Initial Angle Of Attack Research Articles

Galloping Stability and Aerodynamic Characteristic of Iced Transmission Line Based on 3-DOF

A new calculation method of critical wind speed based on three degrees of freedom (3-DOF) is proposed for galloping problem of iced transmission line. Based on the quasistatic theory, the aerodynamic load of iced transmission line is obtained, which considers the influence of transverse and torsional motion on the relative wind angle of attack. Finally, the equivalent galloping model of 3-DOF iced transmission line is established. At the initial angle of attack, the aerodynamic load is expanded by Taylor, and the unsymmetrical linear aerodynamic coefficient matrix is obtained. The Routh–Hurwitz criterion is used to judge the stability of iced transmission line system, and then the critical wind speed is calculated. The in-plane and out-plane frequencies corresponding to the first-order mode of the transmission line are solved by the analytical method and numerical simulation method. The results obtained by the two methods are compared and verified. The influence of dimensionless transmission line parameterλon the in-plane and out-of-plane frequencies is discussed. The aerodynamic coefficients of the iced transmission line are measured by wind tunnel test and the aerodynamic characteristics are analyzed. According to the theoretical formula, the critical wind speed is calculated by MATLAB. The critical wind speed determined in this paper is compared with the critical wind speed determined by Den Hartog and Nigol theory. The influences of torsional vibration frequency, ice thickness, and ice shape on critical wind speed are analyzed. The research results of this paper have important theoretical significance for the stability judgment of iced transmission lines.

Critical angle of attack and the corresponding impact cavity for non-circuitous trajectory of water entry of circular cylinder

The critical initial angle of attack that the cylinder can move along the non-circuitous trajectory in water entry is investigated experimentally and numerically. A primary focus is on characterizing the relationship between the initial inclined angle α0 and horizontal velocity u0 in the parameters space of α0∼u0/v0, where u0/v0 is the horizontal velocity u0 to vertical velocity v0 ratio. To build a dense initial parameter space to capture the accurate critical curvature boundary (CCB), which is the critical parameters of the cylinder moving along a non-circuitous trajectory, a substantial water entry experiment is performed for the cylinder with different initial inclined angles and horizontal velocities. In this experiment, a high-speed camera is employed to record the position and the inclined angle of the cylinder. In addition, a large eddy simulation method is used to simulate some particular water entry cases that the cylinder initial parameters are near the CCB space. After the analysis, the initial parameter space for the oblique water entry almost locates in the left circuitous space. Based on the equation of the oblique water entry in the parameter space of α0∼u0/v0, an empirical equation of CCB is established by giving a correction item of deviation angle of attack. When the initial parameters of the cylinder satisfy the equation of CCB, the cylinder first moves along the initial line Lu and then along the offset line Ld. The offset distance doffset between these two lines first increases with α0 and u0/v0 and then gradually tends to a constant about doffset/D = 1, where D is the diameter of the cylinder. Moreover, the simulation results indicate that the pressure distribution on the cylinder surface is affected by the impact velocity, the wettability of the cylinder surface, the initial angle of attack and the distance between the cylinder surface and the pinch-off location.

Study on Galloping Oscillation of Iced Twin Bundle Conductors considering the Effects of Variation of Aerodynamic and Electromagnetic Forces

Due to the occurrence of whole-span and subspan vibration at the same time during the galloping of the iced bundle conductors, the distance between the subconductors changes, resulting in the variation of the aerodynamic parameters for the leeward subconductors. The existing conventional studies usually only consider the whole-span galloping, ignoring the relative motion and the electromagnetic force between the subconductors. Based on considering the above two factors at the same time, a new numerical simulation method to study the galloping behaviors of the iced conductors is presented. Then, the galloping behaviors of the iced twin bundle conductors’ transmission line with different current intensity, initial wind attack angle, and spacer layout are studied. The galloping oscillation behaviors include time histories of vibration displacements, mode, amplitude, frequency, motion traces, and the distance between two subconductors. The simulation results showed that the whole-span and subspan vibration appear at the same time during galloping oscillation and two subconductors may collide with each other when affected by the varying electromagnetic and aerodynamic forces. The effects of varying electromagnetic and aerodynamic forces on galloping behavior cannot be ignored. The new method presented in this work can contribute to the galloping study of the iced bundle conductors.

