Showing papers on "Leading edge published in 2007"

The Forces Behind Cell Movement

Abstract: Cell movement is a complex phenomenon primarily driven by the actin network beneath the cell membrane, and can be divided into three general components: protrusion of the leading edge of the cell, adhesion of the leading edge and deadhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each of these steps is driven by physical forces generated by unique segments of the cytoskeleton. This review examines the specific physics underlying these phases of cell movement and the origins of the forces that drive locomotion.

Effects of Leading-Edge Protuberances on Airfoil Performance

Abstract: Lift, drag, and pitching moments of airfoils with leading-edge sinusoidal protuberances were measured in a water tunnel and compared with those of a baseline 63 4 -021 airfoil. The amplitude of the leading-edge protuberances ranged from 2.5 to 12% of the mean chord length; the spanwise wavelengths were 25 and 50% of the mean chord length. These ranges correspond to the morphology found on the leading edge of humpback whales' flippers. Flow visualization using tufts was also performed to examine the separation characteristics of the airfoils. For angles of attack less than the baseline stall angle, lift reduction and drag increase were observed for the modified foils. Above this angle, lift of the modified foils was up to 50% greater than the baseline foil with little or no drag penalty. The amplitude of the protuberances had a distinct effect on the performance of the airfoils, whereas the wavelength had little. Flow visualization indicated separated flow originating primarily from the troughs and attached flow on the peaks of the protuberances at angles beyond the stall angle of the baseline foil.

Experimental Evaluation of Sinusoidal Leading Edges

Abstract: P REVIOUS studies on increasing airfoil lift and improving stall characteristics have addressed various passive and active approaches to modifying the leading and trailing edge shapes. The passive approaches have covered such methods as rippling the trailing edge, applying serrated-edge Gurney flaps, or modifying the leading-edge (LE) profile [1,2]. Other efforts have effectively eliminated the dynamic stall of an NACA 0012 airfoil by perturbing the LE contour as little as 0.5–0.9%of the chord [3]. Levshin et al. [4] demonstrated that sinusoidal LE planforms on an NACA 63-021 airfoil section decreased maximum lift, but extended the stall angle by almost 9 deg. The larger amplitude sinusoids created “softer” stall characteristics by maintaining attached flow at the peaks despite separated flow in the troughs. These tests were performed to simulate the effects of LE tubercles on humpback whale (Megaptera novaeangliae) flippers. Prior work by the authors also reported wind tunnel measurements for idealized scale models of humpback whale flippers [5]. One model had a smooth leading edge and a secondmodel had sinusoidal bumps (tubercles) along the leading edge for the outer 2 3 of the span. It was found that the addition of tubercles to a 3-D idealized flipper increased the maximum lift coefficient while reducing the drag coefficient over a portion of the operational envelope. It is thought that the tubercles on the flipper leading-edge enhance the whale’s ability to maneuver to catch prey [6]. Though the work to date regarding sinusoidal or serrated leading-edge planforms is largely motivated by marine mammal locomotion, the effects of extending the stall point for lifting surfaces at similar Reynolds numbers (Re) may have application to small-UAV (unmanned aerial vehicle) design and the inevitable laminar stall problems [7]. However other relevant applications might benefit from the effects of simulated tubercles such as stall alleviation/separation control on sailboat centerboards or wind turbines, where an expanded operating envelope could improve the overall effectiveness of the blade [8,9]. In the present work, a better understanding is sought of the mechanism of the improvements measured in previous experiments, with a greater applicability in mind. The authors seek to determine whether the performance improvements resulted from enhancements to the sectional characteristics of wings with tubercles (i.e., essentially 2-D effects), or from Reynolds number effects on a tapered planform, or from other 3-D effects such as spanwise stall progression.

