实验流体力学

2023, 37(3)

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Abstract:

Mitigation of micro-pressure wave at high-speed maglev tunnel exit by resonant cavity structure

SONG Junhao, YAO Shuanbao, CHEN Dawei, DING Sansan, YANG Mingzhi

2023, 37(3): 1-8. doi: 10.11729/syltlx20220114

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Abstract:
During the passage of the high-speed maglev train through the tunnel, the air flow in front is compressed due to the limitation of annular space formed by the inner wall of the tunnel and the surface of the car body, forming an initial compression wave. The initial compression wave propagates to the tunnel portal at the local sound speed, and some of it radiates outward to form a micro-pressure wave, which seriously affects the tunnel portal environment. This problem is even more pronounced when high-speed maglev trains reach speeds of 600 km/h. Therefore, a tunnel with a resonant cavity structure is proposed, and the three-dimensional, unsteady, compressible N–S equation and SST k–ω turbulence model are used to study the aerodynamic effect mitigation characteristics of the maglev train passing through the tunnel at high speed. The simulation comparison and dynamic model test verification of different resonator schemes are carried out. The results show that the resonant cavity structure installed in the redundant space in the tunnel can dissipate the compressed wave energy, reduce the rate of pressure change of the compression wave, and have a significant slowing effect on the micro-pressure wave at the tunnel opening. Compared with the existing tunnel, the resonator structure has a micro-pressure wave mitigation effect of 41.87% and 40.05% at 20 m and 50 m to the tunnel portal, respectively. The micro-pressure wave mitigation effect is linearly related to the number of resonators in the tunnel. The results of the moving model test show that the slowdown effect of the micro-pressure wave is positively correlated with the operating speed.

The effect of track structure on the aerodynamic characteristics of evacuated tube maglev train

WANG Xiaofei, HU Xiao, LI Zongpeng, LIU Jianru, DENG Zigang, ZHANG Weihua

2023, 37(3): 9-18. doi: 10.11729/syltlx20220140

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Abstract:
The emergence of evacuated tube maglev transportation makes it possible for ground ultra-high-speed rail transit. However, limited by the demand for high-power propulsion motors and low vacuum environment, it is difficult to carry out experimental research. In this paper, the numerical research on the aerodynamic layout of the magnetic track and motor is carried out in the preliminary design of the Dynamic Model Test Platform for Multistate Coupled Rail Transit. Based on the geometric structure of the dynamic model test platform, considering the actual arrangement of the motor platform and the permanent magnet track in the tube, the three-dimensional, compressible RANS method and SST k–ω turbulence model are used to calculate the three-dimensional flow field structure and the shock wave reflection, propagation law of the superconducting maglev train in the low-pressure tube at ultra-high speed. The influence of the rectangular channel on the aerodynamic loads of the train and the flow field in the tube is compared and analyzed. The differences of the pressure and velocity change trend at the bottom of the train, and the shock wave strength at the tail and the wake structure are mainly explored. It is found that the step of the magnetic track and the motor can cause more flow separation and shock reflection in the wake region, resulting in tail pressure fluctuations. When the rectangular channel exists, the shock wave intensity at the tail of the train decreases, the shock wave phenomenon is more obvious, the aerodynamic drag coefficient decreases by 8.855%, and the aerodynamic lift coefficient increases by 14.312%. The research results can provide reference for the design of the magnetic track and motor platform of the multi-state coupling rail transit dynamic model test platform.

Preliminary study on system configuration of ultra high-speed maglev train aerodynamic problem in the low vacuum tube

