Positive Angles Of Attack Research Articles

Vortex wakes of tip loaded rotors at low Reynolds numbers

The effect of tip tabs on the flow field of a three bladed rotor/propeller is investigated experimentally. The experiments are run at chord Reynolds numbers of Rec=0.4–2.9×105. The tab angles of attack of 0°, ∓3°, and ∓5° with respect to the rotation of the rotor are used to vary the tab loading. The rotor wakes and thrust characteristics at positive angles of attack, when the tip loading is outward, are qualitatively similar to those with no-tabs. In contrast, when the tip loading is inward at zero and negative angles of attack, the vortex wake is radically altered; the thrust nearly vanishes, even reverses with increasing inward loading. The key factors influencing the behavior of the wake are the vortex systems of the tabs and their associated downwash. The downwash is inward for the outward tab loading and causes increased volume and momentum flux in the rotor wake, and it is outward for the inward tab loading and causes expansion of the wake and nearly complete loss of thrust. When the tab loading is inward, a quasi-steady bound ring vortex system forms around at the rim of the rotor disk. At tab angle of −5°, the flow direction on the pressure side of the rotor disk reverses.

Aerodynamic characteristics of an aerostat under unsteady wind gust conditions

An analysis of the effect of wind gusts on the aerodynamic characteristics of an aerostat is presented in this study. A three dimensional computational fluid dynamics based flow analysis of an aerostat using the realizable k−ϵ model is presented. The performance of the aerostat is analyzed by varying the angle of attack, gust amplitude, gust duration, gust velocity profile and the aerostat shape. Actual wind data is used to formulate the gust parameters used for the study. It is found that the aerostat drag is unaffected by the angle of attack because of the streamlined structure. A trade-off between the drag and lift values suggests the use of a slight positive angle of attack for the stable operation of the aerostat. A gust with a short rising or falling time with small amplitude is severe than the symmetric gust with larger amplitude and a slower rising or falling profile. A scaling methodology for the aerodynamic characteristics of the aerostat for different angles of attack, gust amplitudes, gust duration and aerostat shapes is presented. The scaling obtained for different angles of attack and gust amplitudes is linear, whereas for different gust duration the scaling is not linear.

Lagrangian Analysis of Unsteady Partial Cavitating Flow Around a Three-Dimensional Hydrofoil

Abstract This study employs an incompressible homogeneous flow framework with a transport-equation-based cavitation model and shear stress transport turbulence model to successfully reproduce the unsteady cavitating flow around a three-dimensional hydrofoil. Cavity growth, development, and break-off during the periodic shedding process are adequately reproduced and match experimental observations. The predicted shedding frequency is very close to the experimental value of 23 ms. By monitoring the motions of the seeding trackers, growth-up of attached cavity and dynamic evolution of U-type cavity are clearly displayed, which indicating the trackers could serve as an effective tool to visualize the cavitating field. Repelling Lagrangian coherent structure (RLCS) is so complex that abundant flow patterns are highlighted, reflecting the intricacy of cavity development. The formation of cloud cavities is clearly characterized by the attracting Lagrangian coherent structure (ALCS), where bumbling wave wrapping the whole shedding cavities indicates the rotating transform of cavities, and stretching of the wave eyes shows the distortion of vortices. Generation of the re-entrant jet is considered to be not only associated with the adverse pressure gradient due to the positive attack angle but also the contribution of cloud cavitating flow, based on the observation of a buffer zone between the attached and cloud cavities.

Effects of lateral space and angle of attack on ventilated partial cavity

Ventilated partial cavity (VPC) is a drag reduction method with great potential for marine vehicles. In applications of VPC ships or yachts, the cavity usually undergoes flow non-uniformity and perturbations induced by hull and pitching motions. This study presents the experimental and numerical investigation on effects of flow non-uniformity and additional pressure gradient on the VPC caused by lateral space and angle of attack (AOA). The flat plate with sufficient lateral space and variable AOA is applied in water tunnel experiments. The computational fluid dynamics (CFD) coupled with the volume of fluid (VOF) are used for the in-depth understanding of VPC flow. Cavity shape, friction, pressure and velocity across a wide range of flow conditions are analyzed. The results reveal that the non-uniform flow induces the non-uniform pressure distribution and leads to the spanwise convection around plate. The positive AOA alters the cavity length and air leakage mode. Besides negative effects of lateral space and AOA, a positive synergetic effect is revealed. The positive AOA changes the direction and position of vortices, and weakens the negative effect of flow non-uniformity. Thus, a critical AOA (2° in current experiment) can balance these effects and achieve the best performance for drag reduction.

