1,354,440 research outputs found
Compressed Sensing: a new framework for signals recovery and its application in Digital Holography
No full textIn olografia digitale l'informazione di ampiezza e fase del fronte d'onda di un oggetto può essere ottenuta quantitativamente e analizzata in digitale attraverso la simulazione numerica delle leggi di diffrazione. Partendo da questo, sono state realizzate molte applicazioni di grande interesse in campo ottico, come l'imaging microscopico a contrasto di fase, la recognition di oggetti 3D e il display dinamico di ologrammi in 3D. In ogni caso, la qualità di queste elaborazioni dipende fortemente dalla qualità degli ologrammi da processare, in particolare dipende dal rumore che il processo di acquisizione di un ologramma introduce sul fronte d'onda registrato.
Le tecniche che vengono, attualmente, applicate sugli ologrammi e sulle loro ricostruzioni numeriche per ottimizzarne la qualità, sono tipiche dell'image processing. Molte di esse permettono di ridurre o sopprimere il rumore, che nel caso specifico di ologrammi digitali, è composto da una mistura di due componenti: un rumore additivo Gaussiano, non correlato all'ologramma, e un rumore moltiplicativo di tipo speckle. Ad esempio i metodi classici basati su trasformata di Fourier e trasformata wavelet sono stati ampiamente studiati, ma possono essere applicati solo nei rispettivi domini trasformati. Inoltre, tali metodi sono fortemente influenzati dal cambiamento di alcuni parametri del fronte d'onda da elaborare, come ad esempio la frequenza, l'ampiezza etc. Al fine di superare efficacemente i limiti evidenziati da i più comuni algoritmi di denoising, risulta fondamentale la realizzazione di algoritmi di ricostruzione che siano robusti nelle diverse condizioni in cui un ologramma può essere acquisito. A tal fine, questo lavoro di tesi propone un nuovo metodo di denoising per ologrammi digitali, molto robusto ed efficiente, basato sulla tecnica del Compressed Sensing (CS).
Il CS è una procedura per l'acquisizione e ricostruzione di segnali che possono essere rappresentati in forma sparsa in un opportuno dominio trasformato. Dopo un inizio lento, questo tema ha acquisito un'elevata risonanza, ed attualmente, ha un impatto scientifico molto forte grazie al riconoscimento che, in natura, molti segnali, se proiettati in un opportuno dominio, ammettono una rappresentazione sparsa. Sfruttando questa sparsità, si può ottenere una migliore approssimazione della funzione distorsione-tasso di un segnale, e fornire linee guida per realizzare un’efficiente ed efficace fase acquisizione e di denoising del segnale, come verrà dimostrato in questa tesi. Nonostante il grande impatto che questo framework ha avuto nella comunità scientifica, la chiave per una comprensione più profonda di questa tecnica è l'identificazione dei limiti fondamentali nei processi di ricostruzione e denoising.
Questa tesi si propone di analizzare questi aspetti per segnali particolari, quali sono gli ologrammi digitali, fornendo una procedura standard per un’efficiente ricostruzione numerica a valle di un processing mirato all'eliminazione delle componenti di rumore. L'algoritmo di denoising progettato ed implementato, risulta molto efficace e robusto nella rimozione del rumore che corrompe gli ologrammi digitali, come dimostrano da diversi esempi sperimentali. Infatti il metodo è stato testato per diversi ologrammi, acquisiti in diverse condizioni sperimentali, ed in ogni caso i risultati si sono rivelati eccellenti, il che dimostra la bontà e la robustezza del metodo proposto. Inoltre è stato analizzato l'effetto di denosing attraverso la proiezione degli ologrammi processati, per vedere e quantificare i miglioramenti che questa tecnica produce nel display 3D di ologrammi
Scrutiny of buffet mechanisms in transonic flow
No full textPurpose: This paper aims to show results of numerical simulations of transonic flow around a supercritical airfoil at chord Reynolds number Rec= 3 × 106, with the aim of elucidating the mechanisms responsible for large-scale shock oscillations, namely, transonic buffet. Design/methodology/approach: Unsteady Reynolds-averaged Navier–Stokes simulations and detached-eddy simulations provide a preliminary buffet map, while a high fidelity implicit large-eddy simulation with an upstream laminar boundary layer is used to ascertain the physical feasibility of the various buffet mechanisms. Numerical experiments with unsteady RANS highlight the role of waves travelling on pressure side in the buffet mechanism. Estimates of the propagation velocities of coherent disturbances and of acoustic waves are obtained, to check the validity of popular mechanisms based on acoustic feedback from the trailing edge. Findings: Unsteady RANS numerical experiments demonstrate that the pressure side of the airfoil plays a marginal role in the buffet mechanism. Implicit LES data show that the only plausible self-sustaining mechanism involves waves scattered from the trailing edge and penetrating the sonic region from above the suction side shock. An interesting side result of this study is that buffet appears to be more intense in the case that the boundary layer state upstream of the shock is turbulent, rather than laminar. Originality/value: The results of the study will be of interest to any researcher involved with transonic buffet
A general framework for the evaluation of shock-capturing schemes
