实验流体力学

2021, 35(6): .

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Abstract:

Experimental study on high frequency unstable waves in hypersonic boundary layer with sharp cone at 6° angle of attack

LIU Shuyi, CHEN Jianqiang, YUAN Xianxu, XIONG Youde, WU Jie

2021, 35(6): 1-7. doi: 10.11729/syltlx20210059

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Abstract:
Transition plays an important role in the aerodynamic and aerothermal design of hypersonic vehicles. Generally, crossflow instability plays the dominant role in inducing transition in the three-dimensional boundary layer. However, in the noisy environment the influence of the second mode of the instability cannot be ignored. In order to obtain a better understanding of the development of crossflow in the hypersonic boundary layer, the measurement of instability waves in the boundary layer of a 7° half-angle sharp cone model with 6° angle of attack was carried out using focused laser differential interferometer (FLDI) and high frequency pressure fluctuation sensor (PCB) in Mach 6 Ludwieg tube tunnel. The experimental results show that the high-frequency unstable waves are detected at different circumferential positions. Through power spectrum and bi-spectrum analysis, the variation of the unstable wave along the generatrix and the nonlinear interaction between the high-frequency unstable wave and the low-frequency signal (20–40 kHz) are obtained.

Study on oscillation behavior of cone jet under unsteady electric field

FU Qingfei, GE Fei, CHENG Jinbo, YANG Lijun, REN Junxue, TANG Haibin

2021, 35(6): 8-16. doi: 10.11729/syltlx20200152

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Abstract:
In view of the lack of systematic research on electro-atomization under the action of unsteady electric field in the existing literature, combined with the actual demand of the space engine for the pulse operation of the thruster, the experimental study on the oscillation behavior of the cone jet under the action of the unsteady electric field is carried out, using the high-speed camera which records the pulsating deformation process of the cone jet under the action of unsteady voltage disturbance. This paper discusses the matching relationship between the jet oscillation frequency and the voltage disturbance frequency, and analyzes the influence of different disturbance voltage parameters on the jet shape. Studies show that when the voltage disturbance frequency is in the low and high frequency range under the action of an unsteady electric field, the jet oscillation frequency is close to the voltage disturbance frequency, and the jet is in the oscillating cone jet mode; when the disturbance frequency is in the middle frequency range, the jet oscillation frequency is in the range of 300–500 Hz, and the jet is in the intermittent jet mode; as the amplitude of the unsteady disturbance voltage increases, the frequency range of the jet in the intermittent jet mode gets larger, and the range in the oscillating cone jet mode becomes smaller; The greater the voltage disturbance frequency is, the greater the cone jet angle of the jet in the oscillating cone jet mode is.

Study of morphological characteristics and gravitational potential energy of crater formed by droplet impact

GUO Tong, YUAN Dekui, ZHAO Fengze

2021, 35(6): 17-27. doi: 10.11729/syltlx20200128

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Abstract:
The processes of single droplets with different diameters and different speeds impacting on the liquid pool with different depths were captured using a high-speed camera. The morphological characteristics of the craters were analyzed. For the deep liquid pools, when the volume of the crater reaches the maximum, its shape is approximately a hemi-sphere. However, for the shallow liquid pools, due to the restriction of the pool bottom, the crater cannot fully develop. Thus, when its volume reaches the maximum, the shape of the crater looks like a hemi-sphere with the bottom being cut-off. For the deep liquid pools, the liquid jet is thick and short, and the number of the secondary liquid droplets is less. For the shallow liquid pools, the liquid jet is fine and high, and the number of the secondary liquid droplets is more. The dimensionless maximum horizontal length and the dimensionless maximum depth of the crater increase with the increase of the Weber number. The larger the diameter of the droplet is, the smaller the dimensionless maximum horizontal length and the dimensionless maximum depth are. For the deep pools, both the dimensionless maximum horizontal length and dimensionless depth increase with the decrease of the pool depth. However, when the pool depth is decreased to a certain value, the dimensionless maximum horizontal length and dimensionless maximum depth decrease significantly. The existing gravitational potential energy model of the crater was extended, and the law of the gravitational potential energy of the crater was analyzed. The gravitational potential energy of the crater increases with the increase of the initial kinetic energy of the droplet. For the deep liquid pools, the gravitational potential energy of the crater increases as the depth of the crater decreases. For the shallow liquid pools, the ratio of the gravitational potential energy to the initial kinetic energy of the droplet is lower.

