Positive Angles Of Attack Research Articles

Aerodynamics of a Flapping-Perturbed Revolving Wing

At low Reynolds numbers, revolving wings become less efficient in generating lift for hovering flight due to the increasing adverse viscous effects. Flying insects use reciprocating revolving wings that exploit unsteady aerodynamic mechanisms for lift augmentation. Here, the aerodynamics of an alternative that introduces unsteadiness to the revolving wings through vertical flapping and its potential to improve aerodynamic performance are investigated. The force production and the flow pattern of such flapping-perturbed revolving wing are analyzed via combined experimental and computational investigations. The results show that drag reduction can be produced consistently by a flapping-perturbed revolving wing at zero angle of attack. The reduction is linearly dependent on Strouhal number, and the critical Strouhal number at equilibrium rotating state is similar to that of two-dimensional heaving plates. At positive angles of attack, the flapping perturbation leads to substantial lift augmentation, accompanied by relatively small increase of drag or even minor drag reduction, depending on the Strouhal number and flapping amplitude. Though slightly less efficient at these angles of attack in terms of power loading, flapping perturbations can be used to improve the maximum lift coefficients attainable by revolving wings and thus have potential applications in micro air vehicle designs.

Swim bladder morphology changes with female reproductive state in the mouth-brooding African cichlid Astatotilapia burtoni.

Mouth brooding is an extreme form of parental care in which the brooding parent carries the developing young in their buccal cavity for the duration of development. Brooding fish need to compensate for the brood weight on the anterior portion of their body. For fishes with a compartmentalized swim bladder, gas distribution between the chambers may aid in regulating buoyancy during brooding. To test this hypothesis, we took radiographs of Astatotilapia burtoni to compare the swim bladder morphology of gravid, mouth-brooding and recovering females. Following spawning, females carry developing fish in their buccal cavity for ∼2 weeks, resulting in a larger and rounder anterior swim bladder compartment. Comparatively, the swim bladder of gravid females is long and cylindrical. Using small beads to mimic brood weight and its effects on female buoyancy, swim bladder changes were induced that resembled those observed during brooding. Immediately after releasing their fry, brooding females swim at a positive angle of attack but correct their swimming posture to normal within 5min, suggesting a rapid change in swim bladder gas distribution. These data provide new insights into how swim bladder morphology and swimming behavior change during mouth brooding, and suggest a compartmentalized swim bladder may be a morphological adaptation for mouth brooding.

Experimental study on the thermal performance of loop heat pipe for the aircraft anti-icing system

The objective of this research is to experimentally investigate the start-up and operating characteristics of the Double Compensation Chamber Loop Heat Pipe (DCCLHP) anti-icing system. The loop was made of stainless steel with nickel wick, and the working fluid was ethanol. Experiments were carried out with the heat loads from 10W to 180W. The angle of attack (AOA) is from −10° to 5°. It was found that: (1) in order to obtain a good anti-icing result, the DCCLHP anti-icing system should be turned on approximately ten minutes before the aircraft encountered cloud with super-cooled droplets; (2) temperature oscillations happened in the start-up process when the AOA is not 0°; (3) the positive AOA can increase the surface temperature, as a result of which the anti-icing effect could also be enhanced.

Analiza numeryczna wpływu efektu przypowierzchniowego na siłę nośną samolotu i siłę docisku pojazdu drogowego

The paper presents the mechanism of wing in ground effect formation, also shows the structure of wing in ground craft, and race car, which uses wing in ground effect. Results of numerical analysis are as expected, which means, the lift coefficient is higher for positive angles of attack, and lower for negative angles of attack. The paper details also characteristics of lift and drag coefficients in angle of attack function. The numerical research was conduct in Ansys Fluent 15.0 academic license. Keywords: airfoil, aircraft, wing in ground effect, numerical analysis

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a positive angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relates the velocity field to pressure fluctuations, suggests that the contribution of the serrations to far-field broadband noise is a function of the streamwise location. This observation is congruent with the spectra of the wall-normal and spanwise velocity fluctuations, which typically show low intensity close to the tips of the individual serrations. It follows that analytical models must take into account the local contribution to the far-field noise induced by the streamwise variation of the hydrodynamic pressure on the serration surface.

