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This study presents an aerodynamic analysis of the 30P30N high-lift three-element wing under varying the angle of attack (α = 5°, 9°, and 23°) at a constant Reynolds number (Rec = 750, 000) by means of large eddy simulations (LES). Baseline results were validated against existing literature, demonstrating good agreement. The increase of the angle of attack entails higher values of lift but also, increased values of drag. At α = 23°, a recirculation area appears above the flap, this being the signature of the stall conditions. This is related to the adverse pressure gradient that develops along the main suction side and the separation of the streamlines in its wake. Actuation strategies using synthetic jets were explored to mitigate this flow separation at this angle of attack, and thus, enhance the efficiency of the wing. Three actuation cases with the jets located on the main wing, on the flap, and on both elements at the same time were investigated. While some improvements are observed with the actuation, particularly in the combined actuation case, the overall wing efficiency was only marginally enhanced. Challenges in achieving significant improvements were attributed to complex flow interactions and the dominance of pressure forces, which affect both the lift and the drag coefficients at the same time. [ABSTRACT FROM AUTHOR]

The increase in emissions associated with aviation requires deeper research into novel sensing and flow-control strategies to obtain improved aerodynamic performances. In this context, data-driven methods are suitable for exploring new approaches to control the flow and develop more efficient strategies. Deep artificial neural networks (ANNs) used together with reinforcement learning, i.e., deep reinforcement learning (DRL), are receiving more attention due to their capabilities of controlling complex problems in multiple areas. In particular, these techniques have been recently used to solve problems related to flow control. In this work, an ANN trained through a DRL agent, coupled with the numerical solver Alya, is used to perform active flow control. The Tensorforce library was used to apply DRL to the simulated flow. Two-dimensional simulations of the flow around a cylinder were conducted and an active control based on two jets located on the walls of the cylinder was considered. By gathering information from the flow surrounding the cylinder, the ANN agent is able to learn through proximal policy optimization (PPO) effective control strategies for the jets, leading to a significant drag reduction. Furthermore, the agent needs to account for the coupled effects of the friction- and pressure-drag components, as well as the interaction between the two boundary layers on both sides of the cylinder and the wake. In the present work, a Reynolds number range beyond those previously considered was studied and compared with results obtained using classical flow-control methods. Significantly different forms of nature in the control strategies were identified by the DRL as the Reynolds number R e increased. On the one hand, for R e ≤ 1000 , the classical control strategy based on an opposition control relative to the wake oscillation was obtained. On the other hand, for R e = 2000 , the new strategy consisted of energization of the boundary layers and the separation area, which modulated the flow separation and reduced the drag in a fashion similar to that of the drag crisis, through a high-frequency actuation. A cross-application of agents was performed for a flow at R e = 2000 , obtaining similar results in terms of the drag reduction with the agents trained at R e = 1000 and 2000. The fact that two different strategies yielded the same performance made us question whether this Reynolds number regime ( R e = 2000 ) belongs to a transition towards a nature-different flow, which would only admits a high-frequency actuation strategy to obtain the drag reduction. At the same time, this finding allows for the application of ANNs trained at lower Reynolds numbers, but are comparable in nature, saving computational resources. [ABSTRACT FROM AUTHOR]

The flow topology of the wake behind a circular cylinder at the super-critical Reynolds number of Re = 7.2 × 105 is investigated by means of large eddy simulations. In spite of the many research works on circular cylinders, there are no studies concerning the main characteristics and topology of the near wake in the super-critical regime. Thus, the present work attempts to fill the gap in the literature and contribute to the analysis of both the unsteady wake and the turbulent statistics of the flow. It is found that although the wake is symmetric and preserves similar traits to those observed in the sub-critical regime, such as the typical two-lobed configuration in the vortex formation zone, important differences are also observed. Owing to the delayed separation of the flow and the transition to turbulence in the attached boundary layer, Reynolds stresses peak in the detached shear layers close to the separation point. The unsteady mean flow is also investigated, and topological critical points are identified in the vortex formation zone and the near wake. Finally, time-frequency analysis is performed by means of wavelets. The study shows that in addition to the vortex shedding frequency, the inception of instabilities that trigger transition to turbulence occurs intermittently in the attached boundary layer and is registered as a phenomenon of variable intensity in time. [ABSTRACT FROM AUTHOR]

The present work focuses on the study of noise generation and radiation of an infinite open three-dimensional cavity at low Mach number with laminar upstream conditions that is of interest to understand noise generation mechanisms in wall-bounded separated flows. A particular feature of this configuration is the oscillatory mode: shear layer mode or wake mode. For the parameters considered in the present study it is seen that while in shear layer mode the flow shows a two-dimensional behavior, in the wake mode the flow is three-dimensional, resulting in significantly different sound sources. The influence of the acoustic feedback mechanism in the shear layer mode has also been investigated comparing the results between different momentum thickness values at the cavity inlet. This paper presents results of sound radiated by a three-dimensional infinite open cavity with aspect ratio L / D = 4 at Reynolds number based on the cavity depth of ReD = 1500 and Mach number of M = 0.15, both for shear layer (L / θ = 67) and wake (L / θ = 84) oscillation modes. To do so, Curle integral evaluated as a post-process of an incompressible solution will be used. The results are compared with the resulting Curle post-process of a two-dimensional incompressible simulation. [ABSTRACT FROM AUTHOR]

Abstract: It is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence transition, reattachment of the flow and further turbulent separation of the boundary layer. In the critical regime, the transition to turbulence in the boundary layer causes the delaying of the separation point and, an important reduction of the drag force on the cylinder surface known as the Drag Crisis. In this paper advanced turbulence simulations at Reynolds numbers in the range of 1.4 × 105-8.5 × 105 will be carried out by means of large-eddy simulations. Numerical simulations using unstructured grids up to 70 million of control volumes have been performed on Marenostrum Supercomputer. One of the major outcomes is shedding some light on the shear layer instabilities mechanisms and their role on the drag crisis phenomena. [Copyright &y& Elsevier]

• The Integral Length Scale Approximation Subfilter Scale Model is applied to separated flows. • A procedure to determine the model constant is proposed. • A low-dissipation finite-element method is used. • The model accounts for interactions between numerical error and turbulence modelling errors. • Flows over a sphere and an Ahmed body are computed, and good agreement with the data is obtained. A new model for the unresolved stresses in large-eddy simulations was recently proposed by Piomelli et al. [J Fluid Mech 2015; 766:499–527] and Rouhi et al., [Phys Rev Fluids 2016; 1(4):0444011], in which the length scale is not related to the grid size, but determined based on turbulence properties. This model, the Integral Length-Scale Approximation (ILSA), has a single parameter, s τ , which represents the contribution of the unresolved scales to the momentum transport, and is assigned by the user. We test ILSA in complex geometries using a low-dissipation finite-element method, and propose a rational method to determine s τ on the basis of a grid-convergence study. The interaction of the model with the numerical method and grid topology is studied first; then, two cases are considered: the subcritical flow around a sphere, and the flow over the Ahmed body, a simplified car model. In each case calculations are performed using three grids and varying s τ. With a consistent combination of grid size and s τ the statistical results are in very good agreement with DNS data and experimental measurements. The eddy viscosity is insensitive to sudden variation of the mesh size, and the model adjusts to the different dissipation and diffusion characteristics associated with different grid topologies and numerical techniques. [ABSTRACT FROM AUTHOR]