Rotational Lift Research Articles

Analysis of nonlinear aerodynamic performance and passive deformation of a flexible flapping wing in hover flight

The flexibility of insect wings and model wings in flapping-wing micro air vehicles (FWMAVs) is crucial to guarantee an economic flight. Since the wing deformation is bilaterally coupled with the aerodynamic force, conventional simulations on this fluid–structure interaction (FSI) problem mostly build up their models using computational fluid dynamics (CFD) and computational solid dynamics (CSD) methods, which requires high computational costs. Alternatively, an analytical model is proposed here for a quick estimate of the aerodynamic performance and passive deformation on flexible flapping wings. The deformation is simplified into a rigid-body rotation at the wing root, a spanwise twist, and a shortening along the span. The aerodynamic force is approximated by a predictive quasi-steady model and the spanwise twist is modeled using a quadratic polynomial. Results show that the introduction of wing flexibility can attenuate the lift generation due to the negative rotational lift and adverse translation–rotation coupling effect after each stroke reversal. The wing with moderate stiffness is recommended to achieve a high lift. An increase of aspect ratio and root cutoff can also enhance the dimensional lift generation, while the most economic wing design has an aspect ratio around 5 and no root cut-off. More importantly, the increase of flapping frequency can only result in a lift enhancement within certain limits. However, the power consumption keeps increasing as the frequency goes up, indicating a significant breakdown of performance. Our findings can uncover the role of wing flexibility in both insects and FWMAVs and therefore contribute to the design of FWMAVs.

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.

An Experimental Study on Transient Characteristics of a Nuclear Reactor Coolant Pump in Coast-Down Process Under Power Failure Condition

In order to study the transient characteristics of a nuclear reactor coolant pump in coast-down process under power failure condition, a transient test device for the nuclear reactor coolant pump is set up. This paper investigates the effects of the rotor of the nuclear reactor coolant pump and the circuit characteristics on the nuclear reactor coolant pump during the coast-down process by changing the rotary inertia of the flywheel and the resistance of the pipe system. The results show that the greater the moment of inertia is, the more slowly rotational speed, flow and lift get down in the same coast-down period. The change of the pipe resistance has little effect on the rate of decline in the coast-down process, but it has a great influence on descent speed of the flow rate and lift. When the pipe resistance is small, the corresponding flow rate decreases faster, but it is slower for the corresponding lift. The results are of great significance in controlling the stability and reliability of the nuclear reactor coolant pump in the coast-down process.

Wind tunnel investigation of autorotation of plate: The effects of geometry, Reynolds number and rotation direction

This paper presents a wind tunnel investigation of the autorotation of plates. The effects of geometry, the Reynolds number, and rotation direction on the aerodynamics of the plate, including the rotation rate, lift and drag coefficient, are analyzed. The autorotation rate of plate with smaller aspect ratio is more sensitive to the thickness ratio, whereas when the aspect ratio is larger than 1.5, its effect on the sensitivity of the autorotation rate to the thickness ratio can be ignored. The lift coefficient of plate with larger aspect ratio and smaller thickness ratio is larger when aspect ratio is smaller than 2. Otherwise, the lift coefficient increases first and then decreases with an increase in the thickness ratio. Meanwhile, the lift coefficient of plate decreases as Reynolds number increases, and the relationship between the lift coefficient and the Reynolds number is close to a straight line in logarithmic coordinate. Moreover, the clockwise rotation direction leading to upward lift was found to have a higher absolute value of the lift coefficient than the counterclockwise direction leading to downward lift, especially at low rotation rates. However, the drag coefficient, which fluctuates around a constant of 1.3, is independent of such factors.

A chordwise offset of the wing-pitch axis enhances rotational aerodynamic forces on insect wings: a numerical study.

Most flying animals produce aerodynamic forces by flapping their wings back and forth with a complex wingbeat pattern. The fluid dynamics that underlies this motion has been divided into separate aerodynamic mechanisms of which rotational lift, that results from fast wing pitch rotations, is particularly important for flight control and manoeuvrability. This rotational force mechanism has been modelled using Kutta-Joukowski theory, which combines the forward stroke motion of the wing with the fast pitch motion to compute forces. Recent studies, however, suggest that hovering insects can produce rotational forces at stroke reversal, without a forward motion of the wing. We have conducted a broad numerical parametric study over a range of wing morphologies and wing kinematics to show that rotational force production depends on two mechanisms: (i) conventional Kutta-Joukowski-based rotational forces and (ii) a rotational force mechanism that enables insects with an offset of the pitch axis relative to the wing's chordwise symmetry axis to generate rotational forces in the absence of forward wing motion. Because flying animals produce control actions frequently near stroke reversal, this pitch-axis-offset dependent aerodynamic mechanism may be particularly important for understanding control and manoeuvrability in natural flyers.

