Lift Enhancement Research Articles

The Lift Enhancement Effect of a New Fluidic Oscillator on High-Lift Wings

Fluidic oscillators have emerged as a prominent topic of research in the field of flow control, owing to their broad sweep range and enhanced control efficiency. However, the underlying mechanisms governing the operation of fluidic oscillators remain poorly understood, and the effect of oscillation frequency on flow control performance has yet to be conclusively determined. In this study, a novel fluidic oscillator is proposed that achieves frequency decoupling by replacing the conventional feedback channel with synthetic jets, thereby enabling modulation of oscillation frequency at a constant momentum coefficient. When applied to a high-lift airfoil, results show that at a momentum coefficient of 14.1%, the lift coefficient increase achieved under F+ = 1 control outperforms that under F+ = 10 by more than 0.3. This finding suggests the presence of an optimal frequency for fluidic oscillators, which maximizes their flow control effectiveness. Notably, this optimal frequency is unaffected by variations in the momentum coefficient. A deeper analysis of the fluidic oscillator’s working principle reveals that periodic oscillations dominate the turbulent kinetic energy and Reynolds shear stress, driving enhanced chordwise momentum exchange. This increased energy transfer strengthens the boundary layer’s resistance to separation, effectively mitigating flow detachment and improving lift enhancement. Finally, the periodic flow field on the surface of the high-lift airfoil under fluidic oscillator control was examined. It was observed that, at low frequencies, the fluidic oscillator effectively controls the shedding of separation vortices, ensuring that the frequency of vortex shedding aligns with the oscillation frequency of the fluidic oscillator. This alignment likely contributes to the superior lift enhancement observed under low-frequency conditions.

Investigation of wind tunnel experiments on nonlinear ground effect for flying cars

This research investigates the nonlinear aerodynamic characteristics of ground effect for flying cars, utilizing high-precision balance measurements to determine the lift and drag of flying cars and particle image velocimetry (PIV) in a closed-circuit low-speed recirculating wind tunnel. The ground effect's lift enhancement mechanism was revealed through analysis of aerodynamic results and flow field structures. Force measurement experiments demonstrated that ground effect significantly improves aerodynamic performance, with the lift coefficient showing a nonlinear trend as altitude decreases, undergoing the gentle force phase, the force increase phase, and the force reduction phase. PIV results indicated that the narrowing flow channel between the flying car and the ground at lower heights reduces airflow speed beneath the wing, converting kinetic energy into pressure potential energy, similar to a high-pressure air cushion. Consequently, airflow speed over the upper wing surface increases, enhancing lift. Additionally, ground effect causes the wingtip vortex structure to move upward and outward along the wing span, reducing downward induced velocity, weakening vortex strength, and increasing the effective aspect ratio of the three-dimensional wing. These changes reduce induced drag, thereby improving the overall performance of the flying car.

Enhancement of flying wing aerodynamics in crossflow at high angle of attack using dual synthetic jets

To address the asymmetric flow field of the flying wing under cross-flow conditions at high angles of attack, dual synthetic jet actuators (DSJAs) are positioned at the leading edge of the windward side. DSJ control effectively affects the flow separation structure on the windward side, thereby enhancing leading-edge suction. This leads to both lift enhancement and drag reduction. Furthermore, it strengthens the stabilizing roll moment and improves lateral static stability. Additionally, the effective suppression of the separation zone significantly reduces the aerodynamic load fluctuations of the flying wing after control. In terms of excitation frequency, low-frequency excitation more effectively generates lift, transfers energy downstream, promotes momentum transfer, and results in an 8.2% increase in lift. Moreover, the DSJ excitation exhibits fast response characteristics, with the entire flow establishment process taking only 190 ms. Therefore, through DSJ control, the asymmetric flow under lateral conditions can be effectively corrected, expanding the flight envelope and enhancing the maneuverability and reliability of the aircraft.

