High Lift Coefficients Research Articles

Wing Simulation Using Naca 0018 and 0024 and Aluminum Alloy 7075 T6-SN and 7050-T7451 Materials for Lift and Drag

Analysis of the airfoil's shape on an airplane wing is essential because geometry can determine aerodynamic performance and the forces at work, such as lift and drag. In this research, two types of aluminum materials, namely Aluminum Alloy 7075 T6-SN and Aluminum Alloy 7050-T7451, will be tested to find out which one can produce the highest lifting force and drag force on NACA 0018 and 0024 as well as look at the pressure contour and speed Contour of each type of material using ANSYS Fluent software at a speed of 200 m/s. From the simulation and calculation results, it was obtained that the highest lift coefficient and drag coefficient on the NACA 0018 Airfoil were using Aluminum Alloy 7075-T6 SN material, namely 6.83 N and 6.89 N. Meanwhile, in the NACA 0024 Airfoil simulation, the highest lift coefficient and thrust coefficient were obtained. It also uses aluminum Alloy 7075-T6 SN, namely 3.23 N and 4.64 N.

Study on the aerodynamic characteristics of reentry capsule with obtuse head inverted cone under hypersonic chemical nonequilibrium flow

Abstract During spacecraft reentry, the aerodynamic characteristics of the reentry capsule with a sizeable obtuse head inverted cone (OBHIC) in a hypersonic state are the key factors in the design of controlled reentry trajectory, the design of trim angle of attack (AoA), and the accuracy of fall point of capsules. This paper establishes a numerical simulation algorithm of a hypersonic chemical nonequilibrium flow field by considering high-temperature real gas effects. The reentry capsule’s aerodynamic and flow field characteristics are predicted and analyzed. The component transport equations for chemical reactions are added to the Navier-Stokes equations, enabling precise modeling of gas molecule vibrational excitation and ionization processes. A half-model multi-block structured mesh is employed to construct the aerodynamic profile of the reentry capsule with OBHIC. The aerodynamic characteristics and pressure distribution of the reentry capsule were calculated and analyzed under the typical conditions of flight altitude of 40~80 km, flight velocity of 6~27 Ma, and AoA of 15°~24°. The results of the aerodynamic characteristics prediction show that the aerodynamic characteristics of the reentry capsule are little affected by the incoming flow velocity in a hypersonic state but significantly affected by the AoA. As the AoA increases, the lift coefficient CL increases linearly, the drag coefficient CD decreases linearly, the lift-to-drag ratio K rises linearly, and K increases from 0.19 to 0.29. In its hypersonic state, the reentry capsule exhibits a low lift-to-drag ratio, characterized by high drag and low lift coefficients. The results of this paper play an essential role in the reentry flight dynamics and reentry trajectory design of the reentry capsule. Moreover, it can further ensure the safety and reliability of the reentry flight of spacecraft.

Modeling and Analysis of Kamikaze UAV Design with 3 Different Wing Configurations

Appropriate design parameters need to be determined for unmanned aerial vehicles that can perform kamikaze missions. In this study, a UAV with 3 different wing configurations and a fuselage and tail wings were designed, and the flow around the wing was examined using computational fluid mechanics. Advanced modeling techniques were employed to simulate and analyze the aerodynamic behavior of these configurations. The effect of angle of attack (AoA), wing positioning on the fuselage, and wing configurations were investigated. Due to the effect of the wing sweep angle, high-pressure values in the arrow-angle wing were lower than in rectangular and trapezoidal wings. In a similar situation, the flow separation on the arrow-angle wing is less advanced towards the wing tip. When the wing type and connection location were examined, the highest ${{C}_{l}}/{{C}_{d}}$ ratio was obtained in the trapezoidal model connected to the fuselage in the middle. The results of numerical wing models compared with the theoretical lift coefficient were consistent. Trapezoidal and rectangular wings had a high lift coefficient, but after ${{15}^{\circ }}$ of AoA, the lift coefficient decreased. At angles of attack beyond ${{15}^{\circ }}$, the arrow-angle wing still has an increasing lift coefficient. As the angle of attack increased, the drag coefficient was also enhanced. The lowest drag coefficient occurred in the arrow-angle wing model. Up to ${{5}^{\circ }}$of AoA, all wing models raised the ${{C}_{l}}/{{C}_{d}}$ ratio. The ${{C}_{l}}/{{C}_{d}}$ ratio decreased at higher angles of attack. As a result of the examination, it would be more correct to choose trapezoidal and arrow-angle wings.

