实验流体力学

A review on three-dimensional flame measurements based on tomography

SONG Erzhuang, LEI Qingchun, FAN Wei

2020, 34(1): 1-11. doi: 10.11729/syltlx20190135

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Abstract:
Three-dimensional (3D) measurements for turbulent flame have been long desired in the combustion community. In the past decades, with the advancement in high-speed cameras, high-power lasers and computing algorithms, it becomes possible to achieve 3D combustion measurements with sufficient spatial and temporal resolution. This paper reviews the recent development and applications of such a 3D measurement technology based on tomography, so as to provide valuable references to researchers in this field. This review includes the following four parts:First, the principles and computing algorithms of tomography are introduced and reviewed. Second, the experimental measurement systems involved in the 3D tomographic measurements are classified and summarized. Third, the applications of 3D tomographic measurements are introduced including the tomographic chemiluminescence, tomographic laser-induced fluorescence and tomographic shadowgraph/schlieren. Last, we concludes the review and poses several questions for the potential development of the technique.

A review of the methods of point measurement and spatial measurement on thin liquid film thickness

LI Tianyu, HUANG Bingyao, LIAN Tianyou, LI Songyang, LI Yuyang

2020, 34(1): 12-24. doi: 10.11729/syltlx20190075

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Abstract:
Liquid film phenomena exist widely in nature and in industry processes. In particular, fuel spray in combustion engines can always form the thin liquid film in sub-millimeter or micron scales. The high precision measurement of the liquid film thickness is vital for the design and improvement of combustion engines. In this paper, we briefly review the point measurement and spatial measurement methods that are commonly used in the thin liquid film thickness measurement. Point measurement methods mainly include electric methods and the total internal reflection (TIR) method for measuring the liquid film thickness at a single position, which are low cost and easily operational but have no spatial resolution. Spatial measurement methods mainly include electric methods, fluorescence intensity method and planar laser induced fluorescence (PLIF) method, which can simultaneously measure the liquid film thickness at multiple positions or in continuous regions and obtain the information about the film thickness distribution and film movement and development. Among spatial measurement methods, electric methods are easily operational and highly stable, but the electrodes can disturb the liquid film. On the other hand, the optical methods are always non-intrusive and suitable for the measurement of the liquid film in high speed motion.

Measurements of laminar burning velocity and analysis of its field for the laminar premixed methane-air flames using the Bunsen burner method and schlieren technique

XIANG Longkai, YA Yuchen, NIE Xiaokang, REN Fei, KE Wei, CHU Huaqiang

2020, 34(1): 25-32. doi: 10.11729/syltlx20190087

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Abstract:
The laminar burning velocity is a key parameter for determining the combustion propagation model and verifying the chemical reaction mechanism. In this paper, the Bunsen burner experiment system and the schlieren experiment system are built to carry out research on the laminar premixed combustion characteristics of methane. The laminar burning velocity and the flame external field of methane/nitrogen/air were measured by the Bunsen burner method and the schlieren technique respectively, and the effects of equivalence ratio (ER) and nitrogen blending on the laminar burning velocity and the external field of the flame were investigated. The following conclusions are obtained through experiments: the equivalence ratio has an important influence on the laminar premixed combustion characteristics of the methane; and as the equivalence ratio increases, the laminar burning velocity increases first and then decreases, and the cone flame height decreases first and then increases; as well the external field of the flame starts to stabilize gradually. Nitrogen blending plays a negative role in the laminar burning velocity variation, at the same fime, the more nitrogen is blended, the more the laminar burning velocity decreases. The flame height increases with the increase of N₂ doping ratio, but the external flow field of the flame becomes more disordered and difficult to stabilize.

Structured illumination for Rayleigh scattering imaging to eliminate the stray light interference