Review on flow separation control: effects of excitation frequency and momentum coefficient

This paper presents a comprehensive review of the studies and research conducted on flow separation control on lifting surfaces. In this paper, two critical parameters, namely, the momentum coefficient and excitation frequency, that significantly impact flow separation control are analyzed in detail. Through a comprehensive literature review, experimental and numerical studies are examined in order to gain insight into the underlying mechanisms of momentum injection and excitation frequency on the shear layer and wake dynamics and to quantify the impact of these parameters on the flow separation control. Effective flow control on lifting surfaces can modify the streamlines and pressure distribution, thereby increasing their aerodynamic efficiency. This paper focuses on the control of flow separation on airfoils, with particular attention paid to the benefits of such control, including lift enhancement, drag reduction, aerodynamic efficiency enhancement, performance enhancement, and other important features. This paper presents a review of studies that have employed blowing actuators, as well as zero net mass flux, plasma, and acoustic actuators, in order to provide an appropriate historical context for recent developments. The findings of this review paper will contribute to a better understanding of the optimal conditions for efficient flow separation control on lifting surfaces using unsteady excitation, which can have significant implications for improving the performance and efficiency of various aerodynamic applications. This paper aims to elucidate and emphasize the positive and negative aspects of existing research, while also suggesting new interesting areas for future investigation.

Efficient aerodynamic shape optimization using transfer learning based multi-fidelity deep neural network

Computational efficiency and precision pose a classic contradiction in aerodynamic shape optimization. To address this challenge, this study introduces an effective optimization framework based on multi-fidelity fully connected neural network (MFFCN). The framework utilizes transfer learning (TL) to train a multi-fidelity surrogate model that establishes direct mappings between geometric configuration parameters and aerodynamic performance by adaptively capturing linear or nonlinear relationships concealed between high-fidelity (HF) and low-fidelity (LF) information. The HF and LF data are derived from fine and coarse grids, respectively, evaluated using the same computational fluid dynamics (CFD) model. The MFFCN-TL framework is applied to optimize the National Advisory Committee for Aeronautics 0012 (NACA0012) airfoil (12 design variables) and the Office National d'Études et de Recherches Aérospatiales M6 (ONERA M6) wing (50 design variables). Simulation results demonstrate that the NACA0012 airfoil achieves a 69.47% enhancement in lift–drag ratio in 1.069 s, compared to a 12.76% gain over 24.8 h in single-fidelity CFD-based optimization. The ONERA M6 wing achieves a 24.66% reduction in drag coefficient in 694 ms compared to 18.37% over 237.3 h in the CFD model. Statistical results show that the MFFCN-TL framework can reduce optimization cost by more than 90% compared to the single-fidelity CFD-based model. These findings suggest that the MFFCN-TL framework significantly enhances optimization efficiency and provides superior feasible solutions over single-fidelity methods.

CFD Investigation of Dual Synthetic Jets on an Optimized Aerofoil's Trailing Edge

In fluid dynamics, a flow control device is used to control, manage, or modify the behavior of a fluid flow. Jet actuators work by releasing high-velocity jets of fluid, usually air or gas, into the surrounding environment to control or manipulate the flow of fluids. In this study, the flow control device, which was a dual synthetic jet actuator (DSJA), acted as a lift enhancement device over an optimized NACA 0012 aerofoil with a rounded trailing edge (TE) (Coanda surface approximately 9% of the trailing edge was modified) to enhance the lift at various angles of attack (AOAs). Fluctuating pressure inlets were introduced in two slots. When the dual synthetic jets were in control, the out-of-phase jets from the upper and lower trailing edge jets helped to boost the lift coefficient. The suction stroke from the lower half of the jet made the Coanda effect stronger in the upper half. The upper trailing edge jet deflected downwards merged with the lower one and helped to deflect the flow field closer to the bottom half. An unsteady CFD analysis was performed on optimized airfoils with and without a DSJ, with a driving frequency of 40.6 and a reduced frequency of 0.025 at a Reynolds number of 25000. The results obtained at different angles indicated that the L/D ratio was improved by 13.5% at higher angles of attack in the presence of the DSJA.

The analysis of Gurney flap applications for autonomous underwater gliders

ABSTRACT In this paper, the Gurney flap structure (GF) from aeronautical research was applied to underwater gliders for improving the lift-to-drag ratio. Firstly, the numerical simulation method was used to verify the lift enhancement principle of the GF structure and the feasibility of its application to autonomous underwater gliders (AUG). Secondly, the performance of different GF was simulated and analysed, and a variable GF structure was proposed with the requirements of the operating modes. The GF parameters were optimised by a genetic algorithm. The results show that the performance of GF in water is similar to that in air, so it is feasible to apply GF to AUG. When the height of GF is close to the turbulent boundary layer of the trailing edge of the wing, the lift-to-drag ratio is significantly improved, especially in the range of small angle of attack and the maximum increase is up to 30%.