Effects of knot orientation on the height and drag of a penaeid trawl

The knots used to make fishing nets are asymmetric which, in most cases for trawls, means they present either a positive or negative angle of attack (AOA) in horizontal panels, producing hydrodynamic side forces and changes in system geometry. The extent of associated displacement and drag were quantified in a flume tank using four, full-scale penaeid (shrimp/prawn) trawls with the same generic design, but with all possible combinations of knot orientation in the top and bottom panels. All four trawls had the same distance between the footrope and flume-tank floor at the wing end during a flow of 1.2ms−1, but the trawl with both panels at a positive AOA had a significantly greater footrope height at the centre (by up to 13mm). Further, compared to both trawls with their top panels orientated to have a positive AOA, the two with their top panels producing a negative AOA had up to 27% lower headlines and 10% less total drag. Differences were also observed among vertically partitioned drag at the wing ends—depending on knot orientation in the bottom panel, which led to greater force on the headline or footrope when at negative or positive AOAs, respectively. The magnitudes of observed variations in footrope and headline heights and drag are likely to affect the catching and engineering performances of penaeid trawls, and so knot orientation should be considered during attempts at improving selectivity and reducing drag. The results also reiterate a need for consistency among knot orientations to avoid confounding effects during fishery-dependant/independent surveys.

Numerical Investigation of the Performance of Pitching Airfoils at High Amplitudes

Recently, a renewed interest toward vertical-axis wind turbines has developed, due to the increasing need of a rational utilization of renewable energy resources. In vertical-axis wind turbines, each blade undergoes a wide variation of angle of attack during one rotation, with important dynamic stall phenomena. Neglecting wake-to-wake interactions and curvature effects, a pitching airfoil is a good representation of the flow experienced by the blades of a vertical-axis wind turbine. In the present work, the flow around an Eppler 387 airfoil in pure pitching is investigated, exploring a parametric space tailored to vertical-axis wind-turbine applications. In such a case, the angle of attack varies in the range , resulting in large-amplitude pitching oscillations. The Reynolds number, based on the freestream velocity and the chord length, is set to . In this sense, the present work differs from earlier studies, where typically the combined heaving and pitching or pure pitching for propulsion purposes have been considered, which are characterized by higher frequencies and smaller amplitudes and mainly positive angles of attack. In this paper, a detailed analysis of the force coefficients and how their evolution is affected by the dynamics of the flow structures generated during pitch-up and pitch-down of the airfoil is presented. Furthermore, it is shown that a modified Strouhal number, which takes into account both frequency and amplitude, is an appropriate metric for the performance of an airfoil in these conditions.

Confounding effects of knot orientation in penaeid trawls

This study assessed the potential for confounding engineering and/or catching effects due to different knot (‘sheet bend’) orientations (and therefore positive or negative hydrodynamic lift) in the principal horizontal panels of a penaeid trawl. During 24 paired comparisons, orientating all meshes in the top and bottom belly panels of a trawl to produce a negative angle of attack (AOA) for all knots resulted in greater wing-end spread, and lower catches of school prawns, Metapenaeus macleayi per hectre trawled (means reduced by up to 26%) and three key species of schooling fish (by up to 67%), but more debris (4×) than when the meshes were orientated to produce a positive AOA. By comparison, catches of two other benthic fish remained unaffected by knot orientation. These catch differences were attributed to the negative-AOA trawl potentially assuming a considerably lower position in the water column (and species-specific orientations). The results demonstrate that it is essential to maintain consistency in knot orientation in penaeid trawls when testing gear modifications and/or during fishery-dependent or -independent surveys.