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.

Quasi-Steady versus Navier–Stokes Solutions of Flapping Wing Aerodynamics

Various tools have been developed to model the aerodynamics of flapping wings. In particular, quasi-steady models, which are considerably faster and easier to solve than the Navier–Stokes equations, are often utilized in the study of flight dynamics of flapping wing flyers. However, the accuracy of the quasi-steady models has not been properly documented. The objective of this study is to assess the accuracy of a quasi-steady model by comparing the resulting aerodynamic forces against three-dimensional (3D) Navier–Stokes solutions. The same wing motion is prescribed at a fruit fly scale. The pitching amplitude, axis, and duration are varied. Comparison of the aerodynamic force coefficients suggests that the quasi-steady model shows significant discrepancies under extreme pitching motions, i.e., the pitching motion is large, quick, and occurs about the leading or trailing edge. The differences are as large as 1.7 in the cycle-averaged lift coefficient. The quasi-steady model performs well when the kinematics are mild, i.e., the pitching motion is small, long, and occurs near the mid-chord with a small difference in the lift coefficient of 0.01. Our analysis suggests that the main source for the error is the inaccuracy of the rotational lift term and the inability to model the wing-wake interaction in the quasi-steady model.

Lift forces on axial symmetry particles rotating in a linear shear flow of a rarefied gas

We theoretically investigate the lift forces (including shear lift and rotational lift) experienced by nonspherical convex particles rotating in a linear shear flow of a rarefied gas, based on the gas kinetic theory. In our model, the interactions between the gas molecules and the particle surface are assumed to be the Maxwell combined specular-diffuse scattering. By integrating the general expressions for several simple axisymmetric bodies, including spheres, cylinders, disks, and spheroids, the specific expressions for the lift forces acting on these bodies are obtained. The pitching effect of the nonspherical particles results in a transverse force for the shear lift, but there is no transverse force for the rotational lift. The expressions for the orientation-averaged shear lift forces are obtained under a uniform distribution of the particle orientation.

THE THREE-DIMENSIONAL FLOW SIMULATION OF A FLAPPING WING

This study investigates three-dimensional (3D) flow fields around a flapping wing with a span of 20 cm. Numerical simulations are performed by varying the simulation mesh density for a half-flapping-wing profile with wind tunnel domain for flapping stroke angles of 53° and 90°. Wind velocities ranging from 1 to 3 m/s, variation in flapping frequency of 14-15 Hz, and corresponding lift characteristics are obtained. Comparative evaluation of numerical and experimental lift force data reveals the translational and rotational lift phenomena of Dickinson’s mechanisms. The stream line that flows over the 3D wing profile is found to be in good agreement with the existing smoke trace experimental data. The motions of a flapping wing obtained through 3D stereo photography and COMSOL Multiphysics simulation are compared and the aerodynamic characteristics are exploited. The streamwise and spanwise 3D velocity fields with intercepting planes at three chord sections are explored, and the reverse Karman vortex sheet is observed. The comparison of the simulation and experimental results for a 3D flapping wing provides insight into the motion characteristics of a flapping wing and flapping wing aerodynamic analysis of micro air vehicles.

Adaptive attitude and position control of an insect-like flapping wing air vehicle

This study describes an adaptive sliding mode technique for attitude and position control of a rigid body insect-like flapping wing model in the presence of uncertainties. For this purpose, a six-degrees-of-freedom nonlinear and time-varying dynamic model of a typical hummingbird is considered for simulation studies. Based on the quasi-steady assumptions, three major aerodynamic loads including delayed stall, rotational lift and added mass are presented and analyzed, respectively. Using the averaging theory, a time-varying system is then transformed into the time-invariant system to design the adaptive controller. The controller is designed so that the closed-loop system will follow any desired trajectory without prior information about uncertainties. In the final stage, in order to find the wing kinematic parameters and to ensure the feasibility of the control commands, two feedforward artificial neural networks are developed and trained using aerodynamic forces and moments from the open-loop simulation data. In comparison with non-adaptive technique, it is shown that the adaptive sliding mode controller is able to stabilize the vehicle in the presence of input disturbances and model uncertainties.