No full textWe introduce a standardized procedure for benchmarking shock-capturing schemes which is intended to go beyond traditional case-by-case analysis, by setting objective metrics for cross-comparison of flow solvers. The main idea is that use of shock-capturing schemes yields both distributed errors associated with propagation of wave-like disturbances in smooth flow regions, and localized errors at shocks where nonlinear numerical mechanisms are most active. Our standardized error evaluation framework relies on previous methods of analysis for the propagation error with associated cost/error mapping, and on novel analysis of the shock-capturing error based on a model scalar problem. Amplitude and phase errors are identified for a number of classical shock-capturing schemes with different order of accuracy. Whereas all schemes are found to be, as expected, first-order accurate at shocks, quantitative differences are found to be significant, and we find that certain schemes in wide use (e.g. high-order WENO schemes) may yield substantial over-amplification of incoming disturbances at shocks. Illustrative calculations are also shown for the 1D Euler equations, which support sufficient generality of the analysis, although nonlinearity suggests caution in straightforward extrapolation to other flow cases
Control of transonic buffet through deployable flaps
No full textWe carry out a series of numerical simulations of transonic flow over a
supercritical airfoil at M=0.7, incidence angle alpha=7°, and chord Reynolds
number Rec=300000, with the aim of suppressing the large-scale shock oscillations (buffet) which are expected to occur under these conditions. For that purpose we use a deployable flap, which is placed in the aft part of the airfoil suction side. The main rationale is to mimic the behavior of shock holders, which are frequently used in experiments to stabilize normal shock waves. For preliminary simulations, the flap has been shaped as a bi-convex airfoil. Two sample configurations have been considered, namely a HIGH configuration, with flap chord cf=0.2c, relative thickness of 6%, and distance of about 0.15c from the airfoil, and a LOW configuration, with flap chord cf=0.1c, relative thickness of 12%, and distance of about 0.075c from the airfoil. The main result is that both the HIGH and the LOW configuration are capable of effectively suppressing buffet, after an initial transient. Both types of control have the main effect of pushing the main shock upstream than its time-average uncontrolled location. However, whereas LOW actuation has the effect of slightly extending the region of flow reversal past the airfoil trailing edge, HIGH actuation is effective in reducing its size. Accompanied with HIGH actuation also comes some small flow separation past the flap, whereas in the LOW case the flap is embedded in a low-speed region, and the flow does not separate. The practical implications of control are observed in the time history of the aerodynamic efficiency. While the efficiency levels off in both cases, LOW actuation retains the (time-average) efficiency of the uncontrolled case, whereas HIGH actuation is responsible for additional losses. A series of numerical simulations covering a wide range of control parameters is currently ongoing, a full account of which will be given in the final paper
Drag Reduction on a Transonic Airfoil
No full textFlow control for turbulent skin-friction drag reduction is applied to a transonic airfoil to improve its aerodynamic performance. The study is based on direct numerical simulations (with up to 1.8 billion cells) of the compressible turbulent flow around a supercritical airfoil, at Reynolds and Mach numbers of Re∞=3×105 and M∞=0.7 . Control via spanwise forcing is applied over a fraction of the suction side of the airfoil. Besides locally reducing friction, the control modifies the shock wave and significantly improves the aerodynamic efficiency of the airfoil by increasing lift and decreasing drag. Hence, the airfoil can achieve the required lift at a lower angle of attack and with a lower drag. Estimates at the aircraft level indicate that substantial savings are possible; when control is active, its energy cost becomes negligible thanks to the small application area. We suggest that skin-friction drag reduction should be considered not only as a goal, but also as a tool to improve the global aerodynamics of complex flows
Guided wave propagation and interaction with ice layers in marine structures
No full textReliable and autonomous monitoring systems have been increasingly considered to enhance metal and composite-based maritime structures. The main goal is to continuously assess the overall condition of the structure using several nodes of sensors to ensure safety and cost-effective maintenance during the lifetime. To achieve the detection of small emerging flaws, ultrasound wave propagation in thin sheets is successfully adopted to interrogate both metallic and composite structures. However, ships are operating in water environment and even in polar regions, making the correct extraction of damage parameters from ultrasound signals quite challenging. An interesting and practically important aspect deals with icing problems, where thin layers of ice emerge in direct contact with the ship’s hull. This may lead to misinterpretations of measured signals, inducing false alarms and potentially missing damage detection. Guided wave propagation should be investigated in this contribution accounting for these aspects to prevent misleading interpretation. A finite element model has been considered including ice models to address the effect of an emerging ice layer on wave propagation using both pitch-catch and pulse-echo approaches. Numerical results show that reflections generated at the discontinuity raise with the thickness and length of ice layer. The ice accretion affects the transmitted energy, whose result is even more visible in the direct propagation analysis. The outcomes definitely prove the ability of guided waves to detect ice and the modeling is well suited for preliminary design of further experiments
