Direct numerical simulation of supersonic pipe flow at moderate Reynolds number

We study compressible turbulent flow in a circular pipe, at computationally high Reynolds number. Classical related issues are addressed and discussed in light of the DNS data, including validity of compressibility transformations, velocity/temperature relations, passive scalar statistics, and size of turbulent eddies.Regarding velocity statistics, we find that Huang's transformation yields excellent universality of the scaled Reynolds stresses distributions, whereas the transformation proposed by Trettel and Larsson (2016) yields better representation of the effects of strong variation of density and viscosity occurring in the buffer layer on the mean velocity distribution. A clear logarithmic layer is recovered in terms of transformed velocity and wall distance coordinates at the higher Reynolds number under scrutiny (\Rey_{\tau} \approx 1000), whereas the core part of the flow is found to be characterized by a universal parabolic velocity profile. Based on formal similarity between the streamwise velocity and the passive scalar transport equations, we further propose an extension of the above compressibility transformations to also achieve universality of passive scalar statistics. Analysis of the velocity/temperature relationship provides evidence for quadratic dependence which is very well approximated by the thermal analogy proposed by Zhang et Al.(2014). The azimuthal velocity and scalar spectra show an organization very similar to canonical incompressible flow, with a bump-shaped distribution across the flow scales, whose peak increases with the wall distance. We find that the size growth effect is well accounted for through an effective length scale accounting for the local friction velocity and for the local mean shear

Cross-sectional imaging of sharp Si interlayers embedded in gallium arsenide

We investigate the electronic properties of the (110) cross-sectional surface of Si-doped GaAs using first-principles techniques. We focus on doping configurations with an equal concentration of Si impurities in cationic and anionic sites, such as occurring in a self-compensating doping regime. In particular we study a bilayer of Si atoms uniformly distributed over two consecutive (001) atomic layers. The simulated cross-sectional scanning tunneling microscopy images show a bright signal at negative bias, which is strongly attenuated when the bias is reversed. This scenario is consistent with experimental results which had been attributed to hitherto unidentified Si complexes.Comment: 10 pages, 3 figure

Chirurgia d'urgenza per tumori colorettali: una sfida ai criteri oncologici di resezione?

Gli attuali criteri oncologici di resezione per i tumori del colon-retto comprendono la resezione en-bloc delle strutture e degli organi infiltrati, un adeguata estensione della resezione e dei margini liberi e la rimozione di un appropriato numero di linfonodi. Tuttavia la perfetta applicazione di tali linee guida può passare in secondo piano in condizioni d’urgenza dove l’attenzione del chirurgo è maggiormente rivolta a risolvere la complicanza che mette a repentaglio la vita del paziente. Lo scopo di questa tesi è di valutare se i criteri oncologici di resezione curativa possano essere rispettati in regime d’urgenza. E’ stato costruito un database raccogliendo i dati di 159 pazienti presentatisi presso l’U.O di Chirurgia generale e d’urgenza tra Gennaio 2006 e Aprile 2008 e sottoposti a resezione chirurgica per cancro del colon e del retto intraperitoneale. Di questi 30 (gruppo A) sono stati operati in regime d’urgenza e 129 (gruppo B) in elezione. I due gruppi sono stati comparati per stadio della neoplasia, incidenza di resezioni en-bloc, lunghezza del pezzo operatorio, margine libero distale, linfonodi rimossi, mortalità e morbilità al fine di eseguire una valutazione prospettica dei risultati. Il gruppo A tende ad avere uno stadio più alto di malattia (p=0,53) dovuto alla totale assenza di tumori in stadio I e ad una maggiore prevalenza di tumori in stadio III/IV. Il numero di linfonodi rimossi è maggiore in urgenza (medie 38,0 vs 28,6; p<0,05) così come la lunghezza del pezzo operatorio (media 608,3 vs 329,1; p<0,0001). I due gruppi non differiscono invece per la lunghezza del margine libero distale (media 107,6 vs 94,3; p=ns) e per la percentuale di resezioni en-bloc (16,7% vs 11,6%; p=0,54). Confrontato con il gruppo B, il gruppo A presenta maggiore mortalità (13,3 vs 8,7; p<0,05) ed una tendenza ad una maggior morbilità (23,3 vs 11,6; p=0,14) I criteri oncologici di resezione per la chirurgia del cancro del colon possono essere rispettati anche in condizioni d’urgenza sebbene non sia stato possibile quantificare l’impatto di tale modello comportamentale nell’outcome chirurgico a distanza.. Morbilità e mortalità sono superiori in urgenza rispetto all’elezione. A tale proposito i risultati ottenuti si sono dimostrati in linea con i migliori valori riportati in letteratura

