Influences of aerodynamic loads on hunting stability of high-speed railway vehicles and parameter studies

The influences of steady aerodynamic loads on hunting stability of high-speed railway vehicles were investigated in this study.A mechanism is suggested to explain the change of hunting behavior due to actions of aerodynamic loads：the aerodynamic loads can change the position of vehicle system（consequently the contact relations）,the wheel/rail normal contact forces,the gravitational restoring forces/moments and the creep forces/moments.A mathematical model for hunting stability incorporating such influences was developed.A computer program capable of incorporating the effects of aerodynamic loads based on the model was written,and the critical speeds were calculated using this program.The dependences of linear and nonlinear critical speeds on suspension parameters considering aerodynamic loads were analyzed by using the orthogonal test method,the results were also compared with the situations without aerodynamic loads.It is shown that the most dominant factors a ff ecting linear and nonlinear critical speeds are different whether the aerodynamic loads considered or not.The damping of yaw damper is the most dominant influencing factor for linear critical speeds,while the damping of lateral damper is most dominant for nonlinear ones.When the influences of aerodynamic loads are considered,the linear critical speeds decrease with the rise of cross wind velocity,whereas it is not the case for the nonlinear critical speeds.The variation trends of critical speeds with suspension parameters can be significantly changed by aerodynamic loads.Combined actions of aerodynamic loads and suspension parameters also a ff ect the critical speeds.The effects of such joint action are more obvious for nonlinear critical speeds.

Effects of aspect ratio and inclination angle on aerodynamic loads of a flat plate

Large Eddy Simulations are carried out to analyze flow past flat plate in different configurations and inclinations.A thin flat plate is considered at three inclination angles(α=30°,60°and 90°)and three aspect ratios(AR=0.5,2 and 5).The Reynolds number based on the free stream velocity and chord length of the plate at different inclination angles varies between 75,000 to 150,000.An increase in the inclination angle while the aspect ratio(span to chord)is constant results in higher drag and lower lift on the plate.Increasing the aspect ratio at a constant inclination angle increases the mean aerodynamic loading except for theα=30°and AR=0.5 case where the mean forces are larger than the other aspect ratios for this specific inclination angle.The small aspect ratio suppresses and blocks the separation of the flow from the top and bottom edges causing larger aerodynamic forces relative to AR=2,5.Visualization of the flow structures shows the tip vortices have a significant role in controlling the shedding vortices from the top and bottom edges.Atα=30°and AR=0.5,the two tip vortcies control and suppress the flow separation from the top and bottom edges.A stable wake was found for this case with no fluctuation.As the aspect ratio increases,the influence of the tip vortices on flow separation from the top and bottom edges reduces.As a result,larger fluctuations were found for cases with higher aspect ratios.

Research on stability of laminated composite plate under nonlinear aerodynamic load

In this paper,we propose an finite element approach based on classical plate theory to investigate the dynamic stability of a layered composite plate subject to nonlinear aerodynamic load.This study considers the influence of temperature,nonlinear geometry,and nonlinear aerodynamic load on composite plate structures simultaneously.Specifically,the present work conduct comparison the results of the critical pressure value between the nonlinear aerodynamic load and the linear aerodynamic load,thereby pointing out some necessary cases which must consider the nonlinearity of aerodynamic load for calculating the aerospace structures.We determine the critical pressure value and vibrational amplitude response of the plate by means of calculation.The outcomes of our calculations can be useful in designing and repairing body shells and wings of aircraft equipment.

Experimental investigation on tip vortices and aerodynamics of a wing with ground effect

The tip vortices and aerodynamics of a NACA0012 wing in the vicinity of the ground were studied in a wind tunnel.The wing tip vortex structures and lift/drag forces were measured by a seven-hole probe and a force balance,respectively.The evolution of the flow structures and aerodynamics with a ground height were analyzed.The vorticity of tip vortices was found to reduce with the decreasing of the ground height,and the position of vortex-core moved gradually to the outboard of the wing tip.Therefore,the down-wash flow induced by the tip vortices was weakened. However,vortex breakdown occurred as the wing lowered to the ground.From the experimental results of aerodynamics,the maximum lift-to-drag ratio was observed when the angle of attack was 2.5°and the ground clearance was 0.2.

