实验流体力学

Contents

2023, 37(5)

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Abstract:

A preliminary study on calibration-free hot-wire anemometry method

GAO Nan, LIU Xuanhe

2023, 37(5): 1-8. doi: 10.11729/syltlx20230004

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Abstract:
The study aimed to investigate convective heat transfer from a heated circular cylinder with a diameter of 5 microns and a length-to-diameter ratio of approximately 200 or more using experimental methods. A new relationship between the Nusselt number and the Reynolds number was proposed. The results obtained from this study and those reported in the literature were fitted linearly to a curve. This curve served as the basis for proposing a novel hot-wire velocity measurement technique. In this method, velocities are directly calculated from the voltage outputs of the anemometer without requiring calibration. However, validation measurements indicated that there were errors of up to 23% between the measured and actual values. These errors may be due to measurement uncertainties in the probe's length, cold resistance measurement, the resistance of the lead wire, and connectors.

Aerodynamic pressure field reconstruction from sparse points using data assimilation method

HUANG Jun, GUO Yuxin, JI Jingjing, HUANG Yong'an

2023, 37(5): 9-17. doi: 10.11729/syltlx20230021

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Abstract:
In wind tunnel experiments, the high-precision pressure distribution of models is highly required, but existing measurement methods still have certain shortcomings. In order to obtain the global pressure distribution of the wind tunnel model, this paper assimilated the sparse measured data and numerical calculation data of the wind tunnel experiment by Ensemble Transform Kalman Filter (ETKF), and realized the high-precision reconstruction of the full-space flow field based on the finite measurement points of the model surface. Two-dimensional airfoil RAE 2822 and NACA 0012 were used for experimental verification. Sparse reconstruction of pressure results of RAE 2822 is more consistent with the measured results than the linear theory correction. This effect is especially evident at the shock wave position, and the prediction error of the pressure coefficient is reduced by about 3%. The lift coefficient and moment coefficient of the wing calculated by using the modified ETKF set mean of attack angle and Mach number are less than 1% error from the experimental values. Experiment of NACA 0012 is oriented to the full-field sensing application of wind tunnel experiments and explore the feasibility of pressure reconstruction based on a small number of measurement points. The experimental results show that the relative error of pressure coefficients reconstructed using six measurement points on the wing object surface can be reduced to 2.42%, and the comparison results show that the assimilation effect is highly dependent on the data point locations.

Progress in complex combustion field diagnostics based on on-line mass spectrometry technology

LI Jing, YANG Dong, LI Mei, HOU Keyong

2023, 37(5): 18-33. doi: 10.11729/syltlx20220145

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Abstract:
The combustion field is usually a complex system of gas-solid-liquid triphase coupling, and the results obtained from combustion diagnostic can support the researches to improve combustion efficiency and reduce pollutant emissions. In order to make the measurement results more accurate, it is urgent to develop advanced detection methods and detection systems. After several years of development, on-line mass spectrometry has the advantages of high sensitivity, fast analysis speed and wide detection range. It can be used for combustion flow field diagnostics under severe conditions such as high temperature and pressure, and can obtain more comprehensive and sensitive diagnostic information. Firstly, this paper summarized the development of key technologies, e.g., analyzer, ionization source, and sampling systems for on-line mass spectrometry in recent years. Secondly, the applications of on-line mass spectrometry on the measurement of flame component concentration and flame temperature in combustion field were enumerated. On this basis, the challenges and development prospects of the on-line mass spectrometry in complex combustion field diagnostics were summarized, which could provide reference for the relevant researchers.

