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A fast numerical method for unsteady aerodynamic calculation of 3D wing is established, which is suitable for the preliminary design. Based on the lifting-line method, the aerodynamic data of the 2D aerofoil obtained by the unsteady CFD simulation is used as the model input to solve the aerodynamic force of the 3D wing. Compared with the traditional steady lifting-line method, the augmented method adopts the unsteady Kutta-Jouowski (K-J) theorem to calculate the circulation and improve the accuracy of the method through the circulation correction. The pitching motion of 3D wing at different aspect ratio and reduction frequencies are studied. The results show that the aerodynamic forces obtained by the augmented lifting-line method have good agreement with the 3D unsteady CFD calculations. Compared with 3D CFD calculation, the calculation efficiency of the improved method is increased by more than 12 times. The improved method has extensive applicability and can be used to estimate the unsteady aerodynamic forces of 3D single or multiple wing configurations. [ABSTRACT FROM AUTHOR]

The unsteady vortex evolution of NACA 0012 airfoil is numerically investigated at low angles of attack ranging from 0° to 10° where the separation is performed from the trailing edge. The Reynolds number ranges between 1000 and 4000. The laminar separation bubble at the trailing edge is observed and the main flow features are presented. It is found that, the increase of the angle of attack and Reynolds number result higher lift to drag ratio by an extensive decrease of the drag coefficient below 8° angle of attack. The transition from the steady condition to periodic force evolution has been revealed with a detailed flow field analysis at different angles of attack and Reynolds numbers. The critical angle is defined as the angle of attack where the onset of oscillations starts with a dominant fundamental frequency of oscillation. The angle of attack where the laminar separation bubble (LSB) is observed is also revealed in the current study. Both the LSB formation angle and the critical angle of attack is found to decrease with the increase of the Reynolds number from 1000 to 4000. [ABSTRACT FROM AUTHOR]

A range of quasi-steady and unsteady aerodynamic models are used to predict the aerodynamic forces experienced by a flapping wing and a detailed comparison amongst these predictions in provided. The complexity of the models ranges from the analytical potential flow model to the computational Unsteady Vortex Lattice Method (UVLM), which allows one to describe the motion of the wake and account for its influence on the fluid loads. The novelty of this effort lies in a modification of the predicted forces as a generalisation of the leading edge suction analogy. This modification is introduced to account for the delayed stall mechanism due to leading edge flow separation. The model predictions are compared with two sets of independent experimental data and with computational fluid dynamics (CFD) simulation data available in the literature. It is found that both, the modified analytical model and the UVLM model can be used to describe the time history of the lift force, in some cases with better results than a high-fidelity CFD model. The models presented here constitute a useful basis for the aerodynamic design of bioinspired flapping-wings micro-air vehicles. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A two-dimensional model is built to describe the translation and the rotation of the hovering flapping movement. The equations of motion are derived for insect's flapping movement, and the model is implemented by the computational fluid dynamics (CFD) software FLUENT and it's user defined function (UDF). It is shown that the lift coefficient changes slowly in the intermediate stage, there are two areas in which the lift coefficient changes dramatically, and the drag coefficient behaves quite differently when flapping up and down. The vortex distribution, the pressure distribution, and the velocity vector distribution in the advanced mode at different times follow quite various rules. [ABSTRACT FROM AUTHOR]

This paper reports the results of a wake measurement campaign carried out at Politecnico di Milano wind tunnel to support current studies of the authors about unsteady aerodynamics of floating offshore wind turbines (FOWT) as well as evaluating experimental evidence for wind farm applications of FOWT. The wind turbine scale model adopted for these tests is the 1/75 DTU 10 MW wind turbine developed for the LIFES50+ project. In this study, imposed surge motions, at different frequencies and amplitudes, were provided at the base of the turbine’s tower, to investigate the influence of motion on the wake. Results reported show good agreement with the state-of-the-art of wind turbines wake aerodynamics in steady conditions; concerning the unsteady conditions (imposed motion) interesting data are reported and collected with respect to a ”wake reduced frequency” parameter which is, according to the authors, a straightforward value to estimate the ”level” of unsteadiness connected to a specific dynamic condition due to floating motion. Interesting and consistent phenomena are observed and commented, although opening to the need of further experimental and numerical investigations. [ABSTRACT FROM AUTHOR]