Influence research of profile chords ratio on aerodynamic characteristics of tandem compressor cascade

Efficiency improvement of gas turbine compressors is ensured by the extensive use of tandem blade rows in straightener blades of the last stages of multistage axial compressors. Their installation provides large flow deflection angles in the unstalled flow of blade rows and, consequently, a low level of losses in the design mode of the compressor. The problem of influence research of the first- and second-row blade chords ratio in tandem blade rows in a wide range of angles of attack is relevant and is of practical interest in developing recommendations for ensuring the gas-dynamic stability of gas turbine compressors. To solve the problem, the method of the computational experiment was used and characteristics of tandem compressor cascade with different first- and second-row blade chords ratios were calculated in the paper. The calculation results of the characteristics of tandem cascade were compared with the characteristics of an equivalent single compressor cascade. For the computational domain of the studied tandem and equivalent single cascades, small adaptive unstructured grid and the second-order design scheme with the local use of the first-order design scheme (High resolution) was applied. The Menter's SST model was used to solve the problems. The results showed that the aerodynamic characteristics of the tandem compressor cascades essentially depend on the first- and second-row blade chords ratio. With positive angles of attack, the quality parameter of tandem cascades is higher than that of single, with the first- and second-row blade chords ratio, corresponding to the position of the slot x щ =(0,3 ... 0,4)b ∑ . With negative angles of attack, the quality parameter of tandem cascades is higher than that of single, with the first- and second-row blade chords ratio, corresponding to the position of the slot x щ =(0,5 ... 0,7)b ∑ .

Quasi-steady response of free-to-pivot flat plates in hover

Using force measurements and flow visualization in a water tunnel, we consider motions of rigid flat plates with square edges free to pivot about the leading edge between incidence angles of ±45°. The plate's leading edge undergoes a prescribed periodic rectilinear translation. During most of the translation semi-stroke, the plate rests against its incidence limiter to produce a positive angle of attack; this reverses on the opposite semi-stroke, producing a motion akin to normal-hover with delayed rotation. Three aspect ratios are considered: a nominally 2D, or wall-to-wall plate spanning the tunnel test section, and plates of aspect ratios 3.4 and 5.5. Reynolds number effects in the range of 10000–31000 were not found to be significant. Aerodynamic force coefficient history was found to be aspect-ratio invariant, despite difference in flowfield evolution in the wake, and the force coefficients magnitude decreased for decreasing motion amplitudes. Flow visualization gives qualitative evidence for leading-edge and trailing-edge vortices, but no evidence was found of leading edge vortex stability or for contribution of vortices to aerodynamic loads, for sinusoidal or nonsinusoidal imposed motions of the plate. No evidence is found that the vortex system in the wake interacts with the plate during or after semi-stroke reversal. The main operative parameter governing aerodynamic force history is the ratio of semi-stroke amplitude to plate chord. Especially for the larger semi-stroke to chord ratios, aerodynamic response during the translational phase of motion is nearly quasi-steady, with decay in both thrust and force opposing the motion, in proceeding along each semi-stroke. The rotational phase of each semi-stroke is dominated by a loads spike as the plate approaches its incidence limiter. This spike largely disappears when the free-to-pivot plate is replaced with one with a prescribed rotational history. These findings offer favorable implications for analysis of flapping-wings using quasi-steady blade element models.

Research on the Influence of the Aerodynamic Measure on the Flutter Derivative of the Steel Truss Suspension Bridge

Flutter derivative is a significant index of the structure flutter stability. Identifying flutter derivative precisely contributes to the bridge flutter stability analyzing. In this paper, we take a research on the Liujiaxia Bridge in Gansu Province, China. Different flutter derivatives, which were got via segment model vibration tests with different aerodynamic measures, were classified, and made comparison in order to get the law of how different aerodynamic measures effect on the flutter derivative. The results show that, setting central stabilized plate, Build-in deflector, flange plate all affect flutter derivative significantly, which leads to changes in the flutter critical wind velocity of the structure. Setting central stabilized plate above the deck contributes to identify the flutter derivative of the 0° and positive attack angle, while setting central stabilized plate will contribute to flutter derivative identification at negative angles. It will make it difficult to identify the flutter derivative at 0° and -3° if the built-in deflector was set. Wind plate contributes to the identification of the flutter derivative at +3°, however, it will make it harder to identify the flutter derivative at 0° and -3°.

