Showing papers on "Leading edge published in 2018"
TL;DR: In this article, an experimental investigation was performed in an Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT) to quantify the unsteady heat transfer and dynamic ice accretion process over an airfoil/wing surface under different icing conditions.
83 citations
19 Oct 2018
TL;DR: In this article, the authors support the hypothesis that the vast majority of the damage accumulated in the leading edge is imposed during extreme precipitation condition events, which occur during a very small fraction of the turbine's operation life.
Abstract: . Impact fatigue caused by collision with rain droplets, hail stones
and other airborne particles, also known as leading-edge erosion, is a severe
problem for wind turbine blades. Each impact on the leading edge adds an
increment to the accumulated damage in the material. After a number of
impacts the leading-edge material will crack. This paper presents and
supports the hypothesis that the vast majority of the damage accumulated in
the leading edge is imposed at extreme precipitation condition events, which
occur during a very small fraction of the turbine's operation life. By
reducing the tip speed of the blades during these events, the service life of
the leading edges significantly increases from a few years to the full
expected lifetime of the wind turbine. This life extension may cost a
negligible reduction in annual energy production (AEP) in the worst case, and
in the best case a significant increase in AEP will be achieved.
71 citations
TL;DR: In this article, a two-dimensional simulation was performed for flexible fins propelled in tandem, diagonal, triangular and diamond configurations, and the flow-mediated interactions between the flexible fins were analyzed by using an immersed boundary method.
Abstract: A fish may gain hydrodynamic benefits from being a member of a school. Inspired by fish schools, a two-dimensional simulation was performed for flexible fins propelled in tandem, diagonal, triangular and diamond configurations. The flow-mediated interactions between the flexible fins were analysed by using an immersed boundary method. A transverse heaving motion was prescribed on the leading edge of each fin, and other posterior parts passively adapted to the surrounding fluid as a result of the fluid–flexible-body interaction. The flexible fins were allowed to actively adjust their relative positions in the horizontal direction. The four basic stable configurations are spontaneously formed and self-sustained purely by the vortex–vortex and vortex–body interactions. The hydrodynamic benefits depend greatly on the local positions of the members. For the same heaving motion prescribed on the leading edge, the input power of the following fin in the stable tandem and diagonal configurations is lower by 14 % and 6 %, respectively, than that of the leading fin. The following fin in the diagonal formation can keep pace with the leading fin even for reduced heaving amplitudes because of the help of the leader via their shared fluid environment, where its required input power is reduced by 21 %. The heaving amplitudes of the trailing fins are reduced to optimize the propulsive efficiency, and the average efficiencies in the triangular and diamond configurations increase by up to 14 % and 19 %, respectively, over that of the isolated swimmer. The propulsive efficiencies are enhanced by 22 % for the fins in the second row and by 36 % for the fin in the third row by decreasing the heaving amplitude in the diamond formation.
68 citations
TL;DR: In this paper, the authors investigated the effect of cavity geometry on fuel transport and mixing processes in a scramjet combustor with a single rear-wallexpansion cavity and found that the vertical flow velocity of the region located right after the expansion wave starting from the cavity leading edge was increased significantly towards the cavity when lowering the rear wall height, leading to an enhanced fuel entrainment into the cavity.
61 citations
61 citations
TL;DR: In this article, a flow control method to suppress the flow separation by setting micro-cylinder in front of the blade leading edge is proposed, and the corresponding numerical simulation analysis for the aerodynamic performance of wind turbine is conducted.
49 citations
TL;DR: In this article, two different modes based on steady and unsteady actuation are compared and investigated for flow control in a stalled NACA 0012 airfoil at Re = 3 × 10 6 and a simplified phenomenological model which uses the correct scale of the plasma body force is considered for the modeling of the actuation effects.
48 citations
TL;DR: In this article, a numerical investigation of the perturbation evolution in a Mach 6 flat-plate boundary layer with a local heating or cooling strip is presented, where the position of the temperature strip is varied while the strip length is constant and approximated to the boundary-layer thickness.
45 citations
TL;DR: In this article, a narrow span-wise rectangular channel (referred to as a slot) is introduced and drilled near the leading edge of a finite-span cambered airfoil to study its impact on the overall aerodynamic performance.
44 citations
TL;DR: In this article, a leading-edge suction parameter (LESP) is derived from potential flow theory as a measure of suction at the airfoil leading edge to study initiation of leading edge vortex (LEV) formation.
