实验流体力学

Contents

2023, 37(2)

Abstract(108) PDF(31)

Abstract:

Summary of research on flame 3D reconstruction based on computed tomography of chemiluminescence technology

FENG Xiaoou, JIN Yi, ZHAI Chao

2023, 37(2): 1-15. doi: 10.11729/syltlx20210148

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Abstract:
Due to the characteristics of combustion such as three-dimension, high temperature, turbulence, and unsteady state accurate measurement of the combustion is difficult and is a hot research topic. Computed Tomography of Chemiluminescence (CTC) combines the chemiluminescence and CT technology. By directly shooting flame images from different angles and using reconstruction algorithms to reconstruct the flame, a fine description of the three-dimensional structure of the flame can be achieved quickly and accurately. The self-luminescence of the flame is used in the CTC as the light source, so there is no additional light source equipment required, which makes the system easy to build and can be implemented in a complex environment. These advantages enable the CTC technology to be used for real-time measurement of high temperature and turbulent flames, which is of great significance for studying complex combustion flow fields and improving combustion efficiency. In this paper, the basic principles of the CTC technology are introduced firstly, and then the research progress of the CTC technology in the direction of 3D reconstruction of flame is introduced in four aspects: the imaging model, the reconstruction algorithm, the experimental equipment and its application. Finally, the development trend of the CTC technology is discussed.

Experimental study on the effect of angle of attack on airfoil boundary layer

HAO Dongzhen, JIANG Nan, TANG Zhanqi, MA Xingyu

2023, 37(2): 16-24. doi: 10.11729/syltlx20210117

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Abstract:
In order to deeply understand the influence of the angle of attack on the airfoil boundary layer, a TR–PIV experimental study on the SD7003 airfoil is carried out. The distributions of statistics such as the average velocity of the airfoil suction surface and the Reynolds shear stress under the working conditions of the angle of attack α = 4°, 6° and 8° are compared. Proper orthogonal decomposition (POD) method is adopted for analysis of the experimental data. The flow structure in each mode and the frequency spectrum characteristics of the modes under different working conditions are analyzed in detail. The study finds that: with the increase of the angle of attack, the position of the separation bubble moves to the leading edge of the airfoil, and the thickness of the separation bubble increases; there is intensive shear motion inside the separation bubble and near the reattachment point; there are alternating positive and negative vortex structures near the reattachment point, and the vortex structures change continuously with the development of the boundary layer; the energy of each mode of POD decomposition is related to the scale of the structure contained and the mode frequency; with the increase of the angle of attack, the scale of the flow structures in the flow field increases, and the frequency domain distribution of flow field energy shifts from high frequency to low frequency.

The kinematics and performance of zebrafish C-shaped maneuvering

LIU Yuansen, YU Yongliang, BAO Lin, GAO Mengchen

2023, 37(2): 25-35. doi: 10.11729/syltlx20210172

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Abstract:
It is difficult to obtain the complete kinematics and hydrodynamics from the experiments in the study of fish C-type fast-start, which can be divided into C-start and C-turn by the duration of maneuvers. Using high-speed photography, a series of top-view images of zebrafish's C-shaped maneuvering were obtained on the self-built platform based on machine vision. A simplified three-dimensional (3D) fish model was established according to the outline and midline of the fish body extracted from the images by using a mathematical morphology algorithm. Then, due to the conservation of the linear and angular momentum, the complete kinematic and hydrodynamic data during the whole maneuver motion can be obtained and the translational and rotational energy can be distinguished from the total kinetic energy of the zebrafish body. The modification of the caudal fin length is a highlight in 3D modeling, which is based on the equivalence of the area second-moment of a flapping 3D plate. Through the benchmark validation, the maximum geometric error of the digital image processing is less than 3.1%. The results show that the maximum acceleration in all the C-type fast-start is linearly related to the maximum angular acceleration, and the rotational energy of C-turn is dominant in the kinetic energy while the translational energy of C-start is dominant.

