Showing papers on "Leading edge published in 2004"

Two distinct actin networks drive the protrusion of migrating cells

Abstract: Cell migration initiates by extension of the actin cytoskeleton at the leading edge. Computational analysis of fluorescent speckle microscopy movies of migrating epithelial cells revealed this process is mediated by two spatially colocalized but kinematically, kinetically, molecularly, and functionally distinct actin networks. A lamellipodium network assembled at the leading edge but completely disassembled within 1 to 3 micrometers. It was weakly coupled to the rest of the cytoskeleton and promoted the random protrusion and retraction of the leading edge. Productive cell advance was a function of the second colocalized network, the lamella, where actomyosin contraction was integrated with substrate adhesion.

Mechanisms and Responses of a Dielectric Barrier Plasma Actuator: Geometric Effects

Abstract: The single dielectric barrier discharge plasma, a plasma sustainable at atmospheric pressure, has shown considerable promise as a flow control device operating at modest (tens of watts) power levels. Measurements are presented of the development of the plasma during the course of the discharge cycle, and the relevance of these measurements to the modeling of the actuator's electrical properties is discussed. Experimental evidence is presented strongly pointing to the electric field enhancement near the leading edge of the actuator as a dominant factor determining the effectiveness of momentum coupling into the surrounding air

Force production and flow structure of the leading edge vortex on flapping wings at high and low Reynolds numbers.

Abstract: The elevated aerodynamic performance of insects has been attributed in part to the generation and maintenance of a stable region of vorticity known as the leading edge vortex (LEV). One explanation for the stability of the LEV is that spiraling axial flow within the vortex core drains energy into the tip vortex, forming a leading-edge spiral vortex analogous to the flow structure generated by delta wing aircraft. However, whereas spiral flow is a conspicuous feature of flapping wings at Reynolds numbers (Re) of 5000, similar experiments at Re=100 failed to identify a comparable structure. We used a dynamically scaled robot to investigate both the forces and the flows created by a wing undergoing identical motion at Re of ~120 and ~1400. In both cases, motion at constant angular velocity and fixed angle of attack generated a stable LEV with no evidence of shedding. At Re=1400, flow visualization indicated an intense narrow region of spanwise flow within the core of the LEV, a feature conspicuously absent at Re=120. The results suggest that the transport of vorticity from the leading edge to the wake that permits prolonged vortex attachment takes different forms at different Re.

Localized Ras signaling at the leading edge regulates PI3K, cell polarity, and directional cell movement

Abstract: During chemotaxis, receptors and heterotrimeric G-protein subunits are distributed and activated almost uniformly along the cell membrane, whereas PI(3,4,5)P3, the product of phosphatidylinositol 3-kinase (PI3K), accumulates locally at the leading edge. The key intermediate event that creates this strong PI(3,4,5)P3 asymmetry remains unclear. Here, we show that Ras is rapidly and transiently activated in response to chemoattractant stimulation and regulates PI3K activity. Ras activation occurs at the leading edge of chemotaxing cells, and this local activation is independent of the F-actin cytoskeleton, whereas PI3K localization is dependent on F-actin polymerization. Inhibition of Ras results in severe defects in directional movement, indicating that Ras is an upstream component of the cell's compass. These results support a mechanism by which localized Ras activation mediates leading edge formation through activation of basal PI3K present on the plasma membrane and other Ras effectors required for chemotaxis. A feedback loop, mediated through localized F-actin polymerization, recruits cytosolic PI3K to the leading edge to amplify the signal.

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.