Abstract: A leading-edge suction parameter (LESP) that is derived from potential flow theory as a measure of suction at the airfoil leading edge is used to study initiation of leading-edge vortex (LEV) formation in this article. The LESP hypothesis is presented, which states that LEV formation in unsteady flows for specified airfoil shape and Reynolds number occurs at a critical constant value of LESP, regardless of motion kinematics. This hypothesis is tested and validated against a large set of data from CFD and experimental studies of flows with LEV formation. The hypothesis is seen to hold except in cases with slow-rate kinematics which evince significant trailing-edge separation (which refers here to separation leading to reversed flow on the aft portion of the upper surface), thereby establishing the envelope of validity. The implication is that the critical LESP value for an airfoil–Reynolds number combination may be calibrated using CFD or experiment for just one motion and then employed to predict LEV initiation for any other (fast-rate) motion. It is also shown that the LESP concept may be used in an inverse mode to generate motion kinematics that would either prevent LEV formation or trigger the same as per aerodynamic requirements.
43 citations
TL;DR: In this article, the authors used the Realizable k-e turbulent model combined with the wall enhanced function to evaluate the heat transfer and flow structure in a rotating pin finned channel with different dimple locations.
TL;DR: In this article, the effect of the orientation of diffuser blades on the performance and detailed flow-physics through a centrifugal pump is investigated at design and an off-design conditions, with the latter corresponding to 40% of the nominal flow-rate.
TL;DR: This viewpoint focuses on the key features that contributed to the influential, problem-solving character of LIBS and discusses current achievements in standoff LIBS methods, underwater solids inspection, and single nanoparticle analysis.
Abstract: Laser-induced breakdown spectroscopy (LIBS) is currently considered one of the most active research areas in the field of analytical spectroscopy. Over the years, scientists and engineers have focused on the development of LIBS as a tool for chemical measurements, with significant efforts in developing new devices, methods, and data processing algorithms to improve the analytical performance of LIBS. In combination with tailored excitation methods using multipulse, multiwavelength laser systems, substantial improvements in detection power, representativeness, accuracy, and sampling throughput have been achieved. Development of LIBS instruments with extended capabilities for energy delivery to the sample using ultrashort laser pulses has been undertaken, which has permitted a better understanding of the underlying issues of LIBS; notably, laser interaction with matter, plasma dynamics, and properties. As a result, LIBS has emerged as a powerful alternative for chemical analysis in a wide front of applications, from geological exploration to industrial inspection, from environmental monitoring to biomedical and forensic analysis, from cultural heritage to homeland security. While LIBS certainly has practical utility in many laboratory-based chemical measurements, the true potential of this technology becomes apparent when it is used for applications inaccessible to more conventional analytical techniques. Inspection of the elemental composition of distant objects and LIBS analysis underwater solids constitute examples of the exclusive capabilities of LIBS. Since detailed account of plasma fundamentals, instruments, methods and applications of LIBS can be found in the extensive literature on the topic, this viewpoint focuses on the key features that contributed to the influential, problem-solving character of LIBS. We take the perspective of our own work in the field and discuss current achievements in standoff LIBS methods, underwater solids inspection, and single nanoparticle analysis. We then introduce the strategies used for improving the detection power of LIBS and discuss the use of LIBS in combination with other spectroscopic tools. Finally, we identify challenges that merit further attention, by underlining some possible ways to solve the most common impediments for advancement of LIBS towards an enabling analytical technology. Standoff LIBS
TL;DR: In this article, two variable droop leading edge (VDLE) modes are proposed and a series of investigations are performed on the effect of different modes and parameters on dynamic stall control of NACA 0012 airfoil in pitching oscillation.
TL;DR: In this article, the Euler-Bernoulli beam equation is used to compare the propulsive performance of tethered panels to the forward swimming speed of the same panels that can move forward freely.
Abstract: Many animals that swim or fly use their body to accelerate the fluid around them, transferring momentum from their flexible bodies and appendages to the surrounding fluid. The kinematics that emerge from this transfer result from the coupling between the fluid and the active and passive material properties of the flexible body or appendages. To elucidate the fundamental features of the elastohydrodynamics of flexible appendages, recent physical experiments have quantified the propulsive performance of flexible panels that are actuated on their leading edge. Here we present a complementary computational study of a three-dimensional flexible panel that is heaved sinusoidally at its leading edge in an incompressible, viscous fluid. These high-fidelity numerical simulations enable us to examine how propulsive performance depends on mechanical resonance, fluid forces, and the emergent panel deformations. Moreover, the computational model does not require the tethering of the panel. We therefore compare the thrust production of tethered panels to the forward swimming speed of the same panels that can move forward freely. Varying both the passive material properties and the heaving frequency of the panel, we find that local peaks in trailing edge amplitude and forward swimming speed coincide and that they are determined by a non-dimensional quantity, the effective flexibility, that arises naturally in the Euler–Bernoulli beam equation. Modal decompositions of panel deflections reveal that the amplitude of each mode is related to the effective flexibility. Panels of different material properties that are actuated so that their effective flexibilities are closely matched have modal contributions that evolve similarly over the phase of the heaving cycle, leading to similar vortex structures in their wakes and comparable thrust forces and swimming speeds. Moreover, local peaks in the swimming speed and trailing edge amplitude correspond to peaks in the contributions of the different modes. This computational study of freely swimming flexible panels gives further insight into the role of resonance in swimming performance that is important in the engineering and design of robotic propulsors. Moreover, we view this reduced model and its comparison to laboratory experiments as a building block and validation for a more comprehensive three-dimensional computational model of an undulatory swimmer that will couple neural activation, muscle mechanics and body elasticity with the surrounding viscous, incompressible fluid.
