Lift Enhancement Mechanism Research Articles

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).

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.

Lift enhancement mechanism study of the airfoil with a dielectric elastic membrane skin

Lift enhancement mechanism of an airfoil with the dielectric elastic membrane skin is studied numerically for smart flow control. The flexible membrane is made of dielectric highly elastic polymer material. Such kind of material can deform and oscillate under the prescribed electric potential difference. The dynamic modeling of the dielectric elastic structure is established to describe the electromechanical behaviors. A high-fidelity aero-electromagnetic-structural coupling model is proposed and verified based on CFD/CSD coupling technique. The aerodynamic characteristics of the airfoil with the dielectric elastic membrane skin is analyzed at various angles of attack. The results show that the lift coefficient of the airfoil is 12.33% higher than that of the rigid airfoil at AOA=14°. The effects of coupling oscillation and applied voltages on the aerodynamic performance of the airfoil are emphasized. In the nonlinear coupling, the high-order lock-in frequency plays a significant role in lift enhancement. The lift coefficient is greatly improved when the second-order frequency lock-in occurs and the second-order lock-in frequency is no less than the second-order fundamental frequency of the flow past the rigid airfoil. The corresponding flow pattern is characterized with the formation and maintain of the vortices with similar scale.

Dynamic lift enhancement mechanism of dragonfly wing model by vortex-corrugation interaction

Dynamic lift enhancement mechanism of dragonfly wing model by vortex-corrugation interaction

Passive Poststall Flow Control on Nonslender Delta Wings Using Flags

The lift force on two nonslender delta wings with sweep angles of Λ=40 and 50°, with flags attached to the leading edge, was investigated. Substantial lift enhancement in the poststall regime of the clean wings and the stall delay were observed. With a flag, the flowfield measurements showed the reattachment of the shear layer onto the wing surface and the re-formation of the leading-edge vortex, whereas the clean wing had completely stalled flow at the same angle of attack. Over a wide range of angles of attack, mass ratios, and flag lengths, the maximum lift enhancement was attained when the dimensionless frequency of flag oscillations was in an optimal range and the flag-tip velocity had sufficient amplitude. The optimal dimensionless frequency range corresponded to the natural shear layer instabilities of the clean wing. The excitation in this range substantially increased the coherency of the separated flow due to the flag oscillations. The main mechanism of the lift increase is the excitation of the shear layer, which exhibits convective instability. This is very different from the lift enhancement mechanism on airfoils where the flags lock-in to the wake instability, which is known to have an absolute instability at the poststall angles of attack of the clean airfoil.

On lift enhancement and noise reduction in serrated gurney flap airfoil of wind turbines using proper orthogonal decomposition

Based on the steady simulation results obtained by the SST k-ω turbulence model, large eddy simulation (LES) and proper orthogonal decomposition (POD) are used to examine the static pressure and vorticity modes of the wind turbine airfoil with serrated Gurney flaps (SGFs) and the baseline at α = 12° and 20°. The effect of SGF on the aerodynamic performance and radiated noise is investigated, and the influence of serration's span structure on the vorticity mode distribution is discussed. By analyzing the main modes and mode coefficients of the flow field, the mechanism of lift enhancement and noise reduction of SGF is explained. The results indicate that an airfoil with a SGF(ASGF) of 6.7%c width and 0.8%c height can effectively improve the lift coefficient within the investigated attack angles, delay stall by about 3°, and reduce the radiated noise of the airfoil by 0.51 and 4.40 dB, respectively, at 12° and 20°. Static pressure distribution analysis reveals that the SGF can significantly widen the positive pressure zone of the pressure surface, improve the ultra-low pressure regime of the suction surface, and make the pressure distribution on the suction surface more uniform, which increase the lift-drag ratio of ASGF. The POD method effectively extracts the most relevant mode of the airfoil surface radiated noise. The unsteady feature of the main flow of the airfoil is the vortices on the suction surface. The ASGF can effectively restrain the vortex separation on the suction surface and reduce the vorticity, delaying the stall and reducing the radiated noise of the airfoil surface.