CHEN Dawei, LIU Jiali, YAO Shuanbao, WANG Weibin

2023, 37(3): 19-26. doi: 10.11729/syltlx20220136

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Abstract:
The low vacuum tube ultra high-speed maglev system is the next generation of the ultra high-speed ground transportation system, which combines the low vacuum tube and high-speed maglev technologies, and thus can effectively reduce the aerodynamic resistance and aerodynamic noise of the train running at ultra high-speed, to achieve a running speed of 800～1000 km/h, or even more than 1000 km/h. In the present paper, the aerodynamic numerical simulation method of the ultra high-speed maglev train in the low vacuum tube was discussed. The influence of the tube pressure, tube area, and train speed on the aerodynamic performance of the ultra high-speed maglev train in the low vacuum tube, such as the aerodynamic drag, aerodynamic lift, aerodynamic noise source, tube intersection pressure wave, and heating equipment temperature, was studied. And the typical scenarios of the low vacuum tube ultra high-speed maglev system were preliminarily discussed in engineering. The research shows that, when the train speed is 600 km/h, the tube pressure of 1.0 atm–tube area of 100 m², and the tube pressure of 0.3 atm–tube area of 40 m², have engineering feasibility; the tube pressure of 0.3 atm–tube area of 100 m² has the problem of equipment heat dissipation, and the engineering feasibility has certain challenges. When the train speed is 1000 km/h, the equipment head dissipation under the tube pressure of 0.3 atm–tube pressure of 100 m² is significant, and the engineering feasibility is challenged. If the tube pressure is further reduced, the design difficulty of equipment heat dissipation and airtight strength would be further increased.

Application of typical magnetic suspension system in maglev flight wind tunnel

YU Xinning, JIANG Xintong, ZHANG Jun, ZHOU Tingbo, NI Zhangsong

2023, 37(3): 27-36. doi: 10.11729/syltlx20220149

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Abstract:
The operation principle of the maglev flight wind tunnel is to drive the model to move at high speed in a closed straight pipe through magnetic suspension. The maglev system is particularly important for accurate control of the acceleration/uniform/deceleration process of the model. This study made a comprehensive analysis of the four maglev systems, including normal conductive electromagnetic suspension (EMS), permanent magnet electrodynamic suspension (PM–EDS), high/low temperature superconducting electrodynamics suspension (HTS/LTS–EDS), and high temperature superconducting pinning levitation (HTS–PL). These several maglev systems were comprehensively analyzed from five aspects. EMS system could not meet the specification requirement of the maximum operating speed Ma = 1.0, which could not be used as an alternative maglev system for the maglev flight wind tunnel. Based on the analytic hierarchy process (AHP) and grey relational analysis (GRA), a comprehensive decision-making model of the maglev system was established for the application scenario of the maglev flight wind tunnel. Results show that the HTS–EDS and HTS–PL system have better application potential in the maglev flight wind tunnel.

Progress on fluid-solid coupling of vacuum pipeline train and analysis of key technology

KOU Jie, FU Cheng, GAO Xinglong, SUN Yunqiang

2023, 37(3): 37-49. doi: 10.11729/syltlx20220143

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Abstract:
Theoretically, the speed of vacuum pipe trains can exceed 1000 km/h by using the magnetic suspended technology in a low-pressure operating environment of pumped vacuums within the tubes. The closed pipe creates a complex aerodynamic environment, while the suspension of the train makes it very susceptible to changes in the attitude of the train. The fluid-solid coupling effects of the train are evident which require targeted research. In order to explore the theory and analysis method of fluid-solid coupling of the vacuum pipeline train, a relatively complete review of the progress in aerodynamic studies of the vacuum pipe train and the study of the fluid-solid coupling characteristics of the railway trains is made. And the key technology of fluid-solid coupling of the vacuum pipeline train is analyzed. It is proposed that the vacuum pipe train fluid-solid coupling study should focus on the development of the vacuum pipe train flow analysis techniques, the vacuum pipe train fluid-solid coupling analysis techniques and the vacuum pipe train control technology. This paper provides reference for vacuum pipe train fluid-solid coupling techniques to facilitate the development of the vacuum pipe train technology.

Influences of porous media on propagation of compression wave in wind tunnel

ZHOU Tingbo, YU Xinning, ZHOU Guolong, JIA Lefan, ZHANG Jiazhong

2023, 37(3): 50-58. doi: 10.11729/syltlx20220115

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Abstract:
Generated during the operation of the maglev flight tunnel and reflected at the end of the tunnel, the compression wave may collide with the model at the test section and interfere with the test results. In order to attenuate the influence of compression wave during the tunnel test, the method of laying porous media inside the tunnel pipe is used to reduce the intensity of the compression wave. According to the existing porous media pressure drop equation, the equations describing the compressional wave pressure drop law through porous media in the tunnel pipe were derived. The parameters on which the wave elimination ability of porous media in the equation depend were analyzed by numerical simulation. The results show that the porous media has the same proportion of the pressure drop effect on the compression wave within different strengths. Its wave elimination ability increases with the increase of the compression wave strength, inertial resistance coefficient and porous media thickness. However, with the increase of the inertial resistance coefficient and thickness, a part of the compression wave is reflected instead of passing through the porous media. The strength of the reflected compressional wave also increases with the increase of the inertial resistance coefficient and thickness. Dividing the porous media into multiple layers can reduce the strength of the reflected compressional wave while other parameters of the porous media remain unchanged, so as to improve the overall wave elimination ability of the porous media. Porous media could maintain favourable wave elimination ability under a wide range of ambient pressure changes (0.0001-1 atm).

