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The flying agility demonstrated by dragonflies is accomplished by means of complex aerodynamic forces produced by flapping their four wings arranged in a tandem configuration. The current study presents a novel tandem flapping wing mechanism for a biomimetic air vehicle that was designed and manufactured to experimentally investigate the aerodynamic forces. By optimizing the configuration and using spatial network analysis, it is shown that the designed structure can flap the wings in a linear up–down stroke motion and is capable of maintaining good consistency and aerodynamic performance. Such a mechanism could be used in a future biomimetic micro air vehicle (BMAV) design. The mechanism uses an electromagnetic actuator to flap the wings with a variable beat frequency (30–210 Hz) at various angles of attack (−10°–20°). The results show that the tandem wings generate approximately 50% higher lift than the forewing or hindwing pairs acting alone. Tandem wings also improve stability, which could potentially allow hovering.

For the design of sailing vessels, the use of Dynamic Velocity Prediction Programs is expanding, as naval architects start to consider the effects of waves and varying wind conditions in order to design faster, safer and more efficient vessels. Many models that predict the unsteady hydrodynamic response are available, but for sail aerodynamics, few models have been presented, and the quasi-steady assumption is instead commonly used. The aim of this paper is to develop a time-domain model for unsteady sail aerodynamics that can handle arbitrary motions and requires only limited input. The proposed model is based on the Indicial Response Method, with specific adaptations to handle the additional complexity of sail aerodynamics. The model's predictive performance is evaluated against URANS CFD results for several cases of increasing complexity. This includes a 3D upwind sail plan subjected to pitching motion, where comparisons are also made with the common quasi-steady (Q-S) assumption. Compared to this, the proposed model delivers significantly better predictions for the amplitude of lift, thrust and sideforce. However, the drag amplitude is over-predicted by the model, and as a result, there is a significant misprediction of thrust phase. While there is a need to improve the prediction of unsteady drag, this paper shows that the model represents a significant improvement over the Q-S assumption, for unsteady performance prediction on timescales shorter than the wave period. • A time-domain model for unsteady upwind sail aerodynamics using the Indicial Response Method. • Specific considerations for sail aerodynamics are presented, along with related adaptations of the method/model. • Model performance is compared against URANS CFD results for several cases of varying complexity. • Limitations of the model applicability are qualitatively indicated. • The model is shown to be a considerable improvement over the quasi-steady approximation. [ABSTRACT FROM AUTHOR]

Given the current context of changes in aeronautics to reduce emissions, it is also necessary to modernise the computation methods to anticipate future cases where disciplines which are now calculated separately (i.e. manoeuvers and gusts) should be computed at the same time including flexible effects and using a time-domain approach. In this work, a static aeroelasticity formulation is adapted to compute wind gust loads. This static method uses aerodynamic matrices to calculate an effective angle of attack (used to recover the local pressure coefficients) from a structural deformation. The approach has been to define this deformation including unsteady effects influence in order to use the same formulation as the static case. Three gust cases (two unsteady and one quasi-steady) have been tested in a rectangular wing, and the proposed method has been compared to the aeroelastic high-fidelity solution and to an uncorrected version of the Doublet Lattice Method (Nastran Solution 146). The proposed solution benefits from the use of the lookup tables to accurately estimate the peak lift coefficient value (maximum error of 6.7%) at least 2.5 times faster than the Doublet Lattice Method. Nevertheless, using a limited model with only two degrees of freedom prevents the proposed method from capturing complex dynamics coming from highly unsteady gust excitation or from aerodynamic instabilities. [ABSTRACT FROM AUTHOR]

The spanwise morphing wing can change its aerodynamic shape to suit its flight environment, thereby having the potential to improve the flight performance of the aircraft, especially in gusty conditions. To investigate the potential of morphing wings, the aerodynamic performance of a spanwise morphing wing with a flapping wingtip in a gust environment was analyzed in this paper. The aerodynamic characteristics of the morphing wing are hard to measure accurately, and thus a wind tunnel test was carried out to study the influences of morphing parameters, such as the morphing length, amplitude and frequency on the gust alleviation effect. The flow mechanism of the designed spanwise morphing wing was analyzed in detail by the instantaneous lift results of the wind tunnel test and the flow field results of the CFD method. The results have shown that with appropriate morphing parameters, the spanwise morphing wing designed in this paper can effectively achieve gust alleviation during flight. The conclusions obtained in this paper can be useful guidance for the design of morphing aircraft.

A combined approach of linear convolution and higher-order Volterra series (VS) to reduced-order modeling of unsteady transonic aerodynamic loads is presented. Our framework offers a simple method to determine the memory depth of the system, clearly separates the linear and the nonlinear (NL) contributions, and significantly reduces the effort required to generate a model for a wide range of reduced frequencies. The generated models use step-response data in the linear and the NL ranges, thus separating the model generation from any specific input signal or a particular reduced frequency. The models were compared against and validated using the computational fluid dynamics results of an oscillating two-dimensional NACA 0012 airfoil. [ABSTRACT FROM AUTHOR]

During the subsequent deploying process after the launch, the vehicle experiences dramatic changes in geometry as well as aerodynamics, which cannot be ignored for the flight stability of the aircraft. The investigation on the flight dynamics modeling and stability analysis during the unsteady deploying process is of great significance for the flight control system design and the launch system design. The nonlinear multibody dynamic model of the vehicle, which mainly consists of the airframe and four wings, is established through the Newton–Euler method. Meanwhile, the unsteady vortex lattice method (VLM), which is suitable for tandem wing aircraft and combined with the viscosity effect in light of the strip theory, is proposed to calculate the aerodynamic loads of the vehicle during the deploying process. A series of simulations about the motion of the vehicle after the launch are calculated to analyze the effects of various parameters, including the deploying forms, the start-up time of the power system along with the launching parameters, on the stability of the aerial vehicle in the deployment process. As results indicated, a small difference in the deploying rates of wings and larger deploying rates of wings on the same side are beneficial to the stability of the vehicle during the deploying process. Additionally, it is advantageous to the pitching stability but not the lateral stability of the vehicle when the power system is started earlier. Besides, the attitude control strategy of the vehicle can be formulated in advance by analyzing the attitude of the vehicle with different launch parameters at the launching moment. [ABSTRACT FROM AUTHOR]