Multi-wavelength synthetic aperture rainbow refractometer

WANG Xinhao, WU Yingchun, XU Dongyan, WU Xuecheng

2023, 37(5): 34-40. doi: 10.11729/syltlx20230026

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Abstract:
Against the increasing requirement of large-scale conditions, there is more demand of the measurement instruments. As an efficient approach of measuring cloud droplets, the measurement distance of the rainbow refractometry is generally less than 50 cm owing to the limitation of the aperture of the imaging system. The synthetic aperture rainbow refractometry is proposed to achieve long distance droplet measurement. Laser beams of multiple wavelengths are used to irradiate droplets and multiple rainbow signals are generated. The multiple rainbow signals are collected and combined into one signal and then inversed to obtain the droplet parameters. In this way, the measurement distance of the rainbow refractometry is increased to around 1.5 m, which breaks the aperture limit of the rainbow refractometry and enlarges its applicability. Based on this technique, the multi-wavelength synthetic aperture rainbow refractometer is developed and suitable for parameter measurement of droplets in the central region of large-scale facilities with instruments located outside. The synthetic aperture rainbow refractometry is verified to achieve long distance droplet measurement. The feasibility and accuracy of the multi-wavelength synthetic aperture rainbow refractometer were proved through experimental tests of water and ethanol droplets with different sizes and refractive indices. The sizes of water and ethanol droplets were within the range from 100 to 200 μm. The droplet size and refractive index uncertainties were within 5 μm and 8 × 10⁻⁴, respectively. Therefore, the multi-wavelength synthetic aperture rainbow refractometer is no longer limited by the aperture size and can be applied to larger industrial scenarios, achieving the multi-parameters droplet measurement of size, refractive index, and so on.

Infrared molecular tagging velocimetry

HU Zhen, SONG Zihao, WANG Weitian, ZHU Ning, CHAO Xing

2023, 37(5): 41-48. doi: 10.11729/syltlx20230036

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Abstract:
Molecular Tagging Velocimetry (MTV) and Particle Imaging Velocimetry (PIV) are often used for flow visualization and velocity field imaging. However, the requirement for tracer particles can bring systematic errors to the velocity measurement of PIV method when the tracer particles have poor followability and uneven distribution. In the case of MTV, although particle seeding is not required, the finite fluorescence lifetime of the tracer molecules typically constrains its use only in high-speed and supersonic flows. To develop a velocity field imaging method with no requirements of tracer particles and suitable for low-speed flows, a novel MTV method based on infrared (IR) laser-induced fluorescence is developed and verified in axisymmetric turbulent jet of carbon dioxide. Resonant vibrational transition of the small gas molecule is selectively excited by an infrared pulsed laser to achieve molecular tagging, and the fluorescence distributions of the excited molecules at different instants are then imaged by an infrared camera, from which the velocity distributions are deduced. The effects of the molecular vibrational energy transfer process model, finite fluorescence lifetime, lateral velocity component and molecular diffusion motion on the fluorescence distribution are analyzed to improve the accuracy of the velocity measurement. The proposed method has been successfully verified in carbon dioxide turbulent jets with velocities ranging from 5 m/s to 51 m/s, and the radial distribution of the axial velocity in the main region of the jet is measured. The radial spatial resolution can reach 107 microns, and the velocity distribution is consistent with the theoretical calculation of turbulent jets and previous experimental results. The relative uncertainty of velocity measurement is better than 8%. This method can be used to obtain high-resolution instantaneous velocity imaging of low-speed flow field. Subsequently, by improving the pulse power, excitation efficiency and repetition frequency of the infrared laser, the measurement accuracy, spatial resolution and temporal resolution of this method can be further improved. Therefore, the proposed method bears great potential to provide a quantitative velocity field imaging method in the near-wall flow, micro-scale flow and large gradient flow where it is difficult to introduce tracer particles.

Research on calibration method of aeroengine temperature rise combustion efficiency

ZHAO Jian, LIU Chongyang, WANG Yufang, HU Lintao, WU Zhijun

2023, 37(5): 49-55. doi: 10.11729/syltlx20220139

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Abstract:
Combustion efficiency is a key pneumatic performance parameter of the aeroengine, and its accurate acquisition is important for improving performance quality of aeroengines, saving fuel, reducing emission and overall performance matching of the combustion chamber. For accurate acquistion of the aeroengine temperature rise combustion efficiency, an inlet reference temperature sensor, an outlet reference temperature sensor and a medium temperature sensor with high accuracy are designed,measurementpoint correction factor is proposed and used based on the combination of the outlet reference sensor and medium temperature sensor, and field calibration of the aeroengine temperature rise combustion efficiency is realized. The results show that the adopted temperature rise combustion efficiency calibration method is reasonable, the calibration result is reliable, and the relative deviation between the temperature rise combustion efficiency measured by the reference temperature sensors and computed by the gas analysis combustion efficiency is 0.3% to 2.1%, and thus the method can solve the calibration problem of the aeroengine temperature rise combustion efficiency.

