Unsteady Aerodynamic Characteristics Research Articles

Research on the unsteady flow field and aerodynamic noise characteristics in the circular space of ultra-high-speed elevators

Due to the non-streamlined blunt body shape of the elevator car, intense aerodynamic loads and noise are generated in the annular space when the elevator operates at high speed in a narrow hoistway. This article uses large-eddy simulation and Curle acoustic theory to establish a numerical model of the wellbore flow field under different guiding shapes of the elevator car. First, the mechanism of action between the flow guide shape, vortex characteristics, and aerodynamic load was studied, and the influence of operating speed on aerodynamic load under different flow guide shapes was analyzed. Second, the distribution and spectral characteristics of noise in the annular space under different flow guide shapes were studied from the near field and far field scales. Finally, it can be concluded that increasing the guide surface can significantly reduce the pressure difference between the upper and lower parts of the car, weakening the separation of the tail airflow. The number of guide surfaces, installation position, and vertex offset significantly impact drag and lateral force, with the optimal reduction of 34.65% and 57.6%, respectively. The maximum sound source intensity of near-field noise is mainly concentrated at the arc transition in the upper part of the annular space, and the frequency is mainly concentrated in the range of 500–1000 Hz. The sound pressure level is linearly related to the logarithm of the longitudinal distance, and the noise attenuation speed becomes faster with the increase in the number of guiding surfaces.

Unsteady aerodynamic characteristics of a morphing tail configuration

Morphing aircraft is an important trend in the future development of next-generation aircraft. This paper focuses on aircraft with a small aspect ratio flying wing configuration that includes morphing tails. The unsteady aerodynamic characteristics of the morphing tail configuration are numerically simulated using the unstructured overset grid technique and the dual-time method, focusing on the effects of the tail deflection angle over time, Mach number, and side slip angle on the unsteady aerodynamic characteristics of the entire aircraft and tails. The second derivative of the tail deflection angle with time should be continuous, and the maximum angular velocity and maximum angular acceleration should be minimized. The hysteresis loop area is closely related to the Mach number. The sideslip angle aggravates the nonlinearity of the unsteady aerodynamic characteristics of the entire aircraft. The unsteady aerodynamic characteristics during tail morphing are affected by static (wing downwash effect and projected area effect) and dynamic (flow field hysteresis effect, additional motion effect, and wall implication effect) characteristics. The enclosed research provides a reference for the aerodynamic and control system designs of morphing tail configurations.

Transient aerodynamic behavior of a high-speed Maglev train in plate braking under crosswind

The test speed of high-speed maglev trains (HSMT) exceeds 600 km/h, requiring higher braking performance and technology. Plate braking technology, which is a suitable choice, has been applied for engineering the high-speed test vehicles. However, the unsteady aerodynamic response during the opening process of HSMT under crosswind needs to be studied. This study explores the unsteady aerodynamic characteristics of a HSMT with a train speed of 600 km/h during plate braking at different crosswind speeds. The plate motion is achieved based on the dynamic grid technology, and the unsteady flow field around the train is simulated using the unsteady Reynolds time averaged equation and the shear stress transport k-omega (SST k–ω) turbulence model. This calculation method was verified using wind tunnel test data. The peak aerodynamic drag (AD) of the braking plates overshot during opening. Under a crosswind of 20 m/s, the AD peak of the first braking plate was 11% larger than that without crosswind. The middle braking plates were significantly affected by upstream vortex shedding, and the AD fluctuation was the most severe. The AD of the head and tail coaches is significantly affected by crosswind. With an increase in the crosswind speed, the AD of the head and tail coaches decreased and increased, respectively. Compared with no crosswind, under a crosswind of 20 m/s, the AD of the head coach decreased by 43%, and the AD of the tail coach increased by a factor of approximately 1.1 times. Furthermore, the AD fluctuation of the tail coach was the most severe.