Vertical-axis wind turbines' simpler design and low center of gravity make them ideal for floating wind applications. However, efficient design optimization of floating systems requires fast and accurate models. Low-fidelity vertical-axis turbine aerodynamic models, including double multiple streamtube and actuator cylinder theory, were created during the 1980s. Commercial development of vertical-axis turbines all but ceased in the 1990s until around 2010 when interest resurged for floating applications. Despite the age of these models, the original assumptions (2-D, rigid, steady, straight bladed) have not been revisited in full. When the current low-fidelity formulations are applied to modern turbines in the unsteady domain, aerodynamic load errors nearing 50% are found, consistent with prior literature. However, a set of steady and unsteady modifications that remove the majority of error is identified, limiting it near 5%. This paper shows how to reformulate the steady models to allow for unsteady inputs including turbulence, deforming blades, and variable rotational speed. A new unsteady approximation that increases numerical speed by 5-10× is also presented. Combined, these modifications enable full-turbine unsteady simulations with accuracy comparable to higher-fidelity vortex methods, but over 5000× faster. [ABSTRACT FROM AUTHOR]

In the present work, an analytical study is proposed to investigate the flutter behavior of low-aspect-ratio wings in subsonic flow. An equivalent plate model is used for structural modelling of a semi-monocoque main wing, consisting of ribs, skins, and spars. Legendre polynomials are used in the Rayleigh-Ritz method as trial functions, and the first-order shear deformation theory is utilized to formulate the structural deformation. Boundary conditions are enforced by applying proper artificial springs. A doublet point method is used to calculate the unsteady aerodynamic loads. Chordwise pressure coefficient distribution at the tip and root of a rectangular wing oscillating in pitching motion is calculated. Flutter analysis is performed using the k method. Instead of using the computationally expensive finite element method, the proposed approach is intended to achieve purposes of quick modelling and effective analysis in free vibration and flutter analyses of low-aspect-ratio wings for preliminary design applications. The effects of aspect ratio on the flutter behavior of wings in subsonic flow are investigated. The obtained results are validated with the results available in the literature.

Floating offshore wind turbines (FOWTs) have received great attention for deep water wind energy harvesting. So far, research has been focused on a single floating rotor. However, for final deployment of FOWT farms, interactions of multiple FOWTs and potential impacts of the floating motion on power performance and wake of the rotors need to be investigated. In this study, we employ CFD coupled with an Actuator Disc model to analyze interactions of two tandem FOWTs for the scenario, where the upstream rotor is floating with a prescribed surge motion and the downstream rotor is fixed and influenced by the variations in the incoming flow created by the oscillating motion of the surging rotor. We will investigate three different surge amplitudes and analyze the fluctuations in power performance of the two rotors as well as their wake interactions. The results show a light increase in the mean power coefficient of both rotors for the surging case, compared against the case with no surge motion. The standard deviation of the transient C P of the surging rotor linearly scales with the surge amplitude, while such impact for the downstream rotor is very limited. Surging motion of the upstream rotor is found to enhance flow mixing in the wake, which therefore, accelerates the wake recovery of the downstream rotor. This finding suggests prospects for research into redesigning wind farm layout for FOWTs, aiming for more compact arrangements. [ABSTRACT FROM AUTHOR]

This paper presents two simple output feedback adaptive control laws for the control of a two-dimensional nonlinear aeroelastic system, including unsteady aerodynamics, parameter uncertainties, and wind gust. The objective is to stabilize the plunge-pitch oscillatory motion by the use of a single trailing-edge control surface. Based on the G-passivity theory for nonsquare systems, two adaptive control systems (Cl and (2) are designed. The control law Cl uses the plunge displacement, pitch angle, and their first derivatives for feedback, and the control law 62 is synthesized using only two outputs (pitch angle and pitch rate). For attenuating the effect of gust load and nonlinearities, a switching control signal is designed. In addition, sigma modification is introduced to prevent parameter divergence. By the Lyapunov analysis, the convergence of the trajectories in a region surrounding the origin is established. Simulation results show that in the closed-loop system, each adaptive law suppresses the oscillatory responses, despite uncertainties in parameters and gust load. It is seen that the control law Cl gives little smaller settling time, but requires larger flap deflection compared with the control law 62. [ABSTRACT FROM AUTHOR]

This work presents a novel global reduced-order modeling and parameter estimation of a maneuvering aircraft up to poststall angles of attack using radial basis functions. A computational fluid dynamics approach is adopted to accurately predict the flow field around the maneuvering standard dynamic model. High-amplitude chirp motions are used to excite the aerodynamic system in both longitudinal and lateral-directional axes up to poststall conditions. Subsequently, a radial basis function neural network is employed to construct a nonlinear aerodynamic model from 20% of the numerical simulation data. Next, a continuous wavelet transform is applied to gain insight into the frequency-time behavior of the aerodynamic moments. Based on the results, the network can predict the great unsteady aerodynamic characteristics of the aircraft under deep dynamic stall and coupled yaw-pitch motion over the unseen test data, compared with the entire numerical simulations. Moreover, instantaneous stability derivatives are computed, which are required for design of a maneuvering aircraft flight control system. A great dependency of the stability derivatives on reduced frequency and angle of attack is perceived in the results. In addition, high coupling is seen in lateral-directional derivatives, expressing the strong influence of the angle of attack on the associated moment coefficients. [ABSTRACT FROM AUTHOR]

