实验流体力学

Contents

2023, 37(6)

Abstract(91) PDF(31)

Abstract:

Experimental study on high frame rate characteristics of dynamic flow field of jet in crossflow

WANG Zhen, WANG Yayao, LIU Xunchen

2023, 37(6): 1-14. doi: 10.11729/syltlx20210077

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Abstract:
Despite the decisive influence of various vortex structures of a jet in crossflow on the jet trajectory and scalar mixing, there are few studies related to the high-frequency dynamic characteristics of shear-layer vortexes during transportation. This paper focuses on the high-frequency flow field characteristic, the scalar concentration distribution and the formation and collapse process of the turbulent microstructure of the jet in crossflow with different nozzle diameters and velocity ratios using 40 kHz Particle Image Velocimetry (PIV) and 20 kHz Acetone Planar Laser Induced Fluorescence (Acetone PLIF). The experimental measurements of the velocity and scalar field show that: increasing the velocity ratio promotes the expansion of the circulation zone behind the jet; in the near field of the jet trajectory, power law fitted velocity distribution and shear-layer vortex trajectory shows an exponentially decrease of the jet velocity, the shear-layer vortex strength and vortex motion frequency also show a downward trend, with the frequency of the shear-layer vortex on the windward side slightly lower than that on the leeward side; as the jet velocity increases, the frequency of the shear-layer vortex increases gradually, but the Strouhal number decreases.

Experimental study on the effect of two-stage radial spacing on flow field and atomization in LDI staged combustor

CAI Yanqing, YANG Xiaoli, WANG Kaixing, LIU Fuqiang, LENG Xianyin, WANG Shaolin, LIU Cunxi, MU Yong, XU Gang

2023, 37(6): 15-24. doi: 10.11729/syltlx20220082

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Abstract:
The radial spacings of the primary and pilot staged swirler is an important parameter for the lean direct injection combustor. In this paper, the particle image velocimetry technology, Mie scattering technology and particle size measurement technology were used to study the cold flow and spray characteristics under three different radial spacings of the primary and pilot stage. The experimental results show that, under normal temperature and pressure, with the increase of two-stage radial spacing, the central reflux area changes from narrower in front and wider in back to the same width in front and back, the backflow zone between the two stages keeps increasing, the fuel cone angle of the pilot stage is less affected, the main stage jet deflects gradually from the main stage to the pilot stage, and the main fuel crushing effect continues to deteriorate. When the radial spacing of the two-stage is 20 mm, the atomization effect of the main fuel is the best, and the atomization effect of the secondary fuel is also good.

A bi-weighted-POD and its application on wind pressure field

ZHANG Hao, YANG Xiongwei, LI Mingshui

2023, 37(6): 25-33. doi: 10.11729/syltlx20210146

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Abstract:
Proper Orthogonal Decomposition (POD) is a reduced order modeling (ROM) method based on 2nd-order statics, which simplifies the investigated wind-pressure field in a new coordinate system formed by a set of orthonormal basis. This paper suggests a method of bi-weighted POD (which weights POD by area and at the same time by root-mean-square), and applies this method to the modal reduction of pressure field around buildings. Firstly, we introduce the POD expansion in a mean-square method, which demonstrates that POD is the optimal choice of ROM in the mean-square sense. Furthermore, we modify the original POD by the bi-weighting-method to improve its capacity of identifying coherent structures with lower energy in pressure field. For the last part, the validity of bi-weighted POD is roughly examined by a case study which applies the method to the pressure field of a 5∶1 rectangular cylinder. It turns out that the modified POD method improves the ROM accuracy at the area associated with lower energy in a significant way. In the meantime, a wind-pressure field ROM constructed by bi-weighted POD captures vital information provided by the original wind-pressure field and is spatially accuracy-consistent.

Evolution of high-speed cavity flow based on PIV technology

WU Jifei, ZHOU Fangqi, XU Laiwu, YANG Ke, LIANG Jinmin

2023, 37(6): 34-41. doi: 10.11729/syltlx20210144

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Abstract:
In cavity structure, complex flows and high-intensity noises appear under the high-speed condition, seriously affecting the aerodynamic characteristics and structural safety of the aircraft. Through the methods of the particle image velocimetry technology and dynamic pressure measurement, the cavity with a length-depth ratio of 3 to 10 is experimentally investigated in the range of Mach number 0.4 to 0.8. The influences of the length-depth ratio and Mach number on the flow field structure in the cavity are emphatically analyzed, and the correlations between the noise intensity and the flow velocity are revealed. The results show that: as the length-depth ratio increases, the thickness of the shear layer in the cavity increases rapidly and expands into the cavity, leading the impact position on the cavity to move down from the back wall to the bottom, and causing the flow type in the cavity to change from open to closed. The increase of the Mach number inhibits the shear layer from expanding into the cavity and induces the main recirculation vortex to move back and the flow type to be open. The amplitude of the overall sound pressure level is positively correlated with the flow velocity in the back of the cavity.