Effects of Secondary Elements on Vortex-Induced Vibration of a Streamlined Box Girder

Box girders are aerodynamically favorable, and the paper presents a study on the vortex-induced vibration (VIV) of this girder type at large angles of attack. Taking a box girder as an example, the form of the guardrails and the position of the maintenance tracks were improved to increase the VIV performance. The aerodynamic mechanism was further investigated according to the change in flow field around the girder. Results showed that the VIV performance of the box girder at positive angles of attack is worse. Improving secondary elements is an effective way to increase the VIV performance. Adding four rectangle bars above the original guardrails could achieve the target by preventing the generation of the vortex and blocking its movement. Moving the maintenance tracks inwards with a reasonable distance could also achieve the target as the flow passes through the girder more smoothly. With the combination of the optimal form of the guardrails and the optimal position of the maintenance tracks, the VIV performance of the bridge is higher. The countermeasures provide reference for the inhibition of the VIV of box girders.

Newtons Laws Explain How Frisbees Fly

Newtonian mechanics explain the physics of how frisbees fly. Specifically, a frisbee in horizontal flight with a positive angle of attack (AOA), will fly through a static mass of air each second (‘m/) that it accelerates to a velocity (’a’) downwards, to create a downward force (Force = ma), due to Newtons 2nd law of motion. The ‘equal & opposite’ upward force generated (Newtons 3rd law of motion), provides an upward force and vertical lift. Air goes down and the frisbee goes up. It’s that simple.
 There are two airflows: The underside of the frisbee pushes air down. While the curved topside of the frisbee pulls air downwards due to the Coanda effect. This explanation is different to other theories of lift currently available, such as those based on fluid mechanics (Bernoulli and Navier-Stokes) and vortices.
 The spin on a frisbee enhances the stability of flight due to the gyroscopic effects. In turn, stability of flight allows the frisbee to generate laminar (smooth) airflows, and thus better lift. The spin itself does not directly contribute towards lift. In addition, airflow vortices account for trick throws, where the frisbee appears to defy normal physics.
 So what? This provides new insight into lift and flight not currently found in accepted physics textbooks. The best performing frisbee will be one that maximises the Coanda effect on the topside of the disc; to increase the amount of air displaced down. Newtons laws can explain why a frisbee thrown flat (small AOA) generates better lift and will fly further than a frisbee thrown high, on a parabolic type of path. This explanation of lift can also be applied to airplane wings.

Performance of wavecatcher intakes at angles of attack and sideslip

A wavecatcher type scramjet intake, that reduces the Mach number number from 4 to 1.552, is used as the basis for a study of flow starting/unstarting as affected by freestream angles of attack and sideslip. The intake design is based on a morphed streamtube consisting of two conical flow streamlines using streamline tracing and osculating axisymmetric design theory. Intake flow and performance is modeled using the numerical CFD code and the k-ε turbulence model. The intake unstarts at a sideslip angle of 2°, a positive angle of attack of 1°. Both positive angle of attack and sideslip angle have an adverse effect on the startability of the MBus intake. At negative angles, the intake initially unstarts at −5° angle of attack, due to the thickened shear layer induced by the streamwise vortex. Then it re-starts at −8° angle of attack, mainly due to the expansion fan formed at the leading edge, causing the shock wave structures inside the intake to be re-established.

Wind-Tunnel Study of a Wing-Embedded Lifting Fan Remaining Open in Cruise Flight

It is investigated whether a nonrotating lifting fan remaining uncovered during cruise flight, as opposed to being covered by a shutter system, can be realized with limited additional drag and loss of lift during cruise flight. A wind-tunnel study of a wing-embedded lifting fan has been conducted at the Side Wind Test Facility Göttingen of DLR, German Aerospace Center in Göttingen using force, pressure, and stereoscopic particle image velocimetry techniques. The study showed that a step on the lower side of the wing in front of the lifting fan duct increases the lift-to-drag ratio of the whole model by up to 25% for all positive angles of attack. Different sizes and inclinations of the step had limited influence on the surface pressure distribution. The data indicate that these parameters can be optimized to maximize the lift-to-drag ratio. A doubling of the curvature radius of the lifting fan duct inlet lip on the upper side of the wing affected the lift-to-drag ratio by less than 1%. The lifting fan duct inlet curvature can therefore be optimized to maximize the vertical fan thrust of the rotating lifting fan during hovering without affecting the cruise flight performance with a nonrotating fan.