Technologies inspired by the functioning and behavior of biological beings are commonly developed for aircraft flight. Among the bio-inspired approaches that have grown in interest, particularly for unmanned aerial vehicle flight, is based on the behavior of bird's cover feathers under higher angles of attack. The covert feathers, when activated by separated flows, promote lift increment that helps in certain maneuvers. This work investigates the benefit in the stall and post-stall performance of employing bio-inspired covert feathers devices attached to an airfoil's upper surface. To fill the gaps in the recent technical literature, experimental analysis of an SD7003 airfoil was executed in a wind tunnel with the application of bio-inspired covert feathers of different shapes and tapes in three chordwise positions. The bio-inspired devices were conceived to resemble the feathers' lightness and discrete-distribution along with the wing model. Experiments were carried out measuring the aerodynamic forces and moment at Reynolds number around 170,000 for static and dynamic ramp-up and hold pitching motion. It has been confirmed that the use of bio-inspired covert feathers brought benefits to the stall and post-stall behavior of the airfoil. The maximum lift has increased, and the transition from attached to stalled flow around the airfoil tends to be smoother when the devices were used. Four shapes for the bio-inspired devices and three positions in chordwise direction were considered. The best performance among the case was encountered for a jagged bio-inspired device taped at a quarter-chord position. Indeed, the most forward position for all the devices resulted in higher maximum lift and increment to the respective angle of attack. Ramp-up and hold wind tunnel tests also confirmed the best performance of jagged bio-inspired devices nearer the leading edge. The aerodynamic response to the pitching motion showed that the stall and post-stall regime occur much smoother, indicating that the approach presents good potential for dynamic stall or gust response passive control. [ABSTRACT FROM AUTHOR]

The unsteady lift characteristics of a double-delta wing were studied using both experimental and numerical approaches, which were also compared with a single-delta wing with the same main wing sweep angle. It was found that by increasing the reduced frequency of pitching, the hysteresis effect of lift was magnified. Moreover, in the high reduced frequency case k = 0.48, the difference between the lift coefficients of single- and double-delta wings became rather subtle. The wing surface pressure distribution results indicated the flow phenomenon of dramatic lift losses was due to the effect of lower surface suction during the wing being pitched downstroke. It was observed that, as the reduced frequency became sufficiently high, the virtual camber effect induced by pitching could dominate the flow field, which would mitigate the impact of wing geometry on the lift characteristics. [ABSTRACT FROM AUTHOR]

This paper presents a study on the flow dynamics involving vehicle interactions. In order to do so, this study first explores aerodynamic prediction capabilities of popular turbulence models used in computational fluid dynamics simulations involving tandem objects and thus, ultimately presents a framework for CFD simulations of ground vehicle platooning using a realistic vehicle model, DrivAer. Considering the availability of experimental data, the simulation methodology is first developed using a tandem arrangement of surface-mounted cubes which requires an understanding on the role of turbulence models and the impacts of the associated turbulence model closure coefficients on the prediction veracity. It was observed that the prediction accuracy of the SST k -- w turbulence model can be significantly improved through the use of a combination of modified values for the closure coefficients. Additionally, the initial validation studies reveal the inability of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach to resolve the far wake, and its frailty in simulating tandem body interactions. The Improved Delayed Detached Eddy Simulations (IDDES) approach can resolve the wakes with a reasonable accuracy. The validated simulation methodology is then applied to the fastback DrivAer model at different longitudinal spacing. The results show that, as the longitudinal spacing is reduced, the trailing car's drag is increased while the leading car's drag is decreased which supports prior explanations of vortex impingement as the reason for drag changes. Additionally, unlike the case of platooning involving Ahmed bodies, the trailing model drag does not return to an isolated state value at a two car-length separation. However, the impact of the resolution of the far wake of a detailed DrivAer model, and its implication on the CFD characterization of vehicle interaction aerodynamics need further investigations. [ABSTRACT FROM AUTHOR]

Frequency-domain unsteady lifting-line theory (ULLT) provides a means by which the aerodynamics of oscillating wings may be studied at low computational cost without neglecting the interacting effects of aspect ratio and oscillation frequency. Renewed interest in the method has drawn attention to several uncertainties however. Firstly, to what extent is ULLT practically useful for rectangular wings, despite theoretical limitations? And secondly, to what extent is a complicated wake model needed in the outer solution for good accuracy? This paper aims to answer these questions by presenting a complete ULLT based on the work of Sclavounos, along with a novel ULLT that considers only the streamwise vorticity and a Prandtl-like pseudosteady ULLT. These are compared to Euler CFD for cases of rectangular wings at multiple aspect ratios and oscillation frequencies. The results of this work establish ULLT as a low computational cost model capable of accounting for interacting finite-wing and oscillation frequency effects and identify the aspect ratio and frequency regimes where the three ULLTs are most accurate. This research paves the way towards the construction of time-domain or numerical ULLTs which may be augmented to account for nonlinearities such as flow separation. [ABSTRACT FROM AUTHOR]

Abstract Recently, a novel concept of flapping Micro-Air-Vehicles (FMAVs) with four wings has been proposed, which potentially utilizes the clap-and-fling effect for lift enhancement and agile maneuvers through an adjustment of wing kinematics. However, the application of the clap-and-fling effect in the four-winged FMAVs is underexplored and the dynamic stability is still unclear. In this paper, aerodynamics and flight dynamic stability of the four-winged FMAVs are studied experimentally and numerically. Results show that the clap-and-fling effect is observed when the flapping frequency is above 18 Hz. Due to the clap-and-fling effect, the lift generation and aerodynamic efficiency are both improved, which is mainly attributed to the fling phase. Further studies show that the clap-and-fling effect becomes weaker as the wing root spacing increases and is almost absent at a wing root spacing of 1.73 chord length. In addition, a wing with an aspect ratio of 3 can increase both lift generation and efficiency due to the clap-and-fling effect. Finally, according to the dynamic stability analysis of the four-winged FMAV, the divergence speed of the lateral oscillation mode is about 4 times faster than that of the longitudinal oscillation mode. Our results can provide guidance on the design and control of four-winged FMAVs.