Direct numerical simulation of round turbulent jets in crossflow

Abstract: Direct numerical simulation is used to study a round turbulent jet in a laminar crossflow. The ratio of bulk jet velocity to free-stream crossflow velocity is 5.7 and the Reynolds number based on the bulk jet velocity and the jet exit diameter is 5000. The mean velocity and turbulent intensities from the simulations are compared to data from the experiments by Su & Mungal (2004) and good agreement is observed. Additional quantities, not available from experiments, are presented. Turbulent kinetic energy budgets are computed for this flow. Examination of the budgets shows that the near field is far from a state of turbulent equilibrium – especially along the jet edges. Also – in the near field – peak kinetic energy production is observed close to the leading edge, while peak dissipation is observed toward the trailing edge of the jet. The results are used to comment upon the difficulty involved in predicting this flow using RANS computations. There exist regions in this flow where the pressure transport term, neglected by some models and poorly modelled by others, is significant. And past the jet exit, the flow is not close to established canonical flows on which most models appear to be based.

On the bimodal dynamics of the turbulent horseshoe vortex system in a wing-body junction

Abstract: The turbulent boundary layer approaching a wall-mounted obstacle experiences a strong adverse pressure gradient and undergoes three-dimensional separation leading to the formation of a dynamically rich horseshoe vortex (HSV) system. In a pioneering experimental study, Devenport and Simpson [J. Fluid Mech. 210, 23 (1990)] showed that the HSV system forming at the leading edge region of a wing mounted on a flat plate at Re=1.15×105 exhibits bimodal, low-frequency oscillations, which away from the wall produce turbulent energy and stresses one order of magnitude higher than those produced by the conventional shear mechanism in the approaching turbulent boundary layer. We carry out numerical simulations for the experimental configuration of Devenport and Simpson using the detached-eddy-simulation (DES) approach. The DES length scale is adjusted for this flow to alleviate the well known shortcoming of DES; namely that of premature, laminar-like flow separation. The numerical simulations reproduce with good acc...

Experimental transition delay using glow-discharge plasma actuators

Abstract: In the present work plasma actuators were applied in a flat-plate boundary layer with an adverse pressure gradient to influence the transition of the boundary layer. The first actuator downstream of the leading edge is operated in pulsed mode to introduce perturbations into the boundary layer to promote transition. Two steady operating actuators further downstream damp the perturbations significantly, which results in transition delay.

Edge Flow and Canopy Structure: A Large-Eddy Simulation Study

Abstract: Sharp heterogeneities in forest structure, such as edges, are often responsible for wind damage. In order to better understand the behaviour of turbulent flow through canopy edges, large-eddy simulations (LES) have been performed at very fine scale (2 m) within and above heterogeneous vegetation canopies. A modified version of the Advanced Regional Prediction System (ARPS), previously validated in homogeneous conditions against field and wind-tunnel measurements, has been used for this purpose. Here it is validated in a simple forest-clearing-forest configuration. The model is shown to be able to reproduce accurately the main features observed in turbulent edge flow, especially the “enhanced gust zone” (EGZ) present around the canopy top at a few canopy heights downwind from the edge, and the turbulent region that develops further downstream. The EGZ is characterized by a peak in streamwise velocity skewness, which reflects the presence of intense intermittent wind gusts. A sensitivity study of the edge flow to the forest morphology shows that with increasing canopy density the flow adjusts faster and turbulent features such as the EGZ become more marked. When the canopy is characterized by a sparse trunk space the length of the adjustment region increases significantly due to the formation of a sub-canopy wind jet from the leading edge. It is shown that the position and magnitude of the EGZ are related to the mean upward motion formed around canopy top behind the leading edge, caused by the deceleration in the sub-canopy. Indeed, this mean upward motion advects low turbulence levels from the bottom of the canopy; this emphasises the passage of sudden strong wind gusts from the clearing, thereby increasing the skewness in streamwise velocity as compared with locations further downstream where ambient turbulence is stronger.