YAN Bo, SU Tie, CHEN Shuang, CHEN Li, YANG Furong, TU Xiaobo, MU Jinhe

2020, 34(1): 33-37, 48. doi: 10.11729/syltlx20190093

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Abstract:
The development of the laser sheet imaging technique is introduced in this paper. This technique is one of the most versatile optical imaging techniques and has been frequently applied in several different domains. However, when applied in limited operating space, the interference from indirect reflections and surrounding backgrounds is known to produce errors in the laser imaging. Therefore, a novel 2D imaging technique named Structured Laser Illumination Planar Imaging (SLIPI) is developed to solve this problem. This method is based on planar laser imaging but uses a sophisticated illumination scheme-spatial intensity modulation-to differentiate between the intensity contribution arising from useful signals and stray light. By recording and dealing with images, the SLIPI method can suppress the diffuse light and maintain the useful signals. In this paper, we first use the MATLAB software to simulate the SLIPI method, and the conclusions suggest that the stray light interference can be removed effectively. Furthermore, the SLIPI apparatus, consisted of the continuous laser, Ronchi grating and EMCCD camera is designed. And the Ronchi grating is used to produce the sinusoidal distribution of the laser sheet. At last, SLIPI apparatus is combined with the Rayleigh scattering approach in the McKenna flat burner to generate the Rayleigh scattering signal. And the thesis presents convincing experimental evidence that the SLIPI method is able to remove the diffuse light contribution, thus improving and enhancing the visualization quality.

Review of automotive aerodynamics research based on physical models

LIU Jinsheng, XU Shengjin, WANG Qingyang, BAO Huanhuan, WANG Yong

2020, 34(1): 38-48. doi: 10.11729/syltlx20190081

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Abstract:
Automotive aerodynamics involves fundamental fluid dynamics problems such as turbulence flow past bluff bodies, flow instability, flow separation and control, fluid-structure interactions and noise, and so on. In this paper, we review the research progress of aerodynamics based on physical models at home and abroad, introduce the achievements of previous studies on aerodynamics, flow field research, flow control, calculation and experiment, multi-vehicle aerodynamics, pollution, wind noise, etc., and investigate the shortcomings of the present studies. Finally, we discuss the research directions of vehicle aerodynamics in the future.

Wind tunnel based virtual flight testing research of F-16 fighter

ZHANG Shiyu, ZHAO Junbo, FU Zengliang, LIANG Bin, ZHOU Jiajian

2020, 34(1): 49-54, 86. doi: 10.11729/syltlx20180157

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Abstract:
A three degree-of-freedom (3-DOF) Wind Tunnel Based Virtual Flight Test (WTBVFT) system is developed to study unstable aerodynamic phenomena and maneuver characteristics coupling with motion of high-performance fighters maneuvering at high angle of attack. Based on this system, a series of 3-DOF virtual flight tests of F-16 fighter are performed. As the results, the short-time modes of flight dynamics are simulated from wind tunnel tests with small angle of attack, and control law are validated in those tests. Then an instability phenomenon in pitch is detected from tests with high angle of attack. A group of lateral-directional unstable aerodynamic-coupling motions are detected from tests when angle of attack is less than zero. In these tests, augmentation control of aileron cannot recover lateral-directional stability of flight, but always leading to control coupling motion. It is more effective to adopt elevator control to change angle of attack to recover the lateral-directional stability.

Effect of non-ideal opening behavior of diaphragm on the operation of free piston shock tunnels

ZHU Hao, ZHANG Bingbing, YU Yifu

2020, 34(1): 55-59. doi: 10.11729/syltlx20190023

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Abstract:
In free piston shock tunnels, the non-ideal opening behavior of the main diaphragm may have different effects on the free piston motion and the shock wave formation. Based on the diaphragm shear-strain model and piston dynamic model, the coupled equations involving piston motion and diaphragm rupture are obtained for the first time, and the effects of the diaphragm opening process on piston motion and driving time under constant pressure are described. Research shows that, due to the interaction between the piston front pressure and the piston speed, the speed of diaphragm rupture or rupture ahead of time has little effect on the free piston terminal speed, and the piston can achieve soft landing at the end of the compression tube. This conclusion is confirmed by test results. Moreover, the test results also show that the non-ideal opening behavior of the main diaphragm has a distinct influence on the formation of the shock wave, and it gradually weakens during the propagation of the shock wave.