The maglev flight tunnel is a novel conceptual aerodynamics test facility, in which the complicated aerodynamic characteristics caused by the high-speed translation of a moving model in a long, straight, closed tunnel, and wave propagation and aero-structure single-way coupling problems can be investigated. The unsteady characteristics originating from a high-speed model in the maglev flight tunnel were investigated and evaluated with regard to aero–structure coupling. The new conservation element and solution element method was used to solve the 3-D compressible fluid surrounding a moving model in a tunnel, and the variations in the aerodynamic parameters, wave propagation characteristics, and pressure distribution in the tunnel were obtained. The results provide support for key technical problems, such as a wave-absorbing construction design of the maglev flight wind tunnel.

This work presents an aerodynamic and structural optimization for a Droop Nose Leading Edge Morphing airfoil as a high lift device for the UAS-S45. The results were obtained using three optimization algorithms: coupled Particle Swarm Optimization-Pattern Search, Genetic Algorithm, and Black Widow Optimization algorithm. The lift-to-drag ratio was used as the fitness function, and the impact of the choice of optimization algorithm selection on the fitness function was evaluated. The optimization was carried out at various Mach numbers of 0.08, 0.1, and 0.15, respectively, and at the cruise and take-off flight conditions. All these optimization algorithms obtained effectively comparable lift-to-drag ratio results with differences of less than 0.03% and similar airfoil geometries and pressure distributions. In addition, an unsteady analysis of a Variable Morphing Leading Edge airfoil with a dynamic meshing scheme was carried out to study its flow behaviour at different angles of attack and the feasibility of leading-edge downward deflection as a stall control mechanism. The numerical results showed that the variable morphing leading edge reduces the flow separation areas over an airfoil and increases the stall angle of attack. Furthermore, a preliminary investigation was conducted into the design and sensitivity analysis of a morphing leading-edge structure of the UAS-S45 wing integrated with an internal actuation mechanism. The correlation and determination matrices were computed for the composite wing geometry for sensitivity analysis to obtain the parameters with the highest correlation coefficients. The parameters include the composite material qualities, thickness, ply angles, and the ply stacking sequence. These findings can be utilized to design the flexible skin optimization framework, obtain the target droop nose deflections for the morphing leading edge, and design an improved model.

A frequency-domain lifting-line solution algorithm for the prediction of the unsteady aerodynamics of wings is presented. The Biot–Savart law is applied to determine the normalwash generated by the wake vorticity distribution, whereas steady and unsteady airfoil theories (Glauert’s and Theodorsen’s, respectively) are used to evaluate the sectional aerodynamic loads, namely the lift and pitching moment. The wake vorticity released at the trailing edge derives from the bound circulation through the Kutta condition and is convected downstream with the velocity of the undisturbed flow. The local bound circulation is obtained by the application of the Kutta–Joukowski theorem, extended to unsteady flows. Assuming a bending and torsion wing, this paper provides the aerodynamic matrix of the transfer functions, relating the generalised aerodynamic loads to the Lagrangian coordinates of the elastic deformation. Its rational approximation yields a reduced-order state-space aerodynamic model suitable for an aeroelastic stability analysis and control purposes. The numerical investigation examines the influence of both the wake shed/trailed vorticity modelling and different approximations of the Kutta–Joukowski theorem for unsteady flows on the aerodynamic transfer functions given by the developed frequency-domain lifting-line solver. The accuracy of the solver is assessed by comparison with the predictions obtained by a three-dimensional boundary-element-method solver for potential flows. It is shown that, at least for the frequency range considered, regardless of the approximation of the Kutta–Joukowski theorem applied, the formulation based on the Theodorsen theory provides predictions that are in very good agreement with the results from the boundary element method for a slender wing. This agreement worsens as the wing aspect ratio decreases. A lower level of accuracy is obtained by the application of the sectional loads given by the Glauert theory. In this case, the predictions are more sensitive to the approximation used to express the Kutta–Joukowski theorem for unsteady flows.

A new forward-flight model for bird-like ornithopters is presented. The flight dynamics model uses results from potential, unsteady aerodynamics to characterize the forces generated by the flapping wings, including the effects of the dynamic variables on the aerodynamic formulation. Numerical results of the model, which are validated with flapping flight experimental data of an ornithopter prototype, show that state variables such as the pitch angle and the angle of attack oscillate with the flapping frequency, while their mean values converge towards steady-state values. The theoretical analysis of the system shows a clear separation of timescales between flapping oscillations and transient convergence towards the final forward-flight state, which is used to substantially simplify both the interpretation and the solution of the dynamic equations. Particularly, the asymptotic separation into three timescales allows for dividing the problem into a much simpler set of linear equations. The theoretical approximation, which fits the numerical results, provides a direct look into the influence of the design and control parameters using fewer computational resources. Therefore, this model provides a useful tool for the design, navigation and trajectory planning and control of flapping wing UAVs.