Numerical Simulation and Experimental Study on the Aerodynamics of Propulsive Wing for a Novel Electric Vertical Take-Off and Landing Aircraft

The electric vertical take-off and landing (eVTOL) aircraft offers the advantages of vertical take-off and landing, environmental cleanliness, and automated control, making it a crucial component of future urban air traffic. As competition intensifies, demands for aircraft performance are escalating, including forward flight speed and payload capacity. The article presents a novel eVTOL design with propulsive wings and establishes methodologies for propulsive wing unsteady numerical simulation and wind tunnel experiments, analyzing its aerodynamic characteristics and lift enhancement mechanism. The results indicate that the cross-flow fan (CFF) provides unique airflow control capabilities, enabling the propulsive wing to achieve remarkably high lift coefficients (exceeding 7.6 in experiments) and propulsion coefficients (exceeding 7.1 in experiments) at extreme angles of attack (30°~40°) and low airspeeds. On the one hand, the CFF effectively controls boundary layer flow, delaying airflow separation at high angles of attack; on the other hand, the rotation of the CFF induces two eccentric vortices, generating vortex-induced lift and propulsion. The aerodynamic performance of the propulsive wing depends on the advance ratio and angle of attack. Typically, both lift and propulsion coefficients increase with the advance ratio, while lift and drag coefficients increase with the angle of attack. The propulsive wing shows significant advantages and prospects for eVTOL aircrafts in the low flight velocity range (0–30 m/s).

Computational and experimental study on the aerodynamic performance of NACA 4412 airfoil with slot and groove

Since the Wright Brothers’ first flight in 1903, extensive research has been dedicated to improving the aerodynamic performance of aircraft. This study investigates the effect of two distinct wing geometric modifications on airfoil performance at high angles of attack (AOAs). These two modifications are slot, specifically the NACA 4412 with only a slot, and groove, specifically the NACA 4412 with both a slot and a groove. The investigation combines numerical simulation using ANSYS fluent with experimental evaluations conducted in the VDAS AF1300 subsonic wind tunnel. Since turbulent airflow often results in early stall, the primary objective of this research is to delay the stall angle of the normal NACA 4412 airfoil by mitigating local separation zones, and boundary layer transitions. Numerical simulations are performed at airspeeds of 20 m/s and 43.9 m/s, while experimental investigations are conducted at a speed of 20 m/s. The results indicate that both modified airfoils have higher lift-to-drag ratio than the normal airfoil at high AOAs. Specifically, the NACA 4412 airfoil with only a slot demonstrates the highest lift-to-drag ratio among the modified airfoils at high AOAs. Moreover, the NACA 4412 airfoil equipped with a slot and a groove demonstrates the highest stall angle, measured at 18°, compared to the normal NACA 4412 airfoil with a stall angle of 14°. At high AOA, the NACA 4412 airfoil with a slot generates a nearly 35 % higher lift coefficient than the normal NACA 4412 airfoil, while the NACA 4412 with a slot and a groove achieves almost a 16 % higher lift coefficient than the normal NACA 4412 airfoil.

Aerodynamic performance of a flyable flapping wing rotor with dragonfly-like flexible wings