A robust data-driven model for flapping aerodynamics under different hovering kinematics

Flapping wing micro air vehicles (FWMAVs) are highly maneuverable, bio-inspired drones that can assist in surveys and rescue missions. Flapping wings generate various unsteady lift enhancement mechanisms challenging the derivation of reduced models to predict instantaneous aerodynamic performance. In this work, we propose a robust data-driven, quasi-steady reduced order model (ROM) to predict the lift and drag coefficients within a flapping cycle. The model is derived for a rigid ellipsoid wing with different parameterized kinematics in hovering conditions. The proposed ROM is built via a two-stage regression. The first stage, defined as “in-cycle” (IC), computes the parameters of a regression linking the aerodynamic coefficients to the instantaneous wing state. The second stage, defined as “out-of-cycle,” links the IC weights to the flapping features that define the flapping motion. The training and test datasets were generated via high-fidelity simulations using the overset method, spanning a wide range of Reynolds numbers and flapping kinematics. The two-stage regressor combines ridge regression and Gaussian process regression to provide estimates of the model uncertainties. The proposed ROM shows accurate aerodynamic predictions for a wide range of kinematics. The model performs best for smooth kinematics that generates a stable leading edge vortex (LEV). Remarkably accurate predictions are also observed in dynamic scenarios where the LEV is partially shed, the non-circulatory forces are considerable, and the wing encounters its own wake.

Regularities between kinematic and aerodynamic characteristics of flexible membrane wing

The kinematic characteristics of flexible membrane wing have vital influences on its aerodynamic characteristics. To deeply explore the regularities between them, time-resolved aerodynamic forces and deformations at different aeroelastic parameters and angles of attack (α) were measured synchronously by wind tunnel experiments. The membrane motion can be mainly divided into two states at α > 0° with various lift-enhancement regularities: Deformed-Steady State (DSS) at pre-stall, and Dynamic Balance State (DBS) at around stall and post-stall. Besides, the mean camber, maximum vibration amplitude, and lift coefficient almost reach their maxima simultaneously within the DBS region. By introducing momentum coefficient Cμ of membrane vibration, positive correlation among amplitude, momentum and lift is successfully established, and the lift-enhancement mechanism of membrane vibration is revealed. Moreover, it is newly and surprisingly found that at different vibration modes, the maximum vibration amplitude and root mean square of vibration velocity present positive and linear correlation with different slopes, and their chordwise locations are basically consistent. Therefore, novel ideas for active control of flexible wing are proposed: by controlling the vibration amplitude, frequency, and mode, while selecting the specific chordwise locations for intensive excitation, Cμ can be efficiently increased. Ultimately, the aerodynamic performance will be improved.

Numerical Study of the Lift Enhancement Mechanism of Circulation Control in Transonic Flow

The lift of an aircraft can be effectively enhanced by circulation control (CC) technology at subsonic speeds, but the efficiency at transonic speeds is greatly decreased. The underlying mechanism of this phenomenon is not fully understood. In this study, Reynolds averaged Navier—Stokes simulation with k−ω shear stress transport model was utilized to investigate the mechanism of lift enhancement by CC in transonic flow. For validation, the numerical CC results were compared with the NASA experimental data obtained for transonic CC airfoil. Thereafter, the RAE2822 airfoil was modified with a Coanda surface. The lift enhancement effects of CC via steady blowing with different momentum coefficients were tested at Ma=0.3 and 0.8 at α=3∘, and various fluid mechanics phenomena were investigated. The results indicate that the flow structure of the CC jet is insensitive to the incoming flow conditions because of the similarity to the local static pressure field around the trailing edge of the airfoil. Owing to the appearance of shockwaves on the airfoil surface in the transonic regime, the performance of the CC jet is restricted to the trailing edge of the airfoil. Transonic CC achieved a slight improvement in aerodynamic performance owing to a favorable shift in the shockwave pattern and accelerated flow in the separation region on the airfoil surfaces. Revealing the mechanism of lift enhancement of CC in the transonic regime can facilitate the rational design of new fluidic actuators with high activity and expand the potential applications of CC technology.

