Large-eddy simulation of an open-channel flow bounded by a semi-dense rigid filamentous canopy: Scaling and flow structure

We have carried out a large-eddy simulation of a turbulent open-channel flow over a marginally dense, fully submerged, rigid canopy. The canopy is made of a set of rigid, slender cylinders normally mounted on a solid wall. The flow in the canopy is resolved stem-by-stem by means of an immersed boundary method. It is found that the flow behavior can be categorized according to the velocity distribution into two separate spatial regions: The canopy itself and the outer region above it. Within the region occupied by the canopy elements, the velocity magnitude is found to be related to the local shear stress. Outside the canopy, a logarithmic velocity profile matching the canonical turbulent open-channel flow over rough walls is recovered albeit the strong manipulation exerted by the canopy on the buffer layer. In the innermost layer, the presence of the stems is responsible for redistributing the local momentum fluctuations from a streamwise to a spanwise leading component, inhibiting the survival of the wall streamwise velocity streaks. On the other hand, the outer region presents a structure very similar to the well-known logarithmic boundary layer with the presence of large and energetic streamwise velocity streaks generated by a system of quasistreamwise vortices. These vortices strongly contribute to the intracanopy fluctuations through vigorous sweep and ejection events that affect all the region occupied by the stems. Consistent with the results of previous investigations [H. Nepf, "Flow and transport in regions with aquatic vegetation," Annu. Rev. Fluid Mech. 44, 123-142 (2012)], it is found that the inflection point in the mean velocity profile, produced by the drag discontinuity at the canopy tip, promotes the appearance of another system of spanwise oriented vorticity structures. However, different from previous results reported in the literature [J. Finnigan, "Turbulence in plant canopies," Annu. Rev. Fluid Mech. 32, 519-571 (2000)], in our simulations, the presence of alternating head up-head down hairpin vortices generated by a mutual induction of the counter-rotating spanwise vortices is not observed. Instead, we advocate that the modulation of the spanwise coherent vorticity is due to the action of the external logarithmic layer structures (i.e., the outer streamwise vortices that penetrate the canopy) rather than by upwash and downwash motions induced by the mutual interaction of the spanwise rollers

Passive control of the flow around unsteady aerofoils using a self-activated deployable flap

Self-activated feathers are used by many birds to adapt their wing characteristics to the sudden change of flight incidence angle. In particular, dorsal feathers are believed to pop-up as a consequence of unsteady flow separation and to interact with the flow to palliate the sudden stall breakdown typical of dynamic stall. Inspired by the adaptive character of birds feathers, some authors have envisaged the potential benefits of using of flexible flaps mounted on aerodynamic surfaces to counteract the negative aerodynamic effects associated with dynamic stall. This contribution explores more in depth the physical mechanisms that play a role in the modification of the unsteady flow field generated by a NACA0020 aerofoil equipped with an elastically mounted flap undergoing a specific ramp-up manoeuvre. We discuss the design of flaps that limit the severity of the dynamic stall breakdown by increasing the value of the lift overshoot also smoothing its abrupt decay in time. A detailed analysis on the modification of the turbulent and unsteady vorticity field due to the flap flow interaction during the ramp-up motion is also provided to explain the more benign aerodynamic response obtained when the flap is in use

On the genesis of different regimes in canopy flows: a numerical investigation

We have performed fully resolved simulations of turbulent flows over various submerged rigid canopies covering the wall of an open channel. All the numerical predictions have been obtained considering the same nominal bulk Reynolds number (i.e. Reb=UbH

The PELskin project-part V: towards the control of the flow around aerofoils at high angle of attack using a self-activated deployable flap