A limiting factor in the design of smaller size uncrewed aerial vehicles is their inability to navigate through gust-laden environments. As a result, engineers have turned towards bio-inspired engineering approaches for gust mitigation techniques. In this study, the aerodynamics of a red-tailed hawk's response to variable-magnitude discrete transverse gusts was investigated. The hawk was flown in an indoor flight arena instrumented by multiple high-speed cameras to quantify the 3D motion of the bird as it navigated through the gust. The hawk maintained its flapping motion across the gust in all runs; however, it encountered the gust at different points in the flapping pattern depending on the run and gust magnitude. The hawk responded with a downwards pitching motion of the wing, decreasing the wing pitch angle to between -20 ∘ and -5 ∘ , and remained in this configuration until gust exit. The wing pitch data was then applied to a lower-order aerodynamic model that estimated lift coefficients across the wing. In gusts slower than the forward flight velocity (low gust ratio), the lift coefficient increases at a low-rate, to a maximum of around 2-2.5. In gusts faster than the forward flight velocity (high gust ratio), the lift coefficient initially increased rapidly, before increasing at a low-rate to a value around 4-5. In both regimes, the hawk's observed height change due to gust interaction was similar (and small), despite larger estimated lift coefficients over the high gust regime. This suggests another mitigation factor apart from the wing response is present. One potential factor is the tail pitching response observed here, which prior work has shown serves to mitigate pitch disturbances from gusts., (Creative Commons Attribution license.)

The present research focuses on the timing of wing–wing interaction that benefits the aerodynamic force of a dragonfly in hovering flight at Reynolds number 1350. A 3-D numerical simulation method, called the system coupling, was utilised by implementing a two-way coupling between the transient structural and flow analysis. We further explore the aerodynamic forces produced at different phase angles on the forewing and hindwing during the hovering flight condition of a dragonfly. A pair of dragonfly wings is simulated to obtain the force generated during flapping at a 60° inclination stroke plane angle with respect to the horizontal. The hovering flight is simulated by varying the phase angle and the inter-distance between the two wings. We observe a significant enhancement in the lift (16%) of the hindwing when it flaps in-phase with the forewing and closer to the forewing, maintaining an inter-wing distance of 1.2 cm (where cm is the mean chord length). However, for the same condition, the lift of the hindwing reduces by 9% when the wings are out of phase/counterstroke flapping. These benefits and drawbacks are dependent on the timing of the interactions between the forewing and hindwing. The time of interaction of wake capture, wing–wing interaction, dipole structure and development of root vortex are examined by 2-D vorticity of the flow field and isosurface of the 3-D model dragonfly. From the isosurface, we found that the root vortex elicited at the root of the hindwing in counter-flapping creates an obstacle for the shedding of wake vortices, which results in reduction of vertical lift during the upstroke of flapping. Hence, at the supination stage, a dragonfly uses a high rotation angle for the hovering flight mode. It is observed that the system coupling method was found to be more efficient and exhibited better performance. The present numerical methodology shows a very close match to the previously reported results. [ABSTRACT FROM AUTHOR]

Unsteady pressure fluctuations measured by fast-response pressure transducers mounted in a low-pressure turbine cascade are compared to unsteady simulation results. Three differing simulation approaches are considered, one time-integration method and two harmonic balance methods either resolving or averaging the time-dependent components within the turbulence model. The observations are used to evaluate the capability of the harmonic balance solver to predict the transient pressure fluctuations acting on the investigated stator surface. Wakes of an upstream rotor are generated by moving cylindrical bars at a prescribed rotational speed that refers to a frequency of f 500 Hz. The excitation at the rear part of the suction side is essentially driven by the presence of a separation bubble and is therefore highly dependent on the unsteady behavior of turbulence. In order to increase the stability of the investigated harmonic balance solver, a developed Lanczos-type filter method is applied if the turbulence model is considered in an unsteady fashion. [ABSTRACT FROM AUTHOR]

Unlike fixed-base offshore wind turbine, the soft floating platform introduces 6 more degrees of freedom of motions to the floating offshore wind turbine. This may cause much more complex inflow environment to the wind turbine rotors compared with fixed-base wind turbine. The wind seen locally on the blade changes due to the motions of the floating wind turbine platform which has a direct impact on the aerodynamic condition on the blade such as the angle of attack and the inflow velocity. Such unsteady aerodynamic effects may lead to high fluctuation of the loads and power output. The present work aims to study the high unsteady aerodynamic performance of the floating wind turbine under platform surge motion. The unsteady aerodynamic loads are predicted with a lifting surface method with a free wake model. A preview predict control algorithm is used as the pitch control strategy. A full scale U.S. Department of Energy's National Renewable Energy Laboratory (NREL) 5 MW floating wind turbine is chosen as the subject of the present study. The unsteady aerodynamic performance and instabilities have been discussed in detail under prescribed platform surge motions with different control targets. Both minimizing the power output and rotor thrust fluctuation are set as the control objectives respectively. The theory analysis and the simulation results indicate that the blade pitch control can effectively alleviate the variation of the rotor thrust under platform surge motions. Larger amplitude of the variation of blade pitch is needed to alleviate the variation of the wind turbine power and this leads to high rotor thrust fluctuation. It is also shown that negative damping can be achieved during the blade pitch control process and may lead the floating platform wind turbine system into unstable condition. [ABSTRACT FROM AUTHOR]

