Unsteady Aerodynamic Characteristics Research Articles

Unsteady Aerodynamic Characteristics depending on Reduced Frequency for a Pitching NACA0012 Airfoil at Rec=2.3×10⁴

Most of small air vehicles with moving wing fly at low Reynolds number condition and the reduced frequency of the moving wing ranges from 0.0 to 1.0. The physical phenomena over the wing dramatically vary with the reduced frequency. This study examines experimentally the effect of the reduced frequency at low Reynolds number. The NACA0012 airfoil performs sinusoidal pitching motion with respect to the quarter chord with the four reduced frequencies of 0.1, 0.2, 0.4 and 0.76 at the Reynolds number 2.3×10⁴. Smoke-wire flow visualization, unsteady surface pressure measurement, and unsteady force calculation are conducted. At the reduced frequency of 0.1 and 0.2, various boundary layer events such as reverse flow, discrete vortices, separation and reattachment change the amplitude and the rotation direction of the unsteady force hysteresis. However, the boundary layer events abruptly disappear at the reduced frequency of 0.4 and 0.76. Especially at the reduced frequency of 0.76, the local variation of the unsteady force with respect to the angle of attack completely vanishes. These results lead us to the conclusion that the unsteady aerodynamic characteristics of the reduced frequency of 0.2 and 0.4 are clearly distinguishable and the unsteady aerodynamic characteristics below the reduced frequency of 0.2 are governed by the boundary layer events.

Robust Backstepping Control of Wing Rock Using Disturbance Observer

Wing rock is a highly nonlinear phenomenon when the aircraft suffers undesired roll-dominated oscillatory at high angle of attack (AOA). Considering the strong nonlinear and unsteady aerodynamic characteristics, an uncertain multi-input and multi-output (MIMO) nonlinear wing rock model is studied, and system uncertainties, unsteady aerodynamic disturbances and externaldisturbancesareconsideredinthedesignofwingrockcontrollaw. Tohandletheproblemof multipledisturbances,arobustcontrolschemeisproposedbasedontheextendedstateobserver(ESO) and the radial basis function neural network (RBFNN) technique. Considering that the effectiveness of actuators are greatly decreased at high AOA, the input saturation problem is also handled by constructing a corresponding auxiliary system. Based on the improved ESO and the auxiliary system, a robust backstepping control law is proposed for the wing rock control. In addition, the dynamic surface control (DSC) technique is introduced to avoid the tedious computations of time derivatives for the virtual control laws in the backstepping method. The stability of the closed-loop system is guaranteed via rigorously Lyapunov analysis. Finally, simulation results are presented to illustrate the effectiveness of the ESO and the proposed wing rock control approach.

Aerodynamic characteristics of a novel catapult launched morphing tandem-wing unmanned aerial vehicle

A novel morphing unmanned aerial vehicle with tandem-wing configuration could fold into a tubular catapult and deploy the four airfoils after launching. Because of the rapid deploying process, the aerodynamic characteristics will become largely different. Numerically investigating the aerodynamic characteristics at low Reynolds numbers (Re < 106) is of great significance for control-stability analysis, control laws design, and overall design. By comparing the numerical simulation results of the different airfoils arrangements, the authors have found that model A is more appropriate for folding configuration and model B will generate more obvious unbalanced downwash flow influence. The lift, drag, and hinge moment of the canard and wing vary significantly during the deploying process. This phenomenon is due to the aerodynamic interference and mutual coupling between the canard and wing. Moreover, the rapider deformation rate will cause higher variable quantity of the unsteady aerodynamic characteristics. The additional movement plays a dominant role in the variable quantity compared with the hysteresis effect. The authors have also tested the catapult launching under folded condition and unfolded condition. And the experimental results coincide better with the simulation results.

Research on Unsteady Aerodynamic Characteristics of Pantographs in Different Positions of High-speed Trains

Research on Unsteady Aerodynamic Characteristics of Pantographs in Different Positions of High-speed Trains

Developing trend of design and manufacture technology for large-scale wind turbine blade