Competing Broadband Noise Mechanisms in Low-Speed Axial Fans

Abstract: This paper compares two broadband noise mechanisms, the trailing-edge noise or self-noise, and the leading-edge noise or turbulence-ingestion noise, in several blade technologies. Two previously developed analytical models for these broadband contributions are first validated with well-defined measurements on several airfoils embedded in an homogeneous flow at low-Mach number. Each instrumented airfoil is placed at the exit of an open jet anechoic wind tunnel with or without a grid generating turbulence upstream of it. Sound is measured in the far field at the same time as the wall-pressure fluctuations statistics close to the airfoil trailing edge and the inlet velocity fluctuation statistics impacting the airfoil leading edge. The models are then compared in some practical cases representative of airframes, wind turbines, and automotive engine cooling modules. The airfoil models of the two mechanisms are then extended to a full rotating machine in open space. The model predictions of both mechanisms are compared with in-flight helicopter measurements and automotive engine cooling modules measurements. In both instances, the turbulence-ingestion noise is found to be a dominant source over most of the frequency range. The self-noise only becomes a significant contributor at high angles of attack close to flow separation.

High-Performance Airfoil Using Coflow Jet Flow Control

Abstract: T O ACHIEVE revolutionary aircraft performance, advanced technologies should be pursued to drastically reduce the weight of aircraft and fuel consumption and significantly increase aircraft mission payload and maneuverability. Both the military and commercial aircraft will benefit from the same technology. Flow control is a promising technology to break through the limits of the conventional aerodynamic concepts. Recently, a novel active airfoil flow control concept with zero-net mass flux, the coflow jet (CFJ) airfoil, has been developed by Zha et al. [1–5]. The CFJ airfoil achieves three effects simultaneously in a dramatic fashion: lift augmentation, stall margin increase, and drag reduction. The energy expenditure of the CFJ airfoil is low [1], and the CFJ airfoil concept is straightforward to implement. The CFJ airfoil may create a new concept of an “engineless” airplane, which uses the CFJ to generate both lift and thrust without conventional propulsion systems of propellers or jet engines [6]. A CFJ airfoil [1–5] uses an injection slot near the leading edge (LE) and a suction slot near the trailing edge (TE) on the airfoil suction surface. Similar to tangential blowing, the LE jet is in the same direction of the main flow, but the same amount of mass flow that is injected is removed via suction near the TE, resulting in zeronet mass-flux flow control. A proposed fundamental mechanism [2] is that the severe adverse pressure gradient on the suction surface strongly augments turbulent mixing between the main flow and the jet [7]. The mixing then creates the lateral transport of energy from the jet to the main flow and enables the main flow to overcome the large adverse pressure gradient and remain attached at high angle of attack (AOA). The stall margin is hence significantly increased. At the same time, the high-momentum jet drastically increases the circulation, which significantly augments lift, reduces drag, or even generates thrust (negative drag). The objective of this paper is to demonstrate the high performance of the CFJ airfoil with the windtunnel test results.

Autonomous Sensing and Control of Wing Stall Using a Smart Plasma Slat

Abstract: DOI: 10.2514/1.24057 The concept of a self-governing smart plasma slat for active sense and control of flow separation and incipient wing stall is presented. The smart plasma slat design involves the use of an aerodynamic plasma actuator on the leading edge of a two-dimensional NACA 0015 airfoil in a manner that mimics the effect of a movable leading-edge slat of a conventional high-lift system. The self-governing system uses a single high-bandwidth pressure sensor and a feedback controller to operate the actuator in an autonomous mode with a primary function to sense and control incipient flow separation at the wing leading edge and to delay incipient stall. Two feedback control techniques are investigated. Wind tunnel experiments demonstrate that the aerodynamic effects of a smart actuator are consistent with the previously tested open-loop actuator, in that stall hysteresis is eliminated, stall angle is delayed by 7 deg, and a significant improvement in the lift-to-drag ratio is achieved over a wide range of angles of attack. These feedback control approaches provide a means to further reduce power requirements for an unsteady plasma actuator for practical air vehicle applications. The smart plasma slat concept is well suited for the design of low-drag, quiet, highlift systems for fixed-wing aircraft and rotorcraft.