The blades of a vertical axis wind turbine (VAWT) experience large variations in the angle of attack at low tip-speed ratios and induce blade force oscillation. These unsteady aerodynamic effects must be considered in the VAWT aerodynamic modelling methodology by utilizing a dynamic stall model. The Beddoes-Leishman (B-L) dynamic stall model is a popular method to simulate the unsteady VAWT blade dynamic stall aerodynamics. However, a limitation of the B-L dynamic stall model is the number of the airfoil dependent parameters derived from both steady and unsteady experimental measurements. In this paper, a methodology is described to compute these B-L dynamic stall model airfoil coefficients utilizing a computational fluid dynamics (CFD) model. This method permits the calculation of the blade dynamic stall characteristics over a range of reduced pitch rates by employing a user-defined sliding mesh motion technique. Furthermore, the variation in the blade Reynolds number is accounted for by conducting simulations at the maximum and minimum VAWT envelope operating limits. Aerodynamic blade force experimental measurements are used to compare the predictions from a low-order model with airfoil data extracted CFD and experiments. This approach expands the applicability of the B-L dynamic stall model for large-scale VAWTs. [ABSTRACT FROM AUTHOR]

The current design of vertical axis wind turbines (VAWTs) suffers from inevitable change in tip speed ratio, λ, in variant wind conditions due to fixed rotor speed. At relatively high wind speeds, which are promising due to high wind power potential, VAWTs operate at low λ with poor power coefficient. Morphing airfoils can be a potential solution by modifying the airfoil shape to optimal at each λ. The optimal airfoil shape for VAWTs at low λ, where dynamic stall is present, has not yet been studied in the literature, therefore, the present study addresses this gap by focusing on this regime to serve as a step towards designing morphing airfoils for VAWTs by identifying the optimal airfoil shape at low λ. The present study performs a combined analysis of three shape defining parameters, namely the airfoil maximum thickness and its position as well as the leading-edge radius, to reveal the overall design space. The analysis is based on 252 high-fidelity transient CFD simulations of 126 identical airfoil shapes. The simulations are verified and validated with three experiments. The results show that the three shape defining parameters have a fully coupled impact on the turbine power and thrust coefficients. When λ reduces from 3.0 to 2.5, the optimal airfoil changes from NACA0018–4.5/2.75 to NACA0024–4.5/3.5, that is increasing the maximum thickness from 18%c to 24%c and shifting its position from 27.5%c to 35%c, while the leading-edge radius index, I, remains 4.5. In general, reducing I from the default value of 6.0 to 4.5 is found to increase the turbine C P. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, a thorough 2D unsteady computational fluid dynamic analysis was performed on a pitching airfoil to properly comprehend the dynamic stall and aerodynamic forces. The computational software ANSYS Fluent was used to solve the unsteady Reynolds-averaged Navier–Stokes equations. Low Reynolds number flows were modeled using the k-ω shear stress transport turbulence model. Aerodynamic forces, fluid flow structures, and flow separation delay angles were explored as a function of the Reynolds number, reduced frequency, oscillation amplitude, and mean angle of attack. The maximum aerodynamic forces, including lift, drag, and the onset of the dynamic stall, were all influenced by these variables. The critical parameters that influenced the optimum aerodynamic forces and ended up causing dynamic stall delay were oscillation amplitude and mean angle of attack. The stall angle was raised by 9° and 6°, respectively, and a large increment in the lift coefficient was also noted in both cases. Additionally, for the highest Reynolds number, a considerable rise in the maximum lift coefficient of 20% and a 28% drop in drag coefficient were observed, with a 1.5° delay in the stall angle. Furthermore, a significant increase of 33% in the lift force was seen with a rise of 4.5° in the stall angle in the case of reduced frequency.

Flying insects perform active flight control with flapping wings by continuously adjusting their wing kinematics in stabilizing the body posture to stay aloft under complex natural environment. While the Proportional Derivative (PD) / Proportional Integral Derivative (PID)-based algorithms have been applied to examine specific single degree of freedom (DoF) and/or 3 DoF flight control associated with insect flights, a full 6 DoF flight control strategy remains yet poorly studied. Here we propose a novel 6 DoF PD controller specified for flight stabilization in flapping flights, in which proportional and derivative gains are optimized to facilitate a fast while precise flight control by combing Laplace transformation and root locus method. The vertical position, yaw, pitch and roll are directly stabilized by tuning the wing kinematics while the forward/backward position and lateral position are indirectly stabilized by controlling the pitch and roll, respectively. Coupled with a recently developed flight dynamic model informed by high-fidelity CFD simulation (Cai et al. 2021), this methodology is proven to be effective as a versatile and efficient tool to achieve fast flight stabilization under both small and large perturbations for bumblebee hovering. The 6 DoF PD flight control strategy proposed may provide a useful bioinspired flight-controller design for flapping-wing micro air vehicles (FWMAVs).

Reduced-order models are widely used in aerospace engineering. A model for unsteady aerodynamics is desirable for designing the blades of wind turbines. Recently, sparse identification of nonlinear dynamics with control was introduced to identify the parameters of an input-output dynamical system. In this paper, two models for attached flows and one for separated flows are identified through this technique. For the unsteady lift of the attached flow, Model I is a linear model that presents the dynamic change of an unsteady lift to a static lift. Model II was built based on Model I in order to obtain a more general system with closed-loop control. It has a first-order inert element that delays the overall input of the static lift. The Model II results replicate the training data very well and give an accurate prediction of other oscillating cases with different oscillation amplitudes, reduced frequency or mean angle of attack. For the unsteady lift of the separated flow, Model III is identified as a nonlinear model, which also has a first-order inert element. This model captures the nonlinear aerodynamics of the separated flow and replicates the training cases well. In addition, the prediction of Model III has good agreement with the numerical results. [ABSTRACT FROM AUTHOR]

