1,720,987 research outputs found
Sintesi e caratterizzazione ottica di amilosio modificato con gruppi spiropirano mediante reazione di “click”
No full textCon questo lavoro ci prefiggiamo l’obbiettivo di produrre rivestimenti innovativi a base di polimeri naturali, in grado di fornire risposte fotocromiche quando irraggiati con luce visibile. L’amilosio, un polimero lineare costituito da unità glucosidiche legate 1-4, è stato
scelto quale substrato di partenza poiché è uno dei componenti dell’amido e ne possiede quindi tutti i vantaggi. Esso è stato azidato selettivamente in C6 [1,2] e successivamente modificato con un gruppo responsivo tramite reazione di cicloaddizione di Huisgen azide-alchino, “click”. Il gruppo responsivo scelto è un derivato dello spiropirano avente una funzionalità alchinica (SPCC) [3]. Esso è in grado, mediante la modifica della propria struttura molecolare, di fornire variazioni cromatiche al variare delle condizioni ambientali nel quale il sistema è immerso [3,4]. Le variazioni ambientali includono l’irraggiato con luce UV/Vis, la polarità dell’intorno ed il pH [4].
Sono stati preparati tre derivati dell’amilosio a contenuto crescente di SPCC (DS 0.15, 0.40, 0.80). Essi sono stati caratterizzati mediante spettroscopia infrarossa, NMR ed analisi elementare e successivamente è stata studiata la loro risposta ottica in soluzione e/o dispersione mediante spettroscopia UV/Vis e fluorimetria. In particolare è stata indagata la stabilità delle dispersioni e la risposta al variare della polarità del solvente (solvatocromismo) sia prima che dopo irraggiamento con luce UV.
In fine, è stata valutata la possibilità di produrre coating utilizzando il derivato a contenuto intermedio di SPCC (Ds 0.4). A tale scopo sono stati preparati dei film per "solution casting" su vetro e per deep coating su carta. In tutti i casi il derivato dell’amilosio ha mostrato capacità di filmatura superiore all’amilosio tal quale. Tutti i film sono stati caratterizzati mediante misure di angolo di contatto e quelli su vetro anche mediante spettroscopia UV/Vis, prima e dopo irraggiamento. Il derivato contenente funzionalità spiropiraniche è risultato più idrofobo dell’amilosio di partenza. L’idrofilia viene parzialmente recuperata
irraggiando il film con luce UV/Vis. L’irraggiamento produce anche variaziani cromatiche nette dei coating (da giallo a viola) le quali sono stabili nel tempo a condizione ambiente, ma reversibili per irraggiamento con intensa luce verde
Large Eddy Simulation of the By-pass Transition Process under Different Inlet Turbulence Conditions
No full textThe transition process of the boundary layer developing over a flat plate with elevated inlet Free Stream Turbulence Intensity (FSTI) has been studied by means of Large Eddy Simulation (LES). To this purpose, four cases with different inflow disturbances have been tested varying the magnitude and the length scale of turbulence. LES has been performed by using the finite-volume ANSYS Fluent code. The computational domain, which was constituted by a rectangular domain with a zero thickness plate, was based on an ERCOFTAC test case in order to provide a validation with a well-known set of data by comparing the boundary layer integral parameters and mean and fluctuating streamwise velocity profiles. The four cases were discussed within the paper by looking at classical statistical properties as well as advanced post-processing tools. It was shown that the decrease in the free stream turbulence level postpones the transition location, whereas the variation of the integral length scale has a very low influence on the distribution of the time-mean flow properties. Proper Orthogonal Decomposition (POD) has been applied to the instantaneous LES flow fields in order to provide a statistical representation of the structures responsible for transition and their response to free-stream turbulence intensity and length scale. The presence of vortical filaments parallel to the wall, typically referred as boundary layer streaks, is clearly identified; their characteristic dimensions and how they change as a function of FSTI properties were analyzed within the paper
Identification of coexisting dynamics in boundary layer flows through proper orthogonal decomposition with weighting matrices
No full textA different version of the classic proper orthogonal decomposition (POD) procedure introducing spatial and temporal weighting matrices is proposed. Furthermore, a newly defined non-Euclidean (NE) inner product that retain similarities with the POD is introduced in the paper. The aim is to emphasize fluctuation events localized in spatio-temporal regions with low kinetic energy magnitude, which are not highlighted by the classic POD. The different variants proposed in this work are applied to numerical and experimental data, highlighting analogies and differences with respect to the classic and other normalized variants of POD available in the literature. The numerical test case provides a noise-free environment of the strongly organized vortex shedding behind a cylinder. Conversely, experimental data describing transitional boundary layers are used to test the capability of the procedures in strongly not uniform flows. By-pass and separated flow transition processes developing with high free-stream disturbances have been considered. In both cases streaky structures are expected to interact with other vortical structures (i.e. free-stream vortices in the by-pass case and Kelvin–Helmholtz rolls in the separated type) that carry a significant different amount of energy. Modes obtained by the non-Euclidean POD (NE-POD) procedure (where weighted projections are considered) are shown to better extract low energy events sparse in time and space with respect to modes extracted by other variants. Moreover, NE-POD modes are further decomposed as a combination of Fourier transforms of the related temporal coefficients and the normalized data ensemble to isolate the frequency content of each mode
Dynamic mode decomposition analysis of separated boundary layers under variable reynolds number and free-stream turbulence
No full textNumerical Study of the Interaction between the Sealing Flow Rate and the Main Flow in a Gas Turbine Stage