Droplet spreading on an oblique surface

LU Jie, LI Yalei, Xu Long, HAO Jiguang

2023, 37(6): 42-50. doi: 10.11729/syltlx20220012

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Abstract:
Droplet spreading on a surface is ubiquitous in a variety of applications including aerospace, industry, and agriculture. Majority of these impacts are oblique, while previous studies focused on orthogonal impacts. Oblique impacts cannot be understood directly by previous theories and/or models. Evolution of film formation following a droplet impacting an oblique surface is investigated experimentally. Evolution of the film shape is obtained under various inclination angles and Weber numbers. Based on a new theory of droplet spreading on oblique surfaces, evolution of the film shape is analyzed. It is found that the film shape at small inclination angles can be predicted reasonably, but the error between the predicted maximum lamella width along the inclination direction and the experimental data is relatively big at large inclination angles since the length of the upstream lamella is assumed as a constant in the theory. Modifications of the theory including more detailed analysis of the length of the upstream lamella lead to an analytical model which permits the theoretical determination of the maximum lamella shape. It is shown that the error between the predicted results and the experimental results can be reduced from 61.8% by the previous theory to 3.2%.

Pressure fluctuation experiments of hypersonic boundary-layer on a 7-degree half-angle sharp cone

CHEN Jiufen, XU Yang, XU Xiaobin, ZOU Qiongfen, LING Gang, ZHANG Yifeng

2023, 37(6): 51-60. doi: 10.11729/syltlx20210054

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Abstract:
In a conventional hypersonic wind-tunnel, pressure fluctuations of the boundary layer on a 7-degree half-angle sharp cone are measured by surface sensors and are analyzed by the linear stability theory. The influences of unit Reynolds numbers and Mach number on the stability and transition position of the boundary layer are studied. The length of the test model is 800 mm and the radius of the head is 0.05 mm. Test unit Reynolds numbers range from 0.49 × 10 ⁷ m^–1 to 2.45 × 10⁷ m^–1. Test Mach numbers range from 5 to 8. The angle of attack is 0°. The transition position and the energy spectrum distribution of the disturbance wave in the boundary layer are obtained by the quantitative infrared thermography and high frequency surface pressure fluctuation measurement techniques. The frequency and growth rate of the most unstable wave are analyzed by using the linear stability theory. The experimental results show that the fluctuating pressure signal with obvious characteristics of the unstable wave spectrum can be measured in the transition region. The frequency of the pressure fluctuation is close to that of the second mode instability analyzed by the linear stability theory, and the amplitude variation trend is also similar to that of the theoretical analysis. With the increase of the unit Reynolds number, the instability appears earlier, the dominant frequency is increased, and the transition onset moves forward. The unstable wave in the boundary layer contains the first and second modes. When the free-stream Mach number is equal to 5, the transition is caused by the first mode, and when the Mach number is above 6, the transition is attributed to the second mode.

Experimental study on characteristic calibration of separated exhaust system

LI Qiufeng, LI Mi, GAO Xiang, WANG Dingqi

2023, 37(6): 61-69. doi: 10.11729/syltlx20220056

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Abstract:
In flight testing, the aeroengine flight thrust is indirectly obtained by the gas generator method. In order to improve the calculation accuracy of the flight thrust, it is necessary to accurately obtain the characteristics of the exhaust system. The laboratory calibration test and numerical simulation research were carried out by using the large bypass ratio separated exhaust system scale model. The results show that: the core nozzle characteristics obtained by the two methods are consistent, and the values are close. When the maximum core nozzle pressure ratio is 1.44, the deviations of the mass flow and the thrust are 0.73% and 0.18%, respectively; the characteristics of the separated exhaust system obtained by the two methods have the same trend and close values. When the max bypass nozzle pressure ratio equals 1.46, the deviations of the mass flow and the thrust are 0.64% and 0.18%, respectively; when the physical model and geometric model of the large bypass ratio separated exhaust system are reasonably simplified, the characteristic deviations of the separated exhaust system obtained by the two methods are in good agreement.

