Showing papers on "Leading edge published in 2017"
TL;DR: In this article, the performance of wind turbines is substantially affected by the phenomenon of dynamic stall which is induced by the variations of angle of attack of rotating blades, especially at low tip-speed-ratios (TSRs).
129 citations
TL;DR: In this article, the effect of edge power loading on the shape of the ITER divertor was investigated, and it was concluded that the geometrical approximation for leading edge power load on radially misaligned poloidal leading edges is indeed valid.
Abstract: The key remaining physics design issue for the ITER tungsten (W) divertor is the question of monoblock (MB) front surface shaping in the high heat flux target areas of the actively cooled targets. Engineering tolerance specifications impose a challenging maximum radial step between toroidally adjacent MBs of 0.3 mm. Assuming optical projection of the parallel heat loads, magnetic shadowing of these edges is required if quasi-steady state melting is to be avoided under certain conditions during burning plasma operation and transiently during edge localized mode (ELM) or disruption induced power loading. An experiment on JET in 2013 designed to investigate the consequences of transient W edge melting on ITER, found significant deficits in the edge power loads expected on the basis of simple geometric arguments, throwing doubt on the understanding of edge loading at glancing field line angles. As a result, a coordinated multi-experiment and simulation effort was initiated via the International Tokamak Physics Activity (ITPA) and through ITER contracts, aimed at improving the physics basis supporting a MB shaping decision from the point of view both of edge power loading and melt dynamics. This paper reports on the outcome of this activity, concluding first that the geometrical approximation for leading edge power loading on radially misaligned poloidal leading edges is indeed valid. On this basis, the behaviour of shaped and unshaped monoblock surfaces under stationary and transient loads, with and without melting, is compared in order to examine the consequences of melting, or power overload in context of the benefit, or not, of shaping. The paper concludes that MB top surface shaping is recommended to shadow poloidal gap edges in the high heat flux areas of the ITER divertor targets.
120 citations
TL;DR: In this article, a detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength, and an optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length scale is approximately one-fourth the serral wavelength.
Abstract: This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto three-dimensional aerofoils. The influence on the noise reduction of the turbulence integral length scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length scale is approximately one-fourth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number , where , and are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce aerofoil self-noise. The mechanism for this phenomenon is explored through particle image velocimetry measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.
115 citations
TL;DR: It is proposed that the mechanical signal of membrane tension exerts upstream control in mechanotransduction by periodically compressing and relaxing the lamellipodium, leading to the positioning of adhesions at the leading edge of cells.
Abstract: Cell migration is dependent on adhesion dynamics and actin cytoskeleton remodeling at the leading edge. These events may be physically constrained by the plasma membrane. Here, we show that the mechanical signal produced by an increase in plasma membrane tension triggers the positioning of new rows of adhesions at the leading edge. During protrusion, as membrane tension increases, velocity slows, and the lamellipodium buckles upward in a myosin II-independent manner. The buckling occurs between the front of the lamellipodium, where nascent adhesions are positioned in rows, and the base of the lamellipodium, where a vinculin-dependent clutch couples actin to previously positioned adhesions. As membrane tension decreases, protrusion resumes and buckling disappears, until the next cycle. We propose that the mechanical signal of membrane tension exerts upstream control in mechanotransduction by periodically compressing and relaxing the lamellipodium, leading to the positioning of adhesions at the leading edge of cells.
104 citations
TL;DR: A key mechanism promoting epithelial migration is defined and a different paradigm for planar cell-cell signaling is established, which mediates short-range communication between neighboring cells instead of transmitting long-range information across a tissue.
86 citations
TL;DR: It is proposed that the mechanochemical feedbacks underlying wave generation in keratocytes may constitute a general module for establishing excitable actin dynamics in other cellular contexts.
79 citations
TL;DR: The morphological adaptations of owls leading to silent flight are reviewed and also aerodynamic properties of owl wings are discussed, which have been an inspiration for biologists and engineers for the development of devices with reduced noise production.