Progress of experimental research on axial turbine tip leakage flow

YANG Yi, MA Hongwei

2021, 35(6): 28-43. doi: 10.11729/syltlx20200107

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Abstract:
Based on the public literature and experimental results obtained by our group, the experimental research for the axial turbine tip leakage flow is summarized, and the future development is also prospected. The analysis shows that most researches are still based on the linear turbine cascade, and there are few measurements on the tip leakage flow under rotating conditions. As for the measurement methods, steady-state, quantitative and qualitative measurements which focus on the outlet flow field are still dominant, while non-intrusive and transient measurements inside the tip gap are relatively deficient. As for the measurement conditions, many studies are conducted at low speed condition, but only few studies are conducted under transonic and supersonic condition. Meanwhile, most analyses focus on the effect of the leakage flow on the turbine performance, but the unsteady flow mechanism of the tip leakage vortex and its breakdown is not fully revealed. In the future, based on the turbine rotor test rig, using the end-wall dynamic pressure measurement array, endoscopic PIV, and LDV to measure the unsteady leakage flow inside and near the tip clearance is an important direction that needs to be studied urgently.

Research on surface measurement method based on multi-view stereo vision and neutral network calibration

HAN Yukun, PAN Chong, WANG Wentao, LIU Wei

2021, 35(6): 44-51. doi: 10.11729/syltlx20210048

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Abstract:
Non-contact optical measurement of three-dimensional surface topography is an important application of the computer multi-view stereo vision technology. However, there are still some technical problems needed to be solved such as limited number of cameras, complex image matching algorithms and insufficient longitudinal measurement accuracy. The article develops an optical method for surface topography measurement based on multi-view stereo vision and neural network calibration. This algorithm includes: applying neural networks for the calibration of the multi-view cameras and the three-dimensional reconstruction of feature points, projecting a laser speckle pattern on the surface as the feature points for image recognition and matching, using the particle tracking velocity technology to match the same single feature point in the multi-view images. As proved by experiments, compared with the traditional stereo vision measurement system, which is calibrated based on the pinhole camera model and matched by the epipolar constraint or cross-correlation algorithm, the method proposed in this paper can be adapted to scenes with large optical distortion, besides, the spatial resolution of the surface topography measurement is effectively improved and the measurement error in the depth direction is at the level of 1.0% – 2.0%.

A Bernoulli sampling based image real-time compression method for high-speed camera

WANG Bin, LIANG Jie, WANG Pan, GU Zhenghua, GAI Wen

2021, 35(6): 52-57. doi: 10.11729/syltlx20200052

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Abstract:
High-speed cameras are widely used in explosive mechanics, hydrodynamics, trajectory observation and other scientific research experiments. Due to massive data and the limit of storage and bandwidth of electronic system, high-speed cameras cannot shoot for long periods of time. Compression of captured images is an effective way to extend the recording time of the high-speed cameras. However, the existing image compression methods can not solve high-speed camera mass image compression task caused by the large calculation complexity and long calculation time. In this paper, a low hardware cost, real-time image compression method with Bernoulli sampling is developed. The method randomly samples the pixels with Bernoulli distribution directly in the process of CCD or CMOS image acquisition. Then in usage, the raw image is reconstructed with the unknown pixel estimating method. The simulation results show that the main visual information of the image can still be observed from the reconstruction image, even when the image compression rate reaches 99%, which is superior to the super-resolution enhancement result of the TOM (Thin Out Mode) with interpolation.

Experimental study on simulation of unsteady downburst outflow in atmospheric boundary layer wind tunnel

ZHONG Yongli, YAN Zhitao, LI Yan, YANG Xiaogang, JIANG Sen

2021, 35(6): 58-65. doi: 10.11729/syltlx20200131

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Abstract:
In order to realize large-scale unsteady simulation of the downburst outflow in the atmospheric boundary layer wind tunnel, a set of large-scale test facility for the transient downburst outflow was designed and manufactured based on the plane wall jet method. The vertical profile of the horizontal mean wind velocity of the downburst was obtained by adding the wall jet device, and then the unsteady wind field was formed by using a fast-opening valve. The wind velocity record from the Andrews Air Force Base downburst is employed for verification of the experimental procedure. The results show that the simulation device of downburst based on wall jet can effectively simulate the wind field of downburst outflow. Under steady-state conditions, the test device can form a relatively uniform two-dimensional wall jet flow field, and in the fully developed area, the results are consistent with the vertical mean wind profile of the typical downburst flow. The time-varying mean wind velocity time history which is very close to the actual downburst record can be obtained by using the unsteady wall jet device.

Free oscillation dynamic derivative test technology of a short-blunt shape vehicle

LIU Jin, SONG Yuhui, CHEN Lan, WANG Fangjian, QIN Han, DONG Lei

2021, 35(6): 66-72. doi: 10.11729/syltlx20210010

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Abstract:
In order to study the dynamic stability characteristics of the short-blunt shape vehicle, the free oscillation dynamic derivative test method is used to establish the dynamic derivative measurement test technology in the 1.2 m trisonic wind tunnel. The newly designed elastic hinges and bearing hinges solve the problem of elastic support and low-frequency oscillation simulation of the short-blunt shape vehicle, and the newly established test device is used to study the influence of the Mach number, angle of attack, reduced frequency, and shape on dynamic stability characteristics. The newly designed elastic hinge can meet the model support and oscillation requirement under the aerodynamic characteristic condition of the short-blunt shape vehicle in the wind tunnel, and the bearing hinge support can simulate the oscillation frequency close to the actual reduced frequency in the wind tunnel. In the trisonic wind tunnel, the measurement of the pitch derivative of a short-blunt shaped vehicle was completed, and the unstable state points of the pitching were obtained, which provides experimental support for future research on the dynamic stability characteristics of this type of vehicle.