Numerical study on aerodynamic performance during the process of entering the formation flight

. Two or more planes can fly in close formation similar to migratory birds, making use of the wing-tip vortex of the leader plane to increase lift and reduce drag, thereby effectively improving the flying range. By conducting wind tunnel tests and numerical simulations, the aerodynamic performance of formation flight at different relative positions can be obtained, the optimal formation position can thereby be solved. However, significant nonlinear and unsteady aerodynamic characteristics, induced by the interference between the follower plane and the wing-tip vortices of the leader plane, will affect the flight safety of the whole formation. At present, there is no effective prediction methods. Numerical simulations adopting adaptive grid refinement and dynamic overset grid were conducted for the dynamic entering process of the formation flight containing two Ty-154 planes. The aerodynamic characteristics and vortex interferences were analysed considering the effects of approaching direction and speed. The results indicate that there is deterioration of stability at the position where maximum lift gain is reached; Compared to the entering speed, the impact of directions on dynamic aerodynamic characteristics is more significant.

Modeling of the Unsteady Aerodynamic Characteristics of the NACA 0015 Airfoil from the Data of Numerical Calculations of the Flow

Modeling of the Unsteady Aerodynamic Characteristics of the NACA 0015 Airfoil from the Data of Numerical Calculations of the Flow

An Investigation on the Steady and Unsteady Wake Flow Regimes and Aerodynamic Characteristics of the Trapezoidal Cylinders Using Immersed Boundary-Lattice Boltzmann Method

Abstract In this study, the flow physics of the forward-facing (FF) and backward-facing (BF) trapezoidal cylinders (TC) subjected to two-dimensional, incompressible, and laminar flow is investigated using an in-house developed flexible forcing immersed boundary-lattice Boltzmann solver. The Reynolds number (Re) is defined based on the cylinder's characteristic length D. For the steady and unsteady flow regimes, Re is varied in the ranges of 10–40 and 75–125, respectively. The TCs shape is varied by modifying its nondimensional axial H/D and transverse Y/D length scales, between 0.5 to 2 and 0 to 1, respectively. Here, TCs horizontal central axis is always aligned along the incoming flow direction. It is observed that the flow separation points on the FF-TC and BF-TC are strongly influenced by the geometric (H/D and Y/D) and flow parameters (Re). Based on the boundary layer separation point, we have categorized the wake flow regimes behind the FF-TC and BF-TC into four types. In addition, the effect of the geometric and flow parameters on the drag coefficient (Cd), vortex shedding frequency, and steady and unsteady wake characteristics are thoroughly investigated here. Furthermore, by performing nonlinear regression analysis, we have proposed a set of correlation equations for the Cd and Strouhal number (St), using which the aerodynamic characteristics of differently shaped TC can be derived in the considered Re range without performing rigorous numerical simulations or experiments.

Aerodynamic performance analysis of a floating wind turbine with coupled blade rotation and surge motion

The unsteady aerodynamic characteristics and interference effects of a floating wind turbine are significantly influenced by the six-degree-of-freedom (6DoF) motions. In this paper, the computational fluid dynamics (CFD) method using the overset grid technique and the improved delayed detached eddy simulation (IDDES) model was adopted to investigate the effects of harmonic enforced surge motions on the aerodynamic performance and wake characteristics of wind turbines. The aerodynamic load coefficients including the constant term, damping coefficients (surge velocity-induced), and additional mass coefficients (surge acceleration-induced), were fitted by using the least squares method at different surge frequencies and surge amplitudes. The blade pressure distributions and wake characteristics were analysed in detail. The Ω ~ R of the third-generation vortex identification method was applied to visualize vortex evolution. The numerical results are evaluated against the data obtained by wind tunnel experiments, indicating an acceptable relative error of 3.45 % for the power coefficient, C p , at the rated tip speed ratio (TSR). The higher frequency and greater amplitude surge motions lead to more dramatic fluctuations for the power coefficient than the axial load coefficient. The neglect of the additional mass coefficient only results in an acceptable error. Furthermore, the mean damping coefficient is on average approximately 3.5 times the mean constant term coefficient for C p , approximately 1.5 times for C z . In terms of the blade pressure distributions and wake characteristics, a higher comprehensive induced velocity, V c , is induced to expand the wider high-pressure area and shrink the low-pressure area at the pressure surface. Meanwhile, toroidal vortex ropes were induced by surge motion. Besides, the surge motion at high frequency and greater amplitude positively impacts enhancing wake deficit recovery.