Abstract: Owls are an order of birds of prey that are known for the development of a silent flight. We review here the morphological adaptations of owls leading to silent flight and discuss also aerodynamic properties of owl wings. We start with early observations (until 2005), and then turn to recent advances. The large wings of these birds, resulting in low wing loading and a low aspect ratio, contribute to noise reduction by allowing slow flight. The serrations on the leading edge of the wing and the velvet-like surface have an effect on noise reduction and also lead to an improvement of aerodynamic performance. The fringes at the inner feather vanes reduce noise by gliding into the grooves at the lower wing surface that are formed by barb shafts. The fringed trailing edge of the wing has been shown to reduce trailing edge noise. These adaptations to silent flight have been an inspiration for biologists and engineers for the development of devices with reduced noise production. Today several biomimetic applications such as a serrated pantograph or a fringed ventilator are available. Finally, we discuss unresolved questions and possible future directions.
73 citations
TL;DR: Results indicate that especially the camber angle and aspect ratio have a critical impact on the force production and efficiency of the flapping wing MAV.
Abstract: Flapping wing micro air vehicles (MAVs) take inspiration from natural fliers, such as insects and hummingbirds. Existing designs manage to mimic the wing motion of natural fliers to a certain extent; nevertheless, differences will always exist due to completely different building blocks of biological and man-made systems. The same holds true for the design of the wings themselves, as biological and engineering materials differ significantly. This paper presents results of experimental optimization of wing shape of a flexible wing for a hummingbird-sized flapping wing MAV. During the experiments we varied the wing 'slackness' (defined by a camber angle), the wing shape (determined by the aspect and taper ratios) and the surface area. Apart from the generated lift, we also evaluated the overall power efficiency of the flapping wing MAV achieved with the various wing design. The results indicate that especially the camber angle and aspect ratio have a critical impact on the force production and efficiency. The best performance was obtained with a wing of trapezoidal shape with a straight leading edge and an aspect ratio of 9.3, both parameters being very similar to a typical hummingbird wing. Finally, the wing performance was demonstrated by a lift-off of a 17.2 g flapping wing robot.
68 citations
TL;DR: In this paper, an analytical model is developed for the prediction of noise radiated by an aerofoil with leading edge serration in a subsonic turbulent stream, which makes use of the Fourier expansion and Schwarzschild techniques in order to solve a set of coupled differential equations iteratively and express the far-field sound power spectral density in terms of the statistics of incoming turbulent upwash velocity.
Abstract: An analytical model is developed for the prediction of noise radiated by an aerofoil with leading edge serration in a subsonic turbulent stream. The model makes use of the Fourier Expansion and Schwarzschild techniques in order to solve a set of coupled differential equations iteratively and express the far-field sound power spectral density in terms of the statistics of incoming turbulent upwash velocity. The model has shown that the primary noise reduction mechanism is due to the destructive interference of the scattered pressure induced by the leading edge serrations. It has also shown that in order to achieve significant sound reduction, the serration must satisfy two geometrical criteria related to the serration sharpness and hydrodynamic properties of the turbulence. A parametric study has been carried out and it is shown that serrations can reduce the overall sound pressure level at most radiation angles, particularly at downstream angles close to the aerofoil surface. The sound directivity results have also shown that the use of leading edge serration does not particularity change the dipolar pattern of the far-field noise at low frequencies, but it changes the cardioid directivity pattern associated with radiation from straight-edge scattering at high frequencies to a tilted dipolar pattern.
65 citations
TL;DR: In this article, a deswirler geometry was obtained using the genetic algorithms and its effects on the flow field, pressure losses and cyclone collection efficiency were numerically investigated.
62 citations
TL;DR: In this article, the aerodynamic characteristics of a NACA0021 aerofoil with a sinusoidal wavy leading edge (WLE) at chord-based Reynolds number Re∞= 1.2×105 and angle of attack α =20
Abstract: A numerical investigation on the stalled flow characteristics of a NACA0021 aerofoil with a sinusoidal wavy leading edge (WLE) at chord-based Reynolds number Re∞= 1.2×105 and angle of attack α =20◦ is presented in this paper. It is observed that laminar separation bubbles (LSBs) form at the trough areas of the WLE in a collocated fashion rather than uniformly/periodically distributed over the span. It is found that the distribution of LSBs and their influence on the aerodynamic forces is strongly dependent on the spanwise domain size of the simulation, i.e. the wavenumber of the WLE used. The creation of a pair of counter-rotating streamwise vortices from the WLE and their evolution as an interface/buffer between the LSBs and the adjacent fully separated shear layers are discussed in detail. The current simulation results confirm that an increased lift and a decreased drag are achieved by using the WLEs compared to the straight leading edge (SLE) case, as observed in previous experiments. Additionally, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding. The beneficial aerodynamic characteristics of the WLE cases are attributed to the following three major events observed in the current simulations: (i) the appearance of a large low-pressure zone near the leading edge created by the LSBs; (ii) the reattachment of flow behind the LSBs resulting in a decreased volume of the rear wake; and, (iii) the deterioration of von-Karman (periodic) vortex shedding due to the breakdown of spanwise coherent structures.