Since twenty-first century, the global wind power industry has been developing rapidly. The trend of the blade growing large-scale is obvious. In order to meet the developing requirements of large-scale blade, new airfoils, materials, design methods and manufacturing processes are shown up. Advantages and disadvantages of current technologies are analyzed in this paper. Combined with the new problems and requirements brought from the blade getting longer, it is found that the large-scale blade has the following developing trends in the aerodynamic design, structure design and manufacture technology. (1) In the aerodynamic design, developing high performance airfoils under high Reynolds number, researching high precision and high efficient aerodynamic analysis methods especially for solving the unsteady aerodynamic characteristics of large-scale blade, and constructing multidisciplinary coordination design method will be the key points of aerodynamic design. (2) In the structure design, developing environment-friendly materials with superior performance, studying on the structure optimization design technology of large-scale blade, and building the inverse problem design method will be the mainstreams of structure design. In addition, new types of blade structure for solving different problems have constantly arisen. Modular design and intelligent design, with their advantages in weight loss, transportation and improving the operational reliability, have great potential in application. (3) In the manufacture process, the molding process with high molding quality, high production efficiency, low production cost and low environmental pollution is the developing direction in the future. Integration and automation manufacture technologies, which have great advantages in moulding quality and efficiency, will become the manufacture trends of large-scale blades. At the same time, the manufacture technologies used for thermoplastic composites have great developing potential, and the development of low viscosity thermoplastic resin is very crucial.

Local Piston Theory with Viscous Correction and Its Application

This paper introduces a local piston theory with viscous correction for the prediction of hypersonic unsteady aerodynamic loads at high attitudes and large Mach numbers. A semi-empirical relation accounting for the viscous interaction effects to determine the effective shape is proposed. The method is validated by applying to thin airfoils at various Mach numbers, angles of attack, and operating altitudes. Sample two- and three-dimensional aerodynamic forces calculations are conducted demonstrating this method. Furthermore, flutter boundary predictions for a two-dimensional airfoil and pitching-in-damping derivative evaluations for a three-dimensional waverider configuration are performed with this unsteady aerodynamic model. Compared with the Euler-based local piston theory, this model performs much better at high altitudes for a wide range of Mach numbers, angles of attack, and shapes. Results suggest the feasibility of using the effective shape of hypersonic vehicles to efficiently and accurately obtain the unsteady aerodynamic characteristics in hypersonic flow environment.

Unsteady aerodynamic characteristics investigation of rotor airfoil under variational freestream velocity

Numerical investigations based on CFD method have been accomplished to illustrate the unsteady aerodynamic characteristics of the SC1095 airfoil under conditions of coupled freestream velocity/pitching oscillation. In order to simulate the variational freestream velocity of airfoil, the moving-embedded grid method is employed to calculate the fluctuating velocity. The unsteady RANS equations coupling with the dual time-stepping approach are chosen as the governing equations to predict flowfield of airfoil under variational freestream velocity condition. The implicit Lower–Upper Symmetric Gauss–Seidel scheme and Spalart–Allmaras turbulence model are employed to improve the calculation efficiency and accuracy, respectively. Based on the present CFD method, the unsteady aerodynamic characteristics of the airfoil with variational freestream velocity are simulated under conditions of fixed angle and variational angle. The simulated results indicate that the peaks of lift coefficient, drag coefficient and pitching moment coefficient of airfoil are enlarged with the increasing of fluctuating velocity, and the start of airflow reattachment process on airfoil is delayed with the increasing of fluctuating velocities and reduced frequencies due to the effects of the airflow hysteresis and compressibility. Additionally, it is illustrated in the case of different phase angles that these peaks would be reduced by increasing phase angle.

Optimization for rotor blade-tip planform with low high-speed impulsive noise characteristics in forward flight

Rotor noise is one of the most important issues for helicopter designer, and high-speed impulsive noise is particularly intense among the various rotor noise sources due to compressibility. Based on Computational Fluid Dynamics/Ffowcs Williams and Hawkings equations with Penetrable Data Surface (CFD/FW-Hpds) methods and hybrid optimization technique, a new optimization design procedure for rotor blade planform with low high-speed impulsive noise characteristics is established. First, in order to accurately capture the unsteady aerodynamic characteristics of rotor, based on the moving-embedded grid methodology, a CFD simulation is developed by solving the compressible Reynolds-average Navier–Stokes equations with Baldwin–Lomax turbulence model. The low dissipation Roe-Monotone Upwind-centered Scheme for Conservation Laws (MUSCL) scheme and highly efficient implicit lower–upper symmetric Gauss–Seidel scheme are used for spatial and temporal discretization, respectively. Second, taking the CFD results as sound pressure information input, the high-speed impulsive noise characteristics generated by transonic rotor are analyzed through a robust numerical method based on FW-Hpds equations. Third, the genetic algorithm and surrogated model based on radial basis function are combined as a hybrid optimization technique; during the optimization process, the blade grids are generated by a highly efficient parameterized method. Aiming at the minimization of the sound pressure level of rotor in forward flight, the parametric effect analyses of blade-tip shapes on transonic noise have been conducted first. Then, optimization analyses based on the rotor blade with double-swept and tapered tip have been accomplished with the aerodynamic performance as constraints. Compared with the baseline blade, it shows that the sound pressure level of rotor with optimized blade-tip shape can be decreased obviously at the present calculating condition due to its weaker transonic delocalization phenomenon in the region of blade tip. In the rotor plane, absolute peak value of sound pressure produced by the optimized blade planform is reduced about 59.1% of that by the baseline one, and the reduced value in sound pressure level is up to 5.6 dB.