The objective of this study is to determine the two-dimensional unsteady aerodynamic forces and moment acting on a heaving wing in a uniform flow using a wind tunnel. However, it is difficult to measure the aerodynamic forces acting on the heaving wing due to measuring device oscillation and the large inertial force of the wing model. In this study, a new type of wind tunnel test, named "heaving wind tunnel," was developed. Here, the wing model remains stationary as the wind tunnel oscillates with a heaving motion. The advantage of this experimental method is that the measurement results are unaffected by the large inertial force acting on the oscillating wing model. Therefore, the wing model can be used in the same way as in steady state experiments. The normal force, thrust and pitching moment coefficients of a heaving airfoil were measured using the heaving wind tunnel test developed in this study. Through flow visualizations and pressure measurements, we found that the rapid drop in normal force coefficient after it reached its maximum value was due to a large growing leading-edge vortex. [ABSTRACT FROM AUTHOR]

In this study, we explore the problem of an airfoil in a harmonic and arbitrary periodic pitching motion and seek to understand how theoretical estimation with first-order potential theory match experiments. We adopt Jones' approximation for the Wagner function for lift computations in the time domain and Theodorsen's model in the frequency domain. We experimentally investigate a two-dimensional symmetric NACA 0018 airfoil in pitching motion under attached flow with reduced frequencies up to 0.25 and Reynolds number varying from 1. 5 × 1 0 5 to 3 × 1 0 5 . To eliminate the influence of the laminar separation bubble, free-stream turbulence intensity was elevated above the baseline level by placing turbulence generating grid, thus, obtaining better compatibility between the flow conditions in the experiments and the theoretical assumptions. Time-resolved sectional lift is determined by simultaneous static pressure measurement with miniature pressure transducers. The transient response is captured with accuracy in the time domain by quantifying the non-circulatory contribution. Excellent agreement is achieved in the time and frequency domains due to the highly accurate measurement of the instantaneous magnitude and phase of the time-resolved surface pressures. We further explore the applicability of the theory for high-pitching amplitudes, which results in a temporary deviation of the measured lift from the theoretical predictions. As the frequency domain unsteady theory is more widely studied than its time domain counterpart, this study provides a unique opportunity to highlight the significance of time-domain analysis in estimating instantaneous lift in non-harmonic motion. [Display omitted] • Explored the application of the Wagner function for lift estimation in real airfoils. • Increased free-stream turbulence level to obtain compatibility with the theory. • Explored experimentally applicability of first-order potential theory beyond stall. • Examined Wagner function applicability in non-harmonic pitching motion. [ABSTRACT FROM AUTHOR]

The effects of unsteady motions of flapping flat plates and corrugated structures in different parameters are studied using fluid–solid coupling and overlapping grid methods. Based on the dragonfly's right forewing and right hindwing model, these actions include sweeping and pitching, take-off acceleration, and tandem wings cruising in the reverse phase at 180 °. The results show that when the advance ratio J = 0. 36 , the "inflow deflection" improves the aerodynamic force in two degrees of freedom compared to simple flapping. When considering only the impact of flexibility, the aerodynamic forces of flexible flat plates and corrugated structures are better than those of the rigid wing models. Considering the effect of corrugated structures, the lift of flexible corrugated wings diminishes, but more thrust is generated. From the perspective of vortex street, vortex rings materialize only in the downstroke stage, while the attachment effect of leading-edge vortices is noticeable in several models. In the same phase flapping, the two wings combine to form a giant wing, which generates significant forward flight momentum. In out-of-phase flapping mode, the series wings generate two lifts and two or three thrust peaks to attain the required forward flight speed while sustaining a high lift. [Display omitted] • Explore the aerodynamics of the flexible corrugate structure dragonfly wing. [ABSTRACT FROM AUTHOR]

An aerodynamic analysis of the Ahmed body in motion is presented in this work. The goal is to investigate the effect of motion on aerodynamic forces with the aim of generating useful information for reducing drag energy consumption and improving stability and comfort. The research is carried out using finite volume computational simulation with the OpenFOAM(R) libraries. Using the well-known benchmark for a fixed Ahmed body geometry, the numerical model is validated with experimental data. A hybrid RANS-LES turbulence model is employed and achieves an excellent level of agreement. The Ahmed body is then simulated under dynamic conditions, using a spectrum of frequencies and amplitudes corresponding to typical heaving motion observed in passenger cars. Furthermore, a quasi-dynamic model is proposed and evaluated to reduce the computational cost of dynamic simulations. The results of the dynamic study differ significantly from those of the static study, demonstrating that vertical velocity, position, and flow inertia have a major impact. The evaluated quasi-dynamic model may not precisely capture quantitative values, but it provides insight into the general trend of aerodynamic forces throughout the oscillation cycle. • Aerodynamics of a moving Ahmed body explored through numerical simulations. • Model validation achieved using experimental force and velocity profile data. • Heaving motion shown to amplify the drag force on the body. • Lift forces exhibit marked sensitivity to the direction of vertical velocity. • Quasi-dynamic approach offers robust qualitative predictions. [ABSTRACT FROM AUTHOR]

A low-cost prediction method that effectively captures the detailed dynamics of unsteady motions is necessary for turbomachinery design. This paper applies two data-driven methods for unsteady cascade flow of a T106 low-pressure turbine with periodic incoming wakes under unknown conditions and unknown instants, respectively. For the prediction under unknown conditions, the dominant coherent structures of the unsteady flows are obtained using sparsity-promoting dynamic mode decomposition. An arbitrary case's sparse promoting modes serve as a basis for prediction and polynomial interpolation is employed to predict the characteristic parameters, including the amplitude and frequency. Based on the predicted characteristic parameters and sparse promoting modes, the flow fields within a range of Reynolds numbers can be reconstructed. The results show that the large-scale flow structure at different Reynolds numbers can be predicted with reasonable error. The detailed flow structure of cascade flow fields also agrees well with simulation results, demonstrating the method's excellent applicability. For the prediction under unknown instants, the dominant coherent structures of the unsteady flows are obtained using proper order decomposition. The extreme gradient boosting method is utilized to construct a surrogate model and predict the time coefficients at unknown times. Based on the dominating modes and predicted time coefficients, the flow fields within a range of unknown instants can be reconstructed. The cascade flow fields can be recovered with a relative error of less than 3 %. The vortex structures and blade surface pressure agree well with simulation results. The present work provides a promising approach for turbomachinery design and investigating underlying mechanisms with low data requirements. [ABSTRACT FROM AUTHOR]