TL;DR: In this article, a cavity is created into the blade profile of a VAWT to enhance its aerodynamic performance, and the authors used computational fluid dynamics to simulate the turbine in turbulent flow and finally calculate the forces affecting it.
TL;DR: In this paper, the authors present numerical and experimental analysis of diesel engine spray dynamics in the region very close to the nozzle exit, where liquid fuel is injected through a single solid cone injector with sharp-edged nozzle inlet.
TL;DR: Jeong et al. as mentioned in this paper investigated the vortex-body interaction problem of a rudder placed downstream of a single-blade marine rotor by numerical simulations and found that the topology of the wake is characterized by strong interaction with the boundary layer on the rudder and is followed by reconnection and merging in the middle and far wake.
Abstract: The vortex–body interaction problem, which characterizes the flow field of a rudder placed downstream of a single-blade marine rotor, is investigated by numerical simulations. The particular topology of the propeller wake, consisting of a helicoidal vortex detached from the blade tips (tip vortex) and a longitudinal, streamwise oriented vortex originating at the hub (hub vortex), embraces two representative mechanisms of vortex–body collisions: the tip vortices impact almost orthogonally to the mean plane, whereas the hub vortex travels in the mean plane of the wing (rudder), perpendicularly to its leading edge. The two vortices evolve independently only during the approaching and collision phases. The passage along the body is instead characterized by strong interaction with the boundary layer on the rudder and is followed by reconnection and merging in the middle and far wake. The features of the wake were investigated by the -criterion (Jeong & Hussain, J. Fluid Mech., vol. 285, 1995, pp. 69–94) and typical flow variables (pressure, velocity and vorticity) of the instantaneous flow field; wall pressure spectra were analysed and related to the tip and hub vortices evolution, revealing a non-obvious behaviour of the loading on the rudder that can be related to undesired unsteady loads.
TL;DR: The results indicate that the owl-inspired leading-edge serrations may be a useful device for aero-acoustic control in biomimetic rotor designs for wind turbines, aircrafts, multi-rotor drones as well as other fluid machinery.
Abstract: Owls are widely known for silent flight, achieving remarkably low noise gliding and flapping flights owing to their unique wing morphologies, which are normally characterized by leading-edge serrations, trailing-edge fringes and velvet-like surfaces. How these morphological features affect aerodynamic force production and sound suppression or noise reduction, however, is still not well known. Here we address an integrated study of owl-inspired single feather wing models with and without leading-edge serrations by combining large-eddy simulations (LES) with particle-image velocimetry (PIV) and force measurements in a low-speed wind tunnel. With velocity and pressure spectra analysis, we demonstrate that leading-edge serrations can passively control the laminar-turbulent transition over the upper wing surface, i.e. the suction surface at all angles of attack (0° 15° where owl wings often reach in flight. Our results indicate that the owl-inspired leading-edge serrations may be a useful device for aero-acoustic control in biomimetic rotor designs for wind turbines, aircrafts, multi-rotor drones as well as other fluid machinery.
TL;DR: In this article, high-accuracy numerical simulations are performed to study aeroacoustic source mechanisms of wavy leading edges (WLEs) on a thin aerofoil undergoing vortical disturbances.