Computational aerodynamic modeling for flight dynamics simulation of ram-air parachutes

This work presents a step toward bridging the gap between flight dynamics simulation of ram-air parachutes and high-fidelity computational fluid dynamics. Today's parachute design codes mainly rely on the empirical or semi-empirical methods generated from wind tunnel experiments and drop tests. The outcome of this study will hopefully help to reduce the cost of experiments and drop testing in the design of future canopies and to better understand the aerodynamic characteristics of these geometries. In this work, the parachute geometries were modeled as rigid rectangular wings with an aspect ratio of two and zero anhedral angle. The wings have seven opening cells and the trailing edge is deflected or not deflected. To validate computational methods, the aerodynamic predictions of similar wings, but with closed and round inlets, are compared with experimental data available from the Subsonic Wind Tunnel at United States Air Force Academy. Total lift and drag force coefficients were measured at a Reynolds number of 1.4 million. The results show that computational predictions of fine (closed-inlet) grids match the experimental data very well up to the stall angle. Both experiments and simulations show that closed wings have sharp stalling characteristics. The aerodynamics of closed wings up to stall can be approximated by linear functions and their derivatives. The closed wings show a negative static stability with respect to changes in the angle of attack. The open wings, on other hand, have positive static stability in the longitudinal and lateral directions. The open wings exhibit highly nonlinear unsteady aerodynamic characteristics; they also stall earlier and have higher drag values than the closed wings. The aerodynamic derivatives of open and closed wings were estimated using a linear regression method and training data simulated in small-amplitude oscillations in pitch, yaw, and roll directions. While the open wings have large oscillations in aerodynamic coefficients over the yawing and rolling hysteresis loops, lateral aerodynamic derivatives of the open and closed wings are similar. Finally, the results show that model predictions are reasonably accurate for use in flight-dynamics simulations.

Unsteady aerodynamics simulation of a full-scale horizontal axis wind turbine using CFD methodology

The aerodynamic performance of wind turbines is significantly influenced by the unsteady flow around the rotor blades. The research on unsteady aerodynamics for Horizontal Axis Wind Turbines (HAWTs) is still poorly understood because of the complex flow physics. In this study, the unsteady aerodynamic configuration of a full-scale HAWT is simulated with consideration of wind shear, tower shadow and yaw motion. The calculated wind turbine which contains tapered tower, rotor overhang and tilted rotor shaft is constructed by making reference of successfully commercial operated wind turbine designed by NEG Micon and Vestas. A validated CFD method is utilized to analyze unsteady aerodynamic characteristics which affect the performance on such a full-scale HAWT. The approach of sliding mesh is used to carefully deal with the interface between static and moving parts in the flow field. The annual average wind velocity and wind profile in the atmospheric border are applied as boundary conditions. Considering the effects of wind shear and tower shadow, the simulation results show that the each blade reaches its maximum and minimum aerodynamic loads almost at the same time during the rotation circle. The blade–tower interaction imposes great impact on the power output performance. The wind turbine produces yaw moment during the whole revolution and the maximum aerodynamic loads appear at the upwind azimuth in the yaw computation case.

A CFD study into the influence of unsteady aerodynamic interference on wind turbine surge motion

To improve knowledge of the unsteady aerodynamic characteristics and interference effects of a floating offshore wind turbine (FOWT), this article focuses on the platform surge motion of a full configuration wind turbine with the rotating blades, hub, nacelle, and tower shapes. Unsteady aerodynamic analyses considering the moving motion of an entire configuration wind turbine have been conducted using an advanced computational fluid dynamics (CFD) and a conventional blade element momentum (BEM) analyses. The present CFD simulation is based on an advanced overset moving grid method to accurately consider the local and global motion of a three-dimensional wind turbine. The effects of various oscillation frequencies and amplitudes of the platform surge motion have been widely investigated herein. Three-dimensional unsteady flow fields around the moving wind turbine with rotating blades are graphically presented in detail. Complex flow interactions among blade tip vortices, tower shedding vortices, and turbulent wakes are physically observed. Comparisons of different aerodynamic analyses under the periodic surge motions are summarized to show the potential distinction among applied numerical methods. The present result indicates that the unsteady aerodynamic thrust and power tend to vary considerably depending on the oscillation frequency and amplitude of the surge motion.

