High Angles Of Attack Research Articles

Experimental investigation of the flow control over an airfoil with owl-inspired trailing-edge modification: On the material, length, and spacing sensitivity

We experimentally investigate the effect of material, length, and spacing of trailing-edge extensions on controlling the flow over an airfoil based on our recent experimental work. Force measurements and flow field quantifications were carried out to investigate the aerodynamic performance and flow structures in the wake of an airfoil and, thus, to reveal differences in control effectiveness and mechanisms. Moreover, multi-scale proper orthogonal decomposition and spectral proper orthogonal decomposition are employed to extract coherent flow structures in the flow field. The results indicate that the owl feather can improve the aerodynamic performance, while artificial materials lead to decreased lift-to-drag ratio. However, nylon has optimal adaptability and robustness in controlling turbulent fluctuations, including Reynolds stress and turbulent kinetic energy at different angles of attack (AOAs). The length sensitivity is highly associated with the AOA, i.e., the optimal length increases with the increase in AOA. In addition, the spacing sensitivity correlates with the Reynolds number (Re), i.e., the optimal spacing decreases with higher Re at high AOA. These differences root in the competition effect between the increasing adverse pressure gradient and the interference on regular vortex shedding. It is concluded that nylon with mediate length (L = 0.2D) and relatively large spacing (S = 0.5B) is recommended for wake control and noise attenuation of the S833 airfoil.

Rotational effects on the blade flow of a horizontal axis wind turbine under axial and yawed inflow conditions

Unsteady aerodynamic loads and fluctuating output power of wind turbines remain challenging to predict accurately due to the incomplete understanding of blade flow mechanisms. Rotational effects can significantly impact the wind turbine aerodynamic performance. Therefore, this paper presents a careful investigation into rotational flow characteristics of the NREL Phase VI blade under axial and yawed inflow conditions based on the DDES method. Rotational effects change the flow regime from the massive 2D trailing-edge separation into the moderate 3D leading-edge separation on the NREL Phase VI blade. Flow separation is found sufficient but not necessary for the radial flow. Under the axial inflow, rotational effects can effectively suppress the flow separation and increase the nonlinear aerodynamic loads. Under the yawed inflow, rotational effects also effectively suppress the unsteady separated flow and accelerate the flow reattachment, particularly on the downwind side. Sectional lift coefficient is therefore greatly increased at the high angles of attack due to the pronounced rotational effects. Increasing the yaw angle and wind speed may also strengthen the rotational effects and widen the difference between the 2D airfoil flow and 3D sectional flow. These findings imply that flow regime and aerodynamic hysteresis on the inboard blade can be substantially changed in comparison with the 2D airfoil flow due to strong rotational effects. This study might deepen the understanding of rotational effects on the near-wall flow of wind turbine blades and also improve the aerodynamic modelling.

Three-dimensional separation over unswept cantilevered wings at a moderate Reynolds number

We conduct experiments on the structure of the 3D flow fields for unswept cantilevered wings at high angle of attack. Stall cell counter-rotating vortices on the suction surface form for a range of aspect ratios (AR), though AR changes the angle of attack at which the flow separates. Analysis of mean flow volume over the suction surface and near wake shows that the arch vortex forms in the wake and connects to the surface at the stall cell foci. Reynolds stress peaks indicate a strong reliance on the arch vortex location in the mean flow. Spectral analysis of the velocity field at select spanwise planes shows that shedding is coherent but intermittent, and varies strongly along the wingspan.

Effect of leading-edge protrusion shapes for passive flow control measure on wind turbine blades