UNSTEADY TRANSONIC AERODYNAMIC LOADINGS ON THE AIRFOIL CAUSED BY HEAVING, PITCHING OSCILLATIONS AND CONTROL SURFACE

An implicit upwind finite volume solver for the Euler equations using the improved flux - splitting method is established and used to calculate the transonic flow past the airfoils with heaving, pitching oscillations and the control surface. Results are given for the NACA64A - 10 airfoil which is in harmonic heaving and pitching oscillation and with the control surface in the transonic flow field. Some computational results are compared with the experiment data and the good agreements are shown in the paper.

Operational Safety Assessment of a High-Speed Train Exposed to the Strong Gust Wind

The operational safety characteristics of trains exposed to a strong wind have caused great concern in recent years.In the present paper,the effect of the strong gust wind on a high-speed train is investigated.A typical gust wind model for any wind angle,named“Chinese hat gust wind model”,was first constructed,and an algorithm for computing the aerodynamic loads was elaborated accordingly.A vehicle system dynamic model was then set up in order to investigate the vehicle system dynamic characteristics.The assessment of the operational safety has been conducted by means of characteristic wind curves(CWC).As some of the parameters of the wind-train system were difficult to measure,we also investigated the impact of the uncertain system parameters on the CWC.Results indicate that,the descending order of the operational safety index of the vehicle for each wind angle is 90°-60°-120°-30°-150°,and the worst condition for the operational safety occurs when the wind angle reaches around 90°.According to our findings,the gust factor and aerodynamic side force coefficient have great impact on the critical wind speed.Thus,these two parameters require special attention when considering the operational safety of a railway vehicle subjected to strong gust wind.

Airfoil Roof in Buildings

In this paper an airfoil that is used on roofs was analyzed:Circular Arc Airfoil.The JavaFoil program for the calculation of aerodynamic parameters of the simulated wing airfoil and small AR(aspect ratio)was used.A wing roof scale model was constructed,and it was tested in the wind tunnel of the Laboratory of Environmental Fluid Dynamics,Universidad Nacional del Comahue.In the model,the AR was equal to 1.46.Thickness of the model was 32%.The tests were conducted at a Reynolds number of 1×10^(5).The curves of the lift coefficient versus angle of attack were obtained,and the pressure coefficient Cp was determined for each surface.The lift coefficients and the Cp values differ from the theoretical profile;this shows the importance of using the wind tunnel to obtain experimental data to achieve a good structural design.

On-line control of surface quality for an axially moving steel strip partially immersed in fluid under tension

An on-line control method of surface quality for continuous hot-dip galvanized steel strip after cooling is presented, which combines analytical dynamics theory of a thin plate with the finite element method. The inherent characteristics of the non-immersed and partially immersed strip in liquid zinc were calculated on the basis of theoretical analysis and numerical simulation, respectively. Multi-parameter fitting of the deviation between results using different methods was performed. To optimize the strip excitation frequency away from the resonance region, on-line vibration control of the strip near the air knife under full product conditions was achieved by changing the field production parameters based on the field test results. The results indicate that although the axial velocity has little effect on the inherent characteristics of the strip compared with other manufacturing parameters such as the steel specifications and tension, it induces external excitations,including moving the aerodynamic load and bearing vibration. To some degree, the vibration near the air knife can be reduced by strengthening the support stiffness of the contact rolls. A total on-line control program of surface quality for continuous hot-dip galvanized pure Zn and galvannealed steel sheet in the cooling section is proposed.

Aerodynamic and structural characteristics of helicopter rotor in circling flight

To investigate the distinct properties of the helicopter rotors during circling flight,the aerodynamic and dynamic models for the main rotor are established considering the trim conditions and the flight parameters of helicopters.The free wake method is introduced to compute the unsteady aerodynamic loads of the rotor characterized by distortions of rotor wakes,and the modal superposition method is used to predict the overall structural loads of the rotor.The effectiveness of the aerodynamic and the structural methods is verified by comparison with the experimental results,whereby the influences of circling direction,radius,and velocity are evaluated in both aerodynamic and dynamic aspects.The results demonstrate that the circling condition makes a great difference to the performance of rotor vortex,as well as the unsteady aerodynamic loads.With the decrease of the circling radius or the increment of the circling velocity,the thrust of the main rotor increases apparently to balance the inertial force.Meanwhile,the harmonics of aerodynamic loads in rotor disc change severely and an evident aerodynamic load shock appears at high-order components,which further causes a shift-of-peak-phase bending moment in the flap dimension.Moreover,the advancing side of blade experiences second blade/vortex interaction,whose intensity has a distinct enhancement as the circling radius decreases with the motion of vortexes.