The numerical simulation of the wing kinematics effects on near wake topology and aerodynamic performance in hovering Drosophila flight

The parallel large-scale unstructured finite volume method based on an Arbitrary Lagrangian–Eulerian (ALE) formulation in Erzincanli and Sahin (2013) has been employed in order to investigate the effects of the wing kinematics on the near wake topology produced by a pair of flapping Drosophila wings in hover flight. The three-dimensional wing shape and the baseline wing kinematics are the same as those of the robotic fly by Dickinson et al. (1999). The simulations are used to quantify the important wing kinematic parameters for the wake topology and as well as their correlations with the force production. The angle-of-attack is shown to be very effective for producing lift during the wing translational motion. However, for larger values the angle-of-attack limits the angle during the stroke reversal and reduces the rotational lift. The angle-of-attack at which the maximum lift coefficient occurs is in excellent agreement with the earlier experiments in the literature. In addition, the numerical results confirm that the increase in the wing stroke amplitude leads to a prolonged attachment of the leading edge vortex (LEV) over a relatively large distance and increases force production. The variations in the heave angle include the figure-of-eight pattern, figure-of-O pattern and figure-of-U pattern. The constant heave angle and figure-of-eight pattern are found to have a more profound influence on the magnitude of force production. In addition, some asymmetric variations in the wing kinematics are introduced in order to assess the important parameters determining forward and backward flights. The paddling wing motion is shown to be very effective to initiate forward and backward acceleration.

Effects of aerodynamic modeling on the optimal wing kinematics for hovering MAVs

The effects of aerodynamic model assumptions on the optimal wing-kinematics for hovering micro-air-vehicles are determined. Specific kinematic functions for the wing motion are specified and the parameters of these functions are considered as the design variables for the optimization problem. Four aerodynamic models having different levels of fidelity that capture various physical aspects of hovering aerodynamics are considered to assess the effects of these different aspects on the optimal wing kinematics. These physical aspects include the leading edge vortex, rotational lift, non-circulatory contributions, and flow unsteadiness. Conventional models for pitching wings are not adequate as they predict considerably high rotational lift and too little power requirements, which makes the optimizer, unrealistically, leans toward almost pure rotational motion with little flapping. In addition, quasi-steady modeling overestimates the generated lift and, as such, leads to a more optimal, but unrealistic, performance. Therefore efficient unsteady modeling is essential in design optimization of flapping-wing micro-air-vehicles.

Low-Order Modeling of the Unsteady Aerodynamics in Flapping Wings

Reduced-order modeling of the unsteady aerodynamics generated by three-dimensional flapping wings undergoing three-dimensional motion is investigated. A low-order quasi-steady model based on rotational lift and a revised version incorporating dynamic stall are compared with experimental data generated with matching kinematics. The dynamic-stall-based model in general showed the ability to produce force curves with scale and shape similar to the experimental data, capturing most salient features but not properly resolving some of the peaks and troughs that appear post-half-cycle. The rotational lift-based model in general closely resembled the static force curves at the midcycle and overestimated the salient features around the half-cycle. This varying behavior between models is due to the primary differences in the influencing factors of the rotational lift and dynamic stall terms. These differences are exploited in a combined model that shows greater agreement with the experimental data.

Reynolds number dependency of an insect-based flapping wing

SummaryAerodynamic characteristics depending on Reynolds number (Re) ranges were studied to investigate the suitable design parameters of an insect-based micro air vehicle (MAV). The tests centered on the wing rotation timing and Re ranges, and were conducted to understand the lift augmentations and unsteady effects. A dynamically scaled-up flapping wing controlled by a pair of servos was installed underwater with a micro force/torque sensor. A high-speed camera and a laser sheet were also put in front of the water tank for the time-resolved digital particle image velocimetry (DPIV). The lift augmentations clearly appeared at low Re and were well reflected on the insect’s flight range. In the case of the high Re, however, the peak standing for the wing–wake interaction was delayed, and the pitching-up rotation was not able to lead to another lift enhancement, i.e., rotational lift. In such Re, the mean CL and the L/D of the advanced rotation were substantially decreased from those of the other rotations. The DPIV results at high Re well described turbulent characteristics such as the irregular, unstable, and high-intensity vortex structures with a short temporal delay. In the advanced rotation, the LEV in the rotational phase could not maintain the attachment. Thus, the rotational lift was not able to work. On the contrary, the temporal response delay benefitted the wing in the delayed rotation. Therefore, the wing in the delayed rotation had both a similar level of the mean CL and a higher marked L/D than those of the advanced rotation. Such results indicate that the high Re could interrupt lift augmentation mechanisms, and these augmentations would not be suitable for a heavier MAV. In conclusion, using adequate wing kinematics to acquire estimations of the weight and range of the Re is highly recommended at the aerodynamic design step.

Nonlinear Dynamic Modeling and Simulation of an Insect-Like Flapping Wing

The main objective of this paper is to present the modeling and simulation of open loop dynamics of a rigid body insect-like flapping wing. The most important aerodynamic mechanisms that explain the nature of the flapping flight, including added mass, rotational lift and delayed stall, are modeled. Wing flapping kinematics is described using appropriate reference frames and three degree of freedom for each wing with respect to the insect body. In order to simulate nonlinear differential equations of motion, 6DOF model of the insect-like flapping wing is developed, followed by an evaluation of the simulation results in hover condition.