During the flight of birds, it is often possible to notice that some of the primaries and covert feathers on the upper side of the wing pop-up under critical flight conditions, such as the landing approach or when stalking their prey (see Fig. 1) . It is often conjectured that the feathers pop up plays an aerodynamic role by limiting the spread of flow separation . A combined experimental and numerical study was conducted to shed some light on the physical mechanism determining the feathers self actuation and their effective role in controlling the flow field in nominally stalled conditions. In particular, we have considered a NACA0020 aerofoil, equipped with a flexible flap at low chord Reynolds numbers. A parametric study has been conducted on the effects of the length, natural frequency, and position of the flap. A configuration with a single flap hinged on the suction side at 70 % of the chord size c (from the leading edge), with a length of (Formula presented.) matching the shedding frequency of vortices at stall condition has been found to be optimum in delivering maximum aerodynamic efficiency and lift gains. Flow evolution both during a ramp-up motion (incidence angle from (Formula presented.) to (Formula presented.) with a reduced frequency of (Formula presented.), (Formula presented.) being the free stream velocity magnitude), and at a static stalled condition ((Formula presented.)) were analysed with and without the flap. A significant increase of the mean lift after a ramp-up manoeuvre is observed in presence of the flap. Stall dynamics (i.e., lift overshoot and oscillations) are altered and the simulations reveal a periodic re-generation cycle composed of a leading edge vortex that lift the flap during his passage, and an ejection generated by the relaxing of the flap in its equilibrium position. The flap movement in turns avoid the interaction between leading and trailing edge vortices when lift up and push the trailing edge vortex downstream when relaxing back. This cyclic behaviour is clearly shown by the periodic variation of the lift about the average value, and also from the periodic motion of the flap. A comparison with the experiments shows a similar but somewhat higher non-dimensional frequency of the flap oscillation. By assuming that the cycle frequency scales inversely with the boundary layer thickness, one can explain the higher frequencies observed in the experiments which were run at a Reynolds number about one order of magnitude higher than in the simulations. In addition, in experiments the periodic re-generation cycle decays after 3–4 periods ultimately leading to the full stall of the aerofoil. In contrast, the 2D simulations show that the cycle can become self-sustained without any decay when the flap parameters are accurately tuned

Mechanisms of airfoil noise near stall conditions

The focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈2×10⁵–4×10⁵. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered by the trailing edge. To this aim, we use simultaneous noise and surface pressure measurement in addition to velocimetric measurements by means of hot wire anemometry and time-resolved particle image velocimetry. Three possible mechanisms for the so-called “separation-stall noise” have been identified in addition to a clear link between far-field noise, surface pressure, and velocity fields in the noise generation

Numerical Investigation of Regime Transition in Canopy Flows

We have carried out highly resolved simulations of turbulent open channel flows. The channel wall is covered with different filamentous layers sharing the same thickness (h=0.1H, where H is the open channel height) and bulk Reynolds number (i.e., Reb=UbH/ν, , Ub is the bulk velocity and ν the kinematic fluid viscosity). The layers are composed of rigid, slender cylindrical filaments mounted perpendicular to the bottom wall. We have selected two layer configurations characterised by filament spacing ratios of ΔS/H=π/24≃0.13 and ΔS/H=π/32≃0.098. The geometrical features of the two layers, allow to classify them as transitional canopies (λ=dh/ΔS2≃0.15, where d is the filament diameter, i.e. dh is the filament frontal area) (Monti et al. 2020), which is defined as the separation between the sparse-dense asymptotic regimes, proposed by Nepf (2012). While the physical characterisation of the two asymptotic regimes is fairly understood, the transitional conditions remain an open question since the physical characteristics unique to the sparse and dense scenarios coexist in the transitional regime. By resolving every single filament with the aid of an immersed boundary technique in the framework of a Large Eddy formulation, we report the physical mechanisms that emerge at the onset of different regimes (chosen values of λ fall on the verge between a dense and a sparse condition) and verify the criterion associated with the inception of the transition regime

Numerical Simulation of a Passive Control of the Flow Around an Aerofoil Using a Flexible, Self Adaptive Flaplet