Infrared thermography experiments of hypersonic boundary-layer transition on a 7° half-angle sharp cone

CHEN Jiufen, LING Gang, ZHANG Qinghu, XIE Futian, XU Xiaobin, ZHANG Yifeng

2020, 34(1): 60-66. doi: 10.11729/syltlx20180172

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Abstract:
In order to promote the in-depth research on the hypersonic boundary layer transition and provide basic wind tunnel experimental data for the study of the boundary layer transition mechanism, the physical model validation, and the transition database construction, infrared thermography experiments of boundary layer transition are carried out in the Φ1 m hypersonic wind tunnel at CARDC. The effects of different unit Reynolds numbers, angles of attack and Mach numbers on the transition positions are studied on a 7° half-angle sharp cone. Test unit Reynolds numbers range from 0.49×10⁷/m to 2.45×10⁷/m. Test angles of attack range from -10° to 10°. Test Mach numbers range from 5 to 7. The head radius of the test model is 0.05mm. The quantitative infrared thermography technique is employed to obtain the temperature distribution photos of the model surface. By this way, the accurate transition positions and the effects of transition factors are obtained. Test results of the global temperature distribution show that an earlier transition occurs with the increase of Mach number. This is due to the larger Reynolds number and stronger flow field noise brought by the higher Mach number. As the unit Reynolds number increases, the transition position of the boundary layer moves forward and the transition Reynolds number remains constant about 3.0×10⁶. When the angle of attack is small, a delayed transition occurs on the windward side and an earlier transition occurs on the leeward side with the increasing angle of attack. When the angle of attack is 10°, an earlier transition occurs at the center line of the windward side and reversed transition with angle of attack takes place, accompanied with a low heat flow strip induced by the flow separation on the leeward side.

Research on gas replenishment for submersed superhydrophobic surface by electrolysis

ZHANG Mengzhuo, HU Haibao, DU Peng, HUANG Xiao

2020, 34(1): 67-71. doi: 10.11729/syltlx20190097

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Abstract:
The variation of current with electrode pole distance and number during artificial seawater electrolysis is studied by using the graphite electrode device. The plastron state on the superhydrophobic surface replenished by the electrolysis device under different working voltages in the rectangular pipeline is observed, which proves the feasibility of this method. The results show that:There is a linear relationship between the voltage and the current are linear in the working process of the electrolyzer. Under a constant voltage, the current increases with the electrodes number and decreases with the pole distance. Study on the electrolysis efficiency shows that increasing the electrodes number is advantageous for increasing the current, but decreases the electrolysis efficiency. It is observed that the superhydrophobic surface plastron disappears under the scouring of turbulent flow. When the electrolyzer is operated at a low voltage, the gas production is not enough and the gas supply device works intermittently. The gas-liquid interface recovers partly on the superhydrophobic surface; When the voltage is increased, the gas production increases and a more obvious specular phenomenon can be observed, which proves the feasibility of the superhydrophobic electrolytic gas replenishment.

Experimental study of air/ethanol mono-injector gas generator

ZHAO Fang, REN Zebin, LI Xianfeng, GUO Longde, TAO Yu, SHI Yu, LUO Zhifeng

2020, 34(1): 72-78. doi: 10.11729/syltlx20180186

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Abstract:
With regard to the safety, economy and miniaturization of the ejector system, a mono-injector air/ethanol gas generator based on the aero-engine combustor structure was designed. A series of ignition tests were carried out. The results show that:the technical design scheme of the gas generator is feasible, and reliable ignition of the gas generator can be realized by direct ignition of the high energy spark plug. The pressure curve indicates a rapid ignition of the generator, and the main working parameters basically meet the design index. The low limit of the excess air coefficient of the gas generator for reliable ignition is 3.9, showing a wide mass flow-rate range. The combustion efficiency is high, decreasing obviously with the increase of the excess air coefficient.

Experimental study on trailing edge with gradual flow resistance suppressing noise diffraction of BWB engine