Assessment of wind-induced nonlinear post-critical performance of bridge decks

A quantitative analysis of the wind-induced post-critical performance of bridge decks is presented in order to highlight the underlying wind-resistant capacity after the critical wind speeds. Two simple indices, i.e., the wind speed extension (after the critical state) with acceptable post-critical vibrations and the relative post-critical capacity (i.e., the ratio between the aforementioned wind speed extension and the critical speed value), are utilized to quantify the post-critical performance of bridge decks. A short review on available limit states of vibration in literatures and design codes is presented in order to define a suitable threshold for acceptable post-critical vibrations. The post-critical performances of three typical cross-sections are quantitatively analyzed with the proposed indices, in which the limit states of vibration are reasonable defined considering either structural normal operation or structural safety. The effects of various mechanical and aerodynamic properties (e.g., mechanical damping ratio, natural frequency, initial angle of attack, and aerodynamic derivatives) on the post-critical behaviors of the selected cross-sections are highlighted. The quantitative analysis of the post-critical performance in the present work may deepen the understanding of the wind-resistant performance of bridges and potentially serve as a building block for a more economic wind-resistant design framework for future long-span bridges.

The effects of leading-edge tubercles on dynamic stall

The effects of leading-edge tubercles, based on the flippers of humpback whales, on the flow field around an airfoil undergoing large-amplitude dynamic changes in the angle of attack have been studied experimentally. Airfoils were pitched from an initial angle of attack of to at constant pitch rates with a chord Reynolds number of 12 000. Velocity and vorticity fields around a standard NACA 0012 airfoil and NACA 0012 modified with leading-edge tubercles were quantified using molecular tagging velocimetry. Vortex dynamics were characterized by tracking the location, core radius and circulation. The resulting velocity fields showed that the dynamics of the formation and separation of the leading-edge vortex were fundamentally different between the straight leading-edge airfoil and the tubercled airfoil. The tubercled airfoil also showed spanwise variation in dynamics of the dynamic stall vortex (DSV) formation. The characteristics of the DSV, specifically the circulation and proximity of the DSV to the airfoil suction surface, are known to have an impact on lift during dynamic pitching. The results showed that the DSV was stronger and remained closer to the airfoil longer for the modified airfoil. The baseline DSV convected away from the airfoil faster than the DSV on the tubercled airfoil once it began to separate from the airfoil. Shear-layer vortices, which form during dynamic stall near the mid-cord region, appeared to affect the convective behaviour of the DSV. The results suggest that the leading-edge tubercles observed on Humpback whale flippers act as passive flow-control mechanisms to control or delay dynamic stall.

Numerical simulation of dynamic multi-body separation flowfields

In this paper, three-dimensional unsteady Navier-Stokes equations coupled with six-degree-of-freedom equations are solved numerically to simulate the process of multi-body separation in the condition of high speed and high altitude. Overset grids are adopted to deal with the relative motion between two different parts. In order to study the security of separating process, the trajectories and attitudes of different parts are calculated and the characteristics of flow field during separation are analyzed carefully. Different initial angles of attack are considered during the simulation and its influence on the separation process is analyzed. The result shows that the initial angle of attack has significant impact on the flow field. Large angle of attack would cause shock/shock interactions between the two parts, threatening the safety of the separating process.

Numerical Study on RC Multilayer Perforation with Application to GA-BP Neural Network Investigation

The finite element model of projectile penetrating multi-layered reinforced concrete target was established via LS-DYNA solver. The penetration model was validated with the test data in terms of residual velocity and deflection angle. Parametric analyses were carried out through the verified penetration model. Seven influential factors for penetration conditions, including the initial velocity of projectile, initial angle of attack of projectile, initial dip angle of projectile, the first layer thickness of concrete target, the residual layer thickness of concrete target, target distance and the layer number of concrete target, were put emphasis on further analysis. Furthermore, the influence of foregoing factors on residual velocity and deflection angle of projectile were numerically obtained and discussed. Based on genetic algorithm, the BP neural network model was trained by 263 sets of data obtained from the parametric analyses, whereby the prediction models of residual velocity and attitude angle of projectile under different penetration conditions were achieved. The error between the prediction data obtained by this model and the reserved 13 sets of test data is found to be negligible.

