1,721,143 research outputs found
Numerical investigation of one-degree-of-freedom vortex-induced vibration of a rigid circular cylinder
Full text linkLe vibrazioni indotte dal fenomeno di distacco di vortici (VIV, Vortex-Induced Vibration)
su un cilindro rigido a sezione, circolare montato elasticamente con un grado di
libertà sono state simulate numericamente. Il sistema meccanico considerato è caratterizzato
da un basso rapporto di massa e un basso rapporto di smorzamento. Nello studio
è stata presa in considerazione un’ampia gamma di regimi di velocità del flusso incidente,
corrispondenti a un numero di Reynolds subcritico.
Il distacco di vortici generato a valle da un cilindro è stato simulato numericamente tramite
gli approcci Unsteady Reynolds-Averaged Navier-Stokes (URANS) e Scale Adaptive
Simulation (SAS). I risultati delle diverse simulazioni numeriche sono stati confrontati
tra loro e con i dati sperimentali di letteratura. La maggiore differenza nei risultati
ottenuti, tramite l’applicazione del modello URANS a due differenti griglie di calcolo -
bidimensionale (2D) e tridimensionale (3D) - si trova nella regione del ramo superiore
(upper branch) dove si verifica la massima ampiezza di oscillazione trasversale.
Dall’analisi della scia in direzione assiale al cilindro, condotta tramite simulazioni tridimensionali
URANS e SAS, è stato scoperto che la maggiore tridimensionalità del flusso
si trova nella zona di transizione tra i diversi tipi di risposta. Queste zone di transizione
sono caratterizzate da una risposta di tipo isteretico. Una risposta di tipo intermittente
è stata osservata nella transizione tra i rami superiore (upper branch) e inferiore (lower
branch). Si è osservato come non vi sia formazione di flusso tridimensionale nel ramo
inferiore (lower branch). Si può considerare che, in questa regione, la semplificazione
numerica bidimensionale sia giustificata.
Si è eseguita la quantificazione dell’incertezza totale e l’analisi di sensibilità del modello
di turbolenza SST, dovuta ai suoi coefficienti di chiusura, applicato al caso del VIV. Per
l’analisi di quantificazione, sono state simulate tre diverse velocità di flusso incidente nel
regime del ramo inferiore (lower branch) con l’approccio URANS bidimensionale. I risultati
ottenuti hanno identificato l’insieme di coefficienti di chiusura che maggiormente
contribuiscono all’incertezza nei risultati.
Viene infine presentato un modello multi-frequenza a singolo grado di libertà (sdof-mf,
single-degree-of-freedom multi-frequency model) per la previsione del fenomeno del VIV.
Il modello sdof-mf proposto si basa sulla decomposizione della forza idrodinamica totale
in una forza di inerzia/resistenza, convenzionalmente associata al movimento del cilindro
nel fluido fermo e ad una forza di portanza aggiuntiva associata al distacco di vortici. I
parametri di questo modello sono stati identificati usando una procedura di identificazione
parametrica (PI, Parameter Identification) applicata ai risultati ottenuti nel dominio
del tempo delle forze indotte dal distacco di vortici, ottenute tramite simulazioni CFD.
Dai risultati complessivi, il modello mostra capacità promettenti nella riproduzione delle
forze di distacco dei vortici e del movimento del cilindro, sia in termini di ampiezze che
di frequenze.Vortex-Induced Vibrations of an elastically mounted rigid circular cylinder with one-degree-
of-freedom (1-DOF) are numerically investigated. The mechanical system considered
is characterised by low mass ratio and low damping ratio. A wide range of flow
regimes, corresponding to a sub-critical Reynolds number, are taken into consideration.
The vortex shedding around the cylinder is investigated numerically by the Unsteady
Reynolds-Averaged Navier-Stokes (URANS) and by the Scale Adaptive Simulation (SAS)
approaches. The results of the different numerical simulations have been compared to
each other and to the experimental data from literature. The difference between bidimensional
(2D) and three-dimensional (3D) URANS mesh models are in the upper
branch region where the maximum amplitude of transverse oscillation occurs. From
the analysis of the wake flow in span-wise direction, conducted with three-dimensional
URANS and SAS simulation, it has been found that the three-dimensionality of the
flow is stronger in the transition zone between the different branches. These zones are
characterised by a hysteretic transition. Intermittency is also observed in the transition
between upper and lower branches. It has been observed that three-dimensional flow
behaviour is absent in the lower branch. In this region, the bi-dimensional simplification
may be justified.