LI Wenjian, CHEN Peng, WANG Yong, LU Xiangyu, WANG Junwei, ZHAO Kun

2020, 34(1): 79-86. doi: 10.11729/syltlx20180175

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Abstract:
In the configuration of Blended Wing Body (BWB) aircraft, the noise effect of engine on the ground can be reduced by using the shielding effect of engine components on engine noise sources. According to the mechanism that the sound-absorbing materials with gradual distribution of flow resistance can effectively suppress the vibration velocity of particles caused by the sudden change of sound pressure in the edge effect, a new type of trailing edge with gradual flow resistance is proposed to further suppress the noise of BWB engine on the ground. Therefore, the simplified model of NACA0012 airfoil + side cylinder is used to simulate the configuration between the fused wing-body airframe and the back engine in a 0.55m×0.4m acoustically guided wind tunnel with full anechoic environment, and the effects of the trailing edge of airfoil filled with three kinds of sound-absorbing materials with different flow resistivity on the diffraction noise of side noise sources are discussed from the perspective of classical acoustics and aeroacoustics respectively. The different influence of different airfoil trailing edges on the sound field noise of the other side is analyzed under different incoming wind speeds. The results show that the sound pressure level of the noise can be reduced by about 5 dB at most when the side noise source is shielded by airfoil; When the standard trailing edge is replaced by three different trailing edges with gradual flow resistance, the diffraction noise in the noise composition is additionally suppressed to some extent, thus further reducing the noise. The noise reduction effect is positively related to the flow resistivity, and the sound-absorbing material with the largest flow resistivity has the best noise reduction effect, which can further reduce the noise sound pressure level by about 3 dB. It is also speculated that in the range of flow resistivity of sound-absorbing materials from 0 to ∞, the noise reduction effect increases firstly, and then decreases with the increase of flow resistivity.

Displacement and deformation measurements of helicopter rotor blades based on binocular stereo vision

ZUO Chenglin, MA Jun, YUE Tingrui, SONG Jin, WANG Xunnian

2020, 34(1): 87-95. doi: 10.11729/syltlx20190071

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Abstract:
To realize the displacement and deformation measurements of helicopter rotor blades in a non-contact way, the binocular stereo vision based three-dimensional measurement method is presented. First, coded targets with unique identifying information are pasted on the rotor blades. Then, with nanosecond illumination provided by the high-frequency laser, high-speed CCD cameras are triggered synchronously to acquire the transient images of rotor blades. Finally, based on the binocular stereo principle, three-dimensional coordinates of coded targets are obtained, which are used to calculate the displacement and deformation parameters of rotor blades further. The test results show that the accuracy of our proposed method is better than 0.3 mm, which is capable of meeting the high precision displacement and deformation measurement requirements of helicopter rotor blades.

Application of ANSYS in piezoelectric balance design

ZHAO Rongjuan, HUANG Jun, LIU Shiran, LYU Zhiguo, LI Guozhi

2020, 34(1): 96-102. doi: 10.11729/syltlx20190005

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Abstract:
A finite element analysis method with coupled mechanical and electric analysis is adopted to evaluate the performance of a three component piezoelectric balance. Static force, modal and transient response analyses are done in this research. The relationship between the balance output and the applied load is obtained by static force analysis to evaluate the main coefficient and interaction coefficient of the balance. The vibration frequency and vibration mode are obtained by modal analysis to evaluate the balance frequency response characteristics. The transient response analysis is done to evaluate the characteristics of the balance with impulse force, and to evaluate the compensation characteristics of accelerometers. The ANSYS analysis results show that, the piezoelectric balance has a large main coefficient, a small interaction coefficient, and high response frequency; the accelerometers can be used to compensate the balance signal, which can meet the requirements of the shock tunnel aerodynamic force test. The result of the balance calibration and shock tunnel test shows the same balance characteristic with that of the simulation results. Through this research, the characteristic of the piezoelectric balance is evaluated for the balance design, which is useful in the balance structure optimal design and determining the position of the sensitive element.

Investigation on target jet mill based on the entrainment of the annular supersonic flow

ZHANG Zhao, LIN Jun, TAO Yang, GUO Qiuting, ZUO Jin, LU Bo

2020, 34(1): 103-108. doi: 10.11729/syltlx20180200

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Abstract:
The jet mill widely used in many industrial fields is a kind of apparatus to produce superfine powder. A new supersonic jet mill, whose design is based on numerical simulations and collision experiments, is presented and manufactured. In our design an annular supersonic flow produced by an annular converging-diverging nozzle is used to entrain the central subsonic flow with particles. After the supersonic flow's entrainment, the particle beam concentrated in the flow center impacts on the target accurately with a great kinetic energy. The multiphase numerical studies show that the collision speed of particles of 25 μm to 1 μm in diameter can reach 440 m/s to 530 m/s approximately in the entrainment of the supersonic flow with the Mach number of 3.0 and the total pressure of 1.5 MPa, and particles can precisely collide on the target. The collision experiments implemented in the new-made jet mill manifest the huge collision energy and the precise supersonic collision in the jet mill by the target erosion and the iron particles size reduction.

2020 Vol. 34, No. 1