Influence of thrust fins on hydrodynamic performance of pod propeller

Based on the viscous flow theory, the influence of the arrangement and geometrical changes of the thrust fins on the hydrodynamic performance of the pod propeller is studied by using the sliding grid technology and the commercial fluid calculation software FINE / marine.The results show that the extension of the thrust fin can slightly increase the resistance performance and propulsion efficiency of the pod propeller while significantly reducing the lateral force of the pod.Changing the circumferential position of the thrust fin can change the magnitude and even the direction of the total lateral force of the nacelle without affecting the resistance of the propeller. The thruster is the most efficient when the circumferential angle of the thrust fin is 60 degrees.When the initial angle of attack of the fin is in the range of -4 to 8, the angle of attack is larger, the efficiency of the thruster is higher, the total lateral force is smaller, and the cabin resistance is smaller. However, after the angle of attack is greater than 4 degrees, the cabin resistance is no longer reduced.

Instability and the post-critical behaviour of two-dimensional inverted flags in axial flow

Inverted flags – clamped–free elastic thin plates subjected to a fluid flowing axially and directed from the free end towards the clamped end – have been observed experimentally and computationally to exhibit large-amplitude flapping beyond a critical flow velocity. The motivation for further research on the dynamics of this system is partly due to its presence in some engineering and biological systems, and partly because of the very rich dynamics it displays. In the present paper, the goal is to develop a nonlinear analytical model for the dynamics and stability of high aspect ratio (i.e. height to length ratio) flags. The inviscid fluid flow is modelled via the quasi-steady version of Theodorsen’s unsteady aerodynamic theory, and the Polhamus leading-edge suction analogy is utilized to model flow separation effects from the free end (leading edge) at moderate angles of attack. Gear’s backward differentiation formula and a pseudo-arclength continuation technique are employed to solve the governing equations. Numerical results suggest that fluidelastic instability may be the underlying mechanism for the flapping motion of high aspect ratio heavy inverted flags. In other words, flapping may be viewed as a self-excited vibration. It was found from numerical results that the undeflected static equilibrium of the inverted flag is stable at low flow velocities, prior to the occurrence of a supercritical pitchfork bifurcation. The pitchfork bifurcation is associated with static divergence (buckling) of the flag. At higher flow velocities, past the pitchfork bifurcation, a supercritical Hopf bifurcation materializes, generating a flapping motion around the deflected static equilibrium. At even higher flow velocities, flapping motion becomes symmetric around the undeflected static equilibrium. Interestingly, it was also found that heavy flags may exhibit large-amplitude flapping right after the initial static equilibrium, provided that they are subjected to a sufficiently large disturbance. Moreover, inverted flags with a non-zero initial angle of attack were found to be less stable than their perfectly flow-aligned counterparts.

Prediction method for galloping features of transmission lines based on FEM and machine learning

Parameter study on galloping of iced transmission lines with different structural, ice and wind parameters is carried out by means of the finite element method (FEM), and a dataset is then created based on the numerical results. The random forests algorithm is used to set up a classification model to predict the galloping mode and vertical vibration mode. Using the span length, initial conductor tension, ice thickness, initial wind attack angle, wind velocity and the galloping mode as well as the vertical vibration mode obtained by the classification model as the input variables, a back-propagation (BP) neural network model is created to predict the galloping features including the frequencies, vibration amplitudes and the maximum conductor tension. The dataset created based on the parameter study is divided into two sub-datasets for training and testing of the two machine learning models respectively. The effects of the input variables on the output variables are quantified with a feature importance analysis for the classification model and a global sensitivity analysis for the feature prediction model respectively. The two models are finally integrated into one to predict galloping features efficiently and quickly. Making use of the proposed method, an early warning system for galloping of transmission lines may be able to be set up in the future.