Unsteady Aerodynamic Characteristics and Safety of High-speed Trains under Crosswinds

Unsteady Aerodynamic Characteristics and Safety of High-speed Trains under Crosswinds

Analysis of Unsteady Aerodynamic Characteristics of Transonic Compressor Cascade in Torsional Oscillation Mode by One-Blade Oscillation Method

Analysis of Unsteady Aerodynamic Characteristics of Transonic Compressor Cascade in Torsional Oscillation Mode by One-Blade Oscillation Method

Transitions in the unsteady wakes and aerodynamic characteristics of the flow past three square cylinders aligned inline

A numerical study is performed to analyze the effect of low Reynolds numbers (Re) and gap spacings (g) on the flow around three square cylinders aligned inline using the multi-relaxation-time lattice Boltzmann method (MRT LBM). The Reynolds number is varied from Re=90 to 175 while the spacing between the cylinders is taken in the range 0.5≤g≤6. It is found that MRT LBM captures the flow characteristics efficiently. Seven different flow patterns are revealed in this study at different Re and g values. Variation of different aerodynamic force coefficients, like mean drag coefficient, Strouhal number, root mean square values of drag and lift coefficients, with flow patterns are discussed in detail in order to develop a link between flow patterns and force characteristics. The drag inversion (DI) spacing (or critical spacing) for the middle cylinder lies within the spacing range 2≤g≤3 and depends on Re. The effect of Re on DI is also studied and it is concluded that the DI spacing decreases with increment in Re. At all chosen Re it is found that the Strouhal number decreases up to DI spacing value while it has increasing behavior after that. The negative values of average drag coefficient for third cylinder are also observed either at low spacing or at high spacing values. Re=90 and 140 are found to be critical for different wake characteristics. Generally, it is observed that the upstream cylinder has higher drag force compared to the middle and downstream one but at some Re and g values the middle or downstream cylinder also found to have higher drag force than the upstream one. The root mean square value of lift coefficients for all the cylinders is higher than the corresponding root mean square value of drag coefficients.

Influence of wake dynamics on the performance and aeroelasticity of wind turbines

Wind turbines are currently a rapidly expanding form of renewable energy. However, there are numerous technological challenges that must be overcome before wind energy provides a significant amount of power in the United States. One of the primary challenges in wind turbine design and analysis is accurately accounting for the aerodynamic environment. This study is focused on a comprehensive verification and validation of the NREL FAST code, which is enhanced to include a free vortex wake model. The verification and validation is carried out through a comparison of blade lift distribution, wind turbine power and force and moment coefficients using a combination of CFD and experimental data. The results are also compared against Blade Element Momentum theory, and results from a 2001 double-blind NREL study on the prediction capabilities of wind turbine modeling tools. Results indicate that the enhanced aeroelastic code generally provides improved predictions. However, in several notable cases the predictions are only marginally improved, or even worse, than those generated using Blade Element Momentum theory aerodynamics. It is concluded that modeling of the aerodynamic environment remains incomplete, even after inclusion of wake effects. One important aspect identified is modeling of the unsteady aerodynamic lift characteristics of the rotor. Finally, the aeroelastic response in the combined presence of wake effects and inflow turbulence is examined. Significant differences are observed in loads, power, and structural response between results computed using the free wake model or simpler models, such as Blade Element Momentum theory.

Unsteady aerodynamic characteristics of a translating rigid wing at low Reynolds number

Rectilinearly surging wings are investigated under several different velocity profiles and incidence angles. The primary wing studied here was an aspect ratio 4 rectangular flat plate. Studies on acceleration distance, ranging from 0.125c to 6c, and incidence angles 5°–45° were performed to obtain a better understanding of the force and moment histories during an extended surge motion over several chord-lengths of travel. Flow visualization and particle image velocimetry were performed to show the flow structures responsible for variations in force and moment coefficients. It was determined that the formation and subsequent shedding of a leading edge vortex correspond to oscillations in force coefficients for wings at high angle of attack. Comparing unsteady lift results to static force measurements, it was determined that for cases with large flow separation, even after 14 chords traveled at a constant velocity, the unsteady forces do not converge to the fully developed values. Forces were then broken up into circulatory and non-circulatory components to identify individual contributors to lift. Although it was observed that the “fast” and “slow” cases produced nearly identical vortex trajectories, circulation measurements confirmed that the faster acceleration case generates more vorticity in the form of a tighter, more coherent vortex and produces significantly more circulation than the slower acceleration case, which is consistent with the difference in force production.