Abstract: SUMMARY Here we show, by qualitative free- and tethered-flight flow visualization, that dragonflies fly by using unsteady aerodynamic mechanisms to generate high-lift, leading-edge vortices. In normal free flight, dragonflies use counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing downstroke, attached flow on the forewing upstroke, and attached flow on the hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats with highly separated downstroke flows, with a single LEV attached across both the fore- and hindwings. We use smoke visualizations to distinguish between the three simplest local analytical solutions of the Navier–Stokes equations yielding flow separation resulting in a LEV. The LEV is an open U-shaped separation, continuous across the thorax, running parallel to the wing leading edge and inflecting at the tips to form wingtip vortices. Air spirals in to a free-slip critical point over the centreline as the LEV grows. Spanwise flow is not a dominant feature of the flow field – spanwise flows sometimes run from wingtip to centreline, or vice versa – depending on the degree of sideslip. LEV formation always coincides with rapid increases in angle of attack, and the smoke visualizations clearly show the formation of LEVs whenever a rapid increase in angle of attack occurs. There is no discrete starting vortex. Instead, a shear layer forms behind the trailing edge whenever the wing is at a non-zero angle of attack, and rolls up, under Kelvin–Helmholtz instability, into a series of transverse vortices with circulation of opposite sign to the circulation around the wing and LEV. The flow fields produced by dragonflies differ qualitatively from those published for mechanical models of dragonflies, fruitflies and hawkmoths, which preclude natural wing interactions. However, controlled parametric experiments show that, provided the Strouhal number is appropriate and the natural interaction between left and right wings can occur, even a simple plunging plate can reproduce the detailed features of the flow seen in dragonflies. In our models, and in dragonflies, it appears that stability of the LEV is achieved by a general mechanism whereby flapping kinematics are configured so that a LEV would be expected to form naturally over the wing and remain attached for the duration of the stroke. However, the actual formation and shedding of the LEV is controlled by wing angle of attack, which dragonflies can vary through both extremes, from zero up to a range that leads to immediate flow separation at any time during a wing stroke.

The mechanisms of lift enhancement in insect flight

Abstract: Recent studies have revealed a diverse array of fluid dynamic phenomena that enhance lift production during flapping insect flight. Physical and analytical models of oscillating wings have demonstrated that a prominent vortex attached to the wing’s leading edge augments lift production throughout the translational parts of the stroke cycle, whereas aerodynamic circulation due to wing rotation, and possibly momentum transfer due to a recovery of wake energy, may increase lift at the end of each half stroke. Compared to the predictions derived from conventional steady-state aerodynamic theory, these unsteady aerodynamic mechanisms may account for the majority of total lift produced by a flying insect. In addition to contributing to the lift required to keep the insect aloft, manipulation of the translational and rotational aerodynamic mechanisms may provide a potent means by which a flying animal can modulate direction and magnitude of flight forces for manoeuvring flight control and steering behaviour. The attainment of flight, including the ability to control aerodynamic forces by the neuromuscular system, is a classic paradigm of the remarkable adaptability that flying insects have for utilising the principles of unsteady fluid dynamics. Applying these principles to biology broadens our understanding of how the diverse patterns of wing motion displayed by the different insect species have been developed throughout their long evolutionary history.

Leading-Edge Vortex Lifts Swifts

Abstract: The current understanding of how birds fly must be revised, because birds use their hand-wings in an unconventional way to generate lift and drag. Physical models of a common swift wing in gliding posture with a 60° sweep of the sharp hand-wing leading edge were tested in a water tunnel. Interactions with the flow were measured quantitatively with digital particle image velocimetry at Reynolds numbers realistic for the gliding flight of a swift between 3750 and 37,500. The results show that gliding swifts can generate stable leading-edge vortices at small (5° to 10°) angles of attack. We suggest that the flow around the arm-wings of most birds can remain conventionally attached, whereas the swept-back hand-wings generate lift with leading-edge vortices.

Control of a Nonlinear Wing Section Using Leading- and Trailing-Edge Surfaces

Abstract: The control of nonlinear aeroelastic response of a wing section with a continuous stiffening-type structural nonlinearity is examined through analytical and experimental studies. Motivated by the limited effectiveness of using a single, trailing-edge control surface for the suppression of limit-cycle oscillations of a typical wing section, improvements in the control of limit-cycle oscillations are investigated through the use of multiple control surfaces, namely, an additional leading-edge control surface. The control methodology consists of a feedback linearization approach that transforms the system equations of motion via Lie algebraic methods and a model reference adaptive control strategy that augments the closed-loop system to account for inexact cancellation of nonlinear terms due to modeling uncertainty. Specifically, uncertainty in the nonlinear pitch stiffness is examined. It is shown through simulations and experiments that globally stabilizing control may be achieved by using two control surfaces.