Position detection method and hydrodynamic characteristics of the water entry of a cylinder with multidegree motion

This study experimentally investigates the hydrodynamic characteristics of a cylinder with multidegree motion during water entry with different initial horizontal velocities and inclined angles. A mechanical device is designed to provide certain initial conditions for the cylinder, and the cavity evolution and cylinder position are recorded with a high-speed camera. To obtain precise hydrodynamic forces, a new image-processing method is established to detect the center and attitude angle of the cylinder based on a step edge model and the Grubbs criterion. Additionally, an algorithm is employed to correct the refraction effects in the raw data. Using the above methods, the phenomena of asymmetric cavities, cavity pinch-off, double cavities and cavity ripples are observed in the wake region of the cylinder. Before the cavity pinches off, the hydrodynamic force driving the cylinder motion is generated from the pressure difference between the wet and dry parts. After pinch-off, the hydrodynamic force is mainly dominated by the induced lift, which depends on the angular speed, and a high-pressure region created by the pinch-off cavity causes an instantaneous impulse force that causes quick decreases in the cylinder accelerations. In addition, for the cylinder lift coefficient, the average rate of change increases almost linearly with an increasing initial angle of attack during water entry. For the drag coefficient, the average rate of increase rises nearly linearly with increasing absolute value of the initial angle of attack before pinch-off. After cavity pinch-off, this rate increases almost linearly with increasing |v0|/α0L.

Coupled states of dual side-by-side inverted flags in a uniform flow

The dynamics of dual side-by-side inverted flags with free leading edges and clamped trailing edges in a uniform flow are simulated using the immersed boundary method. The dynamics of the two inverted flags are systematically analyzed in terms of the bending rigidity, the gap distance between flags and the six initial conditions which are set by adjusting the initial angle of attack of flags. Four distinct dynamic regimes are observed: straight, flapping, chaotic and deflected regimes. According to the relative bending directions in which the inverted flags are bent, the deflected regime is further divided into three sub-regimes: outside deflected, inside deflected and one-side sub-regimes. It is found that the initial conditions exert an influence on which sub-regime to occur. Bistable state occurs in transition from the straight regime to the flapping regime, from the chaotic regime to the flapping regime, and from the flapping regime to the deflected regime.

Online speed optimization with feedforward of unmanned sailboat via extremum seeking without steady-state oscillation

Concentrated on the problem of steady-state oscillation of the traditional extremum seeking algorithm (ESA) that it is not beneficial to the speed optimization of unmanned sailboat, a novel speed optimization scheme with feedforward of unmanned sailboat via extremum seeking without steady-state oscillation (ESWSO) is proposed. This scheme consists of feedforward and feedback control: the feedforward part firstly presets the initial sail angle of attack according to an approximate piecewise constant function of the sailing speed polar diagram, its main function is avoid the large deviation of the longitudinal speed from the optimal value. Subsequently, the feedback part fine-tunes the speed to reach the optimal value through the extremum seeking (ES) control. In the process of continuously optimizing longitudinal speed, a functional relationship between the excitation signal amplitude and the extremum estimation error is constructed to eliminate the steady-state oscillation. Besides, the rigorous stability Proof of the scheme is given. Finally, taking a 12-m class unmanned sailboat as the simulation object, the results show that the optimization scheme can effectively eliminate steady-state oscillation and enable the sailing speed to reach the ideal optimal value, as well as has a high control accuracy and strong robustness.

Study on Line Sail in the Process of Double-guided Parachute Deployment

Double-guided deceleration parachute is widely used in large-scale space return activities. Compared with single-guided parachute, it has higher security, but its structure is more complex. This paper aims to study on line sail and analyse the effects of different initial dynamic pressure, separation speeds and angles of attack on the phenomenon during double-guided parachute deployment. The results show that for the double-guided parachute, the bigger the dynamic pressure of the return cabin is, the smaller the separation speed is, the bigger the angle of attack is, the more serious the line sail phenomenon will be.

An experimental study of the dynamic aerodynamic characteristics of a yaw-oscillating wind turbine airfoil