Experimental study on generation of non-Newtonian droplets in dripping mode in a flow focusing microchannel

LIANG Dingxin, XUE Chundong, ZENG Xiao, QIN Kairong

2023, 37(2): 36-45. doi: 10.11729/syltlx20210184

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Abstract:
Droplet microfluidic is an important branch of the microfluidic field and the biological fluids involved in it often have non-Newtonian properties. In order to understand the influence of non-Newtonian properties on droplet formation, four kinds of fluids with different rheological properties were configured to systematically study the non-Newtonian droplet formation in the dripping mode in a flow focusing microchannel. The results show that compared with Newtonian droplet formation, non-Newtonian droplet formation shows a more significant “beads-on-a-string” phenomenon. Different non-Newtonian properties have different effects on droplet formation. Shear thinning effect and elastic effect have opposite effects on the droplet size and formation frequency. The results of liquid column necking dynamics show that the process of liquid column necking is similar to that of Newtonian fluid due to a single shear thinning effect. The single elastic effect makes the capillary drive stage which appears of liquid column necking different from that of Newtonian fluid. The combined effect of the elastic effect and shear thinning effect leads to more significant capillary drive stage in the process of column necking and more significant “beads-on-a-string” after column necking.

Experimental study on influence of liquid depth on morphological characteristics after droplet impact

XU Duoguang, XU Wan, YUAN Dekui

2023, 37(2): 46-55. doi: 10.11729/syltlx20210153

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Abstract:
Characteristics and mechanism of a droplet impacting on the liquid surface were studied experimentally. The process of droplet impinging on the liquid surface with different depths of the liquid pool was recorded with a high-speed camera, and the characteristic phenomena such as liquid crater, liquid crown, central jet and secondary droplet that appeared at different stages were summarized. An image processing program was developed and the automatic extraction of geometric parameters was then realized. The influence of pool depth Weber number, droplet diameter, and falling height on droplet impacts was discussed. The results show that: under the condition of constant Weber number, when the pool depth crosses a certain critical value, the characteristic phenomena such as liquid crater, liquid crown, central jet and secondary droplet change significantly; the ratio of the liquid crown height to the liquid crater depth increases within a certain range with the increase of Weber number; whether the secondary droplet can be separated by the central jet is closely related to the depth of the liquid pool and the diameter of the initial droplet.

Experimental study on drag reduction characteristics of biopolysaccharide solution

MENG Fanzhe, Qin Liping, XIE Luo, SHI Pengfei, HU Haibao

2023, 37(2): 56-61. doi: 10.11729/syltlx20210089

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Abstract:
In order to obtain the underwater drag reduction performance of the biopolysac-charide solution, the drag reduction characteristics of four biopolysaccharide solutions of guar gum, xanthan gum, tragacanth gum and locust bean gum were tested in the gravity circulating water tank experimental system. The influence law of the injection rate, Reynolds number and injection mass fraction on the drag reduction is shown. The results show that the four biopoly-saccharide solutions have significant spray drag reduction effects, and the locust bean gum solution has the highest drag reduction rate (14.3%). At a constant Reynolds number, with the increase of the injection rate, the drag reduction rate of each polysaccharide solution increases significantly, and shows different trends after reaching the peak value of drag reduction. The drag reduction effect of the polysaccharide solution is better when the Reynolds number is small (<2.0×10⁴). With the increase of the Reynolds number, the drag reduction law of the polysac-charide solution shows differentiation. Excessive injection mass fraction would reduce the drag reduction effect of the polysaccharide solution, and increasing Reynolds number would cause the phenomenon of “peak shift” with the increase of the mass fraction. By introducing relative injection mass fraction, the effects of the injection rate, Reynolds number and injection mass fraction on drag reduction are coupled with each other. With the increase of relative injection mass fraction, the drag reduction rate of each polysaccharide solution increases first and then decreases. Finally, based on the injection spray mass fraction, the drag reduction law of the polysaccharide solution was explained preliminarily.