In the present study, protrusions in the leading edge of the NACA 4415 airfoil were incorporated as a passive flow control measure on the wind turbine blades. Three airfoil models: unmodified leading-edge (ULE), spherical leading-edge protrusion (SLEP), and triangular leading-edge protrusion (TLEP), were investigated experimentally. Thereafter, CFD investigations were carried out using ANSYS 14.0 simulation tool to observe the flow characteristics around the airfoils. Further, LEP-based passive design was applied on a horizontal axis wind turbine (HAWT) to investigate its performance. An amplitude (A) of 1% of the chord length (C) was considered as the height of protrusion, while a distance of 0.25C was maintained between the protrusions to fabricate the experimental models. The study was performed at a low Reynolds number (Re) of 1.5 × 105 for a wide angle of attack (α) of 0°–20°. The experimental results demonstrate that the SLEP model has a higher lift coefficient at α ≥ 18°, whereas the TLEP model performs poorly when compared with the ULE model. The instantaneous lift coefficient plot indicates that the SLEP model generates a more stable force at a higher angle of attack. The computational analysis reveals that a larger primary circulation (extended till 0.2C) is observed in the ULE model at a post-stall angle of attack (α = 18°), indicating early flow separation. Whereas, in the SLEP and TLEP models, it is extended to 0.70C and 0.54C, respectively, indicating the best flow controlling measures achieved by using the SLEP model. The investigations of HAWT rotors with LEP revealed that SLEP HAWT exhibit 8.2% more power coefficient than ULE HAWT.

Design of a passive flow control solution for the mitigation of vortex induced vibrations on wind turbines blade sections as a response to extreme weather events

The goal of the present work is to perform exploratory assessments on the suitability of microcylinder devices to mitigate wind turbine blade vortex induced vibration in small cross flow regimes, specially occurring under adverse weather events. For this purpose, the deep stall behavior of the NACA0021 has been evaluated for the first time on a wide range of high angles of attack (50°–130°) by means of CFD assessments, and the effects of microcylinders have been studied in terms of the reduction of aerodynamic coefficient magnitudes and fluctuations. In a first step, the passive flow control solution at 90° has been analyzed for a total number of investigated cases equal to 15, keeping constant the diameter of the microcylinders and varying the relative positions of these devices with respect to the blade. Mitigations of the load standard deviations between 63% and 97% across all the tested angles of attack have been found. Additionally, genetic programming (GP) was used to obtain a more general viewpoint of the effect of the microcylinders on the aerodynamics forces. Assessments of the flow fields confirm that properly located microcylinders disrupt the coherent vortical structures in the wake, responsible for periodic loading on the blade section.

An Intelligent Algorithm for Aerodynamic Parameters Calibration of Wind Tunnel Experiment at a High Angle of Attack

In this paper, an intelligent algorithm for the aerodynamic parameter calibration of wind tunnel experiments of advanced layout aircraft at a high angle of attack is proposed. The proposed algorithm is based on a homologous comparison and tuning algorithm, and it can effectively improve the accuracy of the wind tunnel experiment model. First, based on the analysis of large oscillation wind tunnel experimental data of an advanced layout scaled aircraft, the high angle of attack wind tunnel experimental model composed of static derivative, dynamic derivative, and rotating balance derivative is established. Second, to improve the accuracy of the wind tunnel experimental model effectively, the idea of combing layered calibration and intelligent algorithm for high angle of attack homologous comparison correction is proposed. The proposed method solves the problems of complex structure, a large amount of data, and poor accuracy in homologous comparison of high angle of attack aerodynamic models of advanced layout aircraft. Finally, the homologous comparison interface software is designed based on MATLAB GUI, which integrates the proposed methods and ideas and realizes effective adjustment of aerodynamic parameters of high angle of attack simulation flight wind tunnel test of an advanced layout aircraft. This study provides a reliable engineering and technical means for subsequent high angle of attack flight test verification of the advanced layout aircraft.

Application of wavy leading edge to enhance winglet aerodynamic performance

AbstractThe wavy leading edge (WLE, also known as leading edge protuberances) is a passive flow control device inspired by the humpback whale pectoral flippers. It reduces the flow of three-dimensional effects on wings and increases their aerodynamic performance at high angles of attack. Despite the numerous studies on its aerodynamic benefits, research on its possible applications is still incipient. Therefore, this article addresses an evaluation of the WLE effects on the aerodynamic performance of a winglet. A rectangular wing, a base smooth leading edge winglet, and a winglet with WLE were designed and manufactured for CFD simulations and wind tunnel measurements. The winglet with WLE increased the maximum aerodynamic efficiency, i.e. this configuration reduced the induced drag by increasing wingtip vortex dissipation at a given angle-of-attack. Such results were used in re-evaluations of the aerodynamic performance of an original agricultural aircraft initially configured with a multi-winglet device. The winglet with WLE showed to be effective at increasing the aircraft operational time and range under a simulated actual condition.