TL;DR: In this paper, the authors used DBD plasma actuation to anti-ice on a NACA0012 airfoil model with a chord length of 53.5 cm in a closed-circuit icing wind tunnel.
TL;DR: In this paper, the effects of the simplification of the blade form from twist to straight has been investigated, and the results showed that the simplified blade form has a significant effect on the film cooling performance.
TL;DR: In this article, the heat transfer and flow performance in a vortex cooling configuration with 5 nozzles were investigated at various Reynolds numbers and temperature ratios, and the results showed that the k-ω turbulence model has the best accuracy to predict the vortex cooling behavior.
TL;DR: In this article, a spanwise uniform body force is introduced using dielectric barrier discharge plasma actuators, to either force against or along the local cross-flow component of the boundary layer.
Abstract: Control of laminar-to-turbulent transition on a swept-wing is achieved by base-flow modification in an experimental framework, up to a chord Reynolds number of 2.5 million. This technique is based on the control strategy used in the numerical simulation by Dorr & Kloker (J. Phys. D: Appl. Phys., vol. 48, 2015b, 285205). A spanwise uniform body force is introduced using dielectric barrier discharge plasma actuators, to either force against or along the local cross-flow component of the boundary layer. The effect of forcing on the stability of the boundary layer is analysed using a simplified model proposed by Serpieri et al. (J. Fluid Mech., vol. 833, 2017, pp. 164–205). A minimal thickness plasma actuator is fabricated using spray-on techniques and positioned near the leading edge of the swept-wing, while infrared thermography is used to detect and quantify transition location. Results from both the simplified model and experiment indicate that forcing along the local cross-flow component promotes transition while forcing against successfully delays transition. This is the first experimental demonstration of swept-wing transition delay via base-flow modification using plasma actuators.
TL;DR: In this paper, the impingement of a tractor-propeller slipstream on the leading edge of a pylon was investigated. But the results were limited to two pylon leading edges: a solid and a flow-permeable one, and the results showed that the solid leading edge insert mitigates the amplitude of surface pressure fluctuations only on the pylon-retreating blade side towards the trailing edge, thus reducing structure-borne noise.
Abstract: This manuscript describes an aeroacoustic computational study on the impingement of a tractor-propeller slipstream on the leading edge of a pylon. Both the flow and acoustic fields are studied for two pylon leading edges: a solid and a flow-permeable one. The computational set-up replicates experiments performed at Delft University of Technology. Computational results are validated against measurements. It is found that the installation of the flow-permeable leading-edge insert generates a thicker boundary layer on the retreating blade side of the pylon. This is caused by an aerodynamic asymmetry induced by the helicoidal motion of the propeller wake, which promotes a flow motion through the cavity from the advancing to the retreating blade side of the pylon. The flow-permeable leading-edge insert mitigates the amplitude of the surface pressure fluctuations only on the pylon-retreating blade side towards the trailing edge, thus reducing structure-borne noise. Furthermore, it causes a reduction of the near-field noise only for receiver angles oriented in the upstream direction at the pylon-retreating blade side. In this range of receiver angles, it is found that the flow-permeable leading-edge insert reduces the amplitude of the tonal peaks for the third and fourth blade passage frequency, but strongly increases the broadband noise for frequencies higher that the seventh blade passage frequency.
TL;DR: In this article, a DBD plasma actuator is attached near the leading edge of an airfoil and the mechanism of flow control of this small device is discussed, especially in burst mode, is shown to be very effective for controlling flow separation at Reynolds number of 6.3 × 104.
