then, a parametrical analysis has been carried out with the aim of highlighting the influence of the main parameters on the friction noise. When testing the same set of materials on a full brake disc system, the measured friction noise indexes resulted to be strongly correlated with the squeal occurrence, validating the proposed characterization method for the squeal propensity of lining materials
Contact stiffness estimation for PMMA/STEEL contact pair
No full textModelling of frictional contact systems with high accuracy needs the knowledge of several contact parameters that are mainly related to the properties of the contact interfaces. While the interface parameters cannot be directly obtained by performing local measurements, the values estimated by means of analytical/numerical models are not reliable to describe the contact behavior, which affects in a prominent way the complex contact phenomena. This work presents a newer approach for identifying reliable values of the normal contact stiffness between rough surfaces in both sliding and sticking conditions as a function of contact pressure, surface roughness and materials. The combination of dynamic experimental tests, on a dedicated set-up, with finite element modelling allowed for an indirect determination of the normal stiffness at the contact
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