Abstract: High-accuracy numerical simulations are performed to study aeroacoustic source mechanisms of wavy leading edges (WLEs) on a thin aerofoil undergoing vortical disturbances. This canonical study is based on a prescribed spanwise vortex travelling downstream and creating secondary vortices as it passes through the aerofoil’s leading edge. The primary aim of the study is to precisely understand the relationships between the vortex-induced velocity perturbation and the wall pressure fluctuation on the WLE geometry. It is observed that by increasing the size (amplitude) of the WLE the source strength at the peak region is reduced rapidly to a certain point, followed by a saturation stage, while at the root (trough) it remains fairly consistent regardless of the WLE size. This observation is demonstrated to be the consequence of three-dimensional vortex dynamics taking place along the WLE. One of the most profound features is that a system of horseshoe-like secondary vortices are created from the WLE peak region upon the impingement of the prescribed vortex. It is found that the horseshoe vortices produce a significantly non-uniform velocity perturbation in front of the WLE leading to the disparity in the source characteristics between the peak and root. The alterations to the impinging velocity perturbation are carefully analysed and related to the wall pressure fluctuation in this study. In addition, a semi-analytic model based on Biot–Savart’s law is developed to better understand and explain the role of the horseshoe vortex systems and the source mechanisms.
TL;DR: In this paper, the NREL 5 MW turbine is simulated with clean and eroded blades, which are compared to coated blades equipped with leading edge protection, and the analysis shows that the worse aerodynamic behaviour of strongly eroded blades can lead to power losses of 9 %.
Abstract: During their operation, wind turbine blades are eroded due to rain and hail, or they are contaminated with insects. Since the relative inflow velocity is higher at the outer than at the inner part of the blades, erosion occurs mostly at the outer blade region. In order to prevent strong erosion, it is possible to install a leading edge protection, which can be applied to the blades after the initial installation, but changes the shape of the initial airfoil sections. It is unclear how this modification influences the aerodynamic performance of the turbine. Hence, it is investigated in this work. The NREL 5 MW turbine is simulated with clean and eroded blades, which are compared to coated blades equipped with leading edge protection. Aerodynamic polars are generated by means of Computational Fluid Dynamics, and load calculations are conducted using the blade element momentum theory. The analysis in this work shows that, compared to clean rotor blades, the worse aerodynamic behaviour of strongly eroded blades can lead to power losses of 9 % . In contrast, coated blades only have a small impact on the turbine power of less than 1 % .
TL;DR: In this article, the effect of multiple leading-edge protuberances on the performance of a two-dimensional airfoil is investigated through experimental measurement of aerodynamic forces, surface tuft visualization, and numerical simulation.
Abstract: Recently leading-edge protuberances have attracted great attention as a passive method for separation control. In this paper, the effect of multiple leading-edge protuberances on the performance of a two-dimensional airfoil is investigated through experimental measurement of aerodynamic forces, surface tuft visualization, and numerical simulation. In contrast to the sharp stall of the baseline airfoil with large hysteresis effect during AOA (angle of attack) increasing and decreasing, the stall process of the modified airfoil with leading-edge protuberances is gentle and stable. Flow visualization revealed that the flow past each protuberance is periodic and symmetric at small AOAs. Streamwise vortices are generated on the shoulders of the protuberance, leading to a larger separation around the valley sections and a longer attachment along the peak sections. When some critical AOA is exceeded, aperiodic and asymmetric flow patterns occur on the protuberances at different spanwise positions, with leading-edge separation on some of the valley sections and non-stalled condition elsewhere. A combined mechanism, involving both the compartmentalization effect of the slender momentum-enhanced attached flows on the protuberance peaks and the downwash effect of the local stalled region with low circulation, is proposed to explain the generation of the aperiodic flow patterns. The influence of the number of protuberances is also investigated, which shows similar aperiodic flow patterns. The distance between the neighboring local stalled valley sections is found to be in the range of 4–7 times the protuberance wavelength. According to the proposed mechanism, it is speculated that the distance between the neighboring local stalled valley sections is inclined to increase with a smaller protuberance amplitude or at a larger AOA.
TL;DR: In this article, a 3D finite element model of a turbine blade coated with multilayer-structure TBCs is developed, in which conjugate heat transfer analysis and the decoupled thermal-stress calculation method are adopted.