Ground effect on flow past a wing with a NACA0015 cross-section

Wing in ground (WIG) effects is an important aerodynamics phenomenon as it affects the aerodynamics of aircraft (during take off and landing), marine crafts like wing in ground effect (WIG) crafts and racing cars. In the present investigation, wind tunnel experiment was conducted on a wing with a NACA0015 cross-section, a chord of 0.24m and an aspect ratio of 1.46. The “ground” was simulated by a vertically mounted 0.6m (streamwise)×0.457m (same height as the wind tunnel test section) perspex plate. The wing was mounted vertically on a load cell, and the load cell on a mechanism that could traverse the wing/load cell set-up in a direction transverse to the free stream, thereby varying the distance between the wing and the ground. Measurements carried out include the pressure distribution on the wing surface at mid-span and forces on the wing, and pressure distribution on the ground. Measurements were carried out at various angles of attack, at a constant wind speed of 12m/s. The corresponding chord based Reynolds Number (Rec) is approximately 1.872×105.Mid-span surface pressure distributions were measured on the wing at both positive and negative angles of attack α, and at different ground clearance h/c. It was observed that for both positive and negative angles of attack, the pressure on the upper surface shows a much stronger angle of attack dependence, and only weak h/c dependence when the angle of attack is in the negative range. On the other hand, the lower surface pressure is influenced by both the angle of attack and h/c. Within a certain range of positive angle of attack and h/c, the “ramming effect” which is associated with a local increase in surface pressure that had been reported in the literature was observed.Streamwise pressure distribution on the ground that corresponds to the wing mid-span shows that when the wing is out of ground effect, the effects of wing angle of attack on the pressure distribution on the ground is not significant. When the ground effect are present (small to moderately small h/c), pressure distribution on the ground especially within the range −0.3<x/c<1 is affected. Different pressure versus x/c trends were observed, depending on the magnitude and sign of the wing angle of attack.Both lift coefficients obtained from integrating surface pressure distribution and from direct load cell measurements show that as h/c reduces, the sign of the lift coefficient remains unchanged but its magnitude increases. A temporary reduction in the lift coefficient is observed when h/c reduces from about 0.3 to 0.15 in the α=0–6° range, for both the pressure distribution integration and load cell measurement cases. This is believed to be the consequence of the “convergent–divergent channel effect” reported in surface pressure distribution. Generally speaking the lift estimated from integrating surface pressure distribution is slightly larger than the one measured directly from the load cell. This is consistent with the slightly more 2-D nature of the flow at the wing mid-span.The lift slope from both the pressure and load cell data show h/c dependence, with the former consistently the larger of the two. Their magnitudes generally lie within the magnitude estimated from the Thin Aerofoil Theory (2-D flow) and Prandtl’s Lifting Line Theory (3-D flow).

Effect of the angle of attack on the wind convection coefficient

In this study the effect of the positive angle of attack (angle between flat plate surface and incoming uniform flow) on the convective heat transfer coefficient was investigated numerically. In the case of inviscid flow, this effect was also presented analytically and was found to be in good agreement with the corresponding numerical results. From the obtained numerical data, an accurate correlation equation of Nusselt number was proposed by introducing the effect of the angle of attack in terms of a new factor A f . The variation of the convective heat transfer coefficient as a function of the angle of attack was found not behaves in the same manner for both small and large values of Prandtl number at small angles of attack.

Numerical Simulation of Flow over an Airfoil with a Cavity

Two-dimensional direct numerical simulation of the flow over a NACA0018 airfoil with a cavity is presented. The low Reynolds number simulations are validated by means of flow visualizations carried out in a water channel. From the simulations, it follows that there are two main regimes of flow inside the cavity. Depending on the angle of attack, the first or the second shear-layer mode (Rossiter tone) is present. The global effect of the cavity on the flow around the airfoil is the generation of vortices that reduce flow separation downstream of the cavity. At high positive angles of attack, the flow separates in front of the cavity, and the separated flow interacts with the cavity, causing the generation of smaller-scale structures and a narrower wake compared with the case when no cavity is present. At certain angles of attack, the numerical results suggest the possibility of a higher lift-to-drag ratio for the airfoil with cavity compared with the airfoil without cavity.