© 2018 The Author(s) Self-activated feathers are used by almost all birds to adapt their wing characteristics to delay stall or to moderate its adverse effects (e.g., during landing or sudden increase in angle of attack due to gusts). Some of the feathers are believed to pop up as a consequence of flow separation and to interact with the flow and produce beneficial modifications of the unsteady vorticity field. The use of self adaptive flaplets in aircrafts, inspired by birds feathers, requires the understanding of the physical mechanisms leading to the mentioned aerodynamic benefits and the determination of the characteristics of optimal flaps including their size, positioning and ideal fabrication material. In this framework, this numerical study is divided in two parts. Firstly, in a simplified scenario, we determine the main characteristics that render a flap mounted on an aerofoil at high angle of attack able to deliver increased lift and improved aerodynamic efficiency, by varying its length, position and its natural frequency. Later on, a detailed direct numerical simulation analysis is used to understand the origin of the aerodynamic benefits introduced by the flaplet movement induced by the interaction with the flow field. The parametric study that has been carried out, reveals that an optimal flap can deliver a mean lift increase of about 20% on a NACA0020 aerofoil at an incidence of 20 o degrees. The results obtained from the direct numerical simulation of the flow field around the aerofoil equipped with the optimal flap at a chord Reynolds number of 2 × 10 4 shows that the flaplet movement is mainly induced by a cyclic passage of a large recirculation bubble on the aerofoil suction side. In turns, when the flap is pushed downward, the induced plane jet displaces the trailing edge vortices further downstream, away from the wing, moderating the downforce generated by those vortices and regularising the shedding cycle that appears to be much more organised when the optimal flaplet configuration is selected

On the manipulation of flow and acoustic fields of a blunt trailing edge aerofoil by serrated leading edges

This paper employs serrated leading edges to inject streamwise vorticity to the downstream boundary layer and wake to manipulate the flow field and noise sources near the blunt trailing edge of an asymmetric aerofoil. The use of a large serration amplitude is found to be effective to suppress the first noise source—bluntness-induced vortex shedding tonal noise—through the destruction of the coherent eigenmodes in the wake. The second noise source is the instability noise, which is produced by the interaction between the boundary layer instability and separation bubble near the blunt edge. The main criterion needed to suppress this noise source is related to a small serration wavelength because, through the generation of more streamwise vortices, it would facilitate a greater level of destructive interaction with the separation bubble. If the leading edge has both a large serration amplitude and wavelength, the interaction between the counter-rotating vortices themselves would trigger a turbulent shear layer through an inviscid mechanism. The turbulent shear layer will produce strong hydrodynamic pressure fluctuations to the trailing edge, which then scatter into broadband noise and transform into a trailing edge noise mechanism. This would become the third noise source that can be identified in several serrated leading edge configurations. Overall, a leading edge with a large serration amplitude and small serration wavelength appears to be the optimum choice to suppress the first and second noise sources and, at the same time, avoid the generation of the third noise source

Large-eddy simulation of two-dimensional dunes in a steady, unidirectional flow

We performed large-eddy simulations of the flow over a typical two-dimensional dune geometry at laboratory scale (the Reynolds number based on the average channel height andmean velocity is 18,900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The results are validated by comparison with simulations and experiments in the literature. The flowseparates at the dune crest, generating a shear layer that plays a crucial role in the transport of momentum and energy, and the generation of coherent structures. The turbulent kinetic energy budgets show the importance of the turbulent transport and mean-flow advection in the bulk flow above the shear layer. In the recirculation zone and in the attached boundary layers production and dissipation are the most important terms. Large, coherent structures of various types can be observed. Spanwise vortices are generated in the separated shear layer due to the Kelvin-Helmholtz instability; as they are advected, they undergo lateral instabilities and develop into horseshoe-like structures, are tilted downward, and finally reach the surface. The ejection that occurs between the legs of the vortex creates the upwelling and downdrafting events on the free surface known as "boils." Near-wall turbulence, after the reattachment point, is affected by large streamwise Taylor-Görtler vortices generated on the concave part of the stoss side, which affect the distribution of the near-wall streaks

The impact of non-equilibrium flow on the structure of turbulence over river dunes

This piece of research expands our description of how rivers flow over dunes on a river bed. Most of the scientific communities' research to date has used unnaturally steady conditions to measure how water moves over dunes. Yet these flow conditions are not strictly true to the variety of conditions nature produces, most importantly during floods. This research is the first detailed description of a wide range of flow states over dunes, and changes our present understanding of the structure of flow over dunes in rivers. Consequently, the scientific community will be able to use this new information to better model and simulate how rivers work, how they flood, and how they transport sediment towards the worlds deltas