Drawing inspiration from insect flapping wings, a Flapping Wing Rotor (FWR) has been developed for Micro Aerial Vehicle (MAV) applications. The FWR features unique active flapping and passive rotary kinematics of motion to achieve a high lift coefficient and flight efficiency. This study thoroughly investigates the aerodynamic performance and design of a bio-inspired flexible wing for FWR-MAVs, emphasizing its novel backward-curved wingtip and variable spanwise stiffness resembling a dragonfly's wing. The research departs from previous aerodynamic studies of FWR, which focused predominantly on rectangular and rigid wings, and delves into wing flexibility. Employing Computational Fluid Dynamics (CFD), Computational Structural Dynamics (CSD), and experimental measurements, the study demonstrates the aerodynamic benefits of the dragonfly-inspired FWR wingtip shape and its reinforced structure. Fluid-Structure Interaction (FSI) analysis is used to examine the effects of elastic deformation encompassing twist and bending on aerodynamic forces. The results underscore the importance of bending deformation in enhancing lift and power efficiency and propose a method for analysing variable stiffness along the wingspan using a vortex delay mechanism that is induced by delayed flapping motion. By comparing modelled and measured stiffness, the study validates the flexibility of the FWR wing, revealing optimal aerodynamic efficiency is achieved through moderate flexibility and spanwise stiffness variation. The curving leading-edge beam forming the sweep-back wingtip offers a practical approach to obtaining variable stiffness and aerodynamic benefits for FWR-MAVs. Using the same pair of dragonfly-like flexible wings, FWR-MAVs have effectively exhibited VTOL and hovering flight capabilities, spanning from a 25-g single-motor drive model to a 51-g dual-motor drive model. This research provides valuable insights into flexible wing design for FWR-MAVs, leveraging biomimicry to improve flight efficiency.

The Effects of Rotation and Solidity on the Aerodynamic Behavior of Low-Pressure Axial Flow Fans

Abstract Implicit axial flow fan models are commonly used for the numerical analysis of industrial heat exchanger systems. However, these fan models perform poorly within the often complex off-design flow environments characteristic of these systems, as their low-order formulations lose applicability under highly 3D flow conditions. At off-design flow conditions, rotation and blade solidity effects cause physical fan blade behavior to deviate from the anticipated 2D behavior characterized by the implicit models. Physical fan blade behavior at off-design flow conditions, however, remains uncertain and the specific effects of rotation and solidity are not yet widely understood. This study investigates off-design fan blade behavior by presenting explicit 3D computations of two low-pressure axial flow fans. The effects of rotation and blade solidity are then identified through a side-by-side comparison of the computed 3D fan blade data against isolated 2D airfoil data. The rotating fan blade lift characteristics are shown to be distinct from the comparative 2D airfoil trends. At low-flow (off-design) operating conditions, rotation establishes large radial flow movements, and the high blade solidities cause standing vortices to develop within the rotating blade passages. The radial outflow energizes the blades' boundary layers, and the vortices inhibit the flow from separating from the blades' surfaces. Consequently, blade stall is avoided and high lift coefficient values are realized. The presented fan blade lift characteristics are unique relative to literature on other turbomachines of lower blade solidity, highlighting the importance of considering solidity effects in implicit rotor model developments.

Deep learning for airfoil aerodynamic-electromagnetic coupling optimization with random forest

Reducing the design variable space is crucial in multi-objective airfoil profile optimization to improve optimization efficiency and reduce computational costs. Based on random forest and deep neural networks (DNNs), this work performs range reduction on ten design variables obtained through a fourth-order class shape transformation parameterization method for subsonic airfoil profiles. Three aerodynamic performance objectives (lift coefficient, drag coefficient, and lift-to-drag ratio) are evaluated using the Reynolds-averaged Navier–Stokes equations, and two radar stealth performance objectives (horizontal and vertical polarization radar cross sections) are assessed through the method of moments. By combining a DNN architecture with an improved regression prediction capability, predictive models are trained for mapping design variables to design objectives. The prediction errors are below 3% for the aerodynamic performance design objectives and below 1% for the stealth performance design objectives. The particle swarm optimization algorithm provides optimized airfoil profiles for three scenarios. First is a higher lift coefficient with a lower radar cross section. Second is a lower radar cross section. Third is a higher lift coefficient. Increasing the airfoil curvature and reducing the maximum thickness improves the lift coefficient by 386 counts and reduces the drag coefficient by 17 counts. By curving the airfoil leading edge, the radar cross section for the transverse electric and transverse magnetic polarizations decreased by 2.78 and 2.09 dBsm, respectively.