Suppression of vortex-induced vibration of single-box girder with various angles of attack by self-issuing jet method

Wind tunnel tests were conducted to investigate the effect of the self-issuing jet flow control on the vortex-induced vibrations (VIVs) of a streamline single-box girder at various angles of attack (AOAs). Spanwise periodic passive slits were symmetrically arranged on the windward and leeward lower inclined panels of the girder. The Reynolds number varied from 4.19×104 to 1.14×105, and the AOA varied from −5.0°to 5.0°with increments of 2.5°. The vibration response characteristics of the girder with different slit arrangements were discussed, and the interactions between the shedding Kármán vortices (KVs) and passive jets were investigated. The results show that the self-issuing jet control method could effectively suppress the VIV; the slits on the upper part of the lower inclined panel showed the best effect (Case I) at various AOAs. In addition, the large shedding KV were modified into two kinds of vortices: a reduced shedding KV and standing vortex at the trailing bottom corner of the girder. When the shedding vortex generation was displaced downstream, a second instability occurred in the near-wake. The self-issuing jets worked better at positive AOAs because more than one slit rows controlled the shedding vortices. Finally, the flutter performance of the single-box girder with self-issuing jets was investigated; Case I had a positive effect on the flutter behaviour at AOAs of −2.5°to 5°; the same flutter performance was observed at −5°.

Multi-objective optimization design for the blade angles of hydraulic torque converter with adjustable pump

To improve the efficiency of a hydraulic torque converter with adjustable pump at low load and thus increase the operation scope of high efficiency, multi-objective optimization design is carried out for the blade angles by incorporating three-dimensional steady computational fluid dynamics numerical simulation, design of experiments, Kriging surrogate model and multi-objective genetic algorithm. The results show that the angle of blade trailing edge in first-stage stator is the main influencing factor of the efficiency of hydraulic torque converter with adjustable pump. All the peak efficiencies of hydraulic torque converter with adjustable pump at three openings of the pump are improved after optimization, and the increased extent increases with decreasing opening of the pump. The operation scope of high efficiency consequently increases from 2.46 to 2.67. Besides, the improvement for the efficiency of hydraulic torque converter with adjustable pump is achieved by increasing the efficiency of the pump. The increase of angle of blade trailing edge in first-stage stator and the decrease of angle of blade leading edge in second-stage turbine after optimization induce the positive angle of attack at the inlet of second-stage turbine, thus realizing the performance optimization of hydraulic torque converter with adjustable pump. This also explains the increased proportion of the torque of second-stage turbine at larger speed ratios after optimization and the fact that the angle of blade trailing edge in first-stage stator is the main influencing factor of the efficiency of hydraulic torque converter with adjustable pump. The established multi-objective optimization method provides a reference solution for the optimization design of blade angles and for the improvement of integrated efficiency of hydraulic torque converter.

Aerodynamic analysis of a generic wing featuring an elasto-flexible lifting surface

A three-dimensional-membrane-type wing is investigated applying fluid-structure-interaction computations and complementary experiments. An analysis for three Reynolds numbers is conducted at various angles of attack. The computations are performed by means of the TAU-Code and the FEM Carat++ solver. Wind-tunnel tests are carried out for performance analysis and to estimate the accuracy of the computations. In the results, the advantages of an elasto-flexible-lifting-surface concept are highlighted by comparing the formvariable surface to its rigid counterpart. The flexibility of the material and its adaptivity to the freestream allow the membrane to adjust its shape to the pressure distribution. For positive angles of attack, the airfoil’s camber increases resulting in an increase in the wing lifting capacity. Furthermore, the stall onset is postponed to higher angles of attack and the abrupt decrease in the lift is replaced by a gradual loss of it.

Lift and Drag Acting on the Shell of the American Horseshoe Crab (Limulus polyphemus).

