1,721,151 research outputs found
Il gruppo di misure meccaniche dell'Universita' di Padova
No full textAbstract. Il panorama delle attività del gruppo padovano si articolano su temi che riguardano sia la ricerca di tipo misuristico e di sviluppo tecnologico nei campi dell’esplorazione planetaria (dove possiamo dire che l’ateneo di Padova ha raggiunto una posizione di eccellenza internazionale), dello sviluppo di propulsori spaziali ed algoritmi di controllo orbitale, della strumentazione per gli impatti iperveloci su satelliti ed altre strutture ed infine nell’automazione e robotica. Il gruppo può vantare questi importanti risultati grazie alle iniziative e alla lungimiranza del prof. Angrilli che è titolare a Padova di Misure Meccaniche e Termiche fin dal 1976
La sua perseveranza e, grazie al supporto di altri docenti dell’Ateneo Patavino, ha portato a fondare il CISAS “G. Colombo” prima e ad istituire il corso di Laurea in Ingegneria Aerospaziale poi, oltre a determinare la formazione di numerosi collaboratori, alcuni dei quali ora docenti presso la Facoltà di Ingegneria, che tengono corsi agli allievi ingegneri per la Laurea triennale e Magistrale in Ingegneria Meccanica, Aerospaziale e Energetica. Il gruppo di ricerca complessivamente consta di 7 docenti e 20 ricercatori che operano nei laboratori del Dipartimento di Ingegneria Meccanica e del CISAS G. Colombo
A system to damp the free piston oscillations in a two-stage light-gas gun used for hypervelocity impact experiments
Full text linkHypervelocity impact experiments that reproduce on-orbit collisions between micrometeoroids or
orbital debris and space structures are commonly performed by means of propellant-driven twostage
light-gas guns. Such devices accelerate projectiles using the thrust of a light propellant gas
that is compressed to high pressure and temperature by a piston running in a pump tube. Though
these guns have the unique capability of accelerating particles up to 9 km/s, many components of
the gun must be checked and/or substituted after each shot making test sessions long and
expensive. In order to have a lot of and many different types of hypervelocity impact data, the
Center of Studies and activities for Space CISAS ”G. Colombo” of Padua University developed a
high-shot-frequency two-stage light-gas gun that can increase the shot repetition rate of standard
facilities by a factor of 5 or more and at the same time reduce the shot cost by a factor of 2 or
more. This is made possible through the use of special mechanical and diagnostic solutions that
were designed to operate the gun for more than 50 shots in sequence without having to carry out
maintenance operations. This article presents the design and operation of the CISAS two-stage
light-gas gun damping system, which is one of the subsystems that makes it possible to achieve
high-shot frequency. The damping system is in charge of controlling the piston oscillations in the
pump tube, making it possible for the piston to withstand more than 100 shots without any damage.
In particular, the damping system avoids piston strikes onto the gun head at the end of each
compression stroke and allows the piston to be positioned at the base of the pump tube after each
shot. The sensitivity of the piston oscillations to the damping operations and main subsystem
design parameters were identified using numerical simulations, carried out according to a model
that describes every working phase of the gun. Moreover, in this paper, the technical solutions for
the damping system implementation are presented and the numerical predictions are compared
with experimental results. For the CISAS high-shot-frequency gas gun, an efficient damping
system proved to be a fundamental requirement to reliably accelerate 100 mg projectiles above 5
km/s and 70 mg projectiles at 5.5 km/s with a shot frequency of 10 shots per day at least, including
the time needed for replacing the target and pumping down the target vacuum chamber
SPH evaluation of out-of-plane peak force transmitted during a hypervelocity impact
Full text linkSmooth particle hydrodynamics (SPH) technique is applied to simulate a hypervelocity impact of
an aluminum sphere on a simple aluminum target and on a honeycomb structure sandwich panel
in order to provide a useful input to Finite Element Model or Statistical Energy Model on evaluating
the vibration environment induced by the projectile on the target. The impact velocity range lays
between 4 and 5 km/s. Different approaches have been analyzed. At first, the application of SPH
technique for direct calculation of the vibration environment is described. Then the calculation of
equivalent force impulse is evaluated. Two strategies have been applied: shear stress analysis and
momentum calculation using drift velocity measurement. Momentum approach revealed to be the
most convenient and reliable method. Results for an aluminum plate and a honeycomb sandwich
panel are reported and compared to results of experiments and Finite Element Analysis
Impact experiments on low-temperature bumpers
Full text linkThis paper presents the results of hypervelocity impact experiments that were carried out at CISAS
Impact Facility onto aluminum bumpers cooled down to −120°C with liquid nitrogen and to −60°C
with solid carbon dioxide. The thickness of the targets was 0.8, 1, 2 and 3.17mm, the diameter of
the spherical projectiles was 1.5, 1.9, 2.3 and 2.9 mm and the impact velocity did span between 4
