Influence of Anteroposterior Symmetrical Aero-Wings on the Aerodynamic Performance of High-Speed Train

The running stability of high-speed train is largely constrained by the wheel-rail coupling relationship,and the continuous wear between the wheel and rail surfaces will profoundly affect the dynamic performance of the train.In recent years,under the background of increasing train speed,some scientific researchers have proposed a new idea of using the lift force generated by the aerodynamic wings(aero-wing)installed on the roof to reduce the sprung load of the carriage in order to alleviate the wear and tear of the wheel and rail.Based on the bidirectional running characteristics of high-speed train,this paper proposes a scheme to apply aero-wings with anteroposterior symmetrical cross-sections on the roof of the train.After the verification of the wind tunnel experimental data,the relatively better airfoil section and extension formof anteroposterior symmetrical aero-wing is selected respectively in this paper,and the aero-wings are fixedly connected to the roof of the train through the mounting column to conduct aerodynamic simulation analysis.The research shows that:compared with the circular-arc and oval crosssections,this paper believes that the crescent cross-section can form greater aerodynamic lift force in a limited space.Considering factors such as aerodynamic parameters,ground effect,and manufacturing process,this paper proposes to adopt aero-wings with arc type extension form and connect them to the roof of the train through mounting columns with shuttle cross-section.When the roof of the train is covered with aero-wings and runs at high speed,the sprung load of the carriages can be effectively reduced.However,there are certain hidden dangers in the tail carriage due to the large amount of lift force,so,the intervention of the aero-wing lifting mechanism is required.At the same time,it is necessary to optimize the overall aerodynamic drag force reduction in the followup work.

Experimental and Numerical Investigation on the Aerodynamic Characteristics of High-Speed Pantographs with Supporting Beam Wind Deflectors

Aiming to mitigate the aerodynamic lift force imbalance between pantograph strips,which exacerbates wear and affects the current collection performance of the pantograph-catenary system,a study has been conducted to support the beam deflector optimization using a combination of experimental measurements and computational fluid dynamics(CFD)simulations.The results demonstrate that the size,position,and installation orientation of the wind deflectors significantly influence the amount of force compensation.They also indicate that the front strip deflectors should be installed downwards and the rear strip deflectors upwards,thereby forming a“π”shape.Moreover,the lift force compensation provided by the wind deflectors increases with the size of the deflector.Alternative wind compensation strategies,such as control circuits,are also discussed,putting emphasis on the pros and cons of various pantograph types and wind compensation approaches.

EFFECT OF UPSTREAM ROTOR ON AERODYNAMIC FORCE OF DOWNSTREAM STATOR BLADES

To study the amplitude and the frequency of the aerodynamic force on stator blades, micro-sensors are embedded on the surface of stator blades of a low-speed single-stage axial compressor rig. The unsteady pressure distribution on stator blades is measured under the conditions of different axial spacing between the rotor and the stator, different rotating speeds and an extensive range of the mass flow. Amplitudes and frequencies of aerodynamic forces are analyzed by the Fourier transform. Experimental results show that under the effect of the rotor wake, the dominant frequencies of pressure fluctuations on stator blades are the rotor blade passing frequency （BPF） and its harmonics. The higher harmonics of the rotor BPF in the fore part of the suction side are more prominent than that in the other parts of the stator blade. Otherwise, fluctuations of the pressure and the aerodynamic force on stator blades vary with the mass flow, the rotating speed and the axial spacing between the rotor and the stator.

Parameter optimization for improved aerodynamic performance of louver-type wind barrier for train-bridge system

To improve the safety of trains running in an undesirable wind environment,a novel louver-type wind barrier is proposed and further studied in this research using a scaled wind tunnel simulation with 1:40 scale models.Based on the aerodynamic performance of the train-bridge system,the parameters of the louver-type wind barrier are optimized.Compared to the case without a wind barrier,it is apparent that the wind barrier improves the running safety of trains,since the maximum reduction of the moment coefficient of the train reaches 58%using the louver-type wind barrier,larger than that achieved with conventional wind barriers(fence-type and grid-type).A louver-type wind barrier has more blade layers,and the rotation angle of the adjustable blade of the louver-type wind barrier is 90–180°(which induces the flow towards the deck surface),which is more favorable for the aerodynamic performance of the train.Comparing the 60°,90°and 120°wind fairings of the louver-type wind barrier blade,the blunt fairing is disadvantageous to the operational safety of the train.