Numerical Investigation on the Aerodynamic Characteristics of a Wing for Various Flow and Geometrical Parameters

The wing is a critical component of an aircraft, responsible for generating the lift force necessary for flight. The aerodynamic behavior of a wing is complex and is influenced by factors such as air speed, angle of attack, wing shape, twist angle, and turbulence. This research investigates the performance of NACA 4412 and NACA 4418 airfoils using 2D CFD, covering a range of angle-of-attack values from -5 to 20 degrees. The superior performing airfoil, NACA 4412, was selected for wing geometry which outperforms NACA 4418 in terms of lower drag and higher lift coefficient at both low and high Reynolds numbers. Then 3D CFD analysis was employed to study how wings perform at different Reynolds numbers and the optimal twist angle for the wing was determined, focusing on a steady-state analysis. The Reynolds number range which corresponds to the flow velocity range, is used in this numerical study for 3D wings. The result shows that as the Reynolds number increases, the wing’s lift coefficient proportionally rises, owing to higher flow velocities. However, this increase in Reynolds number also results in a higher drag coefficient due to a rise in turbulence. The investigation reveals the existence of an optimum twist angle for the wing, which maximizes the lift-to-drag coefficient ratio, thus enhancing overall aerodynamic performance.

Rectangular Cylinder Orientation and Aspect Ratio Impact on the Onset of Vortex Shedding

Rectangular cylinders have the potential to provide valuable insights into the behavior of fluids in a variety of real-world applications. Keeping this in mind, the current study compares the behavior of fluid flow around rectangular cylinders with an aspect ratio (AR) of 1:2 or 2:1 under the effect of the Reynolds number (Re). The incompressible lattice Boltzmann method is used for numerical computations. It is found that the flow characteristics are highly influenced by changes in the aspect ratio compared to the Reynolds number. The flow exhibits three different regimes: Regime I (steady flow), Regime II (initial steady flow that becomes unsteady afterward), and Regime III (completely unsteady flow). In the case of the cylinder with an aspect ratio of 2:1, vortex generation, variation in drag, and the lift coefficient occur much earlier at very low Reynolds numbers compared to the cylinder with an aspect ratio of 1:2. For the cylinder with an aspect ratio of 1:2, the Reynolds number ranges for Regimes I, II, and III are 1 ≤ Re ≤ 120, 121 ≤ Re ≤ 144, and 145 ≤ Re ≤ 200, respectively. For the cylinder with an aspect ratio of 2:1, the Reynolds number ranges for Regimes I, II, and III are 1 ≤ Re ≤ 24, 25 ≤ Re ≤ 39, and 40 ≤ Re ≤ 200, respectively. The cylinder with an aspect ratio of 1:2 is found to have the ability to stabilize the incoming flow due to its extended after-body flatness. Generally, it has been found that a cylinder with an AR of 2:1 is subjected to higher pressures, higher drag forces, higher curvatures of cross-flow rotations, and higher amplitudes of flow-induced drag, as well as higher lift coefficients and lower shedding frequencies, compared to cylinders with an AR of 1:2. In Regime III, elliptic and vertically mounted airfoil-like flow structures are also observed in the wake of the cylinders.

Investigation of coupled aerodynamic characteristics in channel wing propeller configuration

This work studies the lift enhancement effect with a channel wing propeller configuration. Numerical simulations and flow field analysis are performed to investigate the influence (the relative position and spacing, angles of attack (AoA), propeller speed, incoming flow speed) on the lift enhancement effect. The aerodynamic performance is numerically simulated by the method of multi-reference frame (MRF). Numerical results demonstrate that the propeller suction process has a very significant lifting effect. The lifting effect can reach 5 times contrast to the single channel wing. The propeller located at 100% of the chord length of the channel wing achieves the best lift enhancement effect. The intervention of the propeller delays the flow separation at the trailing edge of the channel wing. It holds a high lift coefficient at a high AoA. The lift line slope remains essentially unchanged in the range of 0–30°. It is roughly the same as that of the single channel wing configuration before stall. With the increase of propeller speed or incoming flow speed, the lift coefficient increases significantly. The lift line slope is approximately the same at different speeds. The regularity functions of different coefficient are obtained through mechanism analysis and least squares data fitting methods. The lift coefficient changes with the AoA in a linear relationship and with the propeller speed or incoming flow speed in a quadratic relationship.