and 5 km/s. To establish if any temperature dependence exists in the bumpers’ impact response,
two different features were analyzed: the hole size and the bumper protection capabilities. The
latter property, that is related to the bumper capacity of producing debris cloud composed of
fragments as fine and slow as possible, was assessed through observation of the damage patterns
on witness plates and through measurements of the debris cloud tip velocity. Moreover, qualitative
analyses of high-speed shadowgraphs representing the debris cloud evolution were performed. On
one hand, it was found that low temperature has only minor influence on the hole diameter. On the
other hand, the examination of shadowgraphs showed that the debris cloud structure varies with
bumper temperature, even though it was not proved that such differences correspond to significant
dissimilarities between damage patterns recorded onto witness plates
Application of Wavelet Transform to analyze acceleration signals generated by HVI on thin aluminum plates and all-aluminum honeycomb sandwich panels
Full text linkAmong all possible dangers, hypervelocity impacts on structures produce also a disturbance field,
which results from the superimposition of vibrations originating on the impact point and then
reflected at the target boundaries. Such disturbance environment is composed by waves of
different amplitudes, frequency contents, speeds and directions of propagation. The aim of this
paper is to characterize this complex environment, through the identification of its fundamental
constituents using the Wavelet Transform analysis method, useful for studying transient
phenomena, including wave propagation. Common signal analysis tools, like Shock Response
Spectrum (SRS), cannot provide a good description of the physical behavior of such waves, nor
they can differentiate between them since their frequency decomposition does not retain any time
information. On the contrary, Wavelets associate a time information to the frequency content: each
wave can be characterized by its frequency band and arrival time. Starting from both numerical
and experimental acceleration data from impacts on aluminum plates and aluminum honeycomb
sandwich panels, Wavelet analysis was employed to identify symmetric and antisymmetric waves.
Moreover, reflections and dispersion phenomena were observed, leading to wave distortion due to
different speeds of propagation of wave trains characterized by different frequency content
Huygens atmospheric structure instrument of Huygens probe on Cassini mission
No full textIn the framework of the Cassini-Huygens mission towards the Saturnian system, the Huygens Atmospheric Structure Instrument (HASI) is a multisensor package which has been designed to measure the physical quantities characterizing the atmosphere of Titan, Saturn's largest satellite. HASI sensors are devoted to the study of Titan's atmospheric structure and electric properties, and also to provide information on the surface, whatever its phase, solid or liquid. A description of the experiment, its subsystems and sensors package is reported together with their performances. © 2002 International Astronautical Federation. Published by Elsevier Science Ltd. All rights reserved. © 2002 International Astronautical Federation. Published by Elsevier Science Ltd. All rights resreved
A special design condition to increase the performance of two-stage light-gas guns
Full text linkThis paper reports a theoretical and numerical study aimed at increasing the operating efficiency of
two-stage light-gas guns by appropriately changing their working conditions. In particular, a method
is presented for increasing the projectile speed without any rise of the maximum breech pressure.
The classic design theory of two-stage guns starts from the assumption that the highest velocity is
reached in a gun which constantly maintains the maximum acceptable pressure at the base of the
projectile during the full launching time. The main drawback of this working condition is that it may
require an unfeasible rise of the gun maximum pressure, especially when very high muzzle speed
is requested. To overcome this limitation, a new reference case different from the constant-basepressure
one is presented, based on a novel gas-dynamics solution that can be expressed in exact
form if losses are not accounted for. According to such an approach, it is theoretically shown that
the projectile base-pressure can be appropriately shaped (i) to improve the final speed without
increasing the breech pressure or, in other terms, (ii) to achieve a given muzzle velocity with
reduced maximum gas pressure. The analytical application of the new gas-dynamics condition
showed the capability of obtaining a 1 km/s velocity improvement with no increase of the breech
pressure or, alternatively, a pressure reduction up to 30% with no penalty on the model final speed.
A numerical verification of the calculations was performed through the CISAS light-gas gun full
numerical model, which includes real effects such as friction losses and heat transfer. Finally, an
experimental verification of the numerical test case was attempted and a speed augmentation of
0.8 km/s with no increase in the breech pressure was confirmed in laboratory, highlighting the
agreement with numerical predictions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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