Microjet Flow Control on an Airfoil Flap: Particle Image Velocimetry Characterization

Normal-blowing microjets near the trailing edge of a flap can improve the aerodynamic performance of high-lift systems, potentially without some of the system-integration drawbacks of traditional systems. Previous two-dimensional simulations and a preliminary wind-tunnel test demonstrated the effectiveness of the microjet concept on a deployed flap. To further validate microjet effectiveness, this study uses a specially developed particle image velocimetry technique to estimate the momentum injected by a microjet on a deployed Fowler flap of a two-element NLR7301 airfoil while simultaneously measuring the lift enhancement provided by the microjet via surface pressure taps. The test results are broadly consistent with numerical simulations that predict lift increases proportionally to the square root of momentum injection. Within the experimental uncertainties, the test results show slightly less lift benefit for a given level of estimated momentum injection compared to the simulations, possibly due to three-dimensional effects such as spanwise variability and spanwise velocity components of the injected air, which could not be measured with the current instrumentation.

High lift devices using compliant surfaces

Stall delay and lift enhancement play a crucial role in modern aircraft performance. This is commonly achieved by devices such as slats or flaps located at the leading edge or trailing edge of an aircraft's wing. In this paper, we report a feasibility study of using light-weight compliant surfaces for novel high lift devices. The effects of compliant flags with one end fixed or both ends fixed near the leading edge and trailing edge of an airfoil were studied by force, flag deformation, and flow field measurements in a wind tunnel. When a flag is placed near the leading edge, the excitation of the separated shear layer from the leading edge is the main mechanism in increasing the lift at the post-stall angles of attack. In contrast, the trailing-edge flag with an excess length and both ends fixed could increase the effective camber and the circulation around the airfoil in a time-averaged sense. The mechanism is similar to that of the conventional Gurney flap effect, and equally effective at pre-stall and post-stall angles of attack. When used together, the compliant flags can delay stall angle by 8° and increase the maximum lift coefficient by 67% in the parameter range tested presently. Compliant surfaces require no external power as a passive method. If they are to be used as active methods, they are light weight, and can be stored and deployed easily.

Blade Airfoil Optimization and Its Impact on Vertical Axis Wind Turbine Performance

Abstract Amidst the emergence of crises in petroleum prices and supply, the imperative for the development of clean energy has become increasingly apparent. Vertical axis wind turbines (VAWT), as a type of wind turbine, still grapple with challenges such as low wind energy utilization and sensitivity to dynamic wind speeds. This study focuses on optimizing the airfoil of a two-dimensional vertical axis wind turbine and analyzes the performance variations in a three-dimensional model. Following the optimization, the average power coefficients of the dual-blade two-dimensional VAWT increased by 7.9% and 4%, respectively, while those of the dual-blade three-dimensional model rose by 7.9% and 1.2%. However, the performance of the three-dimensional model was compromised due to the influence of tip vortices, resulting in inferior performance compared to the two-dimensional model. Both three-dimensional optimized blade designs exhibited a substantial reduction in the flow separation region downstream and improved pressure distribution fluctuations caused by the shedding of tip vortices, leading to decreased pressure differentials. Along the spanwise section, the power coefficient gradually decreased from the central segment to the tip segment. Notably, the power coefficient of the optimized blades at the tip segment increased significantly, accompanied by an augmented pressure differential between the upper and lower surfaces. This augmentation contributed to a significant enhancement in lift, thereby facilitating the rotation of the vertical axis wind turbine.

Experimental Investigation of Effect of Corrugations on Delta Wing for Lift Enhancement in Low Reynolds Number Flow

Experimental Investigation of Effect of Corrugations on Delta Wing for Lift Enhancement in Low Reynolds Number Flow

Direct Numerical Simulation of Tandem-Wing Aerodynamic Interactions at Low Reynolds Number

A three-dimensional direct numerical simulation was conducted to investigate the vortex–wing interactions through two NACA 0012 stationary wings placed in tandem at a low Reynolds number Re=10,000. The aerodynamic characteristics and three-dimensional flow structures were analyzed for the tandem wings. The back wing disturbed by the upstream vortices gained an evident increase in aerodynamic performance, where the advantage is related to the suppression of the large-scale vortex formation near the trailing edge. The Liutex method was applied to visualize the vortical structures for investigating the three-dimensional evolution and instability when interacting with the back wing. The upstream wake triggered dual-secondary vortices and intensified the secondary instability on the back wing. The induced vortices contributed to the lift enhancement because they provided an extra low-pressure region when propagating downstream on the suction side of the back wing. Because of the three-dimensional destabilization, the vortex interaction in the evolution process accelerated the transition and injection of the high-momentum flow into the boundary layer attached to the back wing, energizing the turbulent boundary layer and eliminating the large-scale separation near the trailing edge. This study provided a new perspective on the enhanced aerodynamic performance of tandem layout.