Energy considerations and flow fields over whiffling-inspired wings

Some bird species fly inverted, or whiffle, to lose altitude. Inverted flight twists the primary flight feathers, creating gaps along the wing’s trailing edge and decreasing lift. It is speculated that feather rotation-inspired gaps could be used as control surfaces on uncrewed aerial vehicles (UAVs). When implemented on one semi-span of a UAV wing, the gaps produce roll due to the asymmetric lift distribution. However, the understanding of the fluid mechanics and actuation requirements of this novel gapped wing were rudimentary. Here, we use a commercial computational fluid dynamics solver to model a gapped wing, compare its analytically estimated work requirements to an aileron, and identify the impacts of key aerodynamic mechanisms. An experimental validation shows that the results agree well with previous findings. We also find that the gaps re-energize the boundary layer over the suction side of the trailing edge, delaying stall of the gapped wing. Further, the gaps produce vortices distributed along the wingspan. This vortex behavior creates a beneficial lift distribution that produces comparable roll and less yaw than the aileron. The gap vortices also inform the change in the control surface’s roll effectiveness across angle of attack. Finally, the flow within a gap recirculates and creates negative pressure coefficients on the majority of the gap face. The result is a suction force on the gap face that increases with angle of attack and requires work to hold the gaps open. Overall, the gapped wing requires higher actuation work than the aileron at low rolling moment coefficients. However, above rolling moment coefficients of 0.0182, the gapped wing requires less work and ultimately produces a higher maximum rolling moment coefficient. Despite the variable control effectiveness, the data suggest that the gapped wing could be a useful roll control surface for energy-constrained UAVs at high lift coefficients.

Aerodynamic Optimization and Fuel Burn Evaluation of a Transonic Strut-Braced-Wing Single-Aisle Aircraft

This paper presents an assessment of the potential fuel burn savings offered by the transonic strut-braced-wing configuration within the single-aisle class of aircraft relative to a modern conventional tube-and-wing aircraft through aerodynamic shape optimization based on the Reynolds-averaged Navier–Stokes equations. A representative strut-braced-wing aircraft is first developed through conceptual multidisciplinary design optimization based on the Airbus A320neo, with current technology levels assumed. A concept for the conventional tube-and-wing configuration is also developed to represent the Airbus A320neo as a performance baseline. Single-point aerodynamic shape optimization is then performed on wing–body–tail models of each aircraft to address aerodynamic design challenges and to provide more accurate performance estimates. Results indicate that shock formation can be mitigated from the wing–strut junction of the strut-braced wing at Mach 0.78 and a relatively high design lift coefficient of 0.750, providing an 8.2% reduction in block fuel over a 1000 n mile nominal mission when compared to the conventional tube-and-wing aircraft. Multipoint aerodynamic shape optimization is then performed to build toward a more credible estimate of fuel burn performance, with results showing a reduction in the fuel burn savings to 7.8% at the nominal design point relative to the conventional tube-and-wing aircraft to maintain a 7.6–8.0% improvement over the envelope of operating conditions, which includes design points at even higher Mach numbers and lift coefficients. These results demonstrate the viability of the transonic strut-braced-wing configuration for transport aircraft within the single-aisle class and its potential for reducing commercial fleet fuel burn.