Development of high-precision micro-rolling moment gas bearing balance

ZHANG Huangwei, XIANG Guangwei, LYU Binbin, WANG Duowei, YU Li

2023, 37(6): 70-75. doi: 10.11729/syltlx20210182

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Abstract:
During the reentry process of the miniaturized reentry vehicle, small asymmetry of its shape can be produced due to surface ablation, resulting in a small rolling moment. In order to obtain the high-precision micro-rolling moment measurement data of the ablation model of the miniaturized reentry vehicle in the hypersonic wind tunnel, and obtain the other five component aerodynamic data, a six component micro-rolling moment gas bearing balance was developed. The rolling moment design load of the balance is 0.02 N·m, and the axial force design load is 200 N, which are orders different from each other. The overall force measurement scheme of “4+2” balance is proposed, where the four component main balance elements cooperate with the two-component M_x–X elements to complete the extremely mismatched six component aerodynamic measurement. The results of the static calibration and the wind tunnel test show that the balance has good resolution and strong anti-interference ability, and is little affected by temperature. The measurement results of the rolling moment coefficient reach the order of 10^–7. The developed gas bearing balance is little affected by the temperature and can be reused. It can measure the six components of aerodynamic data including the micro-rolling moment at the same time, which greatly improves the test efficiency and reduces the error caused by model disassembly.

Wind tunnel test technique of continuous varying Mach number for air-breathing vehicle

ZHOU Jian, ZHANG Jiang, CHEN Qiang, WEI Wei, LIU Lei, QIAN Dandan

2023, 37(6): 76-85. doi: 10.11729/syltlx20210189

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Abstract:
To study the start/unstart phenomenon of the air-breathing vehicle inlet caused by acceleration or deceleration, which also bring the problem of aerodynamic mutation on vehicle, the test technique of continuous varying Mach number was performed in the 1.2 m supersonic wind tunnel based on the flow mechanism of the 2D wedge shock wave. By developing the shock wave generator system, continuous varying Mach number was realized successfully in one wind tunnel test process. And it has also been confirmed that the technical method can make Mach number vary simply and quickly with high quality and precision. Through the flow-field calibration, the quality of the instantaneous flow-field in variable Mach number region meets the standards of GJB eligibly, and thus the field can be used for force and pressure test compliantly. In addition, the test of inlet starting characteristics was carried out and the dynamic process and critical state from start to unstart were captured, which indicates that the test results agree with the numerical simulation accurately.

The development of the movable solid wall test section for the 5.5 m × 4.0 m acoustic wind tunnel

YANG Wenguo, SHI Yan, WANG Rui

2023, 37(6): 86-91. doi: 10.11729/syltlx20220061

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Abstract:
According to the design indexes and functional requirements of the movable solid wall test section for the 5.5 m × 4.0 m acoustic wind tunnel, this report presents the design project of overall scheme. The simulation calculation is carried out for the strength, stiffness and mode of the frame in the movable solid wall test section. The key technologies of positioning and locking, lifting mechanism, replaceable lower wall panel and adjustable opening side wall panel are studied in detail. At the same time, specific solutions are provided for the key problems in its processing and assembly process. The above research contents have important reference value for the design of the closed test section of the large acoustic wind tunnel of the same type.

Research on wind tunnel test technology of sonic boom measurement based on probe

YANG Yang, QIAN Fengxue, ZHANG Changfeng, LIU Zhiyong

2023, 37(6): 92-100. doi: 10.11729/syltlx20210193

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Abstract:
Wind tunnel test is an indispensable technical means to carry out sonic boom research. Accurately obtaining sonic boom signals with weak signal attributes from complex wind tunnel test environment is the key of wind tunnel test technology. In order to study the influence of the test environment on the accurate measurement of sonic boom signal in the transient supersonic wind tunnel, a small transient trans-supersonic wind tunnel was used to develop a test device with a needle probe with the characteristics of minimizing interference. A dual motion test system with independent motion of probe and model was established, and the sonic boom measurement technology based on probe was developed. Taking a typical cone-cylinder model as the research object, the differences of the complete sonic boom signals obtained by the two test methods of fixed probe and moving model and fixed model and moving probe are compared, and the sonic boom propagation law of cone-cylinder model and the influence of wind tunnel background flow field on sonic boom measurement are studied. The results show that: Compared with the two test methods of fixed model and moving probe, the former will cause the distortion of sonic boom signal, which leads to more serious distortion of sonic boom signal, while the latter is smoother and more accurate, which is a relatively more reliable test method; The background flow field distribution in wind tunnel has a significant influence on sonic boom signal measurement. It is necessary to master the background flow field distribution in wind tunnel in detail and keep it stable as much as possible. On this basis, the positions of model and probe in wind tunnel and the relative positions of model and probe should be strictly selected.

