Unsteady Aerodynamic Performance Research Articles

Unsteady aerodynamic behaviour of high-speed maglev trains during plate braking in tailwind and headwind opening configurations

ABSTRACT High-speed maglev trains are important future rail transportation systems that require high braking performance. One approach to braking in high-speed trains is using aerodynamic plate brakes, which offer improved braking performance at high speeds but can impact the train’s aerodynamic characteristics. In this study, dynamic grid technology was used to achieve plate motion, and unsteady Reynolds-averaged Navier-Stokes (URANS) equations and shear stress transfer (SST) k-ω turbulence model were adopted to simulate the unsteady aerodynamic behaviour of a 5-car maglev train running at 600 km/h. Two wind tunnel experiments validated the method, and grid independence analysis was also conducted. The unsteady aerodynamic performance and transient flow field around the train during plate braking in tailwind and headwind opening configurations were studied and analysed. As the plates opened, there was a rapid increase in both aerodynamic drag and lift, creating a pulse-like change. The tailwind opening configuration generated relatively large peak aerodynamic forces, whereas the peak forces decreased successively downstream along the train length. As the flow stabilized, periodic fluctuations in the aerodynamic forces of the plates occurred. The interference with the convective field and the impact on the aerodynamic force of the tail car were more significant for the tailwind opening configuration. In addition, the tailwind opening plate caused significant fluctuations in the aerodynamic force of the tail car, with similar effects observed in other cars. From an aerodynamic perspective, the headwind opening configuration yielded better results. These findings can guide the design and implementation of plate brakes for high-speed maglev trains.

Experimental investigation on turbulence effects on unsteady aerodynamics performances of two horizontally placed small-size UAV rotors

Wind turbulent disturbance is one of the critical challenges to flying unmanned aerial vehicles (UAVs); especially when the size, weight, and power on board such aircraft are limited. Therefore, to shed light on such turbulent wind effects on UAVs' aerodynamic performances, wind tunnel experiments were conducted on the single- and multi-rotor configurations by varying the turbulence intensity up to 15%, while setting the wind velocity at 5 m/s. The turbulent disturbances were generated inside the wind tunnel using two different passive grids. One is with 29% blockage, and the other is 49%. As the single-rotor experimental tests were conducted, the onset turbulence was found to give rise to a significant change in thrust and the power coefficient in the presence of a lower revolution per minute (rpm). However, at higher rpm, only the wind effect is observed to be the dominant contributor to the aerodynamic performance. Additionally, three different rotor spacings are experimentally examined to measure the propeller wake interactions at the same velocity and different onset turbulence intensities. It was experimentally found that a significant wake interaction occurs between 0.5 D (diameter) and 1.25 D below the propellers, depending on the rotor spacing and the flow conditions. The present work opens up an experimental approach to examine the unsteady aerodynamic performance of multi-rotor UAVs in the presence of wind disturbances, i.e. at different turbulence intensities.

Mesh and model requirements for capturing deep-stall aerodynamics in low-Mach-number flows

The paper presents a comprehensive computational fluid dynamics investigation of the effects of grid resolution and turbulence-model choice for capturing the unsteady three-dimensional aerodynamic performance of NACA 0012 and 0021 airfoils, with specific focus on the deep-stall regime. At high angles of attack (α), wind turbine blades routinely experience vortex-induced vibrations, which can cause significant structural damages. Accurate predictions of post-stall aerodynamics can identify the frequencies at which such vibrations maybe triggered. In this context, the NACA 0012 airfoil simulations are conducted at a chord-based Reynolds number, , with the k-ω Shear-Stress Transport Reynolds-Averaged Navier-Stokes (RANS) and Improved Delayed Detached Eddy Simulation (IDDES) hybrid RANS-Large Eddy Simulation turbulence models. The effect of mesh resolution both in the wall-normal and spanwise directions is investigated. Only the IDDES model with a minimum spanwise resolution of 24 cells per chord length correctly predicts the aerodynamic forces. Spectral analysis shows the peak primary shedding frequency at , which signifies the end of the stall region. In the post-stall regime, both lift and drag frequencies drop asymptotically with increasing α. The Strouhal number, based on normalised chord length, remains nearly constant in this region. Based on this study, NACA 0021 airfoil runs are performed with IDDES for and on the finest wall-normal mesh and three spanwise grids. Simulations conducted on the finer spanwise grids demonstrate grid independence and show good agreement with experiments. The effect of varying on the airfoil frequency statistics is investigated. Additionally, comparison studies are presented to investigate the impact of airfoil thickness on the frequency content at . The results from the study provide guidance on the choice of mesh resolution with the IDDES model to accurately capture aerodynamic quantities for complex industrial applications.