Advances on helicopter brownout

ZHANG Weiguo, TAN Jianfeng, LIU Yakui, YANG Shipeng, WANG Chang

2023, 37(5): 56-75. doi: 10.11729/syltlx20220112

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The interference of the helicopter rotor and the ground causes the lifting of dust, which leads to the phenomenon of brownout. The phenomenon threatens the flight safety of helicopter seriously, and therefore, it is necessary to carry out relevant researches on the aerodynamic basis for the brownout. In this paper, the research progress of helicopter brownout is deeply analyzed from four aspects: numerical methods, experimental techniques, formation mechanism, and suppression methods. Investigations demonstrate that the coupled vortex method or CFD method and the Lagrange sand tracking method can realize simulations of the sand cloud profile and the phenomenon of brownout, but the key factors such as the complex sand bed surface, fluctuating turbulence, migration of sand particles, aggregation, and lifting of sand particles need to be concentrated on. The brownout experimental system built with high-speed PIV techniques and high-speed camera can obtain the morphology data of the sand cloud, but it is still necessary to study the simulation methods of the complex sand bed and the measurement techniques of dust spatial concentration, to explore the driving mechanism of sand particles and the evolution mechanism of brownout, and to develop the design methods and technical approaches to effectively weaken brownout.

A wind tunnel investigation of the helicopter vortex ring state boundary

WANG Chang, MA Shuai, HUANG Zhiyin, WANG Haowen, HUANG Zhiyuan, DENG Haoxuan

2023, 37(5): 76-92. doi: 10.11729/syltlx20220055

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Abstract:
The helicopter vortex ring state boundary is systematically analyzed and studied in this paper. Firstly, the causes of vortex ring state accidents are analyzed, and the physical mechanisms in flight characteristics, rotor performance, rotor inflow, and vortex structure are expounded. The formation of concentrated vortex causes the induced inflow to dominate the vertical inflow on the rotor disk, causing negative damping of rotor thrust and loss of rotor performance, and as a result the instability of subsidence motion. Then, the differences and applicability of various vortex ring state boundaries are compared, the problems of the existing boundary prediction models with strong subjectivity in modeling methods and high dispersion of test data are concluded, and improvement ideas are proposed. Finally, on the basis of the above knowledge, a rotor wind tunnel test to simulate the descending flight was designed and carried out. The test results show that the rotor thrust negative damping, thrust loss and power subsidence phenomenon are presented in the vortex ring state, the rotor thrust loss is up to 30%, and the required power is about 160% of the hovering power when the rotor generates the same thrust as hover; using the negative damping of rotor thrust and the loss of thrust performance, which are concerned in the practice of flight emergency, as the defining index, the discrete points of the critical velocity at vortex ring state boundary are extracted from the test results; in the construction of the vortex ring state boundary model, the strength of the horizontal inflow, the vertical inflow and the induced inflow on the rotor tip vortex is distinguished, and the correction of the momentum theory under different advance ratios and the difference of the rotor tip vortex motion threshold are taken into account. On the basis of the model parameters determined by the least squares method based on the test values, a semi-empirical vortex ring state boundary prediction model is established, and the model is in good agreement with the wind tunnel test results and in line with the trend of flight test results.