Aerodynamics of flexible wing in bees' hovering flight

The aerodynamics of 2-dimensional flexible wings in bees＇ normal hovering flight is studied. Four insect flapping flight coordinate systems, including a global system, a bodyfixed system, a rigid wing-fixed system and a flexible wingfixed system, are established to represent the insects＇ position, gesture, wing movement and wing deformation, respectively. Then the transformations among four coordinate systems are studied. It is found that the elliptic coordinate system can improve the computation accuracy and reduce the calculation complexity in a 2-dimensional rigid wing. The computation model of a 2-dimensional flexible wing is established, and the changes of the force, moment, and power are investigated. According to the computation results, the large lift and drag peaks at the beginning and end of the stroke can be explained by the superposition of the rapid translational acceleration, the fast pitching-up rotation and the Magnus effect; and the small force and drag peaks can be explained by the convex flow effect and the concave flow effect. Compared with the pressure force, pressure moment and translational power, the viscous force, viscous moment and rotational power are small and can be ignored.

Numerical simulation and optimization of aerodynamic uplift force of a high-speed pantograph

Aiming at the problem that aerodynamic uplift forces of the pantograph running in the knuckle-downstream and knuckle-upstream conditions are inconsistent,and their magnitudes do not satisfy the corresponding standard, the aerodynamic uplift forces of pantographs with baffles are numerically investigated, and an optimization method to determine the baffle angle is proposed. First, the error between the aerodynamic resistances of the pantograph obtained by numerical simulation and wind tunnel test is less than 5%, which indicates the accuracy of the numerical simulation method. Second, the original pantograph and pantographs equipped with three different baffles are numerically simulated to obtain the aerodynamic forces and moments of the pantograph components.Three different angles for the baffles are-17°, 0° and 17°.Then the multibody simulation is used to calculate the aerodynamic uplift force of the pantograph, and the optimal range for the baffle angle is determined. Results show that the lift force of the baffle increases with the increment of the angle in the knuckle-downstream condition, whereas the lift force of the baffle decreases with the increment of the angle in the knuckle-upstream condition. According to the results of the aerodynamic uplift force, the optimal angle of the baffle is determined to be 4.75° when the running speed is 350 km/h, and pantograph–catenary contact forces are 128.89 N and 129.15 N under the knuckledownstream and knuckle-upstream operating conditions,respectively, which are almost equal and both meet the requirements of the standard EN50367:2012.

Effects of unsteady deformation of flapping wing on its aerodynamic forces

Effects of unsteady deformation of a＇flapping model insect wing on its aerodynamic force production are studied by solving the Navier-Stokes equations on a dynamically deforming grid. Aerodynamic forces on the flapping wing are not much affected by considerable twist, but affected by camber deformation. The effect of combined camber and twist deformation is similar to that of camber deformation. With a deformation of 6% camber and 20% twist （typical values observed for wings of many insects）, lift is increased by 10% - 20% and lift-to-drag ratio by around 10% compared with the case of a rigid fiat-plate wing. As a result, the deformation can increase the maximum lift coefficient of an insect, and reduce its power requirement for flight. For example, for a hovering bumblebee with dynamically deforming wings （6% camber and 20% twist）, aerodynamic power required is reduced by about 16% compared with the case of rigid wings.

Aerodynamic Characteristics of the Crest with Membrane Attachment on Cretaceous Pterodactyloid Nyctosaurus

The Nyctosaurus specimen K J1 was reconstructed under the hypothesis that there is a membrane attached to the crest; the so-called headsail crest. The aerodynamic forces and moment acting on the headsail crest were analyzed. It was shown that K J1 might adjust the angle of the headsail crest relative to the air current as one way to generate thrust （one of the aerodynamic forces, used to overcome body drag in forward flight） and that the magnitude of the thrust and moment could vary with the gesture angle and the relative location between the aerodynamic center of the headsail crest and body＇s center of gravity. Three scenarios were tested for comparison： the crest with membrane attachment, the crest without membrane attachment and the absence of a cranial crest. It was shown that the aerodynamic characteristics （increasing, maintaining and decreasing thrusts and moment） would have almost disappear in flight for the crest without membrane attachment and was non-existent without the cranial crest. It is suggested from aerodynamics evidence alone that Nyctosaurus specimen KJ1 had a membrane attached to the crest and used this reconstructed form for auxiliary flight control.