Research of Unsteady Aerodynamic Characteristics of Electrically Controlled Rotor Airfoils with Trailing-Edge Flaps

An electrically controlled rotor (ECR), also known as a swashplateless rotor, eliminates the swashplate system to implement the primary control via the trailing-edge flaps (TEFs), which can result in enhancements in rotor performance, as well as substantial reductions in weight, drag, and cost. In this paper, the unsteady aerodynamic characteristics of the airfoil with TEF of a sample ECR under unsteady freestream condition are investigated. The CFD results are obtained with sliding and overset grid techniques that simulate the airfoil pitching and flap deflection. Comparative analysis of the aerodynamic characteristics under steady and unsteady freestream conditions at different advance ratios is conducted. At various advance ratios, the lift and drag coefficients are higher at a small angle of attack under unsteady freestream condition; however, it is the opposite at a large angle of attack. The peak values of the lift and drag coefficients show an increased trend with the increase in the advance ratio. On the contrary, the pitch moment and flap hinge moment coefficients demonstrate minor variation under unsteady freestream condition. Furthermore, the aerodynamic characteristics of airfoils become more unsteady with variation in the freestream. Therefore, the lift and drag coefficients of the ECR airfoil with TEF show significant differences between steady and unsteady freestream conditions; however, the pitch moment and the flap hinge moment coefficients show little difference.

Wing flexibility effect on aerodynamic performance of different flapping wing planforms

Uniform downwash of a revolving or flapping wing is a key indicator of aerodynamic efficiency. Although the slack-angle of flexible wings can significantly alter the downwash due to the inherent aerodynamic twist, its effect on the downwash distribution for a change in planform remains unknown. In this study, the effect of wing flexibility (slack-angle β) on the unsteady aerodynamic characteristics is investigated using three wing planforms. The study reveals that ββ > 5° can suppress the wing planform effect, leading to the generation of the same amount of lift at mid-stroke. To emphasize on the role of wing flexibility, further comparisons are made with rigid cases for each change in planform. A change in planform for the rigid cases significantly affects the position of the tip vortices (TiVs). But, the TiV positions of the flexible wings remained unchanged, helping to generate a wider downwash area to enhance the lift in contrast to that of the rigid-wing counterparts. The study found that the flexible and rigid wings can generate two distinct downwash patterns, which are respectively near uniform and tip-oriented, during the flapping motion. The ability of the flexible wings to sustain the wider near uniform downwash irrespective of the selected planform is paramount for enhanced aerodynamic performance.

Impact of bogie fairing configuration on the aerodynamic performance of high-speed train under crosswind

In this paper, the unsteady aerodynamic characteristics of a three-car grouped high-speed train (HST) with different bogie fairing configurations under the crosswind are studied using improved delayed detached eddy simulation (IDDES) at Re = 1.85 × 106. The numerical method results are validated against the previous wind tunnel experiments. There are three cases with original half-fairing configuration (HFC), non-fairing configuration (NFC) and full-fairing configuration (FFC), respectively. The numerical results show that the bogie fairing configuration has significant effects. Compared to the HFC case, the drag force coefficients of the head, middle and tail cars with NFC increase by 53.13%, 14.87% and 27.04%, and corresponding parts of the FFC case decrease by 49.98%, 24.40% and 12.41%, respectively. The FFC case can also optimize the pressure distribution on the windward and leeward sides of the HST, and the flow structure and vertex distribution around the car body are also optimized. In addition, the increasing encirclement of the fairing configurations reduces the maximum lateral flow velocity inside the bogie cavities by 70% for HFC and 50% for FFC compared to the NFC. Overall, the FFC presents better aerodynamic performance than the NFC and HFC cases under crosswind, which is recommended for the design of the HST.