Abstract: Both computational and experimental studies are conducted for understanding of the flow separation control mechanism of a DBD (dielectric barrier discharge) plasma actuator. Low speed flows over an airfoil are considered. A DBD plasma actuator is attached near the leading edge of an airfoil and the mechanism of flow control of this small device is discussed. The DBD plasma actuator, especially in burst mode, is shown to be very effective for controlling flow separation at Reynolds number of 6.3 × 104, when applied to the flows at an angle of attack higher than the stall. The analysis reveals that the flow structure includes three remarkable features that provide good authority for flow separation control with the appropriate actuator parameters. With proper setting of the actuator parameters to enhance the effective flow features for the application, good flow control can be achieved. Based on the analysis, guidelines for the effective use of DBD plasma actuators are proposed. A DBD plasma actuator is also applied to the flows under cruise conditions. With the DBD plasma actuator attached, a simple airfoil turns out to show higher lift-to-drag ratio than a well-designed airfoil.
TL;DR: In this article, a combined experimental and computational test program, with two low-pressure ratio aero-engine fans, has been used to identify the flow mechanisms at stall inception and the subsequent stall cell growth.
Abstract: © 2019 by ASME. A combined experimental and computational test program, with two low-pressure ratio aero-engine fans, has been used to identify the flow mechanisms at stall inception and the subsequent stall cell growth. The two fans have the same rotor tip clearance, annulus design, and downstream stators, but different levels of tip loading. The measurement data show that both the fans stall via spike-type inception, but that the growth of the stall cell and the final cell size is different in each fan. The computations, reproducing both the qualitative and quantitative behavior of the steady-state and transient measurements, are used to identify the flow mechanisms at the origin of stall inception. In one fan, spillage of tip leakage flow upstream of the leading edge plane is responsible. In the other, sudden growth of casing corner separation blockage leads to stall. These two mechanisms are in accord with the findings from core compressors. However, the transonic aerodynamics and the low hub-to-tip radius ratio of the fans lead to the following two findings: first, the casing corner separation is driven by shock-boundary layer interaction and second, the spanwise loading distribution of the fan determines whether the spike develops into full-span or part-span stall and both types of behavior are represented in the present work. Finally, the axial momentum flux of the tip clearance flow is shown to be a useful indicator of the leakage jet spillage mechanism. A simple model is provided that links the tip loading, stagger, and solidity with the tip clearance axial momentum flux, thereby allowing the aerodynamicist to connect, qualitatively, design parameters with the stall behavior of the fan.
TL;DR: In this paper, a suite of experimental measurements has been performed on a two-dimensional multi-element MD-30P30N airfoil, including PIV, steady and unsteady surface pressure, and phased microphone array measurements.
TL;DR: In this paper, the effect of serrated leading edge serrations on the aero-acoustic and aerodynamic performances of a symmetrical NACA0008 aerofoil was investigated.
TL;DR: In this paper, a new leading edge serration geometry is proposed which provides significantly greater noise reductions compared to the maximum noise reductions achievable by single-wavelength serrations of the same amplitude.
Abstract: Aerofoils operating in a turbulent flow generate broadband noise by scattering vorticity into sound at the leading edge. Previous work has demonstrated the effectiveness by which serrations, or undulations, introduced onto the leading edge, can substantially reduce broadband leading edge noise. All of this work has focused on sinusoidal (single-wavelength) leading edge serration profiles. In this paper, a new leading edge serration geometry is proposed which provides significantly greater noise reductions compared to the maximum noise reductions achievable by single-wavelength serrations of the same amplitude. This is achieved through destructive interference between different parts of the aerofoil leading edge, and therefore involves a fundamentally different noise reduction mechanism from conventional single-wavelength serrations. The new leading edge serration profiles simply comprise the superposition of two single-wavelength components of different wavelength, amplitude and phase with the objective of forming two roots that are sufficiently close together and separated in the streamwise direction. Compact sources located at these root locations then interfere leading to less efficient radiation than single-wavelength geometries. A detailed parametric study is performed experimentally to investigate the sensitivity of the noise reductions to the profile geometry. A simple model is proposed to explain the noise reduction mechanism for these double wavelength serration profiles and shown to be in close agreement with the measured noise reduction spectra. The study is primarily performed on flat plates in an idealized turbulent flow. The paper concludes by introducing the double-wavelength serration on a 10% thick aerofoil, where near-identical noise reductions are obtained compared to the flat plate.
TL;DR: In this paper, the effect of leading edge modifications via fillet shapes near vane/blade-endwall juncture in a transonic environment within the highly loaded high pressure turbine stage was investigated.
TL;DR: In this article, the interaction and coupling between two inverted flags that are placed side-by-side in a uniform flow is investigated in an effort to determine the behavior of systems that are formed by arrays of cantilevered plates.
TL;DR: In this paper, the authors examined the cavitation characteristics and hydrodynamic forces of semi-span hydrofoils with bio-inspired, wavy leading edges in a water tunnel and found that cavitation was largely confined to the regions directly behind the protuberance troughs.