Geometric-control formulation and averaging analysis of the unsteady aerodynamics of a wing with oscillatory controls

Differential-geometric-control theory represents a mathematically elegant combination of differential geometry and control theory. Practically, it allows exploitation of nonlinear interactions between various inputs for the generation of forces in non-intuitive directions. Since its early developments in the 1970s, the geometric-control theory has not been duly exploited in the area of fluid mechanics. In this paper, we show the potential of geometric-control theory in the analysis of fluid flows, exemplifying it as a heuristic analysis tool for discovery of symmetry-breaking and unconventional force-generation mechanisms. In particular, we formulate the wing unsteady aerodynamics problem in a geometric-control framework. To achieve this goal, we develop a reduced-order model for the unsteady flow over a pitching–plunging wing that is (i) rich enough to capture the main physical aspects (e.g. nonlinearity of the flow dynamics at large angles of attack and high frequencies) and (ii) efficient and compact enough to be amenable to the analytic tools of geometric nonlinear control theory. We then combine tools from geometric-control theory and averaging to analyse the developed reduced-order dynamical model, which reveals regimes for lift and thrust enhancement mechanisms. The unsteady Reynolds-averaged Navier–Stokes equations are simulated to validate the theoretical findings and scrutinize the underlying physics behind these enhancement mechanisms.

Can the ground enhance vertical force for inclined stroke plane flapping wing?

The numerical investigation of 2D insect wing kinematics in an inclined stroke plane is carried out using an immersed boundary solver. The effect of vortex shedding and dipole jet on the vertical force generation by the flapping wing due to change in the stroke plane angle is investigated in the vicinity of the ground. The results of instantaneous force and vorticity contours reveal the underlying lift enhancement mechanisms for the inclined stroke plane flapping wing. Moreover, they aid in the understanding of the wake-ground interaction and the associated shear layers. The calculated average vertical force delineates different force trends for the inclined stroke plane flapping near the ground. Furthermore, the dipole jet patterns are analyzed for different heights. These patterns are found to be a better tool to assess the kinematics for the vertical force enhancement and reduction, especially at intermediate heights. Vertical force enhancement is the critical parameter in the design of the micro aerial vehicle (MAV). Through this study, it is certain that the dipole jet has the potential to be used as a lift modification mechanism in MAVs. In summary, the study gives a holistic view of the physics of the inclined plane kinematics near the ground and serves as the basis for the design of MAVs.

Numerical simulation of the aerodynamic performance of a novel micro-aerial vehicle mimicking a locust

Abstract This paper describes the design, micro-fabrication and testing of a novel Micro-Aerial Vehicle (MAV) that mimicking a real locust. Actual parameters of locust insect are used to create a micro-scale MAV that can replace the traditional types that mimicking dragonfly and birds. Based on the obtained results, the novel MAV crucial parameters are its weight and strength to take-off under normal locust performance parameters fashion. Computational Fluid Dynamics (CFD) simulations are carried out at angles of attack of 10°, 20° and 30° and flapping frequencies of 19 Hz, 24 Hz, 30 Hz, 35 Hz and 40 Hz to investigate the aerodynamic performance of this designed MAV and optimize its flapping frequency. The simulation results defined the frequency at which the MAV is capable of hovering and take-off. In addition, the simulation results showed that the MAV is able to utilize some lift enhancement mechanisms that are being actually used by insects. These results enhances the manufacturing process of future MAV’s, especially in the material selection and manufacturing method, and the transmission mechanism for flight.

Data-driven CFD scaling of bioinspired Mars flight vehicles for hover

One way to improve our model of Mars is through aerial sampling and surveillance, which could provide information to augment the observations made by ground-based exploration and satellite imagery. Flight in the challenging ultra-low-density Martian environment can be achieved with properly scaled bioinspired flapping wing vehicle configurations that utilize the same high lift producing mechanisms that are employed by insects on Earth. Through dynamic scaling of wings and kinematics, we investigate the ability to generate solutions for a broad range of flapping wing flight vehicles with masses ranging from insects O(10−3) kg to the Mars helicopter Ingenuity O(100) kg. A scaling method based on a neural-network trained on 3D Navier-Stokes solutions is proposed to determine approximate wing size and kinematic values that generate bioinspired hover solutions. We demonstrate that a family of solutions exists for designs that range from 1 to 1000 g, which are verified and examined using a 3D Navier-Stokes solver. Our results reveal that unsteady lift enhancement mechanisms, such as delayed stall and rotational lift, are present in the bioinspired solutions for the scaled vehicles hovering in Martian conditions. These hovering vehicles exhibit payloads of up to 1 kg and flight times on the order of 100 min when considering the respective limiting cases of the vehicle mass being comprised entirely of payload or entirely of a battery and neglecting any transmission inefficiencies. This method can help to develop a range of Martian flying vehicle designs with mission viable payloads, range, and endurance.