Simulation evaluation of aero-structure interaction for moving model in maglev flight tunnel

GAO Xinglong, WANG Chao, FU Cheng, SUN Yunqiang, KOU Jie

2023, 37(3): 59-68. doi: 10.11729/syltlx20220127

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Abstract:
The maglev flight tunnel is a novel concept aerodynamics test facility, in which the high speed translation of the moving model in the long straight closed tunnel would induce extremely complicated aerodynamic characteristics. The motion of model’s fast acceleration, deceleration and constant speed moving have strong interference to the surrounding fluid field, causing wave propagation problems and the aero-structure single way coupling problems. The unsteady characteristics originated from the high speed moving of the model in the maglev flight tunnel are investigated and evaluated from the view of aero-structure coupling. Based on the new CE/SE method, the 3D surrounding compressible fluid of the moving model in tunnel is solved, and the aerodynamic parameters variations, wave propagation characteristics and the pressure distribution in the tunnel are obtained. Simulation design analysis of parameters for porous medium wave absorbing materials is conducted, which could provide a support for the key technical problems like wave absorbing construction design of the maglev flight tunnel.

Research of dynamic characteristics of maglev train under typical aerodynamic loads

NAN Kaiwei, LIU Mengjuan, HAO Zhanzhou, WU Han, SUN Zhenxu

2023, 37(3): 69-83. doi: 10.11729/syltlx20220108

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Abstract:
This study researched the dynamic characteristics of the TR08 maglev train under strong aerodynamic loads using the sliding grid method. The research investigated the transient aerodynamic load characteristics during open-track single-vehicle operation and meeting scenarios, identified the sources of aerodynamic load oscillation and the train's dynamic characteristics under these loads. Results show that the aerodynamic load of the TR08 maglev train increases with speed, and the pattern is tail train > head train > middle train. The lifting moment is critical for the safe operation of train, and substructure primarily causes load oscillation. The peak value of the pitching moment for a single-segment train shows a hysteresis phenomenon, which is absent in the yawing moment. When single train runs at 600 km/h suspension magnet gap fluctuation exceed safe limits, while when trains meet at 600 km/h instability occurs. This study provides insights into the sources of load oscillation and the train's dynamic characteristics.

Wind tunnel test research on the characteristics of rotor blade-vortex interaction noise

LIU Xiangnan, LIU Shaoteng, ZHOU Guocheng, SHAO Tianshuang, CHEN Bao

2023, 37(3): 84-91. doi: 10.11729/syltlx20210190

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Abstract:
The propagation characteristics of the blade–vortex interaction (BVI) noise were tested in the FL–10 wind tunnel of AVIC Aerodynamics Research Institute. The aerodynamic noise of climb, level flight and descent at medium forward speed was measured on a 40% scale model of BO−105 main rotor. Firstly, the “Heyson” wall interference correction method was used to determine the descent angle of the rotor in the wind tunnel, and the complete noise radiation field under the rotor tip–path–plane was obtained through the movement of the measurement array in the airflow. Furthermore, the BVI noise characteristics of the rotor under different flight condition were studied, and the sound pressure time history, spectrum and sound pressure level contour were given. The results indicate that the BVI phenomena occur on both the advancing and retreating side under the descent flight condition. The noise has strong directivity, and radiates toward upstream under the rotor disk toward the advancing side and the downstream on the retreating side.