Abstract: Prediction of the stress distribution on a 3D turbine blade coated with thermal barrier coatings (TBCs) plays a key role in analyzing the failure of TBCs. In this work, a 3D finite element model of turbine blade coated with multilayer-structure TBCs is developed, in which conjugate heat transfer analysis and the decoupled thermal-stress calculation method are adopted. To obtain a closer to the actual temperature field, the external flow field performed by three turbulence model (RNG k - e , realizable k - e and SST k - ω turbulence model) are analyzed. It is found that the temperature and pressure distribution of the flow field performed by realizable k - e turbulence model are much closer to the experiment data. TBCs present an excellent insulating effect. The heat insulation performance at the leading and trailing edges is relatively better than the suction and pressure sides. Based on the thermal-stress simulation results, the dangerous regions of TBCs are predicted. It is shown that the maximum principal stress of ceramic layer locate at both suction and pressure surfaces near leading edge and trailing edge. Furthermore, the stress level of ceramic layer is higher than that of bond coating, TBCs may spall off at these regions, especially the ceramic layer.
TL;DR: In this article, a biomimetic airfoil featuring leading edge waves, trailing edge serrations and surface ridges is proposed, based on flow control with each section meeting the NACA 0012 air-foil profile.
TL;DR: In order to study the rotor-stator interaction mechanism in impeller and guide vanes of mixed-flow pump under part loading condition, the flow field between the impeller outlet and the guide vane inlet was measured based on Particle Image Velocimetry (PIV) as discussed by the authors.
TL;DR: In this article, a thermal protection system (TPS) is proposed to protect the leading edge of a hypersonic space vehicle from aero-thermal heating and deformation.
Abstract: The leading edges of hypersonic space vehicle experience high temperature and stress due to prevailing aerothermodynamic conditions of extreme heat flux and pressure. The design of thermal protection system (TPS) to protect the metallic airframe structure can ensure longer life and reliability under flight conditions. The effective design of TPS system requires the precise quantitative understanding of thermo-mechanical stresses and deformation, which demands careful computational study under flight simulated conditions. In the above perspective, TPS design for a leading edge exposed to Mach 7 hypersonic flow for 250 seconds has been carried out by performing finite element-based thermo-structural analysis with pressure and heat flux estimated from computational fluid dynamics analysis (CFD). The fidelity and robustness of CFD scheme is established using grid independence and convergence analysis. CFD analysis effectively captures the formation of bow shock around the leading edge and stagnation region near its nose. For finite element analysis, high-quality structural elements have been generated using HyperMesh to precisely model the thermo-structural behavior of TPS. In our computational analysis, TPS is modeled as a three-layered system with outermost layer of ZrB2-SiC, middle layer of phenolic cork and innermost layer of Ti-alloy. The analytical values of spatial variation of temperature, stress components, and displacement across the TPS have been critically analysed to rationalise specific structural configuration for better thermo-structural stability. Together with temporal variation of temperature, the implication of such computational results has led us to propose a new design for TPS. The proposed TPS is capable of containing the stress and displacement within 32 MPa and 0.58 mm, respectively, when the leading edge is exposed to shock induced aero-thermal heating as high as 2.11 MW/m(2) and pressure of 72.8 kPa for a hypersonic cruise flight of 500 km range.
TL;DR: In this paper, the failure of an uncooled turbine blade in an aero gas turbine engine is analyzed to determine its root cause, and the operational condition of the engine was studied and metallurgical investigations are carried out on the fractured blade.
TL;DR: In this paper, the influence of small cylindrical bluntness of the leading edge of a flat plate on formation of spatial structures in a nominally two-dimensional supersonic compression corner flow at the Mach number M∞ ≈ 8 and a laminar state of the undisturbed boundary layer is studied by the method of temperature sensitive paints.
Abstract: The influence of small cylindrical bluntness of the leading edge of a flat plate on formation of spatial structures in a nominally two-dimensional supersonic compression corner flow at the Mach number M∞ ≈ 8 and a laminar state of the undisturbed boundary layer is studied by the method of temperature-sensitive paints. Streamwise vortices are found in the region of reattachment of the separated flow in a wide range of Reynolds numbers (0.15 · 106–2.55 · 106) for various angles of flow deflection and plate lengths. It is demonstrated that the existence of these vortices induces spanwise oscillations of the heat transfer coefficient; the amplitude of these oscillations may reach 30%. The maximum deviations of the Stanton number reaching 80% are observed in the case with significant roughness of the leading edge of the flat plate. Both the maximum Stanton numbers in the reattachment region and the amplitude of spanwise oscillations of the Stanton number induced by streamwise vortices are found to decrease significantly in the case of small bluntness of the leading edge. Solutions of three-dimensional Navier–Stokes equations are obtained for some test conditions. The computed results are in good agreement with experimental data, which points to a significant stabilizing effect of small bluntness on the intensity of streamwise vortices.