Computational investigation of unsteady heat exchange on regenerative cooling structure

ZHANG Ruoling, ZHANG Lei, WANG Xiangyi, JIANG Jin, GU Lei

2023, 37(2): 62-67. doi: 10.11729/syltlx20210102

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Abstract:
In order to support the regenerative cooling structure design and experiment of scramjet, an analytical model of unsteady heat exchange is constructed. The calculated and experimentally measured thermal equilibrium times under water cooling condition are compared. The calculated irregular temperature increasing time is obtained using the analytical model and compared with experimental values. It is found that the thermal equilibrium time can be decreased if the scramjet is preheated to 800 K before test. The research shows that the analytical model is applicable to heat transfer analysis of the regenerative cooling scramjet.

Numerical simulation and experimental research of Lamb wave propagation characteristics in ice

ZHANG Hongjian, ZHANG Yanxin, XIONG Jianjun, ZHAO Zhao, RAN Lin, YI Xian

2023, 37(2): 68-77. doi: 10.11729/syltlx20210170

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Abstract:
As one of the most commonly used ultrasonic guided waves, Lamb wave has the characteristics of concentrated energy, wide propagation range and small probe volume. Its application can be extended to the field of ice detection. In order to explore the propagation law of Lamb wave in ice, this paper constructs a physical model based on the Lamb wave propagation research platform of piezoelectric ceramics, and takes COMSOL Multiphysics software as the calculation tool to simulate the propagation of Lamb wave in ice with different thickness and length. On this basis, the Lamb wave ice detection platform was built, and the Lamb wave propagation experiment of the iced aluminum plate was carried out. Combined with the numerical simulation and experimental results, the effects of temperature, ice geometric characteristics and liquid water on the propagation characteristics of Lamb wave are clarified. The results show that the lower the temperature, the faster the group velocity of Lamb wave propagation; In a certain range of ice thickness, the attenuation of piezoelectric voltage amplitude at the receiver increases with ice thickness; The time delay of Lamb wave B₁ mode wave at the receiver increases linearly with the increase of ice length; Liquid water only affects A₀ mode of Lamb wave, but has little effect on S₀ mode. The experimental and numerical simulation results are in good agreement, which provides a theoretical reference for Lamb wave ice detection technology.

Research on dynamic wind tunnel test technology of rotor airfoil

ZHANG Weiguo, LI Guoqiang, LI Dong, CHE Binghui, GU Yi, WU Linxin

2023, 37(2): 78-93. doi: 10.11729/syltlx20210147

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Abstract:
It is urgent to establish and develop the dynamic wind tunnel test technology for rotor airfoil design optimization and performance determination. Through dynamic simulation and structural optimization design, based on the FL–11 low-speed wind tunnel, a two degree of freedom dynamic test device for rotor airfoil is developed. It can not only complete the single degree of freedom dynamic motion of pitch and plunge, but also realize the coupling operation of pitch/plunge, with the highest oscillation frequency of 5 Hz. Based on the FL–20 continuous transonic wind tunnel, a set of high-frequency and high-speed oscillation test device for rotor airfoil is developed, with the maximum pitch oscillation frequency exceeding 17 Hz and the maximum test Reynolds number exceeding 5 × 10⁶. The simulated parameter envelope meets the parameter requirements of the real helicopter. A large-scale rotor airfoil dynamic test device is developed based on the FL–14 low-speed wind tunnel, with the airfoil model chord length of 800 mm and the maximum test Reynolds number of 4 × 10⁶. The accurate measuring and testing technique of the rotor airfoil dynamic test is developed, and the verification test is carried out. The results show that the dynamic test data of the rotor airfoil are reasonable and reliable, which indicates that the test system and related test technology have high reliability. It can provide important technical support for the research of rotor airfoil dynamic stall.