Influence of curvature distribution smoothing on the reduction of aerofoil self-noise

PurposeThis paper aims to investigate the influence of smooth curvature distributions on the self-noise of a low Reynolds number aerofoil and to unveil the flow mechanisms in the phenomenon.Design/methodology/approachIn this paper, large Eddy simulation (LES) approach was performed to investigate the unsteady aerodynamic performance of both the original aerofoil E387 and the redesigned aerofoil A7 in a time-dependent study of boundary layer characteristics at Reynolds number 100,000 and angle of attack (AoA) 4-degree. The aerofoil A7 is redesigned from E387 by removing the irregularities in the surface curvature distributions and keeping a nearly identical geometry. Flow vorticity magnitude of both aerofoils, along with the spectra of the vertical fluctuating velocity component and noise level, are analysed to demonstrate the bubble flapping process near the trailing edge (TE) and the vortex shedding phenomenon.FindingsThis paper provides quantitative insights about how the flapping process of the laminar separation bubble (LSB) within the boundary layer near the TE affects the aerofoil self-noise. It is found that the aerofoil A7 with smooth curvature distributions presents a 10% smaller LSB compared to the aerofoil E387 at Reynolds number 100,000 and AoA 4-degree. The LES results also suggest that curvature distribution smoothing leads to a 6.5% reduction in overall broadband noise level.Originality/valueThis paper fulfils an identified need to reveal the unknown flow structure and the boundary layer characteristics that resulted in the self-noise reduction phenomenon yielded by curvature distribution smoothing.

Study of unsteady aerodynamic performance of a high-speed train entering a double-track tunnel under crosswind conditions

Significant variation exists in the aerodynamic performances of high-speed trains (HSTs) traveling in different infrastructure scenarios. When high-speed trains travel from one infrastructure scenario to another, the aerodynamic loads acting on the trains change significantly. To investigate the safety as a HST enters a tunnel under crosswind conditions, the unsteady aerodynamic performance of the HST and the flow structures around the train were numerically studied. The results demonstrated that the flow field and the pressure field were symmetrically distributed under conditions with no crosswind, while the distribution of the flow field and the pressure field were clearly asymmetric when crosswinds were present at the tunnel entrance. Furthermore, the flow structures near the train and the pressure distributed on the train surfaces outside the tunnel were most severely affected by the presence of crosswinds. Vortex structures appeared on the windward side surface of the train inside the tunnel and on the leeward side surface of the train outside the tunnel during the entrance of the train into the tunnel. Due to the sudden changes in the flow and the pressure as the train entered the tunnel, the aerodynamic loads changed drastically, and the variations of each vehicle were different, resulting in complex dynamic responses, including lateral vibrations and pitching movement. In particular, the aerodynamic performance of the rear vehicle was the worst as the train entered the tunnel when no crosswind conditions existed, while the head vehicle was the most negatively affected for safe operation of the HST when strong crosswinds were present at the tunnel entrance.