TL;DR: In this paper, a flexible fin flapping near the ground was modeled in a two-dimensional Cartesian coordinate system, and the transverse heaving motion was prescribed at the leading edge and the posterior part of the fin fluttered passively under the fin-fluid interaction.
Abstract: Many animals in nature experience hydrodynamic benefits by swimming near the ground Inspired by near-ground swimmers, a flexible fin flapping near the ground was modeled in a two-dimensional Cartesian coordinate system The transverse heaving motion was prescribed at the leading edge and the posterior part of the fin fluttered passively under the fin–fluid interaction The fin freely moved horizontally in a quiescent flow, which dynamically determined the swimming speed The fluid–flexible fin interaction was considered by using an immersed boundary method The fin could swim up to 14% faster near the ground than in the bulk fluid, and the vortices in the wake moved away from the ground The body kinematics was passively altered by flapping near the ground, and the trailing edge amplitude decreased as the ground proximity increased The benefits or penalties in the thrust and the power input by swimming near the ground were not only the direct results of the hydrodynamic changes, but also the indirect re
TL;DR: In this paper, an improved delayed detached eddy simulation (IDDES) method was adopted to investigate the effects of leading edge protuberances on airfoil stall and post-stall performance.
Abstract: This article presents a numerical investigation of the effects of leading-edge protuberances on airfoil stall and post-stall performance. An improved delayed detached eddy simulation (IDDES) method was adopted. As a result, to clarify the effects of ‘bi-periodic’ phenomenon around stall region, it was found that the flow separation at troughs was the main inducement of aerodynamic lift degradation within pre-stall regime and the flow pattern where vortices diverged was predominant. It was also found that the variations in flow patterns led to the gentle stall process. Furthermore, to study the statistical characteristics of unsteady vortex shedding, corresponding spectrum characteristics were also analyzed from another perspective, suggesting that the vortex shedding frequency was higher where vortices converged. Eventually, the improved performances of tubercled airfoil within post-stall regime could be attributed to the strong streamwise vortices generated by the leading-edge protuberances. Depl...
TL;DR: In this paper, the authors investigated the Reynolds number effects on the flow mechanism induced by a fullspan tubercled wing with the NACA-0021 cross-sectional profile in the transitional and near-turbulent regimes using computational fluid dynamics.
Abstract: Leading-edge modifications based on designs inspired by the protrusions on the pectoral flippers of the humpback whale (tubercles) have been the subject of research for the past decade primarily due to their flow control potential in ameliorating stall characteristics. Previous studies have demonstrated that, in the transitional flow regime, full-span wings with tubercled leading edges outperform unmodified wings at high attack angles. The flow mechanism associated with such enhanced loading traits is, however, still being investigated. Also, the performance of full-span tubercled wings in the turbulent regime is largely unexplored. The present study aims to investigate Reynolds number effects on the flow mechanism induced by a full-span tubercled wing with the NACA-0021 cross-sectional profile in the transitional and near-turbulent regimes using computational fluid dynamics. The analysis of the flow field suggests that, with the exception of a few different flow features, the same underlying flow mechanism, involving the presence of transverse and streamwise vorticity, is at play in both cases. With regard to lift-generation characteristics, the numerical simulation results indicate that in contrast to the transitional flow regime, where the unmodified NACA-0021 undergoes a sudden loss of lift, in the turbulent regime, the baseline foil experiences gradual stall and produces more lift than the tubercled foil. This observation highlights the importance of considerations regarding the Reynolds number effects and the stall characteristics of the baseline foil, in the industrial applications of tubercled lifting bodies.
TL;DR: In this paper, the effect of spanwise waviness on the incompressible flow around infinite wings with a NACA0012 profile was investigated using direct numerical simulations employing the spectral/hp method.