Characteristics of Supersonic Sweeping Jet with Geometry Variation of Actuator Exits

Supersonic sweeping jets (SWJs) have demonstrated their effectiveness across a variety of scenarios, particularly in aeronautic applications (e.g., lift enhancement). An experimental study is conducted to investigate the characteristics of SWJs emitted from actuators with different spreading angle θ and exit length l. Schlieren visualization is used to capture the near- and far-field SWJs at nozzle pressure ratios (NPRs) ranging from 1.6 to 6.9. The results show that as NPR increases the SWJs become underexpanded when l≠0. Different θ have an impact on the shock structure and the law of the spreading angle of jet control area α changing with NPR. When θ is approximately 100 deg, α increases at first and then decreases with increasing NPR, reaching up to 90 deg at high NPRs. When θ is about 50 deg, α remains roughly constant and equal to θ. Internal flow measurements reveal that flow attachment caused by the Coanda effect plays a significant role in the mechanism that leads to a change in α. Proper orthogonal decomposition is applied to analyze the spatial and temporal patterns. Far-field measurements show multiple sound waves propagating upstream and downstream, which generated by the supersonic SWJs.

The flow control mechanism of trailing-edge Gurney flap on a 50°-swept delta wing in forced pitching

The flow control effect of the trailing-edge Gurney flap (TG) on the dynamic lift characteristics for a 50°-swept delta wing during large-amplitude pitching oscillations at various reduced frequencies (k = 0.072, 0.144, 0.287, and 0.575) was investigated via force, particle image velocity, and dye visualization measurements in a water channel facility. Numerical simulations were carried out to further understand the flow control mechanism of the TG in low and high reduced frequency cases (k = 0.072 and 0.575). It was found that as the reduced frequency increases, the lift increments brought by the TG are magnified and abated during the upstroke and downstroke processes, respectively. The breakdown of the leading-edge vortex (LEV) on the upper surface of the wing is promoted by the TG during the early stage of the pitching cycle. The lift enhancement being benefited by the TG is mainly contributed by the recovery of lower surface pressure along the trailing edge due to the blockage effect of TG, which also stimulates the spanwise flow and strengthens the LEV upon the upper surface. The significant lift increment contribution of the upper surface during the upstroke process can be maintained to higher angle of attack as the reduced frequency increases.

Quasi-three-dimensional high-lift wing design approach considering three-dimensional effects of slipstream for distributed electric propulsion aircraft

The efficient utilization of propeller slipstream energy is important for improving the ultra-short takeoff and landing capability of Distributed Electric Propulsion (DEP) aircraft. This paper presents a quasi-three-dimensional (2.5D) high-lift wing design approach considering the three-dimensional (3D) effects of slipstream for DEP aircraft, aiming at maximizing the comprehensive lift enhancement benefit of the airframe-propulsion coupling unit. A high-precision and efficient momentum source method is adopted to simulate the slipstream effects, and the distributed propellers are replaced by a rectangular actuator disk to reduce the difficulty of grid generation and improve the grid quality. A detailed comparison of the 2.5D and 3D configurations based on the X-57 Mod Ⅳ is performed in terms of flow characteristics and computational cost to demonstrate the rationality of the above design approach. The optimization results of the high-lift wing of the X-57 Mod Ⅳ show that the aerodynamic performance of the landing configuration is significantly improved, for instance, the lift coefficient increases by 0.094 at the angle of attack of 7°, and 0.097 at the angle of attack of 14°. This novel approach achieves efficient and effective design of high-lift wings under the influence of distributed slipstream, which has the potential to improve the design level of DEP aircraft.

Effect of a fixed downstream cylinder on the flow-induced vibration of an elastically supported primary cylinder

This paper numerically investigates the influence of a fixed downstream control cylinder on the flow-induced vibration of an elastically supported primary cylinder. These two cylinders are situated in a tandem arrangement with small dimensionless center-to-center spacing (L/D, L is the intermediate spacing and D is the cylinder diameter). The present two-dimensional (2D) simulations are carried out in the low Reynolds number (Re) regime. The primary focus of this study is to reveal the underlying flow physics behind the transition from vortex-induced vibration to galloping in the response of the primary cylinder due to the presence of another fixed downstream cylinder. Two distinct flow field regimes, namely, steady flow and alternate attachment regimes, are observed for different L/D and Re values. Depending on the evolution of the near-field flow structures, four different wake patterns, “2S,” “2P,” “2C,” and “aperiodic,” are observed. The corresponding vibration response of the upstream cylinder is characterized as interference galloping and extended vortex-induced vibration. As the L/D ratio increases, the lift enhancement due to flow-induced vibration is seen to be weakened. The detailed correlation between the force generation and the near-wake interactions is investigated. The present findings will augment our understanding of vibration reduction or flow-induced energy harvesting of tandem cylindrical structures.