Fluid-structure interaction simulation for multi-body flexible morphing structures

The multi-body flexible morphing airfoil can improve the aerodynamic characteristics based on different flight missions continuously. Recently researches have focused on the unsteady aerodynamic characteristics of flexible wings under passive actuation. However, the unsteady aerodynamic characteristics with the fluid–structure interaction effects in the multi-body active actuation process of morphing airfoil deserve further investigation. In this paper, a fluid–structure coupled simulation method for multi-body flexible morphing airfoil with active actuation subsystem was investigated, and the aerodynamic characteristics during deformation were compared with different skin flexibility, flow field environment, actuation mode and actuation time. The numerical results show that for the steady aerodynamic, the skin flexibility can improve the stability efficiency. In the unsteady process, the change trend of the transient lift coefficient and pitching moment are consistent with those of the active drive characteristics, while the instantaneous lift-drag ratio coefficient is greatly affected by the driving mode and can be improved by increasing the driving duration.

Steady and unsteady characteristics of circular arc airfoils for water pumping windmills

Accurate airfoil lift and drag data at low Reynolds number, Re, and high angles of attack, α, are needed to analyze the performance of small wind turbines, particularly their starting. In the current study, the steady and unsteady aerodynamic characteristics of circular arc airfoils (CAAs) with and without spars, as used in water-pumping windmills, were examined in a wind tunnel of 1 m2 cross section at Re<106. The tunnel was configured as an open jet and a closed section. The effects of varying geometrical characteristics on the CAA performance were investigated using a combination of thickness, camber, aspect ratio, and airfoil chord-to-tunnel height ratio. Using force transducers, the aerodynamic forces acting on the airfoils were measured directly for both increasing and decreasing α. The decreasing α measurements produce a higher lift–drag ratio than the increasing measurements, mostly in the post-stall region. In addition, “second stall,” was observed at large α in both the open and closed tunnels depending on the Re and tunnel blockage, but was much more prominent in the closed tunnel due to its wall constraining the wake and preventing the flow from switching from one regime to another. It was shown that the performance of all tested airfoils was sensitive to low Re under steady and unsteady conditions. The latter measurements for an airfoil oscillated at reduced frequencies, k≤0.06, are the first for CAAs. Results from this investigation provide a comprehensive airfoil dataset for the accurate blade element theory modeling of CAAs aerodynamic and starting performances.

A fractional derivative aerodynamic model at high angles of attack

A fractional derivative aerodynamic model is put forward based on Grünwald-Letnikov fractional derivative in this paper. In order to express the hysteresis property of aerodynamic coefficients, a fractional derivative state-space equation is developed from Goman’s differential equation. A fast and high-accuracy calculation method of Grünwald-Letnikov fractional derivative is proposed. Then, the model is demonstrated by several experiments data from literatures. The simulation results show that the presented fractional derivative aerodynamic model can preferably express the nonlinear unsteady aerodynamic characteristics of airfoils and aircrafts. The proposed modelling framework is meaningful to the designation of flight and control system.

The Aerodynamic Performance of Horizontal Axis Wind Turbines under Rotation Condition

The near-surface unsteady incoming flow in the atmospheric boundary layer has a great influence on the aerodynamic performance of horizontal axis wind turbines. To consider the effect of the rotation of the blade on the aerodynamic state of a wind turbine near the ground, the fluid-structure interaction (FSI) method based on the shear stress transfer (SST) turbulence model is applied to analyze the unsteady aerodynamic interaction characteristics including solving the velocity field, pressure field, structural response state, variation of deformation, and output power in the flow field of the wind turbine. The deformation fluctuation points of different blades in the upwind and downwind regions were observed to move towards the blade tips with increasing rotational speed. The variations of flow velocity and pressure that occur along the radial direction of the wind turbine are observed. The velocity increases from the root to the tip of the blade. The tower shadow effect causes the blade deformation in the upper and lower wind areas to fluctuate. It is more obvious when the blade overlaps with the tower; the overall displacement under the effect of rotation has a large increase compared with the shutdown. The peak increments reach 2.1437 mm to 0.8674 mm; under the effect of inter-action wind speed increased, wind turbine output power increased from 68.33 kW to 84.33 kW, respectively. It helps to better understand the aerodynamic performance of wind turbines, prolong the service life, and optimize the design.