The study of ejection mechanism and drop-test in high-speed wind tunnel

WANG Shuai, DONG Jingang, ZHANG Chenkai, ZHAO Xingyu, XIE Feng

2021, 35(6): 73-78. doi: 10.11729/syltlx20200149

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Abstract:
It is necessary to evaluate the separation safety of supersonic embedded weapons for the new generation fighter. According to the requirements of high-speed wind tunnel launching test of embedded weapons, a new type of double cylinder ejection mechanism is designed, which can realize the continuously adjustment of the initial velocity and the initial angular velocity. Firstly, the NX modeling software is used to design the structural and three-dimensional modeling of the ejection mechanism. Secondly, based on the pressure equations of the cylinder rod free cavity and rod cavity, and the flow equation of the rod free cavity and rod cavity connected with the atmosphere and the piston driving force equation, the mathematical model of the ejection process was established, and the design of the ejection mechanism was simulated and analyzed by using the motion simulation software to verify the rationality of the structure. According to the control requirements of the ejection mechanism, the servo control system of the ejection mechanism was designed by using the electric servo valve and related control elements. Finally, the wind tunnel test of Mach 1.5 was carried out. The experiment shows that the maximum ejection speed of the model can reach 5.68 m/s under the condition that the air pressure is not more than 1.0 MPa, and the initial attitude of the carrier is close to the simulation results, which can meet the design and use requirements.

Experimental study of passive control of jet deflection on wing upper surface

WANG Jun, ZHANG Liu, LI Binbin, ZHAO Lei, LI Chang, JIN Yi

2021, 35(6): 79-85. doi: 10.11729/syltlx20210027

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Abstract:
The effect of flap shape on jet deflection on the upper surface of the wing is studied by the static thrust experiment. On this basis, the jet deflection is passively controlled by the vortex generator, and the effects of the installation position, installation angle and height of the vortex generator on the jet deflection performance are studied. The results show that the jet deflection angle reaches the maximum when the flap deflection angle is 30° and increases with the increase of the flap radius. The use of vortex generators helps to promote the jet adhesion and to increase the jet deflection angle. Its height and installation angle are the key parameters that affect the deflection performance of the jet.

Wind tunnel experimental study of the grille-generated turbulence in the short test section

YANG Junwei, YANG Hua, FU Shifeng, ZONG Wangwang, SHA Chenglong

2021, 35(6): 86-93. doi: 10.11729/syltlx20210042

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Abstract:
The objective of the current work is to modulate the local high turbulent field behind grilles through a wind tunnel with a short axial distance. The flow fields formed by five types of grilles (two mesh grilles and three vertical bar grilles, 3 cm × 3 cm sectional aluminum profiles assembly) were experimentally tested by the hot-wire anemometer. The turbulent parameters distribution and isotropic characteristics of the turbulent flows near behind the grilles are obtained. Besides, the fitting formulas for turbulent flows are acquired according to the changing law of parameters, and the goodness of the fitting is 0.96. Meanwhile, the classic spectrum estimation and modern spectrum estimation are adopted to analyze the turbulent power spectrum density (PSD), respectively. It is noted that different spectral estimators can accurately predict the turbulent PSD at the near-field region, and the deviations exist only at low frequencies. Besides, by adjusting the distance between the grilles and the measuring point, the wind speed, and the grille structure, the energy structure behind the grille turbulent field can be changed.

Wind tunnel test investigation on buffeting characteristics of horizontal tail of commercial aircraft in low-speed cruise configuration

CHEN Zhenyu, LIU Yang, XU Liang

2021, 35(6): 94-99. doi: 10.11729/syltlx20210009

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Abstract:
In this paper, a wind tunnel test method is proposed to analyze the buffeting of the horizontal tail. By installing subminiature pulsating pressure sensors on the wing and horizontal tail surface of a commercial aircraft with conventional layout, the time domain and frequency domain data of the pulsating pressure on the wing and horizontal tail surface are measured and analyzed. The test results show the characteristics of fluctuating pressure and surface pressure distribution under the influence of the wing wake and its own flow characteristics on different sections of the horizontal tail. The results show that the structural forced vibration of the horizontal tail is induced by the random fluctuating pressure excitation generated by the strong separated flow of the wing at medium and small angles of attack, and the dominant frequency of the fluctuating pressure on the horizontal tail surface is close to that of the vortex on the main wing. At large angles of attack, the horizontal tail is no longer disturbed by the wake of the main wing, its fluctuating pressure characteristics are related to its own separation characteristics, and the power spectral density of the fluctuating pressure decreases gradually along the spanwise direction of the horizontal tail.

2021 Vol. 35, No. 6