Aerodynamic coefficient of vehicle-bridge system by wind tunnel test

The changes of three components of aerodynamic force were discussed with the attack angle conversion for three kinds of section models. Based on the project of Shanghai Yangtze River Bridge, the wind tunnel test was conducted to obtain its three components of aerodynamic force including 75 conditions of the construction stage, the bridge without vehicles and the bridge with vehicles from - 12 degrees to ＋ 12 degrees. For the bridge with vehicles, the drag force coefficient and the absolute value of both lift coefficient and moment coefficient were decreased by the vehicles. The test resuh shows that the bridge railing and vehicles have much influence on the three components of aerodynamic force of the vehicle-bridge system for Shanghai Yangtze River Bridge.

Time-varying nonlinear dynamics of a deploying piezoelectric laminated composite plate under aerodynamic force

Using Reddy’s high-order shear theory for laminated plates and Hamilton’s principle, a nonlinear partial differential equation for the dynamics of a deploying cantilevered piezoelectric laminated composite plate, under the combined action of aerodynamic load and piezoelectric excitation, is introduced. Two-degree of freedom(DOF)nonlinear dynamic models for the time-varying coefficients describing the transverse vibration of the deploying laminate under the combined actions of a first-order aerodynamic force and piezoelectric excitation were obtained by selecting a suitable time-dependent modal function satisfying the displacement boundary conditions and applying second-order discretization using the Galerkin method. Using a numerical method, the time history curves of the deploying laminate were obtained, and its nonlinear dynamic characteristics,including extension speed and different piezoelectric excitations, were studied. The results suggest that the piezoelectric excitation has a clear effect on the change of the nonlinear dynamic characteristics of such piezoelectric laminated composite plates. The nonlinear vibration of the deploying cantilevered laminate can be effectively suppressed by choosing a suitable voltage and polarity.

A note on the Galilean invariance of aerodynamic force theories in unsteady incompressible flows

As a basic principle in classical mechanics,the Galilean invariance states that the force is the same in all inertial frames of reference.But this principle has not been properly addressed by most unsteady aerodynamic force theories,if the partial force contributed by a local flow structure is to be evaluated.In this note,we discuss the Galilean-invariance conditions of the partial force for several typical theories and numerically test what would happen if these conditions do not hold.

LINEAR AND NONLINEAR AERODYNAMIC THEORY OF INTERACTION BETWEEN FLEXIBLE LONG STRUCTURE AND WIND

In light of the characteristics of the interactions between flexible structure and wind in three directions, and based on the rational mechanical section-model of structure, a new aerodynamic force model is accepted, i.e, the coefficients of three component forces are the functions of the instantaneous attack angle and rotational speed C-t = C-t (beta(t), 0). (i = D, L, M). So, a new method to formulate the linear and nonlinear aerodynamic items of wind and structure interacting has been put forward in accordance with 'strip theory' and modified 'quasi-static theory', and then the linear and nonlinear coupled theory of super-slender structure for civil engineering analyzing are converged in one model, For the linear aerodynamic-force parts, the semi-analytical expressions of the items so-called 'flutter derivatives' corresponding to the one in the classic equations have been given here, and so have the nonlinear parts. The study of the stability of nonlinear aerodynamic-coupled torsional vibration of the old Tacoma bridge shows that the form and results of the nonlinear control equation in rotational direction are in agreement with that of V. F. Bohm's.

Wind tunnel study of aerodynamic wind loading on middle pylon of Taizhou Bridge

Segment sectional model tests are carried out to investigate the wind loading on middle pylon of Taizhou Bridge, which has complicated three-dimensional flow due to its feature of double columns. Through the force measuring tests, aerodynamic force coefficients of every segment of the pylon columns have been obtained. It is found that the tested aerodynamic force coefficients are much smaller than those given by codes. The interference effects of aerodynamic force coefficients between columns of pylon are discussed. The results show that the interference effect is the most evident when the yaw angle is about 30 ° from transverse direction. This kind of interference effect can be described as diminutions in transverse aerodynamic force coefficients and magnifications in longitudinal aerodynamic force coefficients of downstream columns.