Numerical study of the influence of dome shape on the unsteady aerodynamic performance of a high-speed train's pantograph subjected to crosswind

Numerical study of the influence of dome shape on the unsteady aerodynamic performance of a high-speed train's pantograph subjected to crosswind

Unsteady aerodynamic performance of SD7062 airfoil at high Reynolds number

Unsteady aerodynamic performance of SD7062 airfoil at high Reynolds number

Unsteady aerodynamic performance of SD7062 airfoil at high Reynolds number

Unsteady aerodynamic performance of SD7062 airfoil at high Reynolds number

Unsteady aerodynamic performance of a tandem flapping–fixed airfoil configuration at low Reynolds number

In nature, insects with their forewings and hindwings undergoing small-gap flapping motion experience strong aerodynamic interaction. Conventional studies mainly focus on the propulsion performance of tandem flapping wings, while the interaction between a flapping wing and a fixed wing in the tandem configuration at low Reynolds numbers (Re) is unclear. In this paper, we numerically studied the aerodynamic performance and vortex structure of this tandem flapping–fixed airfoil configuration. The effects of horizontal distance (LX), vertical distance (LY), and geometric angle of attack (α) of the fixed wing on the thrust and lift performance are investigated. The results show that LX dominates the propulsion performance, while LY and α control the lift performance. The thrust enhancement of the flapping airfoil is effective only within a small range of LX, and the thrust is mainly determined by the changing rate of the impulse of the vortices directly connected to the airfoils. The lift reaches its peak when LY approaches the plunging amplitude. Compared with a fixed airfoil, the flapping–fixed configuration shows a larger lift-to-drag ratio, indicating a lift enhancement led by the interaction with the upstream flapping airfoil. Moreover, increasing LY and α simultaneously can lead to additional advantages in lift generation. Further analysis shows that changes of LY and α both manifest in a variation of the effective angle of attack of the fixed airfoil, thereby manipulating its lift generation. This paper provides an aerodynamic database and guidance for the design of micro air vehicles using tandem flapping–fixed wings.

Unsteady aerodynamic prediction for iced airfoil based on multi-task learning

Ice accretion on wind turbine blades and wings changes the effective shape of the airfoil and considerably deteriorates the aerodynamic performance. However, the unsteady performance of iced airfoil is often difficult to predict. In this study, the unsteady aerodynamic performance of iced airfoil is simulated under different pitching amplitudes and reduced frequencies. In order to efficiently predict aerodynamic performance under icing conditions, a multi-fidelity reduced-order model based on multi-task learning is proposed. The model is implemented using lift and moment coefficient of clean airfoil as low-fidelity data. Through using few aerodynamic data from iced airfoils as high-fidelity data, the model can achieve aerodynamic prediction for different ice shapes and pitching motions. The results indicate that, compared with single-fidelity and single-task modeling, the proposed model can achieve better accuracy and generalization capability. At the same time, the model can be generalized to different ice shapes, which can effectively improve the unsteady prediction efficiency.

Optimal Design and Aerodynamic Performance Prediction of a Horizontal Axis Small-Scale Wind Turbine

The improved aerodynamic design of a horizontal axis small-scale wind turbine blade is crucial to increasing the efficiency and annual energy production of the turbine. One of the vital stages in aerodynamic design is the selection of the airfoil. Using the existing airfoils for a blade design which results in higher turbine characteristics is tedious. Consequently, this paper provides an optimal design strategy for a horizontal axis small-scale wind turbine blade through the multiobjective optimization of the airfoil using the Nondominated Sorting Genetic Algorithm II (NSGA-II). The latter outperforms the other commonly used genetic algorithms (GAs), as well as the Computational Fluid Dynamics (CFD) investigation of the different airfoil types and the wind turbine rotors on the steady or unsteady state aerodynamic performance. An NACA4412 airfoil with higher aerodynamic efficiency is considered as a baseline for the optimization in order to increase the lift coefficient and lift to drag ratio while avoiding excessive variations in the maximum relative thickness and area. The optimized airfoil (NACA4412-OPT) is used as the cross-sectional profile in the design procedure for a novel 1.15 m diameter three-bladed wind turbine rotor at a wind speed of 11.5 m/s, tip speed ratio of 4.65, and pitch angle of 0.2° by the Wilson design method. The two-dimensional analysis demonstrates that the optimized airfoil outperforms the other airfoils yielding the highest lift coefficient and lift to drag ratio, as well as a larger pitch range. The three-dimensional analysis shows that the time-averaged power coefficient value (0.33) of the new wind turbine is almost 26% higher and more stable than that of the original wind turbine while avoiding a high increase of the axial thrust.