Modelling of Bird Strike on an Aircraft Wing Leading Edge Made from Fibre Metal Laminates – Part 2: Modelling of Impact with SPH Bird Model

Abstract: Fibre Metal Laminates with layers of aluminium alloy and high strength glass fibre composite have been reported to possess excellent impact properties and be suitable for aircraft parts likely to be subjected to impacts such as runway debris or bird strikes. In a collaborative research project, aircraft wing leading edge structures with a glass-based FML skin have been designed, built, and subjected to bird strike tests that have been modelled with finite element analysis. In this second part of a two-part paper, a finite element model is developed for simulating the bird strike tests, using Smooth Particle Hydrodynamics (SPH) for modelling the bird and the material model developed in Part 1 of the paper for modelling the leading edge skin. The bird parameters are obtained from a system identification analysis of strikes on flat plates. Pre-test simulations correctly predicted that the bird did not penetrate the leading edge skin, and correctly forecast that one FML lay-up would deform more than the other. Post test simulations included a model of the structure supporting the test article, and the predicted loads transferred to the supporting structure were in good agreement with the experimental values. The SPH bird model showed no signs of instability and correctly modelled the break-up of the bird into particles. The rivets connecting the skin to the ribs were found to have a profound effect on the performance of the structure.

Leading edge effects in bypass transition

Abstract: The effect of a blunt leading edge on bypass transition is studied by numerical simulation. A mixed direct and large-eddy simulation of a flat plate with a super-ellipse leading edge is carried out at various conditions. Onset and completion of transition is seen to move upstream with increasing bluntness. For sharper leading edges, at lower levels of turbulence, transition usually occurs through instabilities on low-speed streaks as observed by Jacobs & Durbin (2001) and Brandt et al. (2004) whereas increasing either the turbulence intensity or the leading-edge bluntness brings into play another mechanism. Free-stream vortices are amplified at the leading edge because of stretching. In the case of particularly strong vortices, this interaction induces a localized streamwise vortical disturbance in the boundary layer which then grows as it convects downstream and eventually breaks down to form a turbulent spot. These disturbances, which are localized and hence wavepacket-like, move at speeds in the range 0.55 U∞–0.65 U∞ and occur in the lower portion of the boundary layer. Simulations conducted with isolated vortices confirm such a response of the boundary layer.

Magnetic head with stitched top pole layer and single layer coil or solenoidal coil

Abstract: A magnetic head is disclosed that has first and second substantially flat soft magnetic pole layers that are magnetically coupled together in a backgap region that is removed from the medium-facing surface; a soft magnetic pedestal having a leading edge and a trailing edge, the trailing edge adjoining the second pole layer adjacent to the medium-facing surface, the leading edge defining a throat area that is spaced from the first pole layer by a submicron nonferromagnetic gap and defining an apex area that is spaced from the first pole layer by a greater separation than the gap, the throat area meeting the apex area at a throat height; and a plurality of substantially parallel, electrically conductive sections disposed between the first and second pole layers, the conductive sections disposed in a single layer that is aligned along a plane that intersects the pedestal and the backgap region.

Electronic image registration for a scanner

Abstract: A system electronically registers an image on an input document. The system includes a scanner for generating an image data stream representing an electronic image of the image on the input document and an edge detecting circuit for detecting edge data within the image data stream. A circuit detects a first corner of a leading edge of the input document based on the detected edge data and for establishing a first coordinate value therefrom and a second corner of a leading edge of the input document based on the detected edge data and for establishing a second coordinate value therefrom. The system further determines a minimum and maximum location for a leading edge of the scanned document and determines a minimum and maximum location for a trailing edge of the scanned document. An image window is generated representing valid image data to processed and rendered based on the minimum and maximum location for a leading edge of the scanned document, the minimum and maximum location for a trailing edge of the scanned document, the first coordinate value, and the second coordinate value.

Design and verification of the Risø-B1 airfoil family for wind turbines

Abstract: This paper presents the design and experimental verification of the Riso-B1 airfoil family for MW-s ize wind turbines with variable speed and pitch control . Seven airfoils were designed with thickness-to-chor d ratios between 15% and 53% to cover the entire span of a wind turbine blade. The airfoils were designed to have high maximum lift coefficient to allow a slender flexible blade while maintaining high aerodynamic efficiency. The design was carried out with a Riso inhouse multi disciplinary optimization tool. Wind tu nnel testing was done for Riso-B1-18 and Riso-B1-24 in t he VELUX wind tunnel, Denmark, at a Reynolds number of 1.6 ×10 6 . For both airfoils the predicted target characteristics were met. Results for Riso-B1-18 showed a maximum lift coefficient of 1.64. A standa rd case of zigzag tape leading edge roughness caused a drop in maximum lift of only 3.7%. Cases of more severe roughness caused reductions in maximum lift between 12% and 27%. Results for the Riso-B1-24 airfoil showed a maximum lift coefficient of 1.62. The standard case leading edge roughness caused a drop in maximum lift of 7.4%. Vortex generators and Gurney flaps in combination could increase maximum lift up to 2.2 (32%). NOMENCLATURE