Acoustic field modeling and measurement of the parallel graphene based thermo-acoustic actuator

ZHANG Haoyuan, WANG Peng, LIU Yingzheng

2023, 37(2): 94-104. doi: 10.11729/syltlx20210130

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Abstract:
The active flow control technology is an effective method to improve the aerodynamic characteristics of aerospace vehicles, in which actuators are the core of these control technologies such as plasma actuator, synthetic jet actuator and oscillating jet actuator. Present study proposes a parallel graphene actuator based on the thermo-acoustic effect. The actuator has the advantages of simple structure, low input power, wide controlling frequency and strong structural adaptability. It can adapt to various complex curved wall installations of aircraft environment and variable working conditions, resulting in a good application prospect. Specifically, the thermo-acoustic actuator utilizes the extremely low specific heat capacity and high coefficient of heat conductivity characteristics of graphene materials. Through the Joule heating principle, the graphene films can radiate a periodic sound field to the surrounding air for acoustic excitation control. According to this principle, firstly this paper uses the thermo-acoustic theory to model the sound field of the graphene actuator, adds the principle of combined sound source superposition to the modeling, and optimizes the computation method of acoustic wave phase difference and sound field directivity. Secondly, it improves the circuit connection method of the parallel graphene actuator films so that it effectively increases the sound pressure amplitude of the sound field. Finally, by establishing a sound pressure test platform in a semi-anechoic chamber room and analyzing the influence of input power, frequency, test distance and other factors on output sound pressure, this paper studies and verifies the sound field of the parallel graphene thermo-acoustic actuator.

Research on ultra low dew point in-situ on-line measurement technology for cryogenic wind tunnel

WANG Bin, XU Zhenyu, ZHANG Wenqing, KAN Ruifeng, GAI Wen

2023, 37(2): 105-114. doi: 10.11729/syltlx20210062

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Abstract:
To achieve wide temperature domain, high precision and ultra low dew point in-situ on-line measurement in the cryogenic wind tunnel, a technology based on the laser absorption spectrum is developed. In the method, the principles of laser absorption spectroscopic technology for dew point measurement are analyzed firstly. Then the absorption spectroscopic selection, spectral parameter calibration and spectral signal processing are provided. The experiments are carried out on the low temperature platform and in the 0.3 m cryogenic wind tunnel, which are compared to the chilled-mirror dew-point hygrometer measurement. The experimental results show that the developed technology can achieve wide temperature domain, high precision and in-situ on-line dew point measurement. The measurement range is from –100 ℃ to 30 ℃, the error is less than 1 ℃, and the time is less than 1 s. It can be used for ultra low dew point in-situ on-line measurement in the cryogenic wind tunnel.

A temperature measurement technique based on fluorescence intensity ratio of rare earth Dy ion

ZHANG Su, PANG Ran, JIANG Lihong, LI Da, LI Chengyu, ZHANG Hongjie

2023, 37(2): 115-124. doi: 10.11729/syltlx20210176

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Abstract:
Large area surface temperature measurement technology is of great significance in the field of wind tunnel temperature measurement. In order to meet the needs of measurement for higher surface temperature, it is urgent to develop new temperature sensing materials and new temperature measurement technology. Temperature measurement based on the fluorescence intensity ratio of the thermal coupling energy levels of rare earth ions is a new temperature measurement technology. In this work, a temperature sensitive luminescent material (YAG:Dy) was synthesized. The corresponding relationship between the temperature and the ratio of emission intensity of the thermal coupling energy levels of rare earth Dy³⁺ ions (⁴F_9/2→⁶H_15/2, ⁴I_15/2→⁶H_15/2) was investigated in the temperature range from 50 to 1000 ℃. Based on this material, a comparative experiment of two temperature measurements that the fluorescence intensity ratio measurement and the infrared thermometer is carried out. It is shown that the measurement results of the two technologies have a high degree of agreement, which proves that the temperature sensitive luminescent material (YAG:Dy) can be used for temperature measurement in the range of 50–1000 ℃.

2023 Vol. 37, No. 2