This article investigates the applicability of a recently proposed, nonlinear sparse Bayesian learning (NSBL) algorithm to identify and estimate the complex aerodynamics of limit cycle oscillations. NSBL provides a semi-analytical framework for determining the data-optimal sparse model nested within a (potentially) over-parameterized model. This is particularly relevant to nonlinear dynamical systems where modelling approaches involve the use of physics-based and data-driven components. In such cases, the data-driven components, where analytical descriptions of the physical processes are not readily available, are often prone to overfitting, meaning that the empirical aspects of these models will often involve the calibration of an unnecessarily large number of parameters. While an overparameterized model may fit the observed data well, such models may be inadequate for making predictions in regimes that are different from those wherein the data were recorded. In view of this, it is desirable to not only calibrate the model parameters, but also identify the optimal compromise between data fit and model complexity. In this article, we exhibit the optimal model discovery for an aeroelastic system wherein the structural dynamics are well-known and described by a differential equation model, coupled with a semi-empirical aerodynamic model for laminar separation flutter, resulting in low-amplitude limit cycle oscillations (LCO). To illustrate the performance of the algorithm, in this article, we use synthetic data and demonstrate the ability of the algorithm to correctly rediscover the optimal model and model parameters, given a known data-generating model. The synthetic data are generated from a forward simulation of a known differential equation model with parameters selected so as to mimic the dynamics observed in wind-tunnel experiments. Subsequently, we demonstrate the performance of the algorithm for model selection using noisy LCO data from wind tunnel experiments. As there is no ground truth available for the experimental data case, we provide a comparison between NSBL and Bayesian model selection to validate the results, and demonstrate the use of NSBL as an efficient alternative to traditional methods. • NSBL is shown to be an efficient alternative to standard Bayesian model selection. • NSBL provides data-optimal priors for Bayesian inversion in nonlinear aeroelasticity. • NSBL is validated in experiments using synthetic data of a pitching airfoil. • The performance of NSBL is demonstrated using data from wind tunnel experiments. [ABSTRACT FROM AUTHOR]

In this paper, a practical improved dynamic inverse control method is proposed to solve the large control error and control hysteresis of post stall maneuver under unsteady aerodynamics. Firstly, depending on the wind tunnel experimental data of the advanced fighter aircraft model under biaxial coupled large oscillation, an accurate unsteady aerodynamic model is established by using the improved extreme learning machine (ELM) method. Secondly, in terms of the time scale separation, the control error caused by unsteady aerodynamic is reduced by adding integral in the fast loop, and the control delay caused by unsteady aerodynamic is eliminated by applying the lag correction link in the slower loop. The deflections of conventional aerodynamic surface and thrust vector are allocated by the daisy chain method. Finally, the formula of the reduced frequency, which is the key factor in the unsteady aerodynamic modelling process, is derived by analyzing the wind tunnel data. The effectiveness of the present method for the scaled model is verified by herbst post stall maneuver. The present work provides a practical and reliable way for the flight test of post stall maneuver.

In this paper, a vector form of the unsteady Kutta-Joukowski theorem is derived and then used in the formulation of a general Lifting-Line Model capable of analysing a wide range of engineering problems of interest. The model is applicable to investigating lifting surfaces having low to moderate sweep, dihedral, out-of-plane features such as winglets, in both steady-state and unsteady cases. It features corrections of the span-wise circulation distribution based on available two-dimensional aerofoil experimental data, and stable wake relaxation through fictitious time marching. Potential applications include the conceptual and initial design of low-speed Unmanned Aerial Vehicles, the study of flapping flight or Wind Turbine blade design and analysis. Several verification and validation cases are presented, showing good agreement with experimental data and widely-used computational methods.

In order to investigate the influence of ship airwake on aerodynamic characteristics of the carrier-based aircraft, UAV's landings in different winds over deck were simulated by Overset Mesh method. Firstly, mesh factors, steady and unsteady methods were compared based on single aircraft carrier. The results showed that the boundary layer mesh around ship didn't show obvious influence for our simulation, and the calculation results between the steady and unsteady time average showed a similar trend. Then, aircraft carrier's flow fields in three wind directions were analyzed, and ship airwake variations with different direction winds over deck were concluded as well. Next, the reliability of Overset Mesh was verified though single UAV's landing simulation. Finally, the coupled flow fields of UAV/ship were studied. The calculation results indicated that aircraft was always in a low dynamic pressure condition, the lift and pitching moment of UAV had apparent changes in landing. Meanwhile, the aerodynamic fluctuations of UAV also revealed differences in different wind directions. The simulation results can be regarded as a reference for the safety assessment of carrier-based aircraft's landing and its control in the future.

Fast maneuvers at large angles of attack are necessary for modern fighter aircrafts. The aerodynamic coefficients in this situation are unsteady, with the characteristics of hysteresis and fast time-varying. We propose an identification method based on the linear parameter varying (LPV) model. The model takes into account the hysteresis with respect to the angle of attack, and fast time varying with respect to the motion of post-stall maneuvers. By introducing the angular velocity as an exogenous variable, the model can guarantee the tracking ability of fast time-varying variations. We use the instrumental variable least squares (IVLS) algorithm to deal with the estimation problem. Finally, the experimental data obtained from maneuvers at large angles of attack from wind tunnel tests are used to validate the applicability and effectiveness of the proposed method.