The quantification of the total uncertainty in the VIV quantity of interest due to the
uncertainty in the value of closure coefficients of the SST turbulence model is performed.
For the quantification analysis, three different flow velocities in the lower branch regime
have been simulated with 2D URANS approach. The results identify a set of closure
coefficients which most contribute to the uncertainty in the results.
A novel single-degree-of-freedom multi-frequency model (sdof-mf ) for prediction of VIV
is presented. The proposed sdof-mf model relies on the decomposition of the total hydrodynamic
force in an inertia/drag force, conventionally associated with the cylinder
motion in still fluid, and an additional lift force associated to pure vortex shedding. The
parameters of this model are identified using a Parametric Identification (PI) procedure
applied to time domain data of vortex induced forces, obtained via CFD simulations.
From the overall results, the model exhibits promising capabilities in the reproduction of
vortex shedding forces and cylinder motion, in terms of both amplitudes and frequencies
Optimizing optimal reduction: A type inference algorithm for elementary affine logic
No full textWe propose a type inference algorithm for lambda terms in elementary affine logic (EAL). The algorithm decorates the syntax tree of a simple typed lambda term and collects a set of linear constraints. The result is a parametric elementary type that can be instantiated with any solution of the set of collected constraints.
We point out that the typeability of lambda terms in EAL has a practical counterpart, since it is possible to reduce any EAL-typeable lambda terms with the Lamping’s abstract algorithm obtaining a substantial increase of performances.
We show how to apply the same techniques to obtain decorations of intuitionistic proofs into linear logic proofs
Teaching Programming in the Age of Generative AI
Full text linkProgramming has been considered the "essence of informatics" since the beginning of computing as a discipline.
Today, however, the availability of machine learning artefacts that produce high-level code from natural language specifications has completely changed the traditional meaning. Several recent papers reviewed the performance of code generators based on large language models on typical CS1 problems (e.g., from the many possible citations and how machine learning impacts K-12 teaching.
Starting from this data, I will argue for the role of programming in the curriculum, distinguishing between programming taught as part of a holistic curriculum (as in some non-technical high schools) or as a vocational tool
Controllability analysis of multi-agent systems using relaxed equitable partitions
Full text linkThis paper investigates how to make decentralised networks,
amenable to external control, i.e., how to ensure that they are
appropriately organised so that they can be effectively ‘reprogrammed’.
In particular, we study networked systems whose interaction dynamics
are given by a nearest-neighbour averaging rule, with one leader node
providing the control input to the entire system. The main result is a
necessary and sufficient condition for the controllability of such systems
in terms of the graph topology. In particular, we give a graph theoretic
interpretation of the controllability properties through the so-called
relaxed equitable partition
Steering a Leader-Follower Team via Linear Consensus
No full textThe paper considers the problem of driving a formation of autonomous mobile agents. Thegroup of mobile devices is represented by a leader-follower network, where the followers updatetheir position using a simple local consensus procedure, while the leaders, whose positionsrepresent the control inputs of the network, are free to move. We characterize the transientbehavior of the network, and we solve the containment problem without relying on auxiliarysensors
An `Executable' Impredicative Semantics for the Ada Configuration.
No full textWe present a translation of Ada configuration constructs, in a higher order, impredicatively typed, functional language (HOTFUL) with subtypes. The aim of this work is to provide an expressive executable semantics for Ada configuration constructs, and to verify the suitability of the chosen HOTFUL for such a task. In particular, we address the practicability of the approach when dealing with the development of a whole complex system, as well as the description of single modular units. After giving the detailed rules for the translation, we compare our approach with what could be obtained selecting a different typed language as ldquotargetrdquo, namely the predicative type system of Standard ML
An invariant cost model for the lambda calculus
No full textWe define a new cost model for the call-by-value lambda-calculus satisfying
the invariance thesis. That is, under the proposed cost model,
Turing machines and the call-by-value lambda-calculus can simulate each other
within a polynomial time overhead. The model only relies
on combinatorial properties of usual beta-reduction, without
any reference to a specific machine or evaluator. In particular, the cost
of a single beta reduction is proportional to the difference between the size
of the redex and the size of the reduct. In this way, the total cost of normalizing a lambda term
will take into account the size of all intermediate results (as well
as the number of steps to normal form)
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