Aerodynamic simulation of effects of one- and two-side windbreak walls on a moving train running on a double track railway line subjected to strong crosswind

High-speed railway lines are widely distributed worldwide. The surrounding wind environment of such railway lines is complex, which significantly affects the safety of high-speed train operations. Using a combination of the computational fluid dynamics method of improved delayed detached eddy simulation and sliding grid technology, the effects of one- and two-side windbreak walls on the unsteady aerodynamic performance of a high-speed train running under a crosswind on a double-track railway line with an embankment and the flow-field characteristics around the train were systematically studied. The grid independence and numerical methods used in this study were verified. The results showed that when the windbreak wall was absent, the aerodynamic performance of the train running on the upstream railway line was the poorest, while the train running on the downstream railway line exhibited large aerodynamic fluctuations. The windbreak wall significantly reduced the aerodynamic force and its fluctuation; the one- and two-side windbreak walls had different effects on the unsteady aerodynamic performance of trains running on the upstream and downstream railway lines under a crosswind. The one-side windbreak wall could restrain the aerodynamic fluctuation of the train more and is thus preferred from the perspective of construction effort and cost.

Numerical Study of the Unsteady Aerodynamic Performance of Two Maglev Trains Passing Each Other in Open Air Using Different Turbulence Models

The strong change in the flow fields around two maglev trains (MTs) passing each other in open air may affect their manoeuvrability and passengers’ comfort. In this study, we evaluated the aerodynamic performance of two MTs passing each other via shear stress transport (SST) k–ω model and improved delayed detached eddy simulations based on the Spalart–Allmaras model (SA−IDDES) and the SST k–ω model (SST−IDDES). The accuracy of the numerical simulation method was verified using experimental data acquired from a moving model test. The results showed that the difference in the amplitude of the transient pressure obtained with the different turbulence models was less than 5%. The wake vortex structures on the intersection side were found to interact, and their intensity consequently decreased. The SST−IDDES model produced smaller-scale vortices than the SA−IDDES model, particularly in the near-wake region. There were large differences in the drag and lift forces obtained using the different turbulence models. Among them, the lift force of the tail car was more sensitive to the turbulence model, and its maximum value obtained with the SST−IDDES model was 11% larger than that obtained with the SA−IDDES model.

Vortex control strategy for unsteady aerodynamic optimization of a plunging airfoil at a low Reynolds number

This study has explored effective flow control strategies to improve unsteady aerodynamic performance of a plunging airfoil at a low Reynolds number of Re = 59 000 through controlling the leading-edge vortex (LEV) formation and development. The leading-edge blowing control is first utilized to directly strengthen the LEV by injecting momentum into the separated leading-edge shear layer. The influence of the momentum coefficient on the vortical evolution and aerodynamic forces is more significant than that of the exit width. As the momentum coefficient increases in a certain range, the LEV can be enhanced, thereby increasing the maximum unsteady lift. However, it is found that an accelerated detachment usually occurs with enhancement of the LEV as the secondary vortex is also promoted by blowing, leading to a reduced duration of high lift compared with the baseline case. In order to solve such a control problem of the LEV, the control strategy of the combined blowing and suction is further proposed, which could increase the LEV strength and concurrently delay the LEV detachment to some extent through inhibiting the growth of the secondary vortex. As a result, the unsteady aerodynamic performance of the plunging airfoil has been further improved with a significant increase in the maximum lift coefficient and a moderately prolonged duration of high lift.

