102,070 research outputs found
G. Iuso and G. Cafiero Finalization of the plate coupons and manufacturing Clean Sky 2 - Regional Innovative Aircraft Development Platform. REG-2.1.6-TN-POLITO-TECH-219169-A
No full textDrag reduction in a turbulent boundary layer with sinusoidal riblets
No full textWe report on an experimental investigation on the effect of sinusoidal riblets on the near-wall characteristics of a turbulent boundary layer. The investigated riblets are characterized by a fixed wavelength and two different values of the amplitude. We comment on the flow field organization via hot wire anemometry, planar and stereoscopic particle image velocimetry experiments; furthermore, we infer on the friction drag, directly measured with a load cell, comparing the sinusoidal riblets to the reference case of riblets aligned with the mean flow (longitudinal riblets) and the Smooth case.We show that the sinusoidal riblets generally yield higher drag reduction, attaining values as large as 10%, compared with the longitudinal riblets that are limited to 8% under the same conditions. We demonstrate that the drag reduction is associated with an overall attenuation of the turbulence intensity in the buffer layer. Furthermore, we provide statistical evidence of the fact that the sinusoidal riblets are responsible for an attenuation of the Reynolds shear stresses that contribute the most to turbulence production. From the detection of the accelerated events in the buffer layer, we show that the sinusoidal riblets lead to a weakening of the intensity of the events in the streamwise plane and an enhancement of the spanwise induced motion. We relate this mechanism to that responsible for drag reduction when using spanwise wall oscillations, suggesting a possible effect of a secondary alternating vorticity in the grooves of the sinusoidal riblets
Le donne il lavoro e la guerra: fratture e trasformazioni del proletariato femminile italiano durante il primo conflitto mondiale, in "Annale 1995 (III) della Fondazione G. Di Vittorio
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Flow regimes in a trapped vortex cell
No full textThis paper presents results of an experimental investigation on the flow in a trapped vortex cell, embedded into a flat plate, and interacting with a zero-pressure-gradient boundary layer. The objective of the work is to describe the flow features and elucidate some of the governing physical mechanisms, in the light of recent investigations on flow separation control using vortex cells. Hot-wire velocity measurements of the shear layer bounding the cell and of the boundary layers upstream and downstream are reported, together with spectral and correlation analyses of wall-pressure fluctuation measurements. Smoke flow visualisations provide qualitative insight into some relevant features of the internal flow, namely a large-scale flow unsteadiness and possible mechanisms driving the rotation of the vortex core. Results are presented for two very different regimes: a low-Reynolds-number case where the incoming boundary layer is laminar and its momentum thickness is small compared to the cell opening, and a moderately high-Reynolds-number case, where the incoming boundary layer is turbulent and the ratio between the momentum thickness and the opening length is significantly larger than in the first case. Implications of the present findings to flow control applications of trapped vortex cells are also discussed
Large-Scale Forcing of a Turbulent Channel Flow Through Spanwise Synthetic Jets
No full textThe investigation focuses on the forcing of a fully developed turbulent channel flow through a linear array of synthetic jets injected tangentially to the wall and orthogonal to the mean flow direction. Forcing configurations are varied by differently combining the number of actuated jets working in an opposing blowing–suction configuration. Instantaneous wall shear stress and streamwise velocity fluctuations evidence drag reductions as well as turbulence attenuation up to 20%. The forcing effects are persistent up to at least a 150 half-channel height downstream of the injection section. Particle image velocimetry investigations in planes perpendicular to the channel axis highlight the presence of a large-scale streamwise vortical structure covering the whole height of the channel. This structure is thought to be responsible for the significant drag reduction, which is similar to the typical behavior evidenced in the case of colliding jets. The nondimensional forcing frequency of the synthetic jets producing the maximum drag reduction and turbulence attenuation is 0.0074 for the investigated Reynolds number (Reτ=180). A statistical analysis of the near-wall structures demonstrates that the control mechanism acts in a way to reduce them in the forced configuration. It is conclude that the effect of the forcing is such that the near-wall structures merge and become less prone to inducing new structures, thus effectively reducing their number, and consequently the near-wall turbulence activity
Militari, Nazione e Patria nella crisi dell'8 settembre. Scenari esperienze e reazioni
No full textLa crisi dell’8 settembre 1943, all’interno della quale secondo alcuni è “morta la Patria”, ha origini lontane. Può essere considerata una somma di elementi che si rintracciano sin dalla costruzione del nuovo italiano fascista, ovvero nell’obbiettivo finale del regime: la conquista di un ruolo egemone da raggiungere – in modo connaturato al regime stesso – attraverso quella prova suprema che era considerata la guerra.
