Changes In Aerodynamic Forces Research Articles

Influence of Structural Types of CRTS I Plate-Type Ballastless Track on Aerodynamic Characteristics of High-Speed Train

In order to improve the running quality of trains on a ballastless track, the influence of the CRTS I ballastless track with different structures (flat-type and frame-type tracks) is investigated with respect to the aerodynamic characteristics of high-speed trains. In the present paper, the aerodynamic force changes on the head, middle, tail, and whole car of the high-speed train were studied under two conditions, with crosswind and without crosswind, and the influence of different crosswind speeds (10, 15, 20, 25, 30 m/s) on the aerodynamic force of the train was analyzed. The pressure and flow field distribution characteristics were also studied, and the reasons for the different aerodynamic characteristics of different track structures and trains running in different wind environments were analyzed, respectively. The results indicate that the ballastless track structure obviously influences the aerodynamic characteristics of the high-speed train. When there is no natural wind, compared with the flat track, the frame track reduces the drag and lateral forces of the train but increases the lift force. The frame track causes the drag force of the whole vehicle to decrease slightly (the maximum ratio is 2.15%), the lift force increases significantly (the maximum ratio is 12.55%), and the lateral force obviously decreases (the maximum ratio is 52.43%). The lift and lateral forces of the middle car are most affected, which is because the frame structure changes the vortex motion state of the middle car. Compared with the flat track, the drag force of each car on the frame track is reduced under the crosswind; the lift force of each car is increased, and the maximum increase in the lift force of the head, middle, and tail cars is 5.60%, 2.55%, and 3.63%, respectively; the lateral force of the tail car increases greatly at a wind speed of 15 m/s, reaching 6.84%. Due to the existence of the frame structure, the space under the vehicle increases, resulting in a decrease in the airflow rate and an increase in local pressure, which leads to changes in the train’s aerodynamic force. Meanwhile, the train’s aerodynamic change under the crosswind is smaller than that when there is no wind.

Unsteady Aerodynamic Characteristics of a High-Speed Train Induced by the Sudden Change of Windbreak Wall Structure: A Case Study of the Xinjiang Railway

Under strong winds, the effect of sudden windbreak transition (WT) on high-speed trains is severe, leading to a deterioration of train aerodynamics and sudden yawing motion of the car body. To address these problems, based on a high-speed train and the specific geometric conditions derived from Xinjiang railway, first, the impact of a WT on the train and reasons for sudden changes in aerodynamic forces were determined by flow structural analysis. Furthermore, based on a multibody system dynamic model, the dynamic responses to WT were analysed. The results show that the impacts of WT were the strongest on the head car. WT had a strong effect on the train due to the unreasonable structural shape and the insufficient height of the windbreak in the transition region. This led to a strong push effect on the train; subsequently, the train’s dynamic characteristics deteriorated.

Effect of Braking Plates on the Aerodynamic Behaviors of a High-Speed Train Subjected to Crosswinds

Using aerodynamic resistance to provide braking force for trains is an economical braking method. It has few components to wear out and requires no energy. But the aerodynamic braking plate will significantly affect train’s aerodynamics behaviors. This paper studies the effect of the braking plates’ layout on the aerodynamic force of head car when a train is running under a crosswind. The results show that the braking plate will not only increase the drag force, but also significantly affect the lift and lateral force of the train’s head car. The installation position of the braking plates will also have a great effect on the aerodynamic force. In order to increase the drag force and weaken other aerodynamic force changes of the head car, we suggest that the first braking plate be arranged at the end of a streamlined shape, and the second braking plate be arranged at the middle of the car body. Compared with trains without braking plates, the head car’s drag force increases by 85.7%, lift force only increases by 7.6%, and side force decreases by 5.9%, when the braking plates are in operation.

A new deformation control approach for flexible wings using moving masses

This paper presents a new deformation control concept based on moving mass control technique for the flexible wing of the HALE vehicles. Such mass actuated wings can actively adjust its aeroelastic deformation to avoid the unexpected large deflection and also function as a “seamless morphing wing” to improve flight performance. This investigation focuses on the modeling, analysis, and primary controller design of a high-aspect-ratio wing controlled by an internal moving mass. The equations of motion are derived first by considering the couplings among the structural dynamics, the unsteady aerodynamics, and the transient motion of the mass. The variations in flutter speed and steady-state deformation with the locations of the mass are presented. The time-domain simulations are presented under various travel profiles for the moving mass. The feasibility of using moving mass for deformation control of the flexible wing is primarily proved. Placing the moving mass in front of the elastic axis is preferable to obtain higher flutter speed and larger deformation control authority. Continuous changes in aerodynamic forces can be realized following the structural deformation. The unsteady aerodynamic effect is beneficial for obtaining the moderate deformation control performance without oscillations and also eliminates the complex dynamic nonlinear coupling effects. Finally, a proof-of-concept control system integrated with incremental and positional PD controllers is designed to achieve the desired configuration of the wing.