Lift enhancement strategy and mechanism for a plunging airfoil based on vortex control

A new flow control strategy based on leading-edge vortex (LEV) manipulation is proposed to improve the aerodynamic performance of a plunging airfoil. It has been found that the low pressure region produced by the LEV contributes to the high lift during dynamic stall, while the growth of the secondary vortex would weaken the LEV and result in a decrease in lift. Accordingly, the vortex control hypothesis is that we change the evolution of the secondary vortex and LEV, thus achieving a higher lift coefficient with a longer duration. The suction actuator is placed at different positions on the upper surface of the airfoil to test the control hypothesis. When the suction actuator is near the leading edge, the LEV detaches from the shear layer earlier and it can only enhance the lift slightly while not delay stall time. When the suction actuator is near the middle region, it could inhibit the growth of the secondary vortex and, thus, reduce its strength greatly. Therefore, the LEV circulation could continue to increase. As a result, the suction control could increase the lift coefficient and also prolong the high-lift duration. When the suction actuator is near the trailing edge, an increase in lift could also be achieved by an increase in the negative pressure over the upper surface as well as the LEV circulation. Thus, we present and validate the lift enhancement strategy for an unsteady airfoil based on vortex control.

Role of Dipole Jet in Inclined Stroke Plane Kinematics of Insect Flight

The two-dimensional (2D) inclined stroke plane kinematics of insect wing is studied for various stroke plane angles using the Immersed Boundary (IB) solver. The numerical results revealed the dominant lift enhancement mechanisms for this class of flows. The generated dipole was analyzed to find the maximum velocity, inclination and spread. The analysis of these dipole characteristics for the different stroke plane angles exposed the alternate method to study the vertical force variation with the stroke plane angles. Lift enhancement mechanisms and dipole characteristics complement the high vertical force coefficient for the stroke plane angle of 60˚ commonly used by dragonflies during hover. The location of the dipole identified a region of influence around the wing and demonstrated the role of the dipole jet in multi-body dynamics and wall effects.

Computational investigation of wing-body interaction and its lift enhancement effect in hummingbird forward flight

A lift enhancement mechanism due to wing-body interaction (WBI) was previously proved to be significant in the forward flight of insect flyers with wide-shape bodies, such as cicada. In order to further explore WBI and its lift enhancement effect in a flapping flight platform with different wing and body shapes, numerical investigations of WBI were performed on the forward flight of a hummingbird in this paper. A high-fidelity computational model of a hummingbird in forward flight was modeled with its geometric complexity. The wing kinematics of flapping flight were prescribed using experimental data from previous literature. An immersed-boundary-method-based incompressible Navier‒Stokes solver was used for the 3D flow simulations of the wing-body system. Analyses on aerodynamic performances and vortex dynamics of three models, including the wing-body (WB), wing-only (WO), and body-only (BO) models, were made to examine the effect of WBI. Results have shown significant overall lift enhancement (OLE) due to WBI. The total lift force of the WB model increased by 29% compared with its WO/BO counterparts. Vortex dynamics results showed formations of unique body vortex pairs on the dorsal thorax of hummingbird where low-pressure zones were created to generate more body lift. Significant interactions between body vortex and leading-edge vortex (LEV) were observed, resulting in strengthened LEVs near the wing root and enhanced wing lift generation during downstroke. Parametric studies showed strong OLEs over wide ranges of body angle and advance ratio, respectively. The contribution of OLE from the hummingbird body increased with increasing body angle, and the wing pair’s contribution increased as advance ratio increased. Results from this paper supported that lift enhancement due to WBI is potentially a general mechanism adopted by different kinds of flapping-wing flyers, and demonstrated the potential of WBI in the design of flapping-wing micro aerial vehicle (MAV) that pursue higher performance.

Scaling Bioinspired Mars Flight Vehicles for Hover.