Numerical investigations into effects of three-dimensional wake patterns on unsteady aerodynamic characteristics of a circular cylinder at Re=1.3×105

In contrast with a wide range of applications concerning flows around a circular cylinder at upper subcritical Reynolds numbers (Re), there is no systematic understanding about the fundamentals of so-called random flow patterns, and their effects on intermittent modulations in the time history of pressure or force, and the decrease in their spanwise correlations. This paper employed the large-eddy simulation (LES) technique to predict flows past a circular cylinder at Re=1.3×105 and to provide images based on flow visualization that can clarify the physical mechanism responsible for these outcomes. A reasonably sufficient spanwise length was adopted for the numerical model by taking into consideration the effect of aspect ratios (the spanwise length to the diameter). We found that even at such high Res, a three-dimensional pattern of vortical field is present in the wake resulting in total force modulation and weak spanwise correlation, e.g., obvious oblique shedding. The whole development process of the three-dimensional wake is exhibited as a universal. The results revealed that local phase variations in primary vortex shedding are the starting points of three-dimensional wake patterns, which are induced by the “irregular” streamwise vortex. The three-dimensional near wake following local phase variations is associated with a successive evolution composed of certain stages in order. Quantitative analyses based on the time series of sectional lift coefficients show that intermittent increase in primary shedding periods and sectional lift streak divisions are closely related to local phase variations and vortex division in the development process of the three-dimensional pattern. In addition to the phase difference along the span, the three-dimensional pattern also weakens vortex shedding in cross sections perpendicular to the axis of the cylinder, resulting in modulation of the sectional lift coefficient.

CFD calculations on the unsteady aerodynamic characteristics of a tilt-rotor in a conversion mode

In order to calculate the unsteady aerodynamic characteristics of a tilt-rotor in a conversion mode, a virtual blade model (VBM) and an real blade model (RBM) are established respectively. A new multi-layer moving-embedded grid technique is proposed to reduce the numerical dissipation of the tilt-rotor wake in a conversion mode. In this method, a grid system generated abound the rotor accounts for rigid blade motions, and a new searching scheme named adaptive inverse map (AIM) is established to search corresponding donor elements in the present moving-embedded grid system to translate information among the different computational zones. A dual-time method is employed to fulfill unsteady calculations on the flowfield of the tilt-rotor, and a second-order centered difference scheme considering artificial viscosity is used to calculate the flux. In order to improve the computing efficiency, the single program multiple data (SPMD) model parallel acceleration technology is adopted, according to the characteristic of the current grid system. The lift and drag coefficients of an NACA0012 airfoil, the dynamic pressure distributions below a typical rotor plane, and the sectional pressure distributions on a three-bladed Branum–Tung tilt-rotor in hover flight are calculated respectively, and the present VBM and RBM are validated by comparing the calculated results with available experimental data. Then, unsteady aerodynamic forces and flowfields of an XV-15 tilt-rotor in different modes, such as a fixed conversion mode at different tilt angles (15°, 30°, 60°) and a whole conversion mode which converses from 0° to 90°, are numerically simulated by the VBM and RBM respectively. By analyses and comparisons on the simulated results of unsteady aerodynamic forces of the tilt-rotor in different modes, some meaningful conclusions about distorted blade-tip vortex distribution and unsteady aerodynamic force variation in a conversion mode are obtained, and these investigation results could provide a good foundation for tilt-rotor aircraft design in the future.

Unsteady Aerodynamic Characteristics of the Pitched Supersonic Biplane

In this work, supersonic biplanes of the Busemann concept have been analysed, focusing on the unsteady aerodynamic characteristic due to flow disturbance using Computational Fluid Dynamics (CFD) codes in viscous flow. Flow disturbance is modelled by sinusoidal pitch motion simulated by mesh morphing using radial basis functions (RBF) method. The results suggest that there are two flow patterns of the Busemann biplane: oblique wave sequences flow (Pattern A) and choke-flow (Pattern B) with higher wave drag. Unsteady aerodynamic disturbance represented by pitch motion may cause flow pattern transformation. We have also obtained that Pattern B is more stable than Pattern A and choke-flow cannot be eliminated even after returning to the initial flight attitude. Moreover, amplitudes and frequencies of sinusoidal pitch motion play important roles in flow pattern transformation and there exist critical amplitudes and frequencies.

Численный расчет нестационарных аэродинамических характеристик цилиндрических моделей в условиях сверхзвукового ламинарного обтекания

Численный расчет нестационарных аэродинамических характеристик цилиндрических моделей в условиях сверхзвукового ламинарного обтекания