Direct numerical simulation of conical shock wave-turbulent boundary layer interaction
No full textDirect numerical simulation of the Navier-Stokes equations is carried out to investigate the interaction of a conical shock wave with a turbulent boundary layer developing over a flat plate at free-stream Mach number and Reynolds number , based on the upstream boundary layer momentum thickness. The shock is generated by a circular cone with half opening angle . As found in experiments, the wall pressure exhibits a distinctive N-wave signature, with a sharp peak right past the precursor shock generated at the cone apex, followed by an extended zone with favourable pressure gradient, and terminated by the trailing shock associated with recompression in the wake of the cone. The boundary layer behaviour is strongly affected by the imposed pressure gradient. Streaks are suppressed in adverse pressure gradient (APG) zones, but re-form rapidly in downstream favourable pressure gradient (FPG) zones. Three-dimensional mean flow separation is only observed in the first APG region associated with the formation of a horseshoe vortex, whereas the second APG region features an incipient detachment state, with scattered spots of instantaneous reversed flow. As found in canonical geometrically two-dimensional wedge-generated shock-boundary layer interactions, different amplification of the turbulent stress components is observed through the interacting shock system, with approach to an isotropic state in APG regions, and to a two-component anisotropic state in FPG. The general adequacy of the Boussinesq hypothesis is found to predict the spatial organization of the turbulent shear stresses, although different eddy viscosities should be used for each component, as in tensor eddy-viscosity models, or in full Reynolds stress closures
Investigation on guided waves propagation across ice layers
No full textIcing conditions are threatening for a variety of engineering applications. When accreting on the aircraft surface, ice may lead to catastrophic accidents while its deposition on turbine blades can overload the components reducing power and inducing damage. The first way to protect engineering constructions from icing issues deals with early detection of ice. Ultrasound has been demonstrated to be effective for detecting changes of the structure which it is propagating through. Guided waves interact with emerging flaw and discontinuity, including ice accreting on the surface. This paper introduces an experimental campaign carried out on composite structures subject to icing. The building up of planar ice onto surface structure is analysed. Furthermore, a parametric investigation is carried out by finite element modelling to look into the interaction between wave and ice while varying its dimension. Experimental findings demonstrate the ability of guided waves to detect ice layers accreting onto surface. In addition, the ice is likely to be identified from the scattering of guided waves resulting in different transmission and reflection characteristics. Instead, numerical outcomes show how the interaction of guided waves depends upon the thickness and length of the ice
Ultrasonic De-Icing System for Leading Edge in Composite Material
No full textThe paper presents a numerical study about a de-icing system using ultrasonic waves. The activity is developed within the project “SMart On-Board Systems” (SMOS), which is part of Italian Aerospace National Research Program, funded by the Italian Ministry of Education (MIUR) and Research and coordinated by Italian Aerospace Research Centre (CIRA). The system is conceived for an aircraft wing leading edge and it shall be extended to other aircraft components, once its efficiency and reliability will be demonstrated. In this work, a numerical study about a 0012 NACA profile in composite material is discussed and the simulations results coming from finite element analyses in frequency and time domains are presented
Drag Reduction on a Transonic Wing
No full textFlow control for reducing skin-friction drag in the turbulent regime is applied to a transonic airfoil to improve its global aerodynamic performance. The study relies on direct numerical simulations of the compressible turbulent flow around a supercritical airfoil at Reynolds and Mach numbers of Re∞ = 3 × 105 and M∞ = 0.7. The control is applied on a portion of the airfoil suction side only. However, besides locally reducing drag, the control modifies the position and the intensity of the shock wave and significantly improves the aerodynamic efficiency of the airfoil by increasing the lift and slightly decreasing the total drag. The increase of the aerodynamic efficiency implies that the airfoil can achieve the desired lift at a lower angle of attack and, therefore, with a much lower drag. Estimates of the benefits on a full aircraft are provided and indicate that substantial savings are possible, even when the energy cost of an active control is considered. These results suggest that skin-friction drag reduction should no be considered as a goal only, but also as tool to modify and control the global aerodynamics of complex flows
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