Unsteady Aerodynamic Characteristics of The Flapping Wing Micro Air Vehicle (MAV)

Flapping wing propulsion is center of attention in the field of micro air vehicle research in recent years. Knowledge in unsteady aerodynamics is insufficient to design an efficient wing that produces the required lift and thrust force for the flapping wing micro air vehicle. The unsteady aerodynamic effects on aerodynamic wing design parameters were discussed in this paper. The effect of aspect ratio and the wing planform shape on lift and thrust force were determined experimentally. The experiments were conducted in an open jet low speed wind tunnel having test section 350 mmx350 mm. A rack and gear type new flapping mechanism was designed and fabricated and this was used as test bed for all the experiments. Also an experiment was conducted to determine the effect of torsional flexibility of the wing. The result shows that the chord wise flexible wing produces more lift and thrust than the other wing.

Numerical investigation of unsteady aerodynamic characteristics induced by the interaction of oblique/bow shock waves during rotation

The interactions between oblique and bow shock waves are significant problems related to the aerodynamic force and thermal performance of hypersonic vehicles, but few studies have considered the dynamic effect of the body's motion on the phenomena. In this work, a numerical study on the oblique and bow shock waves ahead of an elliptic cylinder rotating with a forced-oscillation approach was conducted at Mach 5 by solving the unsteady, two-dimensional Navier–Stokes equations in a non-inertial coordinate system. The hysteresis loops of aerodynamic coefficients were analyzed first, and it was found that the moment is sensitive to rotation. Then, two different hysteresis forms were found at positive and negative angles of rotation (AOR), corresponding to cases with the interference point above or below the wall, respectively. When AOR is positive, the rate-dependent transition hysteresis among various shock interaction types causes the movement of strong flow structures (reflected shock wave, Mach stem, and jet) to always lag behind the body's motion. When AOR is negative, besides the evolution hysteresis of flow structures, two unusual patterns between Edney Types III and VI were observed on different transition paths, which led to very different peak pressures. Also discussed are the driving mechanisms associated with the effect of the subsonic region and the downstream boundary of the interaction zone, as well as the modulating action of the formed virtual Laval flow channel. Additional simulations were performed to study the effect of rotation speed on the transition boundary and the transition structures between Types III and VI.

Improved delayed detached eddy simulation-based investigation of aerodynamic performance and flow field characteristics of high-speed trains with plate brakes

As a key component, the braking system is vital for ensuring safe and comfortable operation of trains moving at high speeds. Because the aerodynamic drag depends linearly on the square of the vehicle speed, plate braking has broad prospects for high-speed trains. The train body interface is altered by the plate opening, which causes a sudden change in the flow field around the train and deteriorates its aerodynamic performance. In this study, dynamic grid technology was used to realize plate opening, and an improved delayed detached eddy simulation (IDDES) was used to simulate the unsteady aerodynamic characteristics of trains during plate braking. The simulations and settings were verified using wind-tunnel test data. The effects of the plate motion on the aerodynamic performance of the train were clarified by analyzing the changes of the aerodynamic loads of the plate and the train body within 0.2 s for an opening plate and 1.8 s for an opened plate. The aerodynamic forces of the different cars were significantly different during the plate-opening process, especially near the maximal opening angle. In particular, for the middle car, the aerodynamic force depended on the number of plates. The plate opening caused lateral aerodynamic fluctuations in the car body, which caused the car body to shake laterally, especially the downstream car, and the degree of the fluctuation of the lateral force of the car body also depended on the number of plates. Owing to the airflow inertia, the aerodynamic drag of the plate at the end of the motion appeared as a pulse peak, and the peak value decreased downstream of the train. The opening of the plates caused the flow field above the roof to develop disordered downstream. Owing to the coupling and superposition of the downstream flow field, the aerodynamic drag of the first plate fluctuated slightly, whereas that of the downstream plate fluctuated significantly, likely affecting the normal operation of the equipment on the top of the train, such as air conditioning and pantograph.