No full textThis paper focuses on the interaction process between the main and cavity flows in a gas turbine. To this purpose, Reynolds-Averaged Navier-Stokes (RANS) simulations are conducted on the full stage with including the cavity system. Different sealing flow rates injected into the cavity are simulated to study different operating conditions. The study employs a robust computational fluid dynamics (CFD) framework (Cadence) to perform the RANS simulations, leveraging on turbulence models to capture the complex interactions due to the cavity. The aim is to validate the numerical approach in order to provide an accurate flow field within the cavity. To this purpose, the numerical model is compared against experimental data and a good agreement is found. The RANS simulations are validated with experimental data and they allow us to study the time-mean flow field and to calculate an estimate of the time-mean performances of the machine. The increase in the mass flow rate injected into the cavity leads to a local increase in losses near the hub and also to a local variation of the flow angle, with the effect of reducing the aerodynamic efficiency of the downstream blade. This study provides additional data about the main channel-cavity interaction phenomena that may be useful to designers to improve gas turbine performance
Chemical modification of polysaccharides and proteins: synthetic strategies and opportunities toward new functional materials
No full textInstability processes in short and long laminar separation bubbles
No full textThis work studies the link between the bursting process of a flat plate laminar separation bubble and the modification of the stability characteristics of the separated shear layer due to changes in the flow parameters. A vast population of short and long laminar separation bubbles was surveyed by means of Particle Image Velocimetry instrumentation for different values of the Reynolds number, the free-stream turbulence intensity and the streamwise pressure gradient. A fine-step variation of the free-stream velocity allowed us to determine the critical Reynolds number at which bursting occurs. Successively, the most amplified wavelength and frequency were computed for both the short and the long bubble regimes. Once scaled with the boundary layer displacement thickness at separation, the average wavenumber of the vortices shed by the bubble was found to be constant and equal to about 0.9 in the short regime, accordingly to previous studies. Differently, this quantity reduces to about 0.6 in the long bubble regime, and a marked change in the Strouhal number of vortex shedding occurs. Also, the temporal growth of spanwise vortices was seen to occur in the recirculation region of long type bubbles, being linked to an absolute instability of disturbances. The currently acquired data demonstrate the existing link between the bursting process of a laminar separation bubble and a marked change in the instability mechanisms driving the transition process of the boundary layer. A simplified correlation for the prediction of bursting is provided in this work as a function of the free-stream turbulence intensity and the streamwise pressure gradient
Investigation of Laminar Separation Bubble on Flat Plate with Adverse Pressure Gradient: Time-Averaged Flow Field Analysis
No full textThe performance of turbomachinery blade profiles, at low Reynolds numbers, is influenced by laminar separation bubbles (LSBs). Such a bubble is caused by a strong adverse pressure gradient (APG), and it makes the laminar boundary layer to separate from the curved profile surface, before it becomes turbulent. The paper consists on a joint experimental and numerical investigation on a flat plate with adverse pressure gradient. The experiment provides detailed results including distribution of wall pressure coefficient and boundary layer velocity and turbulence profiles for several values of typical influencing parameters on the behavior of the flow phenomena: Reynolds number, free stream turbulence intensity, and end-wall opening angle, which determines the adverse pressure gradient intensity. The numerical work consists on carrying out a systematic analysis, with Reynolds Average Navier-Stokes (RANS) simulations. The results of the numerical simulations are critically investigated and compared with the experimental ones in order to understand the effect of the main physical parameters on the LSB behavior. For RANS simulations, different turbulence and transition models are compared at first to identify the adaptability to the flow phenomena; then, the influence of the three aforementioned parameters on the LSB behavior is investigated under a typical aggressive adverse pressure gradient. Boundary layer integral parameters are discussed for the different cases in order to understand the flow phenomena in terms of flow time-mean properties
Identification of free-stream and boundary layer correlating events in free-stream turbulence-induced transition
No full textBoundary layer receptivity to free-stream disturbances plays a crucial role in forming coherent structures, whose breakup drives the laminar to turbulent flow transition. In the present work, an extended proper orthogonal decomposition (E-POD) procedure is applied to particle image velocimetry (PIV) data to identify correlating events between the free-stream velocity field and transitional boundary layers for flow configurations typical of low-pressure turbine blades. Data collected in two wall-parallel planes were ordered along the homogeneous spanwise coordinate so that the dominant POD coefficients provide the most energetic spanwise wavelengths in the free-stream and the near-wall regions. Then, the cross-correlation matrix of the POD spanwise coefficients computed independently in both measuring planes directly identifies the free-stream scales showing the highest degree of correlation with the boundary layer structures. Low-order reconstructions of the original PIV data show that the most correlating events are directly linked to the formation and the successive breakup process of streaky structures. Otherwise, larger-scale structures which are not involved in the transition process are filtered out. Interestingly, free-stream disturbances appear as organized wave packets with significant elongation in the streamwise direction when the velocity fields are reconstructed considering only the most correlating modes. The effect due to the Reynolds numbers, the pressure gradient, and the free-stream turbulence variation on the free-stream modes affecting the formation of coherent structures in the boundary layer is discussed in the paper
Large Eddy Simulation of the by-pass transition process under different inlet turbulence conditions
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