Analysis of small-aspect-ratio lifting surfaces in extreme curved-ground effect

This paper extends Newman’s (1982, Analysis of small-lifting-ratio lifting surfaces in ground effect. J. Fluid Mech., 117, 305) analysis for a small-aspect-ratio lifting surface moving in very close proximity to a rigid plane surface to one near a curved surface. In this 3D problem, the variation of the velocity along the chord is much smaller than in the transverse cross section, while the flow in the gap beneath the wing is dominant over that above it. At first order, the problem reduces to flow in the gap, which can be represented through a simple non-linear solution. The solution has different boundary conditions at the leading and trailing edges of the wing, where the mean normal velocity on the upper surface and lower surface of the wing is inward and outward of the gap, respectively. Calculations for a delta wing reveal that the changeover point between the leading and trailing edges tends to move forward (backward) when the wing moves over convex (concave) ground. If the angle of attack is positive (negative), a lift (attraction) force acts on the wing. The magnitude of the lift or attraction increases (decreases) when the wing moves over convex (concave) ground. When a wing at a positive angle of attack and its rear part is over curved ground, the lift increases (decreases) significantly for convex (concave) ground because the curved-ground effect is strengthened by a shift in the changeover point. The pressure centre shifts slightly forward when the fore part of the wing is over convex ground. It shifts backward at a much larger amplitude when the rear part of the wing is over convex ground. The similar but reverse trends are observed when the wing moves over concave ground.

Effects of corrugation of the dragonfly wing on gliding performance

We investigate the aerodynamic performance of the dragonfly wing, which has cross-sectional corrugation, via a static 2-dimensional unsteady simulation. Computational conditions are Re=150, 1400, and 10,000 with angles of attack ranging from 0° to 40°. From the computational results, lift coefficients are increased by the wing corrugation at all Reynolds number. However, the corrugation has little influence on the drag coefficients. The flows such as vortex in the valley of corrugation and near the edge of the corrugation are locally different from those of an elliptic wing. However, such local flows have little influence on the time averaged wing performance. From the numerical experiment presented in this study, it is determined that suction side corrugations of the wing have very little influence on increase of the lift coefficient at a positive angle of attack.

Experimental Studies on Unsteady Lateral Blowing on NACA 0012

T HE initial study of steady lateral blowing was conducted by Dixon in 1969 [1]. The mechanism of this concept is analogous to the flow over slender delta wing platforms, which produces a leading-edge vortex and nonlinear lift curves. Steady lateral blowing might be thought of as providing the sweep effects such as those of the delta wings, for which the effective sweep is a function of the jet momentum. The lateral blowing jet can be described in terms of the chordwise position along the airfoil section, the nozzle height, and the nozzle size. Dixon et al. [2] andClarke [3] suggested that the best vertical nozzle positionmay be a function of the nozzle diameter and that the jet from a nozzle placed too near the airfoil surface may have a deleterious effect on the flow over the airfoil. Wong and Kontis [4] performed a comprehensive study on the steady spanwise blowing on a NACA 0012 airfoil section. The force measurements showed that the lift coefficient increasedwhen the steady lateral blowingwas applied, and the most effective blowing location was found to be at x=c 0:25 (x is the chordwise location and c is the airfoil chord), because the lift augmentation ratio Cl=C was always above one for all positive angles of attack [ Cl is the increment in the lift coefficient due to blowing, andC is the blowing momentum coefficient; the definition ofC is shown inEq. (1)].However, largemomentumwas required to generate the lift enhancement on the airfoil and this became a major issue in the effectiveness of blowing. As a consequence, Meyer and Seginer [5] performed some initial tests for generating the same lift increase using lower momentum through pulsed blowing. One of the main disadvantages of lateral blowing is due to the fact that air for blowing is bled from the compressor stage of the engine. This reduces the amount of air for combustion, hence reducing thrust. Unsteady blowing provides the beneficial effect of reducing the mass-flow requirement from the engine. Another adverse effect of the technology arises from the need to install plumbing, control valves, regulators, oscillators, etc., to supply and control the compressed air from the engine. This adds weight to the aircraft, and a higher lift is required. C thrust