With the resurgent interest in landing humans on Mars, it is critical that our understanding of the Martian environment is complete and accurate. One way to improve our model of the red planet is through aerial surveillance, which provides information that augments the observations made by ground-based exploration and satellite imagery. Although the ultra-low-density Mars environment has previously stymied designs for achieving flight on Mars, bioinspired solutions for flapping wing flight can utilize the same high lift producing mechanisms employed by insects on Earth. Motivated by the current technologies for terrestrial flapping wing aerial vehicles on Earth, we seek solutions for a 5 gram bioinspired flapping wing aerial vehicle for flight on Mars. A zeroth-order method is proposed to determine approximate wing and kinematic values that generate bioinspired hover solutions. We demonstrate that a family of solutions exists for designs that are O(101) g, which are verified using a 3D Navier-Stokes solver. Our results show that unsteady lift enhancement mechanisms, such as delayed stall and rotational lift, are present in the bioinspired solution for a 5 g flapping wing vehicle hovering in Mars conditions, verifying that the zeroth-order method is a useful design tool. As a result, it is possible to design a family of bioinspired flapping wing robots for Mars by augmenting the adverse effects of the ultra-low density with large wings that exploit the advantages of unsteady lift enhancement mechanisms used by insects on Earth.

Effect of fins attached to a rotating cylinder on aerodynamic characteristics

The lift acting on a rotating cylinder placed in the flow is known as Magnus effect. In the previous studies, it is confirmed that the lift force is enhanced by adding fins to a rotating cylinder. However, the effect of fins on the lift force has not yet been fully clarified. In this study, wind tunnel tests were performed by using the three models (cylinder with no fins, cylinder with straight fins and cylinder with spiral fins) to investigate the mechanism of lift enhancement for a rotating cylinder with fins. The lift force increases for the rotating cylinder with spiral fins. To clarify the reasons why the spiral fins show the high lift, the smoke flow visualization and PIV (particle image velocimetry) method were carried out. For the spiral fins, the downwash was observed in the wake of the cylinder. Therefore, the lift force is enhanced for the rotating cylinder with spiral fins.

Numerical Investigation of Unsteady Flows Past Flapping Wings with Immersed Boundary-Lattice Boltzmann Method

AbstractBiomimetic motions are helpful to underwater vehicles and new conception airplanes design. The lattice Boltzmann method with an immersed boundary method technique is used to reveal the propulsion and lift enhancement mechanism of biomimetic motions. The flow past a sphere and an ellipsoidal flapping wing were validated respectively by comparing with other numerical methods. Then a single flapping wing and three flapping wings in a tandem arrangement are accomplished respectively. It founds that the mean thrust coefficient of three plate wings is bigger than the one of the single plate wing. Three ellipsoidal wings and single ellipsoidal wing are compared. It shows that the single ellipsoidal wing has larger thrust coefficients than the three ellipsoidal wings. Ellipsoidal flapping wing and plate wing were further compared to investigate the influence of wing shape. It indicates the mean thrust coefficient of the ellipsoidal wing is bigger than the plate wing.

Vortex dynamics and new lift enhancement mechanism of wing–body interaction in insect forward flight

The effects of wing–body interaction (WBI) on aerodynamic performance and vortex dynamics have been numerically investigated in the forward flight of cicadas. Flapping wing kinematics was reconstructed based on the output of a high-speed camera system. Following the reconstruction of cicada flight, three models, wing–body (WB), body-only (BD) and wings-only (WN), were then developed and evaluated using an immersed-boundary-method-based incompressible Navier–Stokes equations solver. Results have shown that due to WBIs, the WB model had a 18.7 % increase in total lift production compared with the lift generated in both the BD and WN models, and about 65 % of this enhancement was attributed to the body. This resulted from a dramatic improvement of body lift production from 2 % to 11.6 % of the total lift produced by the wing–body system. Further analysis of the associated near-field and far-field vortex structures has shown that this lift enhancement was attributed to the formation of two distinct vortices shed from the thorax and the posterior of the insect, respectively, and their interactions with the flapping wings. Simulations are also used to examine the new lift enhancement mechanism over a range of minimum wing–body distances, reduced frequencies and body inclination angles. This work provides a new physical insight into the understanding of the body-involved lift-enhancement mechanism in insect forward flight.