This article presents a robust finite-time maneuver control scheme for the longitudinal attitude dynamic of the aircraft with unsteady aerodynamic disturbances and input saturation. To efficiently eliminate the influence of unsteady aerodynamic disturbances, nonlinear finite-time observers are developed. Despite the existence of the nonlinearity and the coupling between aircraft states and unsteady aerodynamic disturbances, the proposed observers can still precisely estimate the unmeasurable unsteady aerodynamic disturbances in finite time. To attenuate the effect caused by input saturation, a finite-time auxiliary system is constructed. With the error between the desired control input and saturation input as the input of the auxiliary system, the additional signals are generated to compensate for the effect of input saturation. Combined with the finite-time observers and the finite-time auxiliary system, a robust finite-time backstepping attitude control design is developed. The finite-time convergence of all closed-loop system signals is rigorously proved via Lyapunov analysis method under the developed robust attitude control schemes. Finally, simulation results are presented to illustrate the effectiveness of the proposed attitude control approaches. [ABSTRACT FROM AUTHOR]

Flapping flight mechanisms offers a power-efficient and highly manoeuvrable basis for the development of micro air vehicles. The aerodynamic knowledge and prediction tools of the flapping wing are quite important for the design of micro air vehicles. In this paper, the unsteady aerodynamics of a flapping wing is investigated by numerical simulations and the generation of reduced-order model based on the Volterra theory for predicting the unsteady aerodynamic of a flapping wing is described. The three dimensional aerodynamic calculation is performed by solving the unsteady Reynolds-averaged Navier-Stokes equations and the user-defined functions were employed to simulate the flapping motion. The training maneuver is a flapping motion with a linearly increasing flapping frequency. Based on the Volterra theory and numerical simulation results, a reduced-order model is generated to predict the unsteady aerodynamics of a flapping wing. The system identification method is employed to generate the Volterra theory reduced-order model. The results show that the Volterra theory reduced-order model agrees well with the training data and the Volterra theory reduced-order model works well to predict the unsteady aerodynamics of a flapping wing. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A Numerical method to predict the dynamic stability derivatives of complex configuration using Computational Fluid Dynamics is presented. Based on the numerical solution of forced sinusoidal oscillating motion around body axis or translational motion, both static and dynamic stability derivatives are concurrently extracted from the history of the unsteady aerodynamic force and moment coefficients. The method is validated for 3-D reference missile and aircraft dynamic test configurations. Comparisons with the wind tunnel data available in the literature are also included. Excellent comparisons with experiments showed that the present method based on unsteady simulation of RANS can provide an alternative to predict the dynamic stability derivatives for the complex configurations. [ABSTRACT FROM AUTHOR]

The growing applications of wind turbine impose the need for accurate flow solutions of airfoils with different pitching motions. In this paper an adjustable parameter K is employed to realize various nonsinusoidal motions and the effects of nonsinusoidal motion on pitching airfoil aerodynamics of a 2-D NACA0012 airfoil are numerically studied at Re = 1.35 × 10 5 . The nonsinusoidal motion effects on the instantaneous force coefficients and flow patterns are studied at different unsteady parameters (amplitude of oscillation, θ ; reduced frequency, k ). The results reveal that the nonsinusoidal motion noticeably affects the instantaneous force coefficients, maximum lift and drag coefficients. Besides, the formation of leading edge vortices and flow structures are also affected by the nonsinusoidal motion. [ABSTRACT FROM AUTHOR]