Numerical simulation of the aerodynamic characteristics of heavy-duty trucks through viaduct in crosswind

In our numerical simulation the hybrid mesh and the SST k-ω turbulence model are adopted to investigate the variations of the aerodynamic loads and the flow field of heavy-duty trucks while crossing a viaduct with 1.1 m high fences in a crosswind at a velocity of 20 m/s. The results show that, with the protection of a fence, the side force is weakened, and the rolling and yaw moments are strengthened while the truck is crossing the viaduct, which relatively reduces the roll-over safety and the driving stability of the truck. Meanwhile, the direction of the side force changes when the truck enters the viaduct, which makes the roll-over safety and the driving stability the lowest during the process.

Aerodynamic design of tractor propeller for high-performance distributed electric propulsion aircraft

Aiming to maximize the aerodynamic performance of the Distributed Electric Propulsion(DEP)aircraft,a hybrid design framework which focuses on the aerodynamic performance of the propeller/wing integration has been developed and validated numerically.Variable-fidelity modelling for propeller aerodynamics has been used to achieve computational efficiency with reasonable accuracy.By optimizing the aerodynamic loading distributions on the tractor propeller disk,the induced slipstream is redistributed into a form that is beneficial for the wing downstream,based on which the propeller blade geometry is generated through a rapid inversed design procedure.As compared with the Minimum Induced Loss(MIL)propeller at a specified thrust level,significant improvements of both the lift-to-drag ratio of the wing and the propeller/wing integrated aerodynamic efficiency is achieved,which shows great promise to deliver aerodynamic benefits for the wing within the propeller slipstream without any additional devices.

Experimental Analysis of the Aerodynamic Performance of an Innovative Low Pressure Turbine Rotor

In the present work the aerodynamic performances of an innovative rotor blade row have been experimentally investigated. Measurements have been carried out in a large scale low speed single stage cold flow facility at a Reynolds number typical of aeroengine cruise, under nominal and off-design conditions. The time-mean blade aerodynamic loadings have been measured at three radial positions along the blade height through a pressure transducer installed inside the hollow shaft, by delivering the signal to the stationary frame with a slip ring. The time mean aerodynamic flow fields upstream and downstream of the rotor have been measured by means of a five-hole probe to investigate the losses associated with the rotor. The investigations in the single stage research turbine allow the reproduction of both wake-boundary layer interaction as well as vortex-vortex interaction. The detail of the present results clearly highlights the strong dissipative effects induced by the blade tip vortex and by the momentum defect as well as the turbulence production, which is generated during the migration of the stator wake in the rotor passage. Phase-locked hot-wire investigations have been also performed to analyze the time-varying flow during the wake passing period. In particular the interaction between stator and rotor structures has been investigated also under off-design conditions to further explain the mechanisms contributing to the loss generation for the different conditions.

Effects of Avian Wings Color Patterns on Their Flight Performance:Experimental and Computational Studies

There are many theories behind the colors of a bird’s feathers.Many of these theories point to the color’s purpose to attract mates and hide from predators.Some recent investigations concluded that the dark colors of birds help in reducing the drag force during flight.A new theory is presented in the current research,which states that a bird's dark color not only reduces the drag,but the color pattern also improves the overall flight performance,and each color pattern has a different type of flight performance improvement.This difference in improvement is a result of variation in hot and cold surfaces on the bird skin as a result of the variation between light and dark feather colors.To prove this new theory,thermal images were captured of real bird wings under the effect of infrared waves.Also,a novel wind tunnel wing with the ability to adjust the temperature in desired locations and patterns on the wing’s surface was manufactured and tested to evaluate the effect of aerodynamics forces as a function in the surface temperature and the hot–cold regions.The collected data from this wing showed potential flight efficiency improvements of 20%,comparing the lift-to-drag ratio for specific heating cases,which could increase the flight range.Individually considering lift and drag,there were specific heating cases with corresponding angles of attack in which these parameters improved by up to 20%and 7%,respectively.Some heating cases could increase the lift at a low angle of attack,which is helpful in cruise flight performance,while some cases could increase the maximum lift coefficient by 6%.This is very helpful in lowering stall and the minimum flight speeds.Furthermore,some cases could increase the lift-to-drag ratio,which led to an increase in the flight range.To better understand the effect of the various patterns,computational fluid dynamics(CFD)simulations were conducted on the wing.The new theory was proved based on the CFD results and verified through the successful results from the wind tunnel experiments.