Blunt leading-edge effect on spanwise-varying leading-edge contours of an UCAV configuration

The vortex system on the SACCON model is studied under low Reynolds number by using the flow visualization technique and the surface pressure testing measurement in a low-speed water channel and a wind tunnel, respectively. A main topic is discussed below. The blunt leading-edge section on this SACCON model induces various flow phenomena under different angles of attack. The attached flow appearing around the upstream leading-edge region at a low angle of attack induces different flow phenomena downstream, so does the branched vortices appearing near the trailing-edge. Although this attached flow is vanished at higher angle of attack, the stability of forming an outboard vortex can be shown in the results of pressure coefficients as a function of an angle of attack. Moreover, how the blunt leading-edge contour on SACCON model and other models affecting flow field can be compared with the findings given in the previous studies. The models with only blunt leading-edge contour can prompt an outboard vortex and an inner vortex, compared to the flow fields on the models with spanwise-varying leading-edge contours. This inner vortex is formed due to the attached flow inboard passing downstream and being affected by the outboard vortex. However, this attached flow is different from the attached flow observed on SACCON model. Therefore, flow phenomena occurring on different blunt leading-edge models can be differentiated.

Investigation of effects of sand particles on aerodynamic performance of NACA 0012 airfoil

The purpose of this paper is to study the effects of turbulence kinetic energy structures formed during flow and particles of different diameters and different flow velocities in the air in different regional areas on aerodynamic performance characteristics in NACA 0012. Single and two phase fluid flows were worked out by using Ansys Fluent Computational Fluid Dynamics (CFD) code. Computational results obtained from Ansys Fluent CFD code for pure air and the air containing sand particles were compared with numerical values gained in the literature for validation. Results obtained from the numerical tests demonstrate good agreement with the value in the literature. These results indicate the turbulence kinetic energy value occurred in the tail region of the airfoil increases with the increase in the angle of attack and shifts towards the upper region of the airfoil at high attack angle. Moreover, the upper region of the airfoil at high attack angle becomes larger at low Reynolds numbers due to viscous effects. The drag and lift coefficients obtained in the numerical tests of the airfoils and in the experimental tests in the wind tunnel will differ from the values in the application area. Because, in the operation of airfoils in different regional environments, there are always particles of various concentrations and diameters in the air. In this case, the drag coefficient increases and the lift coefficient decreases.

Numerical Design Studies on the Roll Stability of a Multi-Delta-Wing Configuration

The current investigations look at the vortical flow physics and aerodynamic performance of a generic sharp leading-edge multi-delta-wing configuration. This work is part of the DLR project Diabolo looking at technologies and design requirements for next-generation fighter aircraft. The core of the investigation is a common planform geometry to demonstrate the design capability needed. Based on the initial planform, several design tasks in different domains have to be conducted. The present paper is dealing with the aerodynamic task to evaluate the effect of vortex interaction over the configuration progressing from the forebody, strakes, and main wing at asymmetric on flow conditions. The focus is on the roll stability at medium to high angle of attack. The aim is to find a planform and control surface setup which effects the vortex interaction on the main wing in a way to provide a stable aerodynamic roll without taking vertical and horizontal tail planes into consideration at this stage.

Influence of free-stream turbulence intensity on static and dynamic stall of a NACA 0018 aerofoil