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).

Integrated design optimization of ducted lift fan for lift enhancement and noise reduction based on nested NFFD control body

In order to enhance the aerodynamic performance and reduce the aerodynamic noise of the coaxial dual-rotor ducted fan, this paper proposes a stepwise optimization strategy based on nested NFFD (Nurbs-Based Free-Form Deformations) control body parameterization. This strategy takes into account the optimization of aerodynamic performance, which involves significant deformations, and noise reduction, which requires fine deformations. By doing so, it effectively addresses the issue of drastically increased computational complexity and time consumption associated with conventional multi-objective optimization methods. By designing the aerodynamic shape of the blades, along with aerodynamic performance parameter calculation and aerodynamic noise calculation modules, it is possible to achieve lift enhancement and noise reduction for the ducted fan. The numerical simulation results demonstrate that the flow field over the optimized blade surface is improved significantly. The lift has increased from 3.45N to 3.54N, resulting in a 2.6% enhancement. Additionally, the noise level has decreased from 72.33dB to 69.52dB, achieving a noise reduction of 2.81dB.

The lift enhancement mechanism caused by the deformation of the surface of the wide-speed waverider

The optimization method provides an effective approach to enhance the low-speed lift of the vortex lift waveriders by deforming the aerodynamic shape refinedly. However, the vortex lift enhancement mechanism of the optimized configuration is unclear. In this study, the flow evolutions of the original and the optimized configurations are studied by employing the delayed detached-eddy simulation. Results indicate that the convex deformation of the leeward surface plays a dominant role in enhancing the vortex lift by enhancing the low-pressure suction at the upstream breakdown location and delaying the vortex breakdown. For the enhancement of the low-pressure suction, the convex deformation intensifies the streamwise vorticity below the axis of the primary vortex of the leading-edge vortex, in turn enhancing the downwash effect and causing the primary vortex to move downward. This reduces the pressure coefficient induced by the primary vortex on the leeward surface, thus enhancing the vortex lift. In terms of the delay of the vortex breakdown, the convex deformation compresses and accelerates the flow between the spanwise convex and the leading edge. These intensities enhance the washing effect along the spanwise direction on the outward wing and cause the primary vortex to deflect toward the outboard wing. Subsequently, the primary vortex and the shedding vortices generated by the shear layer instability merge, which increase the primary vortex intensity, and enhance the streamwise velocity in the vortex axis. Correspondingly, the primary vortex breakdown is delayed. Ultimately, the increased low-pressure region caused by the delay of the vortex breakdown enhances the vortex lift.

Surrogate-based pneumatic and aerodynamic allocation design optimization for flapping-wing micro air vehicles

Determining the mapping relationships between the bionic structures and aerodynamic characteristics of flapping-wing micro air vehicles (FWMAVs), therefore optimizing their comprehensive performances of pneumatic functions and maneuvering capabilities, poses a formidable challenge. In this work, a surrogate-based approach is proposed to tackle the pivotal challenges. Firstly, an experimental surrogate model is established for predicting the pneumatic performances of the flapping wings using multi-fidelity datasets, which incorporates an efficient global optimization strategy for the sample fill-in to enhance predictive precision while minimizing experimental costs. Secondly, to account for the intricate influences of control inputs on aerodynamic characteristics, comprehensive aerodynamic surrogate models are established to precisely anticipate pneumatic performances and torque allocations under different attitude control patterns. Thirdly, by elucidating the mapping relationships between bionic structures and aerodynamic performances, surrogate-based optimizations aim to identify optimal combinations of corresponding parameters of bionic structures, thereby enhancing the pneumatic and toque allocations of the FWMAV. The bionic structures achieve a maximum lift enhancement of 11.5 % and at least a 5.4 % improvement in torque generation through experimental validations. Based on the optimal outcomes of the bionic structure analysis of the FWMAV, a novel prototype family comprising three distinct sub-varieties is successfully fabricated. The findings of our research are hopefully to offer valuable insights for future FWMAV designs and make potential contributions to the advancement of standardized methodologies for evaluating their flight performances.