Synthesis and process optimization of electrospun PEEK-sulfonated nanofibers by response surface methodology

In this study electrospun nanofibers of partially sulfonated polyether ether ketone have been produced as a preliminary step for a possible development of composite proton exchange membranes for fuel cells. Response surface methodology has been employed for the modelling and optimization of the electrospinning process, using a Box-Behnken design. The investigation, based on a second order polynomial model, has been focused on the analysis of the effect of both process (voltage, tip-to-collector distance, flow rate) and material (sulfonation degree) variables on the mean fiber diameter. The final model has been verified by a series of statistical tests on the residuals and validated by a comparison procedure of samples at different sulfonation degrees, realized according to optimized conditions, for the production of homogeneous thin nanofibers

Simulazione di sistemi industriali di combustione attraverso approcci termofluidodinamici reattivi avanzati

Il presente lavoro affronta l’analisi della combustione del polverino di carbone in un sistema costituito da bruciatore e fornace. Inizialmente è stato fornito un quadro generale relativo alla CFD (Computational Fluid Dynamics), approccio utilizzato per l’analisi numerica del dominio oggetto di studio, e al suo utilizzo, ai suoi limiti, ai modelli implementati nei comuni codici utilizzati nell’industria e nella ricerca. Sono state eseguite in una prima fase una serie di simulazioni termofluidodinamiche del sistema attraverso il codice di calcolo CFX, utilizzando parametri e modelli già utilizzati in precedenti lavori nei test numerici con codice FLUENT, al fine di avere un buon confronto con i valori dati dalla pratica sperimentale. Successivamente sono stati utilizzati modelli più avanzati (CPD, soot model, RNA) per verificare il migliore accordo di tali modelli con i dati sperimentali rispetto ai classici modelli

High-Reynolds-number effects on turbulent scalings in compressible channel flow

The effect of the Reynolds number in a supersonic isothermal channel flow is studied using a direct numerical simulation (DNS). The bulk Mach number based on the wall temperature is 1.5, and the bulk Reynolds number is increased up to Reτ ≈ 1000. The use of van Driest velocity transformation in the presence of heated walls has been questioned due to the poor accuracy at low Reynolds number. For this reason alternative transformations of the velocity profile and turbulence statistics have been proposed, as, for instance, semi-local scalings. We show that the van Driest transformation recovers its accuracy as the Reynolds number is increased. The Reynolds stresses collapse on the incompressible ones, when properly scaled with density, and very good agreement with the incompressible stresses is found in the outer layer

Security Programming with High-Level Abstractions: a Tutorial (Extended Abstract)

The specification of security protocols with high-level programming abstractions, suited for security analysis and verification, has been advocated by the formal methods for security research community. Based on these principles of application design, we developed a tutorial to introduce undergraduate students to the foundations of security programming. The main pedagogical goal of this tutorial is to teach, in a simple and effective way, how to build secure distributed applications using common cryptographic primitives abstracting from their low-level details. The tutorial is aimed at helping the students to grasp quickly the main security concepts and to apply them effectively to the coding of distributed programs implementing security properties like authentication and secrecy. As programming is one of the main skills required by the cybersecurity industry, we believe that this tutorial can contribute to the professional development of future graduates

Turbulence and secondary motions in square duct flow

We study turbulent flows in pressure-driven ducts with square cross-section through direct numerical simulation in a wide enough range of Reynolds number to reach flow conditions which are representative of fully developed turbulence. Numerical simulations are carried out over extremely long integration times to get adequate convergence of the flow statistics, and specifically high-fidelity representation of the secondary motions which arise. The intensity of the latter is found to be in the order of 1-2% of the bulk velocity, and unaffected by Reynolds number variations. The smallness of the mean convection terms in the streamwise vorticity equation points to a simple characterization of the secondary flows, which in the asymptotic high-Re regime are found to be approximated with good accuracy by eigenfunctions of the Laplace operator. Despite their effect of redistributing the wall shear stress along the duct perimeter, we find that secondary motions do not have large influence on the mean velocity field, which can be characterized with good accuracy as that resulting from the concurrent effect of four independent flat walls, each controlling a quarter of the flow domain. As a consequence, we find that parametrizations based on the hydraulic diameter concept, and modifications thereof, are successful in predicting the duct friction coefficient