Skin-Friction Reduction on Body of Revolution Using Boundary-Layer Alteration Devices

The effect of boundary-layer alteration devices (BLADEs) on the characteristics of a turbulent boundary layer on an axisymmetric body of revolution in nominally zero pressure gradient incompressible flow is experimentally studied. The Reynolds numbers based on momentum thickness at the BLADEs’ position were varied from 2610 to 6500. All BLADE configurations that were examined, including single-element ones, are shown to be capable of reducing skin friction in the downstream region. A tandem BLADE configuration is found to be more advantageous than the single-element configuration. The tandem configuration, formed by two flat ring elements with a chord length within 75% of the boundary-layer thickness, reduces the net drag by at least 15% compared to the regular flow. An enhanced drag reduction is also observed for small positive angles of attack of the BLADE. Finally it is found that an increase in the chord length of ring elements degrades the BLADE’s effectiveness.

Laminar Incompressible FlowPast Parabolic Bodies at Angles of Attack

Numerical solutions of a two-dimensional steady laminar incompressible flow over semi-infinite parabolic bodies at angles of attack are obtained. All solutions are found by using a modified numerical approach to solve the timedependent Navier‐Stokes equations. The governing equations are written for the stream function and vorticity variables and are solved on a nonuniform body-fitted parabolic grid. A check of our solutions to those that exist in the literature at zero angle of attack showed excellent agreement. In addition, at zero angle of attack far from the leading edge, an expected correspondence to Blasius flow was found. At positive angles of attack, the body became aerodynamically loaded with lower pressures on its upper surface. At large enough angles of attack, the flow separated on the upper surface. In all of the cases examined, the flow separation was followed by a reattachment that defined a separation zone. An almost linear increase in the streamwise extent of the separation zone occurred with increasing angle of attack. The separation location and extent of the separation zone was a function of the nose Reynolds number. The results indicated that the shape factor could be used to provide a criterion for separation and reattachment in these cases. The characteristics of the separation zone for this geometry should prove to be an excellent basic flow to document the effect of leading-edge flow separation on acoustic receptivity of boundary-layer instabilities.

Numerical Simulation and Experimental Measurement of Pressure Pulses Produced by a Pulp Screen Foil Rotor

Pressure screening is an efficient means of removing various contaminants that degrade the appearance and strength of paper. A critical component of a screen is the rotor, which induces a tangential velocity to the suspension and produces pressure pulses to keep the screen apertures clear. To understand the effect of key design and operating variables for a NACA foil rotor, a computational fluid dynamic (CFD) simulation was developed using FLUENT, and the results were compared to experimental measurements. Comparing the pressure pulses obtained through CFD to experimental measurements over a wide range of foil tip speeds, clearances, angles of attack, and foil cambers, general trends of the pressure pulses were similar, but the overall computed values were 40% smaller than the measured values. The pressure pulse peak was found to increase linearly with the square of tip speed for all the angles of attack studied. The maximum magnitudes of negative pressure pulse occurred for the NACA 0012 and 4312 foils at a 5deg angle of attack and for the NACA 8312 foil at 0deg. The stall angle of attack was found to be ∼5deg for NACA 8312, ∼10deg for NACA 4312, and ∼15deg for NACA 0012. The positive pressure peak near the leading edge of the foil was eliminated for foils operating at a positive angle of attack. The magnitude of the negative pressure coefficient peak increased as clearance decreased. Increased camber increases both the magnitude and width of the negative pressure pulse.