Design concepts of an aircraft wing： composite and morphing airfoil with auxetic structures

This paper is categorized into two parts. （1） A frame work to design the aircraft wing structure and （2） analysis ofa morphing airfoil with auxetic structure. The developed design frame work in the first part is used to arrive at the sizes of the various components of an aircraft wing structure. The strength based design is adopted, where the design loads are extracted from the aerodynamic loads. The aerodynamic loads acting on a wing structure are converted to equivalent distributed loads, which are further converted point loads to arrive at the shear forces, bending and twisting moments along the wing span. Based on the estimated shear forces, bending and twisting moments, the strength based design is employed to estimate the sizes of various sections of a composite wing structure. A three dimensional numerical model of the composite wing structure has been developed and analyzed for the extreme load conditions. Glass fiber reinforced plastic material is used in the numerical analysis. The estimated natural frequencies are observed to be in the acceptable limits. Furthermore, the discussed design principles in the first part are extended to the design of a morphing airfoil with auxetic structure. The advantages of the morphing airfoil with auxetic structure are （i） larger displacement with limited straining of the components and （ii） unique deformation characteristics, which produce a theoretical in-plane Poisson＇s ratio of -1. Aluminum Alloy AL6061-T651 is considered in the design of all the structural elements. The compliance characteristics of the airfoil are investigated through a numerical model. The numerical results are observed to be in close agreement with the experimental results in the literature.

FINITE ELEMENT ANALYSIS OF THE FLOW INDUCED BY ROTATING BLADES IN AN INCOMPRESSIBLE VISCOUS FLUID

Aerodynamic loads on a multi-bladed helicopter rotor in hovering flight were calculated by solving the three-dimensional incompressible Navier-Stokes equations. The rotor wake effects were accounted by the correction of local geometric angle of attack according to a free-wake modeling in addition to an empirical modification for the tip flow effect. The validity and efficiency of the present method were verified by the comparisons between numerical results and experimental data.

Aeroelastic optimization design for wing with maneuver load uncertainties

An aeroelastic optimization design methodology for air vehicle considering the uncertainties in maneuver load conditions is presented and applied to a structural design process of low-aspect-ratio wing. An aerodynamic load correction model is developed and used to predict the critical load conditions with the perturbations of theoretical linear aerodynamic forces and experimental aerodynamic forces from wind-tunnel test, when concerning the uncertainties in use of theoretical linear and experimental aerodynamic forces. Three objective functions of critical loads are defined. The load evaluations for three wing sections are investigated in four characteristic maneuvers, and the most critical load conditions are confirmed by using the sequential quadratic programming method. On this basis, the aeroelastic optimization design employing the genetic-gradient hybrid algorithm is conducted, in which the objective is to minimize structural mass subject to the constraints of stress, deformation and flutter speed. The resulting optimal structure is heavier than the one simply based on the theoretical linear or experimental aerodynamic forces. However, it is more robust when encountering the critical load conditions in actual flight due to the consideration of uncertainties in aerodynamic forces in the early design phase, thereby, the risk of structural redesign can be reduced.

DYNAMIC CHARACTERISTICS OF LAG VIBRATION OF A WIND TURBINE BLADE

Lagwise dynamic characteristics of a wind turbine blade subjected to unsteady aero- dynamic loads are studied in this paper. The partial differential equations governing the coupled longitudinal-transverse vibration of the blade with large bending deflection are obtained by ap- plying Hamilton＇s principle. The modal problem of the coupled vibration is handled by using the method of numerical integration of Green＇s function. Influences of the rotating speed, the pitch angle, the setting angle, and the aerodynamic loads on natural frequencies are discussed. Results show that： （I） Lagwise natural frequencies ascend with the increase of rotating speed; effects of the rotating speed on low-frequencies are dramatic while these effects on high-frequencies become less. （2） Influences of the pitch angle on natural frequencies are little; in the range of the normal rotating speed, the first frequency ascends with the increase of the absolute value of the pitch angle, while it is contrary to the second and third frequencies. （3） Effects of the setting angle on natural frequencies depend on the rotating speed; influences are not significant at low speed, while they are dramatic on the first frequency at high speed. （4） Effects of the aerodynamic loads on natural frequencies are very little; frequencies derived from the model considering aerodynamic loads are smaller than those from the model neglecting aerodynamic loads; relative errors of the results corresponding to two models ascend with the increase of the absolute value of the setting angle.