A concise method of determining critical flutter wind speeds for small damping modes

TANG Jianping, HE Jun, WANG Xue, HUANG Xia, XU Binbin, JIN Ling

2023, 37(6): 101-105. doi: 10.11729/syltlx20210071

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Abstract:
In low speed flutter tests, flutter models with small damping modes start continuous vibration usually at low speeds without obvious flutter divergence. Therefore, it's of some uncertainty on determing the critical flutter wind speeds by visual inspection or by “damping method (DM)” of modal parameter identification. Considering the similarity between the vibration phenomenon of a small damping modal flutter test and that of a fighter buffet test, a technique named “amplitude turning point method (ATPM)” similarly to that used in identifying buffet boundaries is proposed to determine the critical flutter wind speeds. The method is based on RMS of vibration amplitudes, the curves of normalized vibration RMS changing with wind speeds are drawn, and critical flutter wind speeds are determined according to the first turning points of curves. In a small damping modal flutter test, the method was applied in the test data processing of the engine hangers with variable parameters. Comparing the ATPM results with the DM results and the numerical results, the following conclusions are made: the results of three methods are in agreement, the ATPM results are more similar to the numerical results than the DM results, and the ATPM is concise and reliable, with good stability and applicability.

Calibration results and analysis of thin-film gauges calibrated with the transfer method

YANG Kai, LIU Jichun, CHEN Suyu, ZHU Xinxin, WANG Hui

2023, 37(6): 106-111. doi: 10.11729/syltlx20210129

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Abstract:
Considering the fact that there are more error sources in the measured heat flux with thin-film gauges when the two-stage approach is applied to determine the thermal product and resistance–temperature factor, the transfer method is applied to directly calibrate thin-film gauges, in which the thermal product and resistance-temperature factor are treated as the sensitivity coefficients. To get the consistent calibration results of different thin-film gauges fabricated in a batch, the sensitivity coefficients are divided by the resistance–temperature factors of the thin-film gauges, and then the correction sensitivity coefficients are consistent. With the transfer calibration technique, the calibration results of thin-film gauges show a good linearity with a relative expanded uncertainty below 6.5%, which is lower than that reported in other researches, in which the two-stage approach is used to calibrate thin-film gauges.

Pressure sensitive paint measurement correction method based on surface spline interpolation

JING Zhiwei, WANG Libo, TANG Chu

2023, 37(6): 112-119. doi: 10.11729/syltlx20220027

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Abstract:
The PSP (Pressure Sensitive Paint) measurements correction after the wind tunnel test is often implemented through the least square method, and it tends to neglect the chordwise and spanwise flow characteristics on the wing. By employing the IPS (Infinite Plate Spline) technique, a hybrid correction method and process for the PSP test results was presented. A full-span airplane model, with PSI (Pressure Scanner Instrument) and PSP on the upper and lower surfaces of the wing, was utilized to carry out the PSP test and PSI test simultaneously in a transonic wind tunnel with the 2.4 m test section. The pressure measurements were conducted at Ma = 0.735 and in the angle of attack range from −6.38° to 10.59°. Test results indicate that the PSP data demonstrate a good agreement with PSI data. However, the PSP test system with only one camera shows a poor capacity to capture the aerodynamic field at the wing leading edge. The hybrid correction method for PSP results has proven to an effective approach, and the corrected data reveal that the hybrid correction can better deal with the pressure distribution characteristics of the full wing.

Engineering approach of compressor test efficiency correction

CAO Chuanjun, ZHU Wei, WANG Jinchun, ZHANG Xiaoshi, LIU Tianyi

2023, 37(6): 120-127. doi: 10.11729/syltlx20220031

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Abstract:
Efficiency is the key parameter for evaluating compressor performance of the aero engine. It is usually acquired by compressor performance tests. Due to the difference in test condition, mechanical configuration, and instrumentation loss, efficiency obtained from compressor rig tests should be corrected for evaluating performance and engine matching in the engineering field. An engineering approach for correcting the compressor efficiency was proposed. The influence factors of the compressor efficiency were classified systematically under different test conditions, and detailed methods of correcting Reynolds number, inlet strut loss, thermo-couple Mach number recovery, air humidity and probe loss were given. Two examples were conducted using this method. The efficiency of a ten-stage compressor component rig test was corrected by 1.85 percent improvement. Besides, in the engine test, the compressor efficiency was corrected by 0.95 percent improvement. This method can be applied in the engineering field of efficiency measurement correction and performance evaluation for core and whole engines.

2023 Vol. 37, No. 6