Characterization of Unsteady Flow Structures Near Leading-Edge Slat. Part 1; PIV Measurements

Abstract: A comprehensive computational and experimental study has been performed at the NASA Langley Research Center as part of the Quiet Aircraft Technology (QAT) Program to investigate the unsteady flow near a leading-edge slat of a two-dimensional, high-lift system. This paper focuses on the experimental effort conducted in the NASA Langley Basic Aerodynamics Research Tunnel (BART) where Particle Image Velocimetry (PIV) data was acquired in the slat cove and at the slat trailing edge of a three-element, high-lift model at 4, 6, and 8 degrees angle of attack and a freestream Mach Number of 0.17. Instantaneous velocities obtained from PIV images are used to obtain mean and fluctuating components of velocity and vorticity. The data show the recirculation in the cove, reattachment of the shear layer on the slat lower surface, and discrete vortical structures within the shear layer emanating from the slat cusp and slat trailing edge. Detailed measurements are used to examine the shear layer formation at the slat cusp, vortex shedding at the slat trailing edge, and convection of vortical structures through the slat gap. Selected results are discussed and compared with unsteady, Reynolds-Averaged Navier-Stokes (URANS) computations for the same configuration in a companion paper by Khorrami, Choudhari, and Jenkins (2004). The experimental dataset provides essential flow-field information for the validation of near-field inputs to noise prediction tools.

Aeroacoustic wind tunnel tests of wind turbine arfoils

Abstract: Aeroacoustic wind tunnel tests were performed for six airfoils that are candidates for use on small wind turbines. One additional airfoil (NACA 0012) was tested for comparison to benchmark data. The acoustic measurements were performed in NLR's Small Anechoic Wind Tunnel, for a range of wind speeds ( U) and angles of attack, with and without boundary layer tripping. Besides the airfoil self-noise measurements in a clean tunnel flow, the models were also tested with a turbulence grid in the nozzle, to investigate airfoil noise associated with inflow turbulence. A 48-microphone out-of-flow acoustic array was used to locate noise sources and to separate airfoil noise from extraneous wind tunnel noise. Special techniques were applied to translate acoustic source plots to absolute airfoil noise spectra. The acoustic results indicate that in a clean tunnel flow trailing edge noise is dominant for all airfoils. In the untripped condition a number of airfoils exhibit intense tones, that disappear after proper tripping is applied. Broadband trailing edge noise levels are found to scale with U 4.5 . The agreement with the benchmark data was generally good. In case of inflow turbulence, leading edge noise is dominant for all airfoils, and no difference is observed between results with and without tripping. The inflow turbulence noise levels are found to scale with U 6 . Comparison of the acoustic results for different airfoils indicates that inflow turbulence noise levels increase with increasing sharpness of the model leading edge. The directivity of both trailing edge and inflow turbulence noise is found to be symmetrical with respect to the chord. With regard to the test set-up, it was found that a treatment of porous material at the model-endplate junctions yields a reduction of broadband extraneous noise up to 10 dB. As a result, the array can look much 'deeper', which enables the detection of very low trailing edge noise levels.

Blade outer seal with micro axial flow cooling system

Abstract: A turbine blade outer air seal assembly includes a hot side exposed to a combustion hot gas flow, and a back side that is exposed to a supply of cooling air. The outer air seal segment includes a trailing edge cavity and a leading edge cavity separated by a divider. The cavities are feed cooling air through a plurality of inlet openings disposed transverse to the gas flow. The cooling air enters the cavities and flows toward a plurality of outlets at the leading edge and a plurality of outlets along the trailing edge. A plurality of pedestals within each of the cavities disrupts cooling air flow to increase heat absorption capacity and to increase the surface area capable of transferring heat from the hot side.