SIMULATION STUDY OF AERODYNAMIC FORCE FOR HIGH-SPEED MAGNETICALLY-LEVITATED TRAINS

Based on Reynolds average Navier-Storkes equations of viscous incompressible fluid and k-ε two equations turbulent model, the aerodynamic forces of high-speed magnetically-levitated （maglev） trains in transverse and longitudinal wind are investigated by finite volume method. Near 80 calculation cases for 2D transverse wind fields and 20 cases for 3D longitudinal wind fields are analyzed. The aerodynamic side force, yawing, drag, lift and pitching moment for different types of maglev trains and a wheel/rail train are compared under the different wind speeds. The types of maglev train models for 2D transverse wind analysis included electromagnetic suspension （EMS） type train, electrodynamic suspension （EDS） type train, EMS type train with shelter wind wall in one side or two sides of guideway and the walls, which are in different height or/and different distances from train body. The situation of maglev train running on viaduct is also analyzed. For 3D longitudinal wind field analysis, the model with different sizes of air clearances beneath maglev train is examined for the different speeds. Calculation result shows that： ① Different transverse effects are shown in different types of maglev trains. ② The shelter wind wall can fairly decrease the transverse effect on the maglev trains. ③ When the shelter wall height is 2 m, there is minimum side force on the train. When the shelter wall height is 2.5 m, there is minimum yawing moment on the train. ④ When the distance between inside surfaces of the walls and center of guideway is 4.0 m, there is minimum transverse influence on the train. ⑤ The size of air clearance beneath train body has a small influence on aerodynamic drag of the train, but has a fairly large effect on aerodynamic lift and pitching moment of the train. ⑥ The calculating lift and pitching moment for maglev train models are minus values.

Characteristics of aerodynamic force and flow structure behind single box girder under isolated slit control

An isolated slit was placed in a single box girder to obtain passive leading-edge suction and trailing-edge jet flow to control the unsteady aerodynamic force and modify the flow structure.The Great Belt East Bridge was used as a physical model at a geometric scale of 1:125.Wind tunnel experiments were conducted at an incoming airflow speed of 10 m/s,and the Reynolds number was calculated as 2.3×104 using the test model height and wind speed.The surface pressure distribution was measured,and the aerodynamic force acting on the test model with and without the isolated slit was calculated by integrating the pressure result.It was found that the control using an isolated slit can dramatically decrease the fluctuating surface pressure distribution and aerodynamic force.An analysis on the power spectral density of the lift force revealed that the isolated slit accelerated vortex shedding.Moreover,high-speed particle image velocimetry was used to investigate the wake flow structure behind the test model.A vortex separated from the upper surface was pushed to a lower location and the wake flow structure was modified by the isolated slit.A proper orthogonal decomposition(POD)of the flow field showed that the first two POD modes in the controlled case contributed less energy than those in the uncontrolled case,indicating that more energy was transferred to higher modes,and small-scale vortices had more energy.A secondary instability structure was found in the wake flow for a nondimensional jet momentum coefficient J of 0.0667.

Side-View Mirror Vibrations Induced Aerodynamically by Separating Vortices

While driving a car at high speed cruising, the mirror surface of side-view mirrors happens to vibrate. The vibration often leads to image blurs of objects reflected in the mirror. Once the phenomena happen, drivers cannot clearly identify the approaching vehicles from the rear. The paper aims to clarify the vibration modes of side-view mirror experimentally and to capture forces on the mirror surface induced by separating vortices around the mirror numerically. Experimental study clarified two findings. One is that the mirror has the primary natural frequencies of 25, 30 and 33 Hz. The other is that vibrations of the mirror increase in proportion to flow velocity and their frequencies have peak values at 120 and 140 km/h. The frequencies of the mirror vibration coincide completely with the primary natural frequencies. In order to capture the external forces vibrating the mirror surface, numerical study was performed by unsteady air-flow analyses. Relationships between flow velocity fluctuations close to the mirror surface and pressure fluctuations on the mirror surface were investigated. It was found that the two power spectra have peak values at the same frequency of 24.4 Hz at 120 km/h. This shows that flow velocity fluctuations with the frequency of 24.4 Hz affect directly pressure fluctuations on the mirror surface. Numerical analyses clarify that the frequencies of shedding vortices are 24.4 Hz at 120 km/h and 28.3 Hz at 140 km/h. The frequencies of mirror vibration are very close to those of flow fluctuations. This shows that the frequencies of the mirror vibration have much to do with the frequencies of the forces induced aerodynamically by vortex shedding. Therefore it follows that image blurs at high speed cruising are caused by resonance phenomena that the mirror surface resonates with the frequencies of shedding vortices around the mirror.