Low-Reynolds Number Behavior of the Leading-Edge Suction Parameter at Low Pitch Rates

Small unmanned aerial vehicles (UAVs) are becoming increasingly popular as low-cost observation and research platforms. However, these aircraft operate low in the atmosphere, exposed to the turbulent wakes of man-made and natural roughness features. As such, it is important to estimate their unsteady aerodynamic performance. The leading-edge suction parameter (LESP) is one criterion used in vortex methods for predicting flow separation, which can arise in both steady and unsteady flows. The critical LESP represents the maximum suction supported in the leading-edge region, equivalent to the flat-plate behavior of thin airfoil theory. This parameter was investigated at low pitch rates for a NACA0012 airfoil at Reynolds numbers applicable to small UAVs, to examine the hypothesis of a constant critical value existing for an arbitrary motion. It is seen that the critical LESP shows a strong dependence on pitch rate, with a piecewise linear fit being obtained. A higher pitch rate induces a greater pressure-field lag, promoting a higher peak LESP for flow separation, and a lower value for reattachment. These peaks in the LESP agree well with the instances of separation and reattachment. Raising the Reynolds number also acts to raise the critical LESP for a given reduced pitch rate.

Load control and unsteady aerodynamics for floating wind turbines

Unlike fixed-base offshore wind turbine, the soft floating platform introduces 6 more degrees of freedom of motions to the floating offshore wind turbine. This may cause much more complex inflow environment to the wind turbine rotors compared with fixed-base wind turbine. The wind seen locally on the blade changes due to the motions of the floating wind turbine platform which has a direct impact on the aerodynamic condition on the blade such as the angle of attack and the inflow velocity. Such unsteady aerodynamic effects may lead to high fluctuation of the loads and power output. The present work aims to study the high unsteady aerodynamic performance of the floating wind turbine under platform surge motion. The unsteady aerodynamic loads are predicted with a lifting surface method with a free wake model. A preview predict control algorithm is used as the pitch control strategy. A full scale U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) 5 MW floating wind turbine is chosen as the subject of the present study. The unsteady aerodynamic performance and instabilities have been discussed in detail under prescribed platform surge motions with different control targets. Both minimizing the power output and rotor thrust fluctuation are set as the control objectives respectively. The theory analysis and the simulation results indicate that the blade pitch control can effectively alleviate the variation of the rotor thrust under platform surge motions. Larger amplitude of the variation of blade pitch is needed to alleviate the variation of the wind turbine power and this leads to high rotor thrust fluctuation. It is also shown that negative damping can be achieved during the blade pitch control process and may lead the floating platform wind turbine system into unstable condition.

Effect of the Coupled Pitch–Yaw Motion on the Unsteady Aerodynamic Performance and Structural Response of a Floating Offshore Wind Turbine

The floating offshore wind turbines (FOWTs) have many more advantages than the onshore wind turbines, but they also have more complicated aerodynamic characteristics due to complex platform motions. The research objective of this paper is to investigate unsteady aerodynamic characteristics of a FOWT under the pitch, yaw, and coupled pitch–yaw platform motions using the computational fluid dynamics (CFD) method in the Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations. The pitch, yaw, and coupled pitch–yaw motions are studied separately to analyze the platform motions’ effects on the rotor blade. The accuracy of the numerical simulation method is verified, and the overall performances, including power and thrust, are discussed. In addition, the comparison of total aerodynamic performance, force coefficients at different spans and structural dynamic response is provided. The numerical simulation results show that the platform pitching is the main influencing factor of power fluctuation, and the average thrust values of the pitch, yaw, and coupled motions are consistent regardless of the frequency value. The angle of attack (AOA) of airfoils is much more sensitive to the yaw motion, while the blade normal and tangential forces are mainly affected by pitch motion. As for the structural response, the results suggest that the aerodynamic loads of the wind turbine are more sensitive to the pitch motion, which is confirmed by the thrust force and torque of each blade during the platform motions.