Aerodynamic Study on a Heavy Truck Passing by a Bridge Pylon under Crosswinds Using CFD

AbstractHigh-sided vehicles experience sudden changes in aerodynamic forces when passing through the wake of towers of bridges under crosswind. This may lead, in the worst case, to vehicle rollover...

Study of Changes in the Aerodynamic Characteristics of the Axisymmetric Supersonic Vehicle in Case of Gas Blowing from the Lateral Surface

We consider the flow around the axisymmetric supersonic vehicle with the use of gas jetblowingfrom the lateral surface. The blowing is made in series of points at different distances from the nose fairing. The aim of the work was to determine the changes in aerodynamic forces and the formation of the moment, when jet of gasblowingin different parts of the supersonic vehicle. The study was conducted by numerical modeling of different cases of injection. As a result, data were obtained which showed the degree of influence not only jet thrust from the jet flow, but same the impact of the redistribution of the flow by body surface on the formation of aerodynamic forces and moments.

Impact of Retrorocket Plumes on Upper-Stage Aerothermodynamics During Stage Separation

In this paper, an extensive investigation of the separation process of the first two stages of a carrier rocket that employs solid rocket motors for the lower stage is presented. As the reference vehicle, the VEGA rocket is used. The effect of the plumes of first-stage retrorockets on upper-stage aerodynamics and aerothermal loads is analyzed by means of wind-tunnel testing in the hypersonic wind tunnel H2K of DLR, German Aerospace Center. Aerodynamic coefficients are determined by force measurements. In addition, pressure distributions on the upper-stage surface and schlieren images for flow visualization are recorded. Infrared thermography measurements are conducted to determine the effect on aerothermal loads. Different flow conditions are achieved by variation of Reynolds number, retrorocket injection pressure ratio, and angle of attack. Results showed extensive flow separation around almost the entire upper stage by the retrorocket plumes already at low injection pressure ratios. During angle-of-attack sweeps, sudden changes in the flow structure occurred accompanied by strong changes in aerodynamic forces at values of . This behavior was found to be influenced by hysteresis effects.

Instantaneous wing kinematics tracking and force control of a high-frequency flapping wing insect MAV

The superior maneuverability of insect flight is enabled by rapid and significant changes in aerodynamic forces, a result of subtle and precise change of wing kinematics. The high sensitivity of aerodynamic force to wing kinematic change demands precise and instantaneous feedback control of the wing motion trajectory, especially in the presence of various parameter uncertainties and environmental disturbances. Current work on flapping wing robots was limited to open-loop averaged wing kinematics control. Here we present instantaneous closed-loop wing trajectory tracking of a DC motor direct driven wing-thorax system under resonant flapping. A dynamic model with parameter uncertainties and disturbances was developed and validated through system identification. For wing trajectory generation, we designed a Hopf oscillator based central pattern generator with smooth convergence. Using the linearized model while treating the nonlinearity as disturbance, we designed a proportional-integral-derivative (PID) controller and a linear quadratic regulator (LQR) for instantaneous wing trajectory tracking at 24 Hz; Using the original nonlinear model, we designed a nonlinear controller to achieve robust performance at over 30 Hz. The control algorithms were implemented and compared experimentally on a 7.5 g Flapping Wing Micro Air Vehicle (MAV). The experiments showed that the PID and nonlinear controls resulted in precise trajectory tracking; while LQR controller tracked with less precision but with smaller input effort. In addition, the nonlinear control algorithm achieved better tracking of wing trajectories with varying amplitude, bias, frequency, and split-cycles while adapting to the variations on wing morphological parameters such as wing geometry and stiffness. Furthermore, the lift force measurements of the nonlinear control results were compared with those of open-loop average wing kinematics control commonly adopted in current designs.