Aerodynamic characterization of two tandem wind turbines under yaw misalignment control using actuator line model

Yaw control has proven to be promising in alleviating the wake effects that plague the efficiency of wind farms. In this work, the actuator line modeling (ALM) method is adopted to simulate the flows over two tandem turbines distanced by 3–7 rotor diameters, with the yaw angle of the upstream rotor varying from γ1=0° to 50°. The aim is to provide a comprehensive aerodynamic characterization of this simple wind farm under yaw misalignment control. With increasing yaw angle, the power generated by the downstream rotor increases, compensating the power loss in the upstream rotor, and resulting in significantly higher total power of the two turbines than that without yaw control. The maximum power output is achieved as the upstream wake of the yawed rotor is redirected away from the downstream rotor plane. Behind the downstream rotor, the secondary steering phenomenon is observed, where the wake is also redirected from the centerline. The use of the actuator line model also reveal unsteady aerodynamic characteristics that cannot be captured by lower-fidelity models. For the upstream rotor, the yaw misalignment results in time-varying change in the local angle of attack on the blade, giving rise to unsteady loading. The downstream rotor is partially submerged in the deflected wake incurred by the yawed upstream rotor. As the blade revolves into and out of the wake deficit, the blade experiences cyclic loading, leading to even stronger fluctuations in the aerodynamic loads than the upstream rotor. These analysis provides a comprehensive understanding of the yaw control effects on the two tandem rotors from the perspectives of aerodynamic performance, wake profiles, and unsteady characteristics. The insights gained from the present study can aid the design of collective yaw control strategies of wind farms, and lay the foundation for assessing the fatigue damage associated with yaw misalignment.

Unsteady aerodynamic characteristics and dynamic performance of high-speed trains during plate braking under crosswind

Unsteady aerodynamic characteristics and dynamic performance of high-speed trains during plate braking under crosswind

A wind tunnel investigation of the unsteady aerodynamic characteristics of bionic flapping wings with three-dimensional complex motion

The bionic flapping wing aircraft realizes flight by imitating the structure and flapping mode of birds. In this paper, a three-dimensional composite motion aerodynamic analysis model of a bionic flapping wing is established. The diverting field and flapping wing grid are divided up using dynamic hybrid grid technology. The flow field of the flapping wing is analyzed by solving the Navier–Stokes (N–S) equation combined with the Boussinesq hypothesis. The lift of the flapping wing under different flutter frequencies and incoming wind speeds is studied under the asymmetric flutter condition. In order to verify the accuracy of the aerodynamic simulation results, a flapping-wing shrinking-ratio model prototype is made, and low-speed wind tunnel experiments are carried out to test the changes in the flight lift of the wing at different flutter frequencies and angles of attack. A comparative analysis of the wind tunnel experiment and the aerodynamic simulation results shows that when the flapping frequency (1–5 Hz) and incoming wind speed (1–5 m/s) increase, the lift force generated by the wing flapping increases. Due to the deviation between the experimental sample airfoil area and the simulated airfoil area, as well as the wing-driven fuselage vibration during the experiment, a sensor error is produced, resulting in a deviation of about 1 N between the experimental result curve and the simulation result curve. However, the aerodynamic characteristics obtained from the aerodynamic simulation analysis are basically consistent with the aerodynamic change law measured in the wind tunnel experiment.