Enhancing the Dynamic Stability of Pylons via Their Drag and Lift Coefficients by Finite Volume Method

This study aimed to estimate the drag and lift coefficients of the long-span bridge pylon using the finite volume method (FVM). The k-ω turbulence model was applied to analyze the behavior of wind flow around the pylon, yielding drag and lift coefficient values with an error of 0.98% compared to a previous tunnel experiment. Four recommended cross-sections were proposed to reduce drag and lift forces acting on the pylon, including concave, convex, crossing, and chamfering cross-sections. The finding indicated that drag and lift coefficient decreased for all cross-sections. Cutting edges of concave, convex, and chamfering cross-sections with a ratio ranging from 0.2 to 0.3 has the greatest impact on reducing drag coefficient, while the crossing cross-section with a cutting ratio ranging from 0.2 to 0.25 has the lowest drag coefficient. The maximum reduction in drag and lift coefficients were 23.69% and 13.14% for concave and chamfering cross-sections. Thus, cutting edges of cross-sections is an effective method to enhance the aerodynamic stability of the pylon. Additionally, we evaluated drag and lift coefficients for different wind direction angles. The angles of 0, 30, and 90 degrees resulted in the highest drag coefficient, while the angle of 0 degrees and the angle of 90 degrees resulted in the lowest and highest lift coefficient, respectively. This study not only provides recommendations for cross-sections that reduce forces acting on the pylon but also provides the intensity of this reduction through corresponding estimation equations. In conclusion, concave and chamfering cross-sections are the most effective in reducing drag and lift coefficients, or, in other words, increasing the aerodynamic stability of the pylon.

The Aerodynamic Characteristics Investigation on NACA 0012 Airfoil with Owl’s Wing Serrations for Future Air Vehicle

Owls have an incredibly unique ability to fly in silence, and this ability has significant potential to reduce noise pollution from aircraft engines. For decades, researchers have been studying the noise abatement system in conventional aircraft, but only a few of them had looked into the owl’s ability to reduce the aircraft noise level, especially on the serrated wing. The present study aims to investigate the lift and drag coefficient of the serrated wings including leading-edge serrations, trailing-edge serrations, both-edge serrations, and the original/baseline wing model from the NACA 0012 airfoil. The experimental testing was performed in a wind tunnel at Reynolds numbers of 20,000 and 40,000 and angle of attack from 2° to 30°, in 2° angle intervals. In general, the results showed that each wing model produced a similar lift and drag coefficient profiles, which were proportional to the angle of attack. At Reynolds number of 40,000, the lift coefficient increases from 0.45 to 1.5, as the angle of attack increases from 2° to 24°. Afterward, it decreased as the angle of attack exceeded 24°. According to the data, the trailing-edge serrations model showed the lowest drag coefficient at a Reynolds number of 40,000. Meanwhile, the highest lift coefficient at Reynolds number of 40,000 had been produced by the baseline wing model. This result is beneficial in reducing the noise generation produced by wing structure and at the same time enhancing aerodynamic efficiency.

Neural network flow optimization using an oscillating cylinder

Flow behaviors of a downstream object can be affected significantly by an upstream object in close proximity. Combined with the neural network algorithms, this concept is used for flow control in this study to optimize the aerodynamic performance of a downstream object. Flow with an oscillating cylinder placed upstream is systematically studied because there are multiple control parameters that influence the flow dynamics around the downstream object. These control parameters are used as the input factors of a back-propagation neural network, and then a revised genetic algorithm is applied to find the optimal set of control parameters. In the current study, we use an airfoil in a low-Reynolds-number flow as an example to investigate the proposed neural network flow optimization concept. The datasets used to train the neural network are from the computational simulation with a previously validated immerse-boundary method to accommodate the motion of the cylinder. The results show that by optimally placing an upstream moving cylinder, it is possible to enhance the aerodynamic performance of the downstream object. Compared to the reference case, the optimized lift/drag ratio of the downstream airfoil can be achieved 2.4 times of its reference value, while maintaining a relatively high lift coefficient.