Spatially and temporally resolved measurements of bead resuspension and saltation in a turbulent water channel flow

AbstractResuspension and saltation of nearly neutrally buoyant polystyrene beads $({d}_{p} = 583\pm 14. 4~\lrm{\ensuremath{\mu}} \mathrm{m} , {\rho }_{p} = 1050~\mathrm{kg} ~{\mathrm{m} }^{\ensuremath{-} 3} )$ in a turbulent boundary layer were studied using time-resolved particle image velocimetry and particle tracking velocimetry in a horizontal water channel facility $({\mathit{Re}}_{h} = 7353)$. The time difference between frames was $ \mrm{\Delta} {t}^{+ } = 0. 297$, comparable to the particle Stokes number, ${ \tau }_{p}^{+ } = 0. 267$. Near-wall coherent structures were visualized using spatial distributions of vorticity and swirling strength in combination with those of the instantaneous ${u}_{1} {u}_{2} $ correlations and ${u}_{1} $. Two case studies, the first on resuspension and the second on saltation, showed that in all cases lift-off coincided with the passage of a vortex core, creating an ejection-sweep cycle (‘burst’) responsible for lift-off. In all cases beads left the wall when immersed in near-wall ejections and exposed to positive shear. As a consequence, a high shear-induced lift force coincided with bead lift-off, while the Magnus force due to bead rotation and translation-induced lift were negligible. The wall-normal component of the drag force mostly opposed lift-off, causing the bead’s deceleration. The difference between resuspension and saltation was governed by the type of coherent flow structures encountered by the beads when lifted out of the viscous sublayer. Resuspension was observed when beads were carried upwards by the combined action of a strong, spatially coherent upstream fast moving $({u}_{1} \gt 0)$ flow structure and a downstream ejection. On the other hand, saltation was accompanied by similar but weaker and spatially less coherent near-wall turbulence structures.

Flapping Flight Stability in Hover: A Comparison of Various Aerodynamic Models

The interest in flapping wing MAVs has been rising progressively in the past years. The complex aerodynamic mechanisms responsible for high manoeuvrability and energy efficient lift production have been modelled by multiple techniques. Here we present a comparison of several models from the perspective of flight stability in hover. We estimated the stability derivatives by aerodynamic models of various levels of complexity, ranging from analytical expressions derived from a quasi steady model to CFD results that were taken as a reference. The stability of the complete 6DOF linearized system, split into longitudinal and lateral part, was evaluated in terms of eigenvalues and eigenvectors. While the pole locations and modes of longitudinal system were consistent for all the models (two stable subsidence modes, one unstable oscillatory mode), the lateral dynamics showed that quasi steady models with only translational force are insufficient and revealed the necessity of including the rotational lift to obtain pole locations that are in accordance with the CFD study (one slow divergence mode, one fast subsidence mode and one stable oscillatory mode).

Model and Auto Take-Off Control Law Design of Unmanned Aerial Vehicle

The Unmanned Aerial Vehicle (UAV) has special dynamical characteristics in ground motion different from that of flight in the airborne. The forces and moments on the ground to the UAV differ during taxiing, according to the different mechanical and aerodynamic characteristics. An all-states nonlinear model is established by studying on a sample UAV. Rotation speed and lift off speed is determined, finally a control law including longitudinal and lateral control for automatic take-off is designed. Results of simulation show that the model and control scheme can not only join the ground taxiing and flight in the airborne smoothly, but also has a high value in realizing engineering project.

Evaluation of Quasi-Steady Theory Applied to Windborne Flat Plates in Uniform Flow

This paper analyzes models based on the quasi-steady theory for the flight of windborne plate debris. It is shown that the effects of the rotational lift, drag, and pitching moment are important and are included in the model. The model was used with success to predict the behavior of thin, square plates in a uniform stream, based on comparisons with existing experimental data. In fact, some of the scatter in the existing experimental data was explained by the numerical results. It was also shown that the buoyancy parameter was the key parameter in determining initial flight speeds and normalization of spatial coordinates with this parameter collapsed the data well.

Numerical Simulation of Particle Saltation Process

A numerical model for simulation of particle single-step saltation was developed. The model includes drag force, shear lift force, rotational lift force, buoyancy force, added mass force, and torque. The governing equations were solved using the fourth-order Runge-Kutta scheme. The model was calibrated and verified using the experimental data. The computational results include particle trajectory, longitudinal velocity of particle and flow, relative velocity of particle and flow, dimensionless drag, and lift forces along the trajectory. Sensivity analysis was performed to determine the influence of various parameters. Saltation characteristics were also calculated for various Reynolds numbers in the range of 2.5 to 7.7. It was found that very close to the bed, drag force decreases as Reynolds number increases. An increase of about three times the Reynolds number has a decreasing effect of three times and two times on the drag and lift force, respectively. The influence of Reynolds number increase on the falling phase was less than that on the rising phase.