Flow Structure on a Delta Wing of Low Sweep Angle

Abstract: The instantaneous and averaged flow structure past a delta wing of low sweep angle is investigated using a technique of high-image-density particle image velocimetry. Emphasis is on crossflow planes, where vortex breakdown and stall occur, and the identification of buffeting mechanisms in these regions. At all values of angle of attack up to the fully stalled condition, the averaged vorticity layer exhibits an elongated form; the classical (single) large-scale concentration of vorticity within the leading-edge vortex of a highly swept wing is not present. At low angle of attack α, this elongated, averaged layer can exhibit, however, well-defined concentrations of vorticity. These elongated vorticity layers are accompanied by narrow recirculation zones adjacent to the wing surface. Furthermore, the averaged streamline topology exhibits, at lower α, a saddle point located slightly outboard of the leading edge, in contrast to a saddle point located on the plane of the symmetry of a highly swept wing. Patterns of velocity fluctuation and Reynolds stress show peaks that are generally coincident with large values of averaged vorticity, which indicates that they arise from unsteady events in regions of high shear

Magnetic heads for perpendicular recording and magnetic recording disk apparatus using the same

Abstract: A magnetic head having at least a main pole having a profile on a magnetic head air bearing surface composed of a first portion having a length in a cross-track direction which continuously increases from a leading edge to a trailing edge, and a second portion located on the side of the trailing edge of the first portion. A length of the second portion in the cross-track direction at the trailing edge is substantially equal to a length in the cross-track direction at the point of contact between the first and second portions. A rate of change in the length of the second portion in the cross-track direction from the leading edge to the trailing edge is different from a rate of increase in the length of the first portion in the cross-track direction.

Scalloped surface turbine stage

Abstract: A turbine stage includes a row of airfoils joined to corresponding platforms to define flow passages therebetween. Each airfoil includes opposite pressure and suction sides and extends in chord between opposite leading and trailing edges. Each platform has a scalloped flow surface including a bulge adjoining the pressure side adjacent the leading edge, and a bowl adjoining the suction side aft of the leading edge.

The leading edge of production wafer probe test technology

Abstract: Microelectronic wafer and die level testing have undergone significant changes in the past few years. This work's first section describes today's leading edge characteristics for numerous areas of this test technology including the minimum I/O pad pitch, advances in contactor technologies, maximum number of l/Os probed, maximum number of die tested in parallel, the largest prober and substrates, and the maximum frequencies being tested at the wafer level. The second section discuss the leading edge practices in three critical areas of wafer test: probe contactor cleaning, I/O pad damage minimization, and sorting good from bad die. The final section present the communication methods between the design and the probe test organizations and some state-of-the-art examples for I/O pad designs.

Role of Tip-Leakage Vortices and Passage Shock in Stall Inception in a Swept Transonic Compressor Rotor

Abstract: The current paper reports on investigations aimed at advancing the understanding of the flow field near the casing of a forward-swept transonic compressor rotor. The role of tip clearance flow and its interaction with the passage shock on stall inception are analyzed in detail. Steady and unsteady three-dimensional viscous flow calculations are applied to obtain flow fields at various operating conditions. The numerical results are first compared with available measured data. Then, the numerically obtained flow fields are interrogated to identify the roles of flow interactions between the tip clearance flow, the passage shock, and the blade/endwall boundary layers. In addition to the flow field with nominal tip clearance, two more flow fields are analyzed in order to identify the mechanisms of blockage generation: one with zero tip clearance, and one with nominal tip clearance on the forward portion of the blade and zero clearance on the aft portion. The current study shows that the tip clearance vortex does not break down, even when the rotor operates in a stalled condition. Interaction between the shock and the suction surface boundary layer causes the shock, and therefore the tip clearance vortex, to oscillate. However, for the currently investigated transonic compressor rotor, so-called breakdown of the tip clearance vortex does not occur during stall inception. The tip clearance vortex originates near the leading edge tip, but moves downward in the spanwise direction inside the blade passage. A low momentum region develops above the tip clearance vortex from flow originating from the casing boundary layer. The low momentum area builds up immediately downstream of the passage shock and above the core vortex. This area migrates toward the pressure side of the blade passage as the flow rate is decreased. The low momentum area prevents incoming flow from passing through the pressure side of the passage and initiates stall inception. It is well known that inviscid effects dominate tip clearance flow. However, complex viscous flow structures develop inside the casing boundary layer at operating conditions near stall.Copyright © 2004 by ASME

Influence of Surface Roughness on Three-Dimensional Separation in Axial Compressors

Abstract: Surface roughness on a stator blade was found to have a major effect on the three-dimensional (3D) separation at the hub of a single-stage low-speed axial compressor. The change in the separation with roughness worsened performance of the stage. A preliminary study was carried out to ascertain which part of the stator suction surface and at what operating condition the flow is most sensitive to roughness. The results show that stage performance is extremely sensitive to surface roughness around the leading edge and peak-suction regions, particularly for flow rates corresponding to design and lower values. Surface flow visualization and exit loss measurements show that the size of the separation, in terms of spanwise and chordwise extent, is increased with roughness present. Roughness produced the large 3D separation at design flow coefficient that is found for smooth blades nearer to stall. A simple model to simulate the effect of roughness was developed and, when included in a 3D Navier-Stokes calculation method, was shown to give good qualitative agreement with measurements.Copyright © 2004 by ASME