Mitigation of transonic shock buffet on a supercritical airfoil through wavy leading edges

Mitigation of the shock buffet phenomenon over a supercritical airfoil by means of wavy leading edges (WLEs) is analyzed with implicit large eddy simulations. A Dassault Aviation's V2C airfoil is simulated in a transonic flow with Re∞=5.0×105 and M∞=0.7 at α=7.0°. This airfoil profile is designed for transonic flows, delaying the onset of wave drag and decreasing the skin friction drag. The results of this upstream flow condition on a straight airfoil are a large oscillation of lift and drag coefficients. In the first part, time-averaged aerodynamic characteristics over a straight leading edge (SLE) airfoil and three modified airfoils with different wavy amplitudes are compared. The results show that overall WLE airfoils are more efficient than SLE ones, and the airfoil with the lowest amplitude (h = 0.0075) is the most efficient, increasing the lift coefficient and decreasing the drag coefficient. Flow unsteadiness plays a key role for airfoils in transonic flows at moderate and high angles of attack. Hence, the second part of the paper is a detailed unsteady analysis of flow phenomena. The starting point is an investigation of unsteady aerodynamic performance. It is observed that WLE airfoils are capable of significantly decreasing low-frequency oscillations' amplitude, identified with shock buffet. The best performance is obtained with h = 0.0125 where a high-frequency oscillation becomes the dominant unsteady phenomenon. High-frequency oscillations are identified through the application of a frequency filtering method to the flow field. It is proved that the oscillation on the SLE airfoil is related to vortex shedding, while the one on WLE airfoils is related to laminar separation bubble (LSB) breathing. Overall, the unsteady analysis shows a connection between shock buffet and LSB breathing phenomena on wavy airfoils and identifies the WLE amplitude as a key parameter to control this relation and mitigate the shock buffet.

Numerical analysis of unsteady aerodynamic performance of floating offshore wind turbine under platform surge and pitch motions

The aerodynamic performance of floating offshore wind turbines is extremely complex due to the motions of the floating platform. The surge and pitch motions are the most influential motions among the six degrees-of-freedom motions. In view of this, the aim of this study is to investigate the unsteady aerodynamic characteristics of a floating offshore wind turbine under single (surge or pitch) and combined motions using computational fluid dynamics simulations, In addition, the coupling technique of dynamic mesh and sliding mesh is employed, as well as the unsteady Reynolds averaged Navier-Stokes method. The numerical simulation method in this paper is first validated by comparison to the results of the blade element momentum method and the vortex method. Then, the aerodynamic characteristics of the floating offshore wind turbine under harmonic platform motions with different periods and amplitudes are investigated. The results show that the increase of amplitude and frequency aggravate the fluctuation of the overall aerodynamic performance of the wind turbine. In addition, the combined surge-pitch motion reduces the average power generation indicating that complex platform motions adversely affect the power generation of floating offshore wind turbines.

Reduced order model for unsteady aerodynamic performance of compressor cascade based on recursive RBF

Based on Recursive Radial Basis Function (RRBF) neural network, the Reduced Order Model (ROM) of compressor cascade was established to meet the urgent demand of highly efficient prediction of unsteady aerodynamics performance of turbomachinery. One novel ROM called ASA-RRBF model based on Adaptive Simulated Annealing (ASA) algorithm was developed to enhance the generalization ability of the unsteady ROM. The ROM was verified by predicting the unsteady aerodynamics performance of a highly-loaded compressor cascade. The results show that the RRBF model has higher accuracy in identification of the dimensionless total pressure and dimensionless static pressure of compressor cascade under nonlinear and unsteady conditions, and the model behaves higher stability and computational efficiency. However, for the strong nonlinear characteristics of aerodynamic parameters, the RRBF model presents lower accuracy. Additionally, the RRBF model predicts with a large error in the identification of aerodynamic parameters under linear and unsteady conditions. For ASA-RRBF, by introducing a small-amplitude and high-frequency sinusoidal signal as validation sample, the width of the basis function of the RRBF model is optimized to improve the generalization ability of the ROM under linear unsteady conditions. Besides, this model improves the predicting accuracy of dimensionless static pressure which has strong nonlinear characteristics. The ASA-RRBF model has higher prediction accuracy than RRBF model without significantly increasing the total time consumption. This novel model can predict the linear hysteresis of dimensionless static pressure happened in the harmonic condition, but it cannot accurately predict the beat frequency of dimensionless total pressure.