The study of a vehicle moving through a lateral wind gust has always been a difficult task due to the difficulties in granting the right similitude. The facility proposed by Ryan and Dominy has been one of the best options to carry it out. In this approach, a double wind tunnel is used to send a lateral moving gust on a stationary model. Using this idea as a starting point, the ISAE has built a dedicated test bench for lateral wind studies on transient conditions. Experimental work has been carried out by means of time-resolved PIV, aiming at studying the unsteady interpenetration of the two flows coming from each wind tunnel. Meanwhile, a 3D CFD model based on URANS was set up, faithfully reproducing the double wind tunnel. Both the experimental and numerical results are compared, and the evolution of the reproduced wind gust is discussed. Conclusions are finally determined about the validity of this kind of test bench for ground vehicle applications. [ABSTRACT FROM AUTHOR]

Abstract: A generic approach is carried out to model the kinematics and aerodynamics of ornithopter to mimic flapping wing to produce lift and thrust for hovering and forward flight, by considering the motion of a three-dimensional rigid thin wing in flapping and pitching motion with phase lag. Basic Unsteady Aerodynamic Approach incorporating salient features of viscous effect and leading-edge suction will be utilized. Parametric study is carried out to reveal the aerodynamic characteristics of ornithopter flight characteristics and for the analysis of various selected simple models in the literature, in an effort to develop a flying ortnithopter model. Further considerations will be given to other important parameters such as flapping frequency and wing geometry for well conceived formulations with realistic and workable assumptions and limitations. [Copyright &y& Elsevier]

This work develops an understanding of the flow mechanisms that induce vibrations on automotive side view mirrors. The unsteady nature of the flow over side view mirrors causes unsteady aerodynamic load distributions and flow-induced vibrations on the mirror assembly. These vibrations generate blurred rear-view images and higher noise levels, affecting the mirror functionality and passenger comfort. Certain geometrical design features of side view mirrors can exacerbate the flow-induced vibration levels of the mirror assembly significantly. This work quantifies the impact of these design features on the vibration amplitude, develops a methodology for testing mirror vibrations in a small, low-speed wind tunnel using only the mirror of interest, and delves into the interactions between the bluff body mirror geometry and its wake. Two similar side view mirror designs were investigated in this work by using laser-based vibrometry, flow visualization, particle image velocimetry, hot film anemometry, and surface stress sensitive film techniques. The magnitude of the vibrations was found to depend on the level of excursion in the dynamic location of flow separation, particularly when characteristic flow frequencies couple with the mirror housing natural frequency. [ABSTRACT FROM AUTHOR]

An energy-based variational approach is used for structural dynamic modeling of the IPMC (Ionic Polymer Metal Composites) flapping wing. Dynamic characteristics of the wing are analyzed using numerical simulations. Starting with the initial design, critical parameters which have influence on the performance of the wing are identified through parametric studies. An optimization study is performed to obtain improved flapping actuation of the IPMC wing. It is shown that the optimization algorithm leads to a flapping wing with dimensions similar to the dragonfly Aeshna Multicolor wing. An unsteady aerodynamic model based on modified strip theory is used to obtain the aerodynamic forces. It is found that the IPMC wing generates sufficient lift to support its own weight and carry a small payload. It is therefore a potential candidate for flapping wing of micro air vehicles. [ABSTRACT FROM AUTHOR]

The current paper describes the development of pressure-sensitive paint (PSP) technology as an advanced measurement technique for unsteady flow fields and short-duration wind tunnels. Newly developed paint formulations have step response times approaching 1 µs, making them suitable for a wide range of unsteady testing. Developments in binder technology are discussed, which have resulted in new binder formulations such as anodized aluminium, thin-layer chromatography plate, polymer/ceramic, and poly(TMSP) PSP. The current paper also details modeling work done to describe the gas diffusion properties within the paint binder and understand the limitations of the paint response characteristics. Various dynamic calibration techniques for PSP are discussed, along with summaries of typical response times. A review of unsteady and high-speed PSP applications is presented, including experiments with shock tubes, hypersonic tunnels, unsteady delta wing aerodynamics, fluidic oscillator flows, Hartmann tube oscillations, acoustics, and turbomachinery. Flowfields with fundamental frequencies as high as 21 kHz have been successfully measured with porous PSP formulations. [ABSTRACT FROM AUTHOR]