Use of Microspoilers for Control of Finned Projectiles

Relative to other air vehicles, the physical control mechanism on a smart projectile plays much more of a central role in the overall system design. Many different smart projectile control mechanisms have been created, including aerodynamic based mechanisms such as movable canards, propellant based mechanisms such as squibs, and inertia based mechanisms such as internal moving masses. The work reported here considers small micro spoilers located between rear fins to create aerodynamic force changes to enable projectile control. In particular, boundary layer shock interaction between the projectile body, fins, and micro spoilers provides a multiplicative effect on controllable forces and moments. A parametric study varying the micro spoiler configuration is conducted to examine the level of control authority possible for this control mechanism concept. Results indicate that relatively small micro spoilers located between fins generate substantial control authority that is capable of eliminating impact errors caused by muzzle jump, aerodynamic uncertainty, and atmospheric winds. These conclusions are based on CFD predictions of the effect of micro spoilers on air loads coupled to a rigid 6-degree-of-freedom projectile trajectory simulation.

Force Measurements on Locusts during Visually-Evoked Collision Avoidance Maneuvers

The collision avoidance behavior of the locust, Schistocerca americana, in response to simulated approaching objects, also called looming stimuli, was investigated in a low speed wind tunnel. The animals were mounted using a sting on a sensitive six-component microbalance custom-designed for the experiments. Forces and moments were measured as a function of time during the simulated approach and interpreted in the context of collision avoidance behaviors. The stimuli presented from the side effectively evoked robust and discernible collision avoidance responses. Locusts attempted to avoid collision by either flying over or under the looming object, or alternatively by steering around it. These efforts appeared in the form of changes in aerodynamic forces and moments as a function of time. It was also observed that the locusts increased the wing flapping frequency in response to the stimulus.

A computational study of the wing–wing and wing–body interactions of a model insect

The aerodynamic interaction between the contralateral wings and between the body and wings of a model insect are studied, by using the method of numerically solving the Navier–Stokes equations over moving overset grids, under typical hovering and forward flight conditions. Both the interaction between the contralateral wings and the interaction between the body and wings are very weak, e.g. at hovering, changes in aerodynamic forces of a wing due to the present of the other wing are less than 3% and changes in aerodynamic forces of the wings due to presence of the body are less than 2%. The reason for this is as following. During each down- or up-stroke, a wing produces a vortex ring, which induces a relatively large jet-like flow inside the ring but very small flow outside the ring. The vortex rings of the left and right wings are on the two sides of the body. Thus one wing is outside vortex ring of the other wing and the body is outside the vortex rings of the left and right wings, resulting in the weak interactions.

SYNTHETIC JETS

▪ Abstract The evolution of a synthetic (zero-net mass flux) jet and the flow mechanisms of its interaction with a cross flow are reviewed. An isolated synthetic jet is produced by the interactions of a train of vortices that are typically formed by alternating momentary ejection and suction of fluid across an orifice such that the net mass flux is zero. A unique feature of these jets is that they are formed entirely from the working fluid of the flow system in which they are deployed and, thus, can transfer linear momentum to the flow system without net mass injection across the flow boundary. Synthetic jets can be produced over a broad range of length and timescale, and their unique attributes make them attractive fluidic actuators for a number of flow control applications. The interaction of synthetic jets with an external cross flow over the surface in which they are mounted can displace the local streamlines and induce an apparent or virtual change in the shape of the surface, thereby effecting flow changes on length scales that are one to two orders of magnitude larger than the characteristic scale of the jets. This control approach emphasizes an actuation frequency that is high enough so that the interaction domain between the actuator and the cross flow is virtually invariant on the global timescale of the flow, and therefore, global effects such as changes in aerodynamic forces are effectively decoupled from the operating frequency of the actuators.

Diffraction of shock waves by a moving thin wing

An analytical solution is obtained for the flow field due to the impinging of a plane shock wave of arbitrary strength by a thin wing moving in the opposite direction. The planform and the thickness distribution of the wing can be arbitrary and the speed of the wing can be either supersonic or subsonic relative to the undisturbed stream ahead of the shock or to that behind the shock. The solution is a generalization of the previous solution of Ting & Ludloff for the diffraction of shock wave by a two-dimensional stationary airfoil to a three-dimensional wing moving with supersonic or subsonic speed relative to the stream ahead of or behind the shock. The solution is employed for the analysis of the changes in aerodynamic forces when an airplane encounters a blast wave or a shock wave of another airplane. It is also used to study the diffraction of a shock wave or anN-wave advancing over flat terrains.