Rotor and Subrotor Dynamics in the Lee of Three-Dimensional Terrain

Abstract: The internal structure and dynamics of rotors that form in the lee of topographic ridges are explored using a series of high-resolution eddy-resolving numerical simulations. Surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel-shear instability breaks this vortex sheet into small intense vortices or subrotors. The strength and evolution of the subrotors and the internal structure of the main large-scale rotor are substantially different in 2D and 3D simulations. In 2D, the subrotors are less intense and are ultimately entrained into the larger-scale rotor circulation, where they dissipate and contribute their vorticity toward the maintenance of the main rotor. In 3D, even for flow over a uniform infinitely long barrier, the subrotors are more intense, and primarily are simply swept downstream past the main rotor along the interface between that rotor and the surrounding lee wave. The average vorticity within the interior of the main rotor is much weaker and the flow is more chaotic. When an isolated peak is added to a 3D ridge, systematic along-ridge velocity perturbations create regions of preferential vortex stretching at the leading edge of the rotor. Subrotors passing through such regions are intensified by stretching and may develop values of the ridge-parallel vorticity component well in excess of those in the parent, shear-generated vortex sheet. Because of their intensity, such subrotor circulations likely pose the greatest hazard to aviation.

Inertia-Gravity Waves Generated within a Dipole Vortex

Abstract: Vortex dipoles provide a simple representation of localized atmospheric jets. Numerical simulations of a synoptic-scale dipole in surface potential temperature are considered in a rotating, stratified fluid with approximately uniform potential vorticity. Following an initial period of adjustment, the dipole propagates along a slightly curved trajectory at a nearly steady rate and with a nearly fixed structure for more than 50 days. Downstream from the jet maximum, the flow also contains smaller-scale, upward-propagating inertia–gravity waves that are embedded within and stationary relative to the dipole. The waves form elongated bows along the leading edge of the dipole. Consistent with propagation in horizontal deformation and vertical shear, the waves’ horizontal scale shrinks and the vertical slope varies as they approach the leading stagnation point in the dipole’s flow. Because the waves persist for tens of days despite explicit dissipation in the numerical model that would otherwise damp th...

Piezoelectric transducer actuated leading edge de-icing with simultaneous shear and impulse forces

Abstract: Piezoelectric actuations for simultaneous generation of shear and impulse forces for effective and energy efficient de-icing applications are proposed. Aircraft leading edge structures are considered for analysis and lab experiments. Piezoelectric actuators are affixed on the inner surface of the leading edge at the locations where highest amount of ice accretion on the outer surface has occurred. Simultaneous shear and impulse force generation is achieved with actuators consisting of two sets of electrodes, one arranged in parallel to the poling direction, and the other perpendicular to it, to generate shear forces and normal forces, respectively. Finite element models of the leading edge structure with ice accretion layer are formulated. Simulations of the de-icing process are performed and the actuator locations, electric charge applied, and impulse duration are optimized to achieve effective ice removal

Modeling and Experiment of Leading Edge Separation Control Using SDBD Plasma Actuators

Abstract: This work presents the study of the single-dielectric barrier discharge aerodynamic plasma actuator. To model the physics of the plasma discharge, a space-time lumpedelement circuit model was developed. The model solution compared well to some of the characteristic features of the discharge such as the dependence of the sweep velocity and maximum extent of the ionized air as functions of the applied voltage and a.c. driving frequency. The time-dependent charge distribution obtained from the model was used to provide boundary conditions to the electric field equation that was used to calculate the time dependent electric potential. The was then used to calculate the space-time distribution of the actuator body force. An application of the plasma actuators to the leading-edge separation control on the NACA 0021 airfoil was studied numerically and experimentally. The results were obtained for a range of angles of attack for uncontrolled flow, and steady and unsteady plasma actuators located at the leading edge of the airfoil. The control of the lift stall was of particular interest. Improvement in the airfoil characteristics were observed in the numerical simulations at post-stall angles of attack with the plasma actuators. The computational results corresponded very well with the experiments.