Measurement of wall shear stress in wind-blown sand environment

HUANG Ning, YANG Bo, HE Wei, ZHANG Jie

2023, 37(5): 93-100. doi: 10.11729/syltlx20230074

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Abstract:
As a key driving force of aeolian sand movement, wall shear stress is of importance in the study of aeolian sand movement. However, at present, the experimental measurement of the surface shear force in the wind-blown sand environment is extremely scarce, which has become a bottleneck for the further development of the wind-blown sand research. This paper introduces a method based on the hot-film technology for accurate measurement of the wall shear stress in the wind-blown sand environment. The wall shear stress was measured under conditions without sand using hot-film probes and two-dimensional hot-wire probes, respectively. The result shows that the hot-film probe has a good response to the wall shear stress, and the difference between the measurement results of the friction velocity of these two probes is less than 6.6%. The measurement results of the hot-film probe for the wall shear stress of flow around the square column are in good agreement with the OpenFOAM numerical simulation results, which further proves the effectiveness of the hot-film probe for the measurement of the wall shear stress. In addition, the particle impact test results of the hot film probes show that the influence of particle impact on the hot-film signal is negligible, so the hot-film probe can be used to effectively measure the wall shear stress in the wind-blown sand environment. Based on this technology, the spatial distribution characteristics of the wall shear stress along the flow direction in the wind-blown sand environment were tested. And for the first time, the spatial distribution of the wall shear stress in the wind-blown sand environment was obtained by means of actual measurement. In view of the high-frequency response capability of the hot-film probe, this technology will play an important role in the experimental research on the mechanism of aeolian sand movement in the turbulent boundary layer.

Experimental study on Rayleigh–Bénard convection during supercritical phase transition of carbon dioxide

ZHAO Yifan, WU Di, WANG Jia, LI Jialiang, DUAN Longsheng, DUAN Li, KANG Qi

2023, 37(5): 101-110. doi: 10.11729/syltlx20230003

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Abstract:
Supercritical fluid is a kind of special fluid under extreme conditions (temperature and pressure are above the critical point). The Rayleigh–Benard (RB) convection of supercritical fluid driven by buoyancy is a new nonlinear thermal convection system. Its buoyancy does not conform to the Boussinesq approximation. Under the action of temperature difference, the physical properties of RB fluid show severe distortion near the critical point, accompanied by abundant flow and phase transition coupling processes. In this experiment, a transparent sapphire pressure vessel capable of carrying supercritical carbon dioxide (SCO₂) was designed to establish RB convection of supercritical fluid under the effect of vertical temperature gradient. The flow structure and supercritical phase transition process under different temperature differences were observed. The velocity field of “atomized” droplets was calculated by image cross-correlation algorithm. In the experiment, platinum resistance temperature measurement was used to accurately control the temperature of the upper and lower ends of the container, and the evolution of various flow modes and velocity fields in the linear cooling process was studied. In the linear cooling process, SCO₂ goes through three typical processes: supercritical flow, transcritical flow and gas liquid two-phase flow. The strong coupling of the transcritical phase transition with buoyancy convection results in the heterogeneous unsteady flow of supercritical carbon dioxide RB convection. It shows that the supercritical RB convection is extremely sensitive to temperature difference, and the larger the temperature difference is, the more intense the convection in the supercritical domain is. With the decrease of temperature, the atomized droplets condense to form abundant structure of multi-layer flows, which finally turn to the two-phase flow.

Experimental research on the mixture properties inside cloud cavitation region

WANG Benlong, ZHANG Hao, LIU Yunqiao

2023, 37(5): 111-121. doi: 10.11729/syltlx20220138

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Cavitating flows are unique phenomena in the research field of hydrodynamics. Within the cavitation region, vapor and water are mixed in a complex state. Special experimental techniques are required to obtain the mixture properties. Both non-intrusive and intrusive techniques for cavitating mixture measurement are reviewed, including ionizing radiation, endoscopy, electrical impedance probes and optical fiber probes. In comparison with the experimentally demanding non-intrusive methods, intrusive probe techniques provide simple and stable single-point measurements for the void fraction, bubble velocity and bubble size. Recent applications of phase-resolved probe techniques are then introduced. The electrical impedance probe and optical fiber probe were used in the mixture property measurements in wedge-plate and hydrofoil cavitation, respectively. The time-averaged void fraction and statistical distribution of the bubble size were obtained. The results indicate that the void fraction within cloud cavitation is closely related to the cavitation number, and the characteristics of the bubble size distribution are quite similar between the two kinds of cloud-cavitating flows (wedge-plate and hydrofoil cavitation).

2023 Vol. 37, No. 5