Un secondo passaggio è rappresentato dalla crisi dell’estate del 1943 allorquando gli italiani precipitano in una crisi dalle caratteristiche del tutto nuove sia in termini di coscienza sia in termini di frammentazione geografica. Tratto importante per i militari diviene il contesto geografico nel quale si trovano ad agire che, si somma alla dimensione politica del crollo. Ambiguità degli ordini, differenze di contesto, capacità di orientamento si pongono quindi come un discriminante fra chi si trova in Italia e chi si trova all’estero.
Il momento dell’Armistizio, di conseguenza, è la fine di un percorso (quello della Nazione fascista) e coincide per molti soldati con quello della “scelta” a fronte di ciò che era accaduto e su come, ai primi di settembre, si interruppe il rapporto con la “nazione fascista” a fronte di un ricongiungimento emotivo e simbolico attraverso la Patria che, quindi, non muore l’8 settembre.
Se il crollo fu così repentino e frammentato, quindi, altrettanto complesso è definire un percorso comune per la ricomposizione dei legami con la Nazione e la Patria. Per quanto tempo queste parole rimangono per così dire “sospese”? Quanto le differenti esperienze incidono nella loro ricongiunzione? Con l’armistizio non era crollata la Patria, era crollata una generazione di giovani italiani: nati, cresciuti, formati sotto il regime.
Non esiste quindi una storia ma tante storie che precipitano nella crisi del 1943 e che da questa riemergono
Deep reinforcement learning for active control of a three-dimensional bluff body wake
No full textThe application of deep reinforcement learning (DRL) to train an agent capable of learning control laws for pulsed jets to manipulate the wake of a bluff body is presented and discussed. The work has been performed experimentally at a value of the Reynolds number Re similar to 10(5) adopting a single-step approach for the training of the agent. Two main aspects are targeted: first, the dimension of the state, allowing us to draw conclusions on its effect on the training of the neural network; second, the capability of the agent to learn optimal strategies aimed at maximizing more complex tasks identified with the reward. The agent is trained to learn strategies that minimize drag only or minimize drag while maximizing the power budget of the fluidic system. The results show that independently on the definition of the reward, the DRL learns forcing conditions that yield values of drag reduction that are as large as 10% when the reward is based on the drag minimization only. On the other hand, when also the power budget is accounted for, the agent learns forcing configurations that yield lower drag reduction (5%) but characterized by large values of the efficiency. A comparison between the natural and the forced conditions is carried out in terms of the pressure distribution across the model's base. The different structure of the wake that is obtained depending on the training of the agent suggests that the possible forcing configuration yielding similar values of the reward is local minima for the problem. This represents, to the authors' knowledge, the first application of a single-step DRL in an experimental framework at large values of the Reynolds number to control the wake of a three-dimensional bluff body. Published under an exclusive license by AIP Publishing
Wall bounded flows manipulation using sinusoidal riblets
No full textWe experimentally investigate the effects of microgrooves on the development of a zero pressure gradient turbulent boundary layer. Starting from the well-known streamwise aligned riblets, we look at the effect of wavy riblets, characterized by a sinusoidal pattern in the mean flow direction. We perform hot wire experiments as well as particle image velocimetry to get some insights on the effect of the sinusoidal shape on the near wall organisation of the boundary layer. The statistical analysis clearly shows that the wavy pattern has a strong influence on the near wall structure of the boundary layer. The statistical analysis performed using the VITA technique reveals that the coherent structures that characterize the turbulent boundary layer are attenuated by the geometry manipulation. Furthermore, the POD reconstructed velocity fields, measured with PIV, reveal that the manipulation tampers with the momentum exchange occurring between the near wall and the outer region of the boundary layer, hence suggesting a modified turbulence production cycle
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