Wind drive apparatus for an aerial wind power generation system

Abstract: Wind driven apparatus for an aerial power generation system include driven elements and controls The driven elements are configured and shaped to provide maximum force from both lift and drag during the downwind phase of operation and minimum force during the upwind phase The driven elements have a sail portion with a leading edge and a trailing edge The controls change the driven elements between high force configurations for downwind operation and low force configurations for upwind operation, adjust the pitch and azimuth angles of the driven elements, and control the camber

Active separation control on the flap of a two-dimensional generic high-lift configuration

Abstract: The paper describes experimental results of controlling flow separation by periodic excitation on the flap of a generic high-lift configuration. The single slotted flap of the two-dimensional test model is equipped with a robust and reliable actuator system that fits inside the flap. The flow is excited using a pulsed wall jet that emanates from the upper surface near the flap's leading edge through a small spanwise-oriented slot By preventing the flow from separating or by reattaching the separated flow, lift and drag are substantially improved, resulting in a lift-to-drag ratio enhancement of 20-25 %. Because of the actuator assembly with spanwise individually addressable segments, the separated flow can be forced to attach only to certain parts of the flap. Local spanwise excitation is thus used to generate a rolling moment without the need to deflect an aileron.

Airfoil having improved impact and erosion resistance and method for preparing same

Abstract: A turbine airfoil having an improved impact and erosion resistance The airfoil comprises: (a) a base segment having an impact resistant leading edge section proximate to the leading edge comprising a material having having a yield strength of least about 250 ksi and an elongation percentage of about 12% or less; and (b) an erosion resistant coating overlaying the base segment at least in the leading and trailing edge portions of the pressure side, the erosion resistant coating comprising at least one ceramic layer having at least one of the following properties: (1) an erosion value of at least about 200 g of erodent to cause a thickness loss of about 15–20 microns; (2) an erosion volume loss value (V) of about 19 or less as defined by the equation V=H −018 ×E 075 ×F −165 where H is hardness, E is elastic modulus and F is fracture toughness; and (3) an F value of at least about 15 MPa*m 1/2 The erosion resistant coating can comprise alternating ceramic and metallic layers and is typically formed by a method involving the step of forming on the pressure side of the base segment of the airfoil in alternating fashion at least one ceramic layer and at least one metallic layer

Replaceable blade outer air seal design

Abstract: A blade outer air seal system includes a body that extends between two circumferential sides, a leading edge and a trailing edge, and a radially inner side and a radially outer side. An attachment section associated with the body and includes at least one engagement surface that is transverse to the radially outer side.

The formation and maintenance of a leading-edge vortex during the forward motion of an animal wing

Abstract: A simple model is presented to explain the observed generation of a quasi-steady vortex at the leading edge of an animal wing that rotates in a horizontal plane about a body-centred axis. Vorticity formed by separation at the leading edge is transported outwards by a spanwise velocity field generated by two sources of spanwise pressure gradient, one induced centrifugally and the other by variations in the vortex size and circulation. The vorticity is then deposited into a trailing vortex system that takes the form of a downward propagating vortex ring. This mechanism appears to apply generally to flying animals but is modelled here for those in hovering flight.

Perpendicular write head having a modified wrap-around shield to improve overwrite, adjacent track interference and magnetic core width dependence on skew angle

Abstract: A magnetic write head for perpendicular magnetic data recording. The write head includes an wrap around trailing shield structure for improved write field strength, reduced skew related adjacent track interference and magnetic core width. The trailing wrap around shield includes a side shield that is separated from sides of the write pole by a side gap that is narrower near the leading edge of the write pole and wider near the trailing edge of the write pole.

Lift enhancement over flexible nonslender delta wings

Abstract: Unsteady aerodynamics of flexible nonslender delta wings is investigated in an experimental study using various techniques Dramatic fluid/structure interactions emerge with increasing wing flexibility and result in substantial lift enhancement in the poststall region This recently discovered phenomenon appears to be a feature of nonslender wings Self-excited antisymmetric vibrations of the wing promote reattachment of the shear layer, which results in the lift enhancement These self-excited vibrations are not observed for a half-model The Strouhal number of the dominant frequency of the structural vibration is on the order of unity for all nonslender wings, which also corresponds to the frequency of the shear-layer instabilities for the rigid wing Velocity measurements demonstrate the striking difference between the flows over the flexible and rigid wings in the poststall region The effect of flexibility is to promote the reattachment of the shear layer near or downstream of the apex, depending on the incidence There are substantial effects on the vortical flow with increasing wing flexibility, which might lead to the axial flow developing within the reattached region The time-averaged vorticity flux increases due to the oscillating leading edge, which leads to increased circulation