Machine learning-assisted sparse observation assimilation for real-time aerodynamic field perception

Accurate aerodynamic distribution perception and real-time flight state evaluation are crucial for flight safety,e.g.,stall detection.However,the observations are usually sparse due to limitations in sensor mounting space and cost,and a reconstruction technology is urgently required.Herein,a machine learning-assisted assimilation method based on sparse observations has been proposed.Different from the traditional reconstruction methods focusing on boundary condition correction,the proposed method formulates the flow field pressure distribution as a linear superposition of flow field modes,thereby forming a real-time reconstruction pattern that combines offline modal extraction using computational fluid dynamics(CFD)with real-time determination of modal weights using a neural network.In this study,CFD simulations were conducted under 800different operating conditions for common modal extraction and model training.The weights of these modes were determined online based on merely five observations for reconstructing the full pressure field.A pressure reconstruction with a relative error of 6.1%and a mean square error of 0.003 was achieved within the prescribed condition range.The computational cost was just2 ms for each reconstruction run,significantly faster than the 20 min required by the classical reconstruction ensemble transform Kalman filter.It also showed that the method maintains almost the same accuracy amidst 1.5%measurement noise.As practical examples,shock waves and the change of lift coefficient were analyzed using the proposed method,providing remarkable evidence for the capability of the method in supporting stall detection.These validate the method’s effectiveness and explore its potential in real-time and accurate monitoring of an aircraft.

Wing/body kinematics measurement and force and moment analyses of the takeoff flight of fruitflies

In the paper, we present a detailed analysis of the takeoff mechanics of fruitflies which perform voluntary takeoff flights. Wing and body kinematics of the insects during takeoff were measured using Based on the measured data, high-speed video techniques. inertia force acting on the insect was computed and aerodynamic force and moment of the wings were calculated by the method of computational fluid dynamics. Subtracting the aerodynamic force and the weight from the inertia force gave the leg force. The following has been shown. In its voluntary takeoff, a fruitfly jumps during the first wingbeat and becomes airborne at the end of the first wingbeat. When it is in the air, the fly has a relatively large ＂initial＂ pitch-up rotational velocity （more than 5 000~/s） resulting from the jumping, but in about 5 wingbeats, the pitch-up rotation is stopped and the fly goes into a quasi-hovering flight. The fly mainly uses the force of jumping legs to lift itself into the air （the force from the flapping wings during the jumping is only about 5%-10% of the leg force）. The main role played by the flapping wings in the takeoff is to produce a pitch-down moment to nullify the large ＂initial＂ pitch-up rotational velocity （otherwise, the fly would have kept pitching-up and quickly fallen down）.

Bifurcation and dynamic behavior analysis of a rotating cantilever plate in subsonic airflow

Turbo-machineries,as key components,have wide applications in civil,aerospace,and mechanical engineering.By calculating natural frequencies and dynamical deformations,we have explained the rationality of the series form for the aerodynamic force of the blade under the subsonic flow in our earlier studies.In this paper,the subsonic aerodynamic force obtained numerically is applied to the low pressure compressor blade with a low constant rotating speed.The blade is established as a pre-twist and presetting cantilever plate with a rectangular section under combined excitations,including the centrifugal force and the aerodynamic force.In view of the first-order shear deformation theory and von-K′arm′an nonlinear geometric relationship,the nonlinear partial differential dynamical equations for the warping cantilever blade are derived by Hamilton’s principle.The second-order ordinary differential equations are acquired by the Galerkin approach.With consideration of 1:3 internal resonance and 1/2 sub-harmonic resonance,the averaged equation is derived by the asymptotic perturbation methodology.Bifurcation diagrams,phase portraits,waveforms,and power spectrums are numerically obtained to analyze the effects of the first harmonic of the aerodynamic force on nonlinear dynamical responses of the structure.

Effects of 3-D deformation of elastic wings on aerodynamic performance of an aircraft model

The wind tunnel experiment is conducted on a simplified aircraft model with rigid and two kinds of elastic wings to investigate the effect of wing 3-D deformation on the aircraft aerodynamic performance.The results show that two elastic wings exhibit different aerodynamic performances,which are classified as the lift-enhancement wing and the drag-reduction wing.For the liftenhancement wing,the stall angle is delayed from 8°to 15°with a corresponding lift increment of 64.3%compared with the rigid wing.It is shown that the lift enhancement of the aircraft model is accompanied by the torsional vibration mode of the wing,which results in the significant improvement of wing circulation.For the drag-reduction wing,the aerodynamic performance is dominated by the time-averaged deformation,which couples the bending and twisting.The wing twist reduces the effective angle of attack,as well as the frontal area,and contributes to the decreased wake deficit.Meantime,the bent wings function as barriers to the cross flow resulting in a reduction of lift-induced drag.As a result,the drag coefficient is reduced from 0.115 to 0.098 with a reduction of 14.8%at angle of attack of 12°.