Wind tunnel experimental study on aerodynamics and noise based on the influence of automobile rearview mirror shapes

FU Wei, WANG Xunnian, LI Yong

2023, 37(3): 92-106. doi: 10.11729/syltlx20210187

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Abstract:
To reduce the aerodynamic noise caused by automobile rearview mirrors, a simplified rearview mirror model was taken as the research object, and three different modeling improvement schemes were proposed: model A tilts the mirror body at 15°; model B tilts the mirror body at 30°; model C changes the cylindrical base to an elliptical base. Wind tunnel experiments were carried out on the models to analyze the variation rules of the flow field, drag, and wall pressure fluctuation with the change of the model. The particle image velocimetry (PIV) and six-component balance were used to measure the aerodynamic characteristics including the flow field and drag, and the wall microphone was applied to measure the acoustic characteristics. The results show that all three schemes can improve the flow quality in the wake area of the rearview mirror, effectively reducing the drag and the generation of aerodynamic noise. Compared to the base model, the drag coefficient and the overall sound pressure level of the wall pressure fluctuation of model B can be reduced by 18.4% and 4.6 dB in the low-mid frequency range. For model C, the corresponding results are 7.5% and 4.3 dB, respectively. The research results are beneficial for engineers in the aerodynamic and acoustic design of automobile rearview mirrors.

Experimental study on aerodynamic noise characteristics of helicopter ducted tail rotor

DING Cunwei, ZHOU Guocheng, CHEN Bao, ZHONG Weigui

2023, 37(3): 107-112. doi: 10.11729/syltlx20210186

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Abstract:
Using the FL–52 aero-acoustic wind tunnel test system of the Aerodynamics Research Institute, the experimental study on the ducted tail rotor noise characteristics is carried out. The influence of the jet shear layer on the test data is corrected, and the noise spectrum and far-field directivity of the culvert tail rotor in hover and forward flight are obtained. The variation laws of the noise with the Mach number of the tail rotor tip are analyzed, and the results show that the aerodynamic noise of the ducted tail rotor conforms to the load noise characteristics. The effects of the blade distribution along the hub on the spectral characteristics of the aerodynamic noise are compared. The shielding noise reduction effect of the duct on the noise propagation under typical working conditions is obtained in hover, the noise in the rotating plane is 2 dB lower than that in other positions in forward flight, the noise reduction in the rotating plane is 5 – 8 dB.

Research on mode switch logic in Unmanned Aerial Vehicle autonomous soaring

CHEN Jie, SHI Zhiwei, YAO Zhangyi, YIN Zhenquan, GE Zengran

2023, 37(3): 113-123. doi: 10.11729/syltlx20210165

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Abstract:
Unmanned Aerial Vehicle(UAV) can obtain energy from the thermal updraft in the natural environment by autonomous soaring to improve its endurance. Mode switch is the key of UAV autonomous soaring. For the mode switch problem in autonomous soaring of UAV, an airflow sensing system was designed based on the seven-hole probe and the embedded technology. The airflow sensing system could measure the direction and speed of airflow up to 72° flow angle. Based on the airflow sensing system, the mode switch logic was designed to control the UAV to enter and leave the soaring mode. Using the wind tunnel virtual flight experiment technology, the situation of UAV encountering thermal updraft was simulated in the wind tunnel to verify the feasibility of the designed mode switch logic. Results of the wind tunnel virtual flight experiment show that under the influence of different sizes of updraft, the mode switch logic designed based on the airflow sensing system could make the UAV enter and leave the soaring mode independently, and the mode switch logic could make the UAV leave the soaring mode under different roll angle commands.

On complete rebound of liquid droplets impacting on soft hydrophobic surfaces

YANG Lei, LIU Ximiao, LI Zhonghong

2023, 37(3): 124-131. doi: 10.11729/syltlx20220072

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Abstract:
With the method of high-speed camera and image recognition, the complete rebound of the liquid droplet with different viscosity impacting on the surface of the soft hydrophobic material (PDMS) with different elastic modulus was obtained. The influence curves of liquid viscosity and PDMS elastic modulus on the rebound Weber number and recovery coefficient of the droplet were also plotted. Due to the influence of liquid viscosity on the spreading process and viscous energy dissipation, the maximum and minimum Weber numbers of droplet complete rebound on PDMS surfaces increase with the increase of droplet viscosity, and the rebound recovery coefficient gradually decreases. With the decrease of the elastic modulus of PDMS, the maximum Weber number of droplet complete rebound on PDMS surface increases and the minimum Weber number decreases, and the elastic modulus of PDMS has no significant effect on the rebound recovery coefficient.

2023 Vol. 37, No. 3