The design of large wind turbines requires a comprehensive and accurate analysis of the dynamic loads of airfoils, so it is of great importance to study the dynamic aerodynamic characteristics of a yaw-oscillating airfoil. In this paper, using “electronic cam” technology and synchronous acquisition of dynamic data, a wind tunnel test of yaw oscillation for the airfoil dynamic “sweep effect” is carried out for the first time, providing previously missing lateral dynamic test data. The results show that the aerodynamic curves of the yaw-oscillating airfoil have an obvious hysteresis effect, induced mainly by a periodic pressure fluctuation on the airfoil suction surface, and the aerodynamic hysteresis characteristics are enhanced with increasing oscillation frequency, initial angle of attack, and amplitude. The hysteresis loops of the lift and pressure drag, as a function of yaw angle, follows a “W” shape, the hysteresis loop of the pitching moment follows an “M” shape, and the hysteresis loop of the unsteady lift increment follows an “∞” shape. The aerodynamic force of the airfoil under negative stroke is higher than that under positive stroke, and the aerodynamic coefficients decrease clearly with increasing oscillation frequency under positive stroke. The pressure fluctuation on the airfoil surface is due to a periodic generation, development, movement, breakdown, dissipation, and reconstruction of shear layer vortices, leading edge vortices, trailing edge vortices, and dynamic separation vortices. The dynamic aerodynamic hysteresis of the yaw-oscillating airfoil occurs essentially because of the dynamic interaction between vortex and vortex, or vortex and airfoil surface boundary layer.

Experimental investigation of flutter characteristics of shallow [formula omitted] section at post-critical regime

The aeroelastic response of a bridge deck at post-critical regime of flutter is featured by limit cycle oscillation (LCO) due to nonlinearities in both fluid and structural-dynamics. In this study, wind tunnel tests are conducted on an elastically-mounted rigid section model with shallow Π cross section, which is free to vibrate in vertical and torsional directions. The aim is to explore the response characteristics of shallow Π cross section at different initial angles of attack (AOA) for wind speeds above the flutter threshold. Firstly, the structural nonlinearities of the elastically-mounted system for section model are carefully characterized in still air to evaluate their effects on aeroelastic responses of the model. Then, post-critical responses of the model subjected to flutter instability are measured at different initial AOA. Experimental phenomena in terms of the aerostatic displacement, critical velocity, steady state amplitude, time history of oscillations, frequency characteristics, phase-angle shift between heaving and torsional motions, and damping evolution are discussed in detail as well as the influence of structural damping on post-critical responses. The results show that the oscillations of Π cross section at post-critical regime of flutter grow slowly with wind speed, instead of building up to large amplitude abruptly. Both the critical flutter velocity and the steady state amplitude of flutter oscillations are sensitive to initial AOA and structural damping. One degree of freedom (DOF) motion approximation is suitable for small amplitude flutter oscillations. For large amplitude oscillation description of coupled 2-DOF motion would be necessary for the bluff Π cross section. The torsional and vertical heaving oscillations tend to be synchronous at high velocity with large amplitude. Compared to slight nonlinear structural damping in the dynamic system, the nonlinear aerodynamic damping related to higher-order terms of displacement is more significant, implying that the aerodynamic nonlinearity is the main source leading to LCO.

Mechanism of suppression of vortex-induced vibrations of a streamlined closed-box girder using additional small-scale components

VIV (vortex-induced vibration) suppression using aerodynamic countermeasures are critical for design of long-span bridges. Strong vertical VIVs were observed in a streamlined closed-box bridge girder subject to wind at an initial attack angle of +3°. CFD simulation, and pressures and displacements measured in wind tunnel tests on a large-scale model (approximately 1:20) were analyzed to reveal the VIV-triggering mechanism for a bridge girder, as well as the VIV-suppression mechanism for bridge girders modified with typical small-scale additional components such as spoilers on crash barriers or guide vanes near maintenance traces. Spoilers can almost eliminate VIVs, while guide vanes can moderately mitigate them. Large-scale vortexes generated from flow separation at leading barriers and leading maintenance trails, referred to as separated vortexes and secondary vortexes, respectively, and called "double vortex mode" together, are responsible for VIVs of an unmodified girder. When spoilers were set on the barriers, the separated vortexes, and then the "double vortex mode" observed on the unmodified bridge girder, could be broken. This greatly reduced the correlation between local aerodynamic forces and general vortex-excited forces (VEFs), and thus the contribution of local aerodynamic forces to general VEFs on the model surface, which caused the VIVs to disappear. As for the unmodified girder, the "double vortex mode" is also responsible for vertical VIVs of a girder modified with guide vanes. This is due to the strong correlation between local aerodynamic forces and general VEFs. However, the phase lags between them have an overall offset of approximately 90° from the approximate synchronous action. Furthermore, a significant decrease in RMS values of pressure coefficients on the model surface dramatically reduce the direct contribution of local aerodynamic forces to general VEFs, especially in the downstream region of the upper surface, thus clearly mitigating VIVs.