Unsteady three-dimensional flow phenomena due to breakdown of tip leakage vortex in a transonic axial compressor rotor

Abstract: Unsteady three-dimensional flow fields in a transonic axial compressor rotor (NASA Rotor 37) have been investigated by unsteady Reynolds-averaged Navier-Stokes simulations. The simulations show that the breakdown of the tip leakage vortex occurs in the compressor rotor because of the interaction of the vortex with the shock wave. At near-peak efficiency condition small bubble-type breakdown of the tip leakage vortex happens periodically and causes the loading of the adjacent blade to fluctuate periodically near the leading edge. Since the blade loading near the leading edge is closely linked to the swirl intensity of the tip leakage vortex, the periodic fluctuation of the blade loading leads to the periodic breakdown of the tip leakage vortex, resulting in self-sustained flow oscillation in the tip leakage flow field. However, the tip leakage vortex breakdown is so weak and small that it is not observed in the time-averaged flow field at near-peak efficiency condition. On the other hand, spiral-type breakdown of the tip leakage vortex is caused by the interaction between the vortex and the shock wave at near-stall operating condition. The vortex breakdown is found continuously since the swirl intensity of tip leakage vortex keeps strong at near-stall condition. The spiral-type vortex breakdown has the nature of self-sustained flow oscillation and gives rise to the large fluctuation of the tip leakage flow field, in terms of shock wave location, blockage near the rotor tip and three-dimensional separation structure on the suction surface. It is found that the breakdown of the tip leakage vortex leads to the unsteady flow phenomena near the rotor tip, accompanying large blockage effect in the transonic compressor rotor at the near-stall condition.Copyright © 2004 by ASME

Adiabatic and Overall Effectiveness for a Film Cooled Blade

Abstract: Laboratory studies of film cooling performance for turbine section airfoils typically quantify adiabatic effectiveness and occasionally the heat transfer coefficient for the film cooling configuration. In this study the normalized airfoil metal surface temperatures are obtained directly by using a test model that has a material conductivity scaled to the external and internal heat transfer coefficients so that the Biot number for the model is similar to that for the actual airfoil. These results provide an experimental test case of the conjugate heat transfer involved in turbine airfoil cooling. In this study, conventional adiabatic effectiveness and the overall cooling effectiveness (normalized surface temperature for the matched Biot model) were measured for a generic blade leading edge using three rows of shaped holes. Distinct differences were found between the adiabatic effectiveness and overall cooling effectiveness. Also included is a practical application of this experimental method for which the degradation of overall cooling effectiveness due to a plugged cooling hole is examined.Copyright © 2004 by ASME

Free convection boundary-layer flow above a nearly horizontal surface in a porous medium with newtonian heating

Abstract: In this paper the steady free convection boundary-layer along a semi-infinite, slightly inclined (both positive and negative) to the horizontal plate embedded in a porous medium with the flow generated by Newtonian heating has been investigated. The asymptotic solution near the leading edge and the full numerical solution along the whole plate domain have been obtained numerically, whilst the asymptotic solution far downstream along the plate has been obtained analytically. For a positive inclination the full numerical solution is in agreement with the asymptotic solutions. However, for a negatively inclined plate, only the small asymptotic solution near the leading edge of the plate can be predicted giving an insight that the model for a negatively inclined plate, whilst mathematically interesting, is not physically realistic.

Algebraic growth in a Blasius boundary layer: optimal and robust control by mean suction in the nonlinear regime

Abstract: Optimal and robust control for the three-dimensional algebraically growing instability of a Blasius boundary layer is studied in the nonlinear regime. First, adjoint-based optimization is used to determine an optimal control in the form of a spanwise-uniform wall suction that attenuates the transient growth of a given initial disturbance, chosen to be the optimal perturbation of the uncontrolled flow. Secondly, a robust control is sought and computed simultaneously with the most disrupting initial perturbation for the controlled flow itself. Results for both optimal and robust control show that the optimal suction velocity peaks near the leading edge. In the robust-control case, however, the peak value is smaller, located farther downstream from the leading edge, and the suction profile is much less dependent on the control energy than in the optimal-control case.