The intertidal zone is a turbulent landscape where organisms face numerous mechanical challenges from powerful waves. A model for understanding the solutions to these physical problems, the American horseshoe crab (Limulus polyphemus), is a marine arthropod that mates in the intertidal zone, where it must contend with strong ambient flows to maintain its orientation during locomotion and reproduction. Possible strategies to maintain position include either negative lift generation or the minimization of positive lift in flow. To quantify flow over the shell and the forces generated, we laser-scanned the 3D shape of a horseshoe crab, and the resulting digital reconstruction was used to 3D-print a physical model. We then recorded the movement of tracking particles around the shell model with high-speed video and analyzed the time-lapse series using particle image velocimetry (PIV). The velocity vector fields from PIV were used to validate numerical simulations performed with the immersed boundary (IB) method. IB simulations allowed us to resolve the forces acting on the shell, as well as the local three-dimensional flow velocities and pressures. Both IB simulations and PIV analysis of vorticity and velocity at a flow speed of 13cm/s show negative lift for negative and zero angles of attack, and positive lift for positive angles of attack in a free-stream environment. In shear flow simulations, we found near-zero lift for all orientations tested. Because horseshoe crabs are likely to be found primarily at near-zero angles of attack, we suggest that this negative lift helps maintain the orientation of the crab during locomotion and mating. This study provides a preliminary foundation for assessing the relationship between documented morphological variation and potential environmental variation for distinct populations of horseshoe crabs along the Atlantic Coast. It also motivates future studies which could consider the stability of the horseshoe crab in unsteady, oscillating flows.

Влияние кинематических параметров летательного аппарата на дроссельную характеристику сверхзвукового воздухозаборного устройства

The article analyzes influence of the aircraft kinematic parameters on the throttle characteristic of the air intake device. Numerical mathematical modeling of the intra-channel flow of a supersonic air intake device is performed with different kinematic parameters. The solid-state model is built in the CAD-NX Unigraphics package, the simulation is carried out in the CAE-package STAR-CCM+. The throttle characteristics obtained by numerical calculation and engineering method are compared to characteristics obtained by experimental blowing in the TSAGI wind tunnel. According to the results of numerical simulation of intra-channel flow in a supersonic air intake device, it is found, for example, that an increase in the speed of aircraft movement leads to a displacement of the throttle characteristic compared with the base curve; at positive angles of attack, the efficiency of the air intake device increases due to an increase in engine thrust.

Parametric Study of a Gurney Flap Implementation in a DU91W(2)250 Airfoil

The growth in size and weight of wind turbines over the last years has led to the development of flow control devices, such as Gurney flaps (GFs). In the current work, a parametric study is presented to find the optimal GF length to improve the airfoil aerodynamic performance. Therefore, the influence of GF lengths from 0.25% to 3% of the airfoil chord c on a widely used DU91W(2)250 airfoil has been investigated by means of RANS based numerical simulations at Re = 2 × 106. The numerical results showed that, for positive angles of attack, highest values of the lift-to-drag ratio CL/CD are obtained with GF lengths between 0.25% c and 0.75% c. Particularly, an increase of 21.57 in CL/CD ratio has been obtained with a GF length of 0.5% c at 2° of angle of attack AoA. The influence of GFs decreased at AoAs larger than 5°, where only a GF length of 0.25% c provides a slight improvement in terms of CL/CD ratio enhancement. Additionally, an ANN has been developed to predict the aerodynamic efficiency of the airfoil in terms of CL/CD ratio. This tool allows to obtain an accurate prediction model of the aerodynamic behavior of the airfoil with GFs.

Negative lift characteristics of NACA 0012 aerofoil at low Reynolds numbers

Numerical investigations on the flow over NACA 0012 aerofoil are carried out to provide better understanding of the unusual lift characteristics exhibited by this aerofoil at low Reynolds numbers. Computations are carried out at Re = 10,000–100,000, for different values of angles of attack and freestream turbulence intensity. There exists a narrow range of these parameters where the net circulation around this symmetrical aerofoil is negative, leading to the generation of negative lift at positive angles of attack. Different flow regimes are identified and physical explanations are given for this unusual behaviour of negative lift, and the influence of different flow parameters is discussed.

A study of coaxial rotor aerodynamic interaction mechanism in hover with high-efficient trim model

A numerical method based on Reynolds Averaged Navier–Stokes (RANS) equations and moving overset mesh technique is developed to simulate the aerodynamics of Harrington rotor-2. A high-efficient trim model for hover is presented and coupled into the numerical method and the “delta trim arithmetic” is implemented to simplify the calculation of Jacobian matrix. Validation cases are computed, showing reasonably coincident with the available experimental data. Hover cases of different thrust coefficients with torque-balanced are conducted. Specially, zero-lift and one-rotor-stay cases are set for a better understanding of the interaction mechanism. Results indicate that the temporal thrusts of the coaxial rotor show a variation of gradually increasing and sharply decreasing. Different from the previous work, the fluctuation features are mainly explained by strong-weak “induction effect” and impulsive “overlap effect” caused by the interaction of wakes and bound vortexes of coaxial rotor. At a positive attack angle, the induction and overlap effects are more evident for the upper rotor because the induced flow is stronger above the blade than that below it. The first impulse of thrust caused by overlap effect takes place at about 3° azimuth after the alignment of the blades for present cases, as the upper and lower blades meet in a rotating way.