Numerical simulation of three-dimensional dynamic stall (DS) of helicopter rotor blades has been undertaken using computational fluid dynamics (CFD) based on the Navier-Stokes equations. The CFD method has been carefully validated against experimental data before being used to study the topology of the three-dimensional DS vortex and the effects of yaw and rotation on its shape and trajectory. The three-dimensional unsteady viscous computations were found to give a wealth of results at the expense of significant amounts of CPU time. To alleviate this problem and to develop a faster model for the aerodynamic loads encountered during three-dimensional DS, a neural network (NN) was put forward. The NN was trained using both CFD and experimental data and was subsequently used as a method for interpolating DS loads between known states for which CFD data were available. The NN was found to work well for such interpolations, although its capability to extrapolate outside the training envelope was somehow limited. Nevertheless, the NN was found to be a very efficient technique for reconstructing three-dimensional DS and has demonstrated great potential as a method for reducing non-linear aerodynamics to a simple computational model. [ABSTRACT FROM AUTHOR]

Traditionally, computational predictions and experimental evaluations of aerodynamic concepts have been conducted separately, with little collaboration other than post priori comparisons of results. This has led to distrust and even antagonism between the computational and the experimental communities. These difficulties probably began when early computational fluid dynamic practitioners boasted that wind tunnels would become secondary in aerodynamic concept development within a few short years, a prediction that has not come true. On the contrary, it is believed that a great deal of synergy can be cultivated when computational and experimental evaluations are conducted in an integrative fashion. A variety of projects where this has been done will be reviewed, including a pitching Unmanned Combat Air Vehicle, a delta wing with periodic suction and blowing for aerodynamic control, a missile with drag brakes that caused excessive unsteady flow, a C-130 aircraft configured for airdrop, and closed-loop flow control. Further evolution of the numerical/experimental collaboration will be discussed showing results from the flow control research where the dividing line between numerical predictions and experimental evaluations is becoming blurred. Suggestions for future directions in collaboration will also be made. [ABSTRACT FROM AUTHOR]

Aeroservoelastic simulation of realistic configurations requires the representation of moving control surfaces. Once a control surface is deflected, for example, at the trailing edge of a wing, the surface is no longer continuous and this is considered here. A sliding grid approach is used to allow control surface deflection within a multiblock-structured grid framework. Two methods of information transfer between patched (non-matching) grids are developed and tested in this paper: the first involving halo interpolation and the second, conservative flux interpolation. Results are presented for a range of test cases involving steady and unsteady flows using non-matching and sliding interfaces at block boundaries. It is shown that both methods can be applied to the example of a wing with a deflected control surface, where no advantage is gained by using a significantly more complex, conservative method. [ABSTRACT FROM AUTHOR]

Micro air vehicles (MAVs) are defined as flying vehicles ca. 150 mm in size (hand-held) weighing 50–100 g, and are developed to reconnoitre in confined spaces (inside buildings, tunnels, etc.). For this application insect-like flapping wings are an attractive solution and hence the need to realize the functionality of insect flight by engineering means. Insects fly by oscillating (plunging) and rotating (pitching) their wings through large angles, while sweeping them forwards and backwards. During this motion the wing tip approximately traces a figure-of-eight and the wing changes the angle of attack (pitching) significantly. This paper presents some aspects of aerodynamic modelling and mechanical design pertinent to engineering realization of insect-like flapping wings on the MAV scale. The aerodynamic modelling uses an indicial Polhamus approach which is implemented in a modular manner. Comparisons are presented with experimental force data from a dynamically scaled insect model and generally show good agreement. The design is then described of a four-bar linkage mechanism to drive an insect-like flapping-wing motion. The indicial Polhamus aerodynamic model was used as part of the design process to predict wing loading. This highlighted some interesting interactions between the position of the wing pitch axis and its loading. The paper concludes with some directions for future work in this challenging field. [ABSTRACT FROM AUTHOR]