In many engineering applications, aerofoils experience elevated free-stream turbulence levels. The present work experimentally investigates the effect of free-stream turbulence on the aerodynamic characteristics of a sinusoidally pitching NACA 0018 aerofoil at the transitional Reynolds number of 2.8×105. Wind tunnel tests are conducted in quasi-static and dynamic conditions at different turbulence intensities between 0.3% and 8.2%, considering reduced frequencies up to 0.1. The dynamic experiments investigate multiple angle-of-attack ranges in order to quantify the influence of free-stream turbulence in the attached-flow regime, in the stalled regime, and in-and-out of stall. The study demonstrates that high free-stream turbulence drastically changes the flow physics over the aerofoil for both static and dynamic conditions, producing large deviations in the lift and moment coefficients. The quasi-static experiment performed at low free-stream turbulence features a large stall hysteresis linked to the breakdown of the leading-edge laminar separation bubble, whereas static hysteresis is not present at higher turbulence levels. Moreover, dynamic experiments at high angles of attack show a strong dependency on the incoming turbulence intensity, which is found to delay flow separation during the upstroke and enhance reattachment during the downstroke. The work also reveals the intrinsic difficulty of predicting the dynamic stall behaviour under different turbulence conditions, and gives insight into why the existing empirical dynamic stall models are unlikely to succeed in accurately predicting the aerodynamic loads across different onset turbulence intensities.

Large Eddy Simulation of Transitional Separated Flow Over a Low Reynolds Number Cambered Airfoil

Abstract The accurate simulation of the aerodynamic behavior of low Reynolds number (Re) cambered airfoils requires the ability to capture the transitional separated boundary layer (BL) that occurs naturally on the surface of the airfoil. In this study, simulations are performed using a modern cambered airfoil designed for use in low Re applications, which are an advancement from previous studies using flat plate geometries or symmetric NACA airfoils. The cambered SD 7037 airfoil is simulated using wall-resolved large eddy simulation (LES) at a modest Re of 4.1×104 and at 1 deg, 5 deg, and 7 deg angles of attack (AOAs), with results validated against experimental data. Simulated predictions of pressure and skin friction coefficients clearly capture the correct location of the laminar separated bubble (LSB) which forms during the natural BL transition process. Sensitivity to elevated inflow turbulence is found to cause early BL reattachment at higher AOAs without impacting the location of BL separation. An integral BL analysis verifies the accuracy of the simulated velocity profiles against experimental values. The scale of horseshoe structures visualized in the transitional BL is larger in comparison to airfoil chord length than what is seen in previous simulations at Re of the order of 105, which highlights the importance of investigating cambered airfoils at a modest Re.

The effect of vortex induced vibrating cylinders on airfoil aerodynamics

Two-dimensional fluid-structure coupling dynamic models of single- and double-cylinder with airfoil are established using fluid-structures dynamics methods. The accuracy of the airfoil and vortex-induced vibration simulation models is verified by comparing them with available experimental data. Coupled simulations are used to study the vortex-induced vibration of two cylinders under uniform incoming flow, and the influence of two cylinders at different positions of the airfoil leading edge on their aerodynamic forces. Vorticity and streamline diagrams provide evidence of the effects of single cylinder and double cylinders on the airfoil when placed at different angles of attack. The vibration response of the cylinder is also observed. The results show that at high angles of attack the flow separation of the airfoil can be effectively controlled by positioning small cylinders in the proximity of the airfoil's leading edge. A small oscillating cylinder can contribute to control the flow around the airfoil, better than fixed cylinder. Indeed, when the airfoil is within stall condition, a single vibrating cylinder can control flow separation and improve airfoil lift-to-drag ratio, better than when no cylinder or double cylinders are considered. For increasing angle of attack post stall condition, the ability of a single oscillating cylinder to control the flow around airfoils decreases gradually, while the one of double cylinders increases.