Measurements of near-wall pressure fluctuations for trailing-edge serrations and slits

Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° angles of attack. At 0° angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at positive angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge.Graphical abstract

Effect of Large Scale 3-D Structures on the Flow Around a Heated Cylinder at Low Reynolds Number

This work presents results of flow around a heated circular cylinder in mixed convection regime and demonstrates that Prandtl number and angle of attack of the incoming flow have a large influence on the characterisation of the flow transition from 2-D to 3-D. Previous studies show that heat transfer can enhance the formation of large 3-D structures in the wake of the cylinder for Reynolds numbers between 75 and 127 and a Richardson number larger than 0.35. This transitional mode is generally identified as “mode E”. In this work, we compare the results for water-based flow (large Prandtl number) with the ones for air-based flows (low Prandtl number). The comparison is carried out at two Reynolds numbers (100 and 150) and at a fixed Richardson number of 1. It shows that at the low Reynolds number of 100 the low Prandtl number flow does not enter into transition. This is caused by the impairment of the baroclinic vorticity production provoked by the spanwise temperature gradient. At low Prandtl number temperature gradients are less steep. For an air-based flow at Reynolds number 150, several Richardson numbers have been simulated. In this situation, the flow enters into transition and exhibits the characteristics of “mode E”, with the development of Λ-shaped structures in the near wake and mushroom-like structures in the far wake. It is also observed that the transition is delayed at Richardson number of 0.5. Simulations are also carried to investigate the effect of the angle of attack on the incoming flow on the development of large coherent structures. When the angle of attack is positive, the development of the wake tends to return to a more bi-dimensional configuration, where large scale coherent structures are impaired. In contrast, when the angle of attack is negative, large scale tri-dimensional structures dominate the flow in the wake, but with a very chaotic behaviour and the regular pattern of zero angle of attack is destroyed. The different behaviour of the flow with the variation of the angle of attack is also related to the baroclinic vorticity production, where new terms appear in the equations, leading to a positive effect of the vorticity production in case of a negative angle of attack and the opposite for a positive angle of attack.

Aeroacoustic analysis of automotive roof crossbars through on-track acoustic measurements

This work concerns the noise radiated by crossbars installed on the roof of automotive vehicles. The Reynolds numbers analyzed range between 98,000 and 147,000 based on the crossbar chord. Through a comparison of results obtained in an aeroacoustic wind-tunnel, we demonstrate that on-track sound pressure measurements from a single in-flow microphone are suitable to study the aeroacoustics of crossbars under realistic operational conditions. The current investigation presents noise measurements by reference profiles such as elliptic and circular crossbars, as well as a NACA 0012 airfoil. Results from the elliptical crossbar show both tonal and broadband components in the noise spectra, combining features observed for the circular bar and the airfoil. For higher Reynolds numbers, the tones observed in the elliptical profile are broad and free of harmonics, similar to those observed for the NACA 0012. The influences of the leading and trailing-edge shapes of the elliptical crossbar on the radiated noise are presented together with the effects of the angle of attack. A thinner trailing edge leads to a reduction in the tonal noise followed by an increase in the broadband content. On the other hand, a blunter leading edge reduces the broadband noise, especially for higher frequencies. Positive angles of attack provide a reduction in both tonal and broadband noise components for the elliptical crossbar, particularly at lower Reynolds numbers.

Wake-Model Effects on Induced Drag Prediction of Staggered Boxwings

For staggered boxwings the predictions of induced drag that rely on common potential-flow methods can be of limited accuracy. For example, linear, freestream-fixed wake models cannot resolve effects related to wake deflection and roll-up, which can have significant affects on the induced drag projection of these systems. The present work investigates the principle impact of wake modelling on the accuracy of induced drag prediction of boxwings with stagger. The study compares induced drag predictions of a higher-order potential-flow method that uses fixed and relaxed-wake models, and of an Euler-flow method. Positive-staggered systems at positive angles of attack are found to be particularly prone to higher-order wake effects due to vertical contraction of wakes trajectories, which results in smaller effective height-to-span ratios than compared with negative stagger and thus closer interactions between trailing wakes and lifting surfaces. Therefore, when trying to predict induced drag of positive staggered boxwings, only a potential-flow method with a fully relaxed-wake model will provide the high-degree of accuracy that rivals that of an Euler method while being computationally significantly more efficient.