Abstract: The use of spanwise waviness in wings has been proposed in the literature as a possible mechanism for obtaining improved aerodynamic characteristics, motivated by the tubercles that cover the leading edge of the pectoral flippers of the humpback whale. We investigate the effect of this type of waviness on the incompressible flow around infinite wings with a NACA0012 profile, using direct numerical simulations employing the spectral/hp method. Simulations were performed for Reynolds numbers of and , considering different angles of attack in both the pre-stall and post-stall regimes. The results show that the waviness can either increase or decrease the lift coefficient, depending on the particular and flow regime. We observe that the flow around the wavy wing exhibits a tendency to remain attached behind the waviness peak, with separation restricted to the troughs, which is consistent with results from the literature. Then, we identify three important physical mechanisms in this flow. The first mechanism is the weakening of the suction peak on the sections corresponding to the waviness peaks. This characteristic had been observed in a previous investigation for a very low Reynolds number of , and we show that this is still important even at . As a second mechanism, the waviness has a significant effect on the stability of the separated shear layers, with transition occurring earlier for the wavy wing. In the pre-stall regime, for , the flow around the baseline wing is completely laminar, and the earlier transition leads to a large increase in the lift coefficient, while for , the earlier transition leads to a shortening of the separation bubble which does not lead to an increased lift coefficient. The last mechanism corresponds to a sub-harmonic behaviour, with the flow being notably different between subsequent wavelengths. This allows the wing to maintain higher lift coefficients in some portions of the span.
TL;DR: It is concluded that under steady conditions the LEV as an ‘absence of stall’ model/mechanism is the most satisfying explanation for observed aerodynamic behaviour.
Abstract: The presence of a stable leading edge vortex (LEV) on steadily revolving wings increases the maximum lift coefficient that can be generated from the wing and its role is important to understanding natural flyers and flapping wing vehicles. In this paper, the role of LEV in lift augmentation is discussed under two hypotheses referred to as 'additional lift' and 'absence of stall'. The 'additional lift' hypothesis represents the traditional view. It presumes that an additional suction/circulation from the LEV increases the lift above that of a potential flow solution. This behaviour may be represented through either the 'Polhamus leading edge suction' model or the so-called 'trapped vortex' model. The 'absence of stall' hypothesis is a more recent contender that presumes that the LEV prevents stall at high angles of attack where flow separation would normally occur. This behaviour is represented through the so-called 'normal force' model. We show that all three models can be written in the form of the same potential flow kernel with modifiers to account for the presence of a LEV. The modelling is built on previous work on quasi-steady models for hovering wings such that model parameters are determined from first principles, which allows a fair comparison between the models themselves, and the models and experimental data. We show that the two models which directly include the LEV as a lift generating component are built on a physical picture that does not represent the available experimental data. The simpler 'normal force' model, which does not explicitly model the LEV, performs best against data in the literature. We conclude that under steady conditions the LEV as an 'absence of stall' model/mechanism is the most satisfying explanation for observed aerodynamic behaviour.
TL;DR: In this paper, an experimental study of unidirectional flow through a model mangrove forest measured both velocity and forces on individual trees, including 24 prop roots distributed in a three-dimensional layout.
Abstract: An experimental study of unidirectional flow through a model mangrove forest measured both velocity and forces on individual trees. The individual trees were 1/12th scale models of mature Rhizophora, including 24 prop roots distributed in a three-dimensional layout. Thirty-two model trees were distributed in a staggered array producing a 2.5-m long forest. The velocity evolved from a boundary layer profile at the forest leading edge to a vertical profile determined by the vertical distribution of frontal area, with significantly higher velocity above the prop roots. Fully-developed conditions were reached at the 5th tree row from the leading edge. Within the root zone the velocity was reduced by up to 50 percent and the TKE was increased by as much as 5-fold, relative to the upstream conditions. TKE in the root zone was mainly produced by root and trunk wakes, and it agreed in magnitude with the estimation obtained using the Tanino and Nepf (2008) formulation. Maximum TKE occurred at the top of the roots, where a strong shear region was associated with the change in frontal area. The drag measured on individual trees decreased from the leading edge and reached a constant value at the 5th row and beyond, i.e. in the fully-developed region. The drag exhibited a quadratic dependence on velocity, which justified the definition of a quadratic drag coefficient. Once the correct drag length-scale was defined, the measured drag coefficients collapsed to a single function of Reynolds number.