Bio-inspired optimization of leading edge slat

PurposeThe high lift devices are effective at high angle of attack to increase the coefficient of lift by increasing the camber. But it affects the low angle of attack aerodynamic performance by increasing the drag. Hence, they have made as a movable device to deploy only at high angles of attack, which increases the design and installation complexities. This study aims to focus on the comparison of aerodynamic efficiency of different conventional leading edge (LE) slat configurations with simple fixed bioinspired slat design.Design/methodology/approachThis research analyzes the effect of LE slat on aerodynamic performance of CLARK Y airfoil at low and high angles of attack. Different geometrical parameters such as slat chord, cutoff, gap, width and depth of LE slat have been considered for the analysis.FindingsIt has been found that the LE slat configuration with slat chord 30% of airfoil chord, forward extension 8% of chord, dip 3% of chord and gap 0.75% of chord gives higher aerodynamic efficiency (Cl/Cd) than other LE slat configurations, but it affects the low angles of attack aerodynamic performance with the deployed condition. Hence, this optimum slat configuration is further modified by closing the gap between LE slat and the main airfoil, which is inspired by the marine mammal’s nose. Thus increases the coefficient of lift at high angles of attack due to better acceleration over the airfoil nose and as well enhances the aerodynamic efficiency at low angles of attack.Research limitations/implicationsThe two-dimensional computational analysis has been done for different LE slat’s geometrical parameters at low subsonic speed.Practical implicationsThis bio-inspired nose design improves aerodynamic performance and increases the structural strength of aircraft wing compared to the conventional LE slat. This fixed design avoids the complex design and installation difficulties of conventional movable slats.Social implicationsThe findings will have significant impact on the fields of aircraft wing and wind turbine designs, which reduces the design and manufacturing complexities.Originality/valueDifferent conventional slat configurations have been analyzed and compared with a simple fixed bioinspired slat nose design at low subsonic speed.

An Optimisation of a Chordwise Slot to Enhance Lateral Flow Control on a UCAV

This research aims to optimise a chordwise slot so that lateral flow control of a flying wing configuration can be enhanced. This was achieved by maximising the airflow rate over the trailing edge control surfaces of the wing. A higher rate of airflow over the trailing edge control surfaces will increase the operationality of control surfaces, resulting in enhanced lateral control of the air vehicle at medium to high angles of attack. Four variables describing the chordwise slot are identified for numerical optimisation. They are: location, width, length, and angle of trajectory of chordwise slot relative to freestream. The results of the airflow rate for the wing with an optimised chordwise slot are compared with a baseline clean configuration. The flying wing configuration with a chordwise slot has shown a higher mass flow rate over the control surfaces of the wing in comparison to the baseline clean configuration, demonstrating that an optimised chordwise slot can be implemented on a flying wing configuration as it successfully controls the lateral flow by maximising the flow rate over the trailing edge control surfaces.

Study on the Asymmetric Separation Characteristics of Slender Body at High Angle of Attack under Lateral Jet Conditions

The slender body is selected as the shape of the study. The effects of lateral jet flow at M∞ = 0.3, 0.6, 0.8 on the asymmetric separation characteristics of the body at large angles of attack are numerically simulated. To obtain the deterministic asymmetric separation results of the slender body, the artificial disturbance is carried on the tip of its head. The aerodynamic characteristics of asymmetric separation in jet-off and jet-on conditions are obtained, and the effects of Mach number, Angle of attack on asymmetric separation flow behaviors of the slender body are analyzed.

Numerical investigation of the wake transition and aerodynamic efficiency of the two-dimensional propulsive wing

The propulsive wing is a new concept wing of automatic propulsion with high lift coefficients and has great application value in plant protection and forest fire control. The propulsive wing wake is a reverse Bénard–von Kármán (RBvK) vortex street, which is considered a thrust-generating wake. The wake structure will change greatly at high angles of attack and lead to changes in the aerodynamic performance of the propulsive wing. To explore the optimal working range and the wake characteristics of the propulsive wing, the wake transition and aerodynamic efficiency of the propulsive wing in cruise are numerically studied. The results indicate that there are three types of structures for the propulsive wing wake. When α ≤ 20°, the wake transits from the RBvK vortex street to the critical state with the increase in cruise speed, and the Strouhal number approaches 1.9. The critical wake region decreases gradually with the increase in the angle of attack. The maximum propulsive efficiency is 0.17 at a cruise speed of 15 m/s. When α > 20°, the wake transits directly from the RBvK vortex street to the Bénard–von Kármán (BvK) vortex street and the Strouhal number approaches 0.34. The maximum propulsive efficiency appears at a cruise speed of 10 m/s, which is close to the BvK vortex street boundary. Before entering the stall state, the lift efficiency of the propulsive wing increases with the increase in cruise speed and angle of attack, up to 3–5.