Switching Control Architecture with Parametric Optimization for Aircraft Upset Recovery

This paper describes a novel finite-state conditional switching structure that enables autonomous recovery for a large envelope of loss-of-control conditions. The switching control architecture mimics human pilot recovery actions through numerous discrete states that are activated individually as the recovery proceeds, based on guidelines inspired by the Federal Aviation Administration’s suggestions for piloted upset recovery. The switching conditions between discrete states are optimized using interpolation algorithms that extend the recovery envelope while decreasing the average recovery time. The presented architecture is tested on a high-fidelity transport aircraft model under various initial angles of attack, sideslip and roll angles, and roll/pitch/yaw rates. Comparisons with previous methods are conducted to demonstrate that the proposed approach achieves a greater recoverability percentage and lower average recovery time. In addition, the proposed switching architecture is implemented on a high-fidelity F-16 model, and an example recovery scenario is given to demonstrate the implementation of our approach for an agile fighter aircraft model.

Effects of types of bridge decks on competitive relationships between aerostatic and flutter stability for a super long cable-stayed bridge

Aerodynamic configurations of bridge decks have significant effects on the aerostatic torsional divergence and flutter for super long-span bridges, which are onset for selection of suitable bridge decks for those bridges. Based on a cable-stayed bridge with double main spans of 1500 m, considering typical twin-box, stiffening truss and closed-box section, which are the most commonly used form of bridge decks and assumed that the rigidity of those section is completely equivalent, are utilized to investigate the effects of aerodynamic configurations of bridge decks on aerodynamic instability performance comprised of the aerostatic torsional divergence and flutter, by means of wind tunnel tests and numerical calculations, including three-dimensional (3D) multimode flutter analysis and nonlinear aerostatic analysis. Regarding the aerostatic torsional divergence, the results obtained in this study show twin-box section is the best, closed-box section the second-best, and the stiffening truss section the worst. Regarding the flutter, the flutter stability of the twin-box section is far better than that of the stiffening truss and closed-box section. Furthermore, wind-resistance design depends on the torsional divergence for the twin-box and stiffening truss section. However, there are obvious competitive relationships between the aerostatic torsional divergence and flutter for the closed-box section. Flutter occur before aerostatic instability at initial attack angle of +3 and 0, while the aerostatic torsional divergence occur before flutter at initial attack angle of −3. The twin-box section is the best in terms of both aerostatic and flutter stability among those bridge decks. Then mechanisms of aerostatic torsional divergence are revealed by tracking the cable forces synchronous with deformation of the bridge decks in the instability process. It was also found that the onset wind velocities of these bridge decks are very similar at attack angle of −3. This indicates that a stable triangular structure made up of the cable planes, the tower, and the bridge deck greatly improves the aerostatic stability of the structure, while the aerodynamic effects associated with the aerodynamic configurations of the bridge decks have little effects on the aerostatic stability at initial attack angle of −3. In addition, instability patterns of the bridge depend on both the initial attack angles and aerodynamic configurations of the bridge decks. This study is helpful in determining bridge decks for super long-span bridges in future.

Numerical study of saddle-node bifurcation for longitudinal flight with CFD/RBD technique

The square cross section missile was found instability in the longitudinal direction at a certain Mach number in a range of angle of attack both in CFD and wind tunnel test results. To deeply understand the flight behavior and the aerodynamics of the missile, the coupled Computational Fluid Dynamic/Rigid Body Dynamics (CFD/RBD) approach was employed to simulate the free flight process, and the predict technique of dynamic stability derivatives was founded to analyze the types of equilibrium points. The study shows that the type of trim angle of attack is turned from stable node to an unstable saddle with the increase of Mach number, known as the saddle-node bifurcation. There are two extra stable nodes are produced and located on the left and right sides of the saddle point when the Mach number is bigger than the transition Mach number. Further simulation results of free pitching motions reveal the decrease of the attraction region of the two extra stable modes, which makes the pitching motion more complicated and sensitive. A slight change of the initial angle of attack may make the missile move from one trim point to another.