The transient response of a vehicle to a wind disturbance is of importance to car drivers since low level inputs can result in poor vehicle refinement, and extreme effects can result in path deviation. This paper investigates the use of an oscillating aerofoil gust generator to simulate the transient aerodynamic effects produced on a car-type bluff body during a simplified sinusoidal side gust interaction. A simplified bluff body was exposed to a range of sinusoidal cross-wind excitations corresponding to a reduced frequency of between 0.09 and 0.71 based on the model length. Unsteady measurements of surface pressure are processed to determine the side force and yaw moment and the aerodynamic magnification (χ[sub a]) is calculated by comparing the transient response with a quasi-steady prediction. The transient yaw moment response is shown to exceed the quasi-steady by as much as 30 per cent. The transient side force is generally significantly less than the quasi-steady value except at the lowest frequency tested. The change in response is attributed to changes in the strength of the front and rear pillar vortices and to changes in phase relative to the quasi-steady response. [ABSTRACT FROM AUTHOR]

A computational analysis is performed of the unsteady aerodynamics associated with the blade sections of helicopter rotors in forward flight. The unsteady flow is studied through solutions of the two-dimensional Reynolds averaged Navier-Stokes equations together with a strongly coupled two-equation model of turbulence. Two motions are studied. The first motion is that of an aerofoil subjected to harmonic in-plane oscillations. The influence of advance ratio and reduced frequency is investigated. It is shown that, in the absence of shock waves, the flow is periodic with a reduced frequency equal to that of the forcing motion. However, the flow development lags behind the forcing motion. Furthermore, for constant reduced frequency the phase lag is independent of advance ratio. In addition to harmonic motion, the aerodynamic response to a step change in Mach number is investigated. Using an assumed form of the response of lift coefficient to a step change in Mach number, a lift transfer operator for step changes in Mach number is obtained in the Laplace domain. An analytical expression for the response to harmonic Mach number oscillations is then obtained from the transfer operator. The resulting formulation for the aerodynamic response confirms that the lag between the forcing motion and the aerodynamic response is independent of advance ratio. [ABSTRACT FROM AUTHOR]

• The Hybrid RANS-LES method for turbulence closure coupled with the velocity fluctuation algorithm (Vortex method) was used in this paper. • The aerodynamic efficiency of the wing model (B737) was affected by the increase in the number of vortices (N) formed. • More than 40% reduction of the wing's aerodynamic performance was observed for N = 300. • A further 10% decrease in the aerodynamic efficiency was attained by varying the density of the air from 1.255 kg/m3 (15 °C) to 1.455 kg/m3 (−10 °C). Steady airflow over the wing of an aircraft in-flight is critical to achieving maximum aerodynamic performance. However, commercial flight routes are most times characterized by fluctuations in the airflow properties. The unsteadiness in the air velocity vectors and mass flow directly affects the aerodynamic efficiency (AE) of the aircraft during flight and could lead to air accidents. These correlations between the variation in the airflow properties and the aerodynamic coefficients of a fixed-wing aircraft are not yet fully established. Therefore, this paper makes use of computational fluid dynamics code to study the link between these functions. Herein, a realistic wing model of the BOEING 737 aircraft was used for the investigation. Simulations were carried out at a Mach number of 0.84, with the nonzonal Hybrid RANS-LES method. The fluctuations in the airflow properties were modeled using the vortex fluctuation algorithm (vortex method) in Fluent software. The vortex number , N formed in the flow field were varied in the range of 100–300. Findings from the numerical study revealed that the wing achieved an optimal AE of 95.1% for the steady case scenario. However, the wing's AE was significantly reduced by 30% when the streamlined velocity was perturbed by the fluctuating velocity component of the flow. Also, a further decrease in the wing performance was observed with an increase in the divergence of the airflow velocity vectors which experienced a stalling effect after an 80 s flow period for N = 300. Moreover, a static increase in the density of the airflow from 1.255 kg/m3 (15 °C) to 1.455 kg/m3 (−10 °C) contributed to approximately a 20% reduction in the lift and moment coefficients for N = 300. [ABSTRACT FROM AUTHOR]