Designing of Slat Cove Filler as a Noise Reduction Device for Leading-edge Slat

Abstract: The purpose of this study is to design noise reduction devices for leading-edge slat which is called as a slat cove filler (SCF), from both aerodynamic and acoustic points of view. From the previous studies, a SCF seems to have an effect on reducing broadband noise by forming substantially continuous shape instead of a slat cusp configuration. However, there are some studies which indicate the decrease of aerodynamic performance when the SCF is attached. Since the primary feature of high-lift-devices is to increase aerodynamic performance, reduction of maximum lift coefficient or stall angle etc. is not acceptable even if the device is effective in reducing noise. In order understand both features, two kinds of flow solver, UPACS and UPACS-LES codes, are used to simulate steady and unsteady flows around slats. The UPACS code is used mainly for aerodynamic force prediction, and UPACS-LES is used to understand the noise generation and reduction mechanism. Based on the simulations, it became apparent that if the SCF is designed while maintaining the geometry around the slat trailing edge and the main element leading edge, the aerodynamic performance will be the same as that of the baseline. Also, to suppress the noise as much as possible, it is important to reduce small separation along the lower surface of a SCF, which seems to be caused by adverse pressure gradient. Additionally, wind tunnel experiments are performed for verification purpose. The SCF designed in this study satisfies both aerodynamic and acoustic performance successfully.

Guide vane and method of fabricating the same

Abstract: A method for fabricating an outlet guide vane includes the steps of fabricating a structural spar from a first material, fabricating a fairing from a second material, and installing the fairing onto the spar so as to at least partially surround the spar and form an airfoil. An outlet guide vane includes an airfoil including a leading edge and a trailing edge, a structural spar formed from a first material located within the airfoil, and a fairing formed from a second material at least partially surrounding the spar. A gas turbine engine assembly includes a core gas turbine engine, a fan assembly including a plurality of fan blades coupled to the core gas turbine engine, and a plurality of outlet guide vanes coupled downstream from the fan blades, at least one of the outlet guide vanes including an airfoil having a leading edge and a trailing edge, a structural spar formed from a first material located within the airfoil, and a fairing formed from a second material at least partially surrounding the spar.

Segmented ring for an industrial gas turbine

Abstract: A blade outer air seal in a gas turbine engine, the BOAS includes a plurality of ring segments secured to a blade ring carrier. An impingement ring forms a pressure vessel within the blade ring carrier and includes a first group of impingement holes positioned over the middle of the blade tip. The ring segments each include a plurality of stiffener ribs forming a plurality of leading edge pockets, trailing edge pocket and middle pockets each with pin fins to enhance the heat transfer effect. A metering plate is secured over the stiffener ribs to form closed pockets over the edge pockets while leaving the middle pockets open. A second group of impingement holes are formed in the metering plate over the edge pockets. Pressurized cooling air supplied to the pressure vessel first passes through the first group of impingement holes and is discharged directly over the middle pockets. The cooling air then flows through the second group of impingement holes and into the closed edge pockets, and then out discharge holes arranged along the leading and trailing edges of the ring segment. The individual pockets can have the impingement holes of varying sizes in order to regulate the amount of cooling air that flows into the specific pocket.

Turbine blade with showerhead film cooling holes

Abstract: A turbine blade used in a gas turbine engine, the blade having a showerhead film cooling hole arrangement along the leading edge of the blade, the showerhead arrangement including three rows of film cooling holes with a middle row being aligned with the stagnation point, a second row on the pressure side of the stagnation point, and the third row on the suction side of the stagnation point. The film cooling holes can be oriented at any angle within each row, and the cooling holes spiral within the airfoil wall to promote heat transfer to the cooling air flow. In large turbine airfoils, very fine cooling passages can be formed in the airfoil wall using the small diameter ceramic core ties used in the present invention.