Flexible blades to improve Darrieus turbine performance and reduce cost

Low cost “sailwing” aerofoils of flexible nylon mounted between a rigid leading edge of circular cross section and a rigid flat trailing edge were investigated in the 1970s and 1980s. Wind tunnel measurements of lift and drag of these aerofoils over the full range of angles of attack were published, but no predictions were made for a complete wind turbine incorporating them. Another paper stated that a sailwing turbine with aerofoils incorporating pivoting aerodynamic profile leading edges was found to exert considerable starting torque. In the present paper, published lift and drag data have been used to predict the performance of a small Darrieus hydrokinetic or wind turbine with sailwing aerofoils, using a standard multiple streamtube model over the full range of tip-speed ratios (TSRs) from 0.5 to 3. To provide a fair comparison, this method is also used to predict the performance of the turbine with symmetrical NACA0018 and cambered Gö420 blade profiles. At all examined TSRs, average torque and power of the turbine with flexible blades are higher than the turbines with rigid blades. For instance, at TSR of 2.5, the flexible blade turbine achieved 55 % and 165 % higher average torque than the turbines with symmetrical and cambered blades, respectively. This finding is attributed to the fact that the flexible blade camber reverses with reversing incidence, producing high lift coefficients at low incidence. Drag is higher for the flexible aerofoil, probably due to separation at the circular cross section leading edge, and measures are suggested to improve this. The flexible aerofoils are very cheap and simple to fabricate, making them an attractive option for affordable hydrokinetic turbines.

Numerical and Experimental Studies on the Aerodynamics of NACA64 and DU40 Airfoils at Low Reynolds Numbers

The aerodynamics of airfoils can be seen in a wide range of applications. To obtain the aerodynamic loads, geometrically-scaled airfoil sections are tested in wind tunnels. However, due to the limited space of the wind tunnel, the mismatch of Reynolds numbers may lead to different aerodynamic loads. Previous works showed that decreased lifts and increased drag coefficients are associated with lower Reynolds numbers, which are accompanied by the changes in ambient flow, such as increased sizes of the separation bubbles and wake vortices. Although insightful, few direct connections between loads, pressures, and ambient flow were presented, leaving a critical knowledge gap for aerodynamic modifications to improve the aerodynamic performances at low Reynolds numbers. To bridge this gap, this work utilizes numerical simulations and wind tunnel experiments to study the aerodynamics of a thin airfoil (NACA64) and a thick airfoil (DU40), at two chord Reynolds numbers, i.e., 4000 and 60,000. The two-dimensional (2D) vortex particle method (VPM) with varying-sized particles is used to simulate the unsteady flow and compared to the steady-state simulations by XFOIL. As the Reynolds number increases, it reveals that the higher lift coefficients are associated with the increased upstream suction and positive pressures on the upper and lower surfaces of the airfoils, respectively. These changes are explained by the increased and decreased normalized wind speeds on the upper and lower surfaces of the airfoils, respectively. Stronger pressure recoveries observed downstream of the reattachment points are the main cause of drag reductions at higher Reynolds numbers. The smaller and more irregular vortices in the roll-up shear layers and wakes observed at the higher Reynolds number are similar to the previous experimental findings, which are shown in this work to make the force fluctuations more irregular at higher frequencies. Possibly due to missing 3D effects, the results obtained from the 2D VPM are observed to ‘overestimate’ the effects of increasing the Reynolds number at ReC = 60,000. Furthermore, both VPM and XFOIL are found to work best in explaining the physics at low angles of attacks, i.e., −10°≤α≤10°, which are similar to the previous numerical works utilizing 2D methods.

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.

A study on separation control mechanisms of tandemly arranged wing with linear leading edge and leading edge protuberance

A study on separation control mechanisms of tandemly arranged wing with linear leading edge (LLE) and leading edge protuberance (LEP) in low Reynolds number was conducted. In this study, to investigate the separation control mechanism, we carried out the visualization experiment using smoke-wire method, the streamwise and spanwise velocity measurements above the upper surface of the backward wing by using hotwire anemometer, and the numerical simulation. From the results of flow visualization experiment, it was found that the high lift coefficient obtained by tandemly arranged wing with LLE and singly and tandemly arranged wing with LEP were caused by reducing the flow separation region. From the results of velocity measurements and numerical simulation, it was found that in the case of wing with LLE, the turbulence generated by forward wing, which had mainly streamwise component with peak frequency of 230Hz and broadband frequency over than 30Hz, controled the flow separation. Whereas in the case of wing with LEP, regardless of the wing arrangement, separation control was brought by the separation bubble that caused by the longitudinal vortices generated by the LEP. It was also found that the turbulence from forward wing affected the position of the valley part where the separation bubble and the strong negative pressure were formed.