Active control of flow past an elliptic cylinder using an artificial neural network trained by deep reinforcement learning

The active control of flow past an elliptical cylinder using the deep reinforcement learning (DRL) method is conducted. The axis ratio of the elliptical cylinder Γ varies from 1.2 to 2.0, and four angles of attack α = 0°, 15°, 30°, and 45° are taken into consideration for a fixed Reynolds number Re = 100. The mass flow rates of two synthetic jets imposed on different positions of the cylinder θ1 and θ2 are trained to control the flow. The optimal jet placement that achieves the highest drag reduction is determined for each case. For a low axis ratio ellipse, i.e., Γ = 1.2, the controlled results at α = 0° are similar to those for a circular cylinder with control jets applied at θ1 = 90° and θ2 = 270°. It is found that either applying the jets asymmetrically or increasing the angle of attack can achieve a higher drag reduction rate, which, however, is accompanied by increased fluctuation. The control jets elongate the vortex shedding, and reduce the pressure drop. Meanwhile, the flow topology is modified at a high angle of attack. For an ellipse with a relatively higher axis ratio, i.e., Γ ⩾ 1.6, the drag reduction is achieved for all the angles of attack studied. The larger the angle of attack is, the higher the drag reduction ratio is. The increased fluctuation in the drag coefficient under control is encountered, regardless of the position of the control jets. The control jets modify the flow topology by inducing an external vortex near the wall, causing the drag reduction. The results suggest that the DRL can learn an active control strategy for the present configuration.

Pressure Feedback Control of Aerodynamic Loads on a Delta Wing in Transverse Gusts

A feedback active flow control (AFC) scheme is studied for control of unsteady aerodynamic loads that act on a generic tailless delta wing during transverse gust encounters. The transverse gust and the roll angle of the wing create an unsteady roll moment. The trailing edge actuators generate unbalanced lift increments on the two sides of the model to control the roll moment. Spatially distributed pressure sensors measure surface pressure on the suction side of the wing, and the real-time aerodynamic loads are estimated through models that are identified by the Sparse Identification of Nonlinear Dynamics algorithm. The aerodynamic load estimation is then used as a surrogate to provide the AFC system with a feedback signal to alleviate the unsteady roll moment due to the gust effect. A physical interpretation of the model identification results is made to locate the source of the lift and roll moment fluctuations in gust encounters. Experimental results show that the AFC effect with a momentum coefficient input of 2% is equivalent to 26° elevon deflection in terms of the roll moment change, and the AFC doubles the control authority relative to elevons. The real-time control results show that the pressure feedback control system reduces the mean roll moment error and suppresses the fluctuations to less than 20% of the uncontrolled fluctuation level at a high angle of attack.

Numerical Investigation of Flow Separation Control over Rotor Blades Using Plasma Actuator

The flow separation control over rotating rotor blades in hover using an alternating current dielectric barrier discharge (AC-DBD) plasma actuator is investigated through numerical simulation, possibly for the first time. Three-dimensional unsteady Reynolds-averaged Navier–Stokes calculation with a Reynolds stress turbulence model is adopted to resolve the airflow. The impact of the AC-DBD plasma discharge on the airflow is modeled by an empirical body force model. The objective of this work is to characterize the separated flow over the rotating blades, resolve the flow control procedure, reveal the flow control mechanism, and evaluate the control authority. Compared with the experiment, the numerical simulation can provide satisfactory predictions of the baseline flow over the rotor for the qualitative flow pattern and quantitative aerodynamic features. The good agreement between the simulation and experiment lends credibility to the observations made from the numerical result. It is found that in the baseline flow, the flow separation occurs in the vicinity of the blade tips and exhibits strong three-dimensional characteristics under the influence of centrifugal force. The plasma excitation can significantly inhibit the flow separation over the blades at large angles of attack (AOAs). The flow control is seen to be dominated by the spanwise vortices induced by the plasma discharges, for which the strong entrainment effect makes the separated flow fully or partially reattach to the blade surfaces. The resulting forced flow pattern is influenced by the AOA of the blades. At each high AOA under consideration, the controlled flow is a significant improvement over the baseline case, and the aerodynamic characteristics of the rotor blades are appreciably enhanced.