实验流体力学

Experimental investigation on the dynamic behaviors of droplets impacting on ultrasonically vibrating curve surfaces

ZHANG Haixiang, HE Feng, ZHANG Xiwen, HAO Pengfei

2020, 34(4): 1-8. doi: 10.11729/syltlx20200036

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Abstract:
The present work experimentally investigates the dynamic behaviors of droplets impacting on ultrasonically vibrating curve surfaces. The complicated experimental phenomena, including edge splash, surface splash, capillary wave, cavitation, and sub-droplet rebound, are observed, and the mechanisms behind each phenomenon are revealed. The critical curve of the edge splash is obtained, and the critical vibration amplitudes on curve surfaces are lower than that on flat surfaces due to the aerodynamic force. Using the image processing technique, the expelling efficiency of the ultrasonic vibrating curve surface and the size distribution of secondary droplets are elucidated and discussed. The expelling efficiency increases linearly with the increase of the ultrasonic vibration amplitude, and the impact velocity has almost no influence on the expelling efficiency. A higher excitation amplitude results in a wider secondary droplet size distribution and a larger average size. It is found that the temperature slightly affects the dynamic collision process of the droplets through comparing the experimental results under room temperature and supercooled conditions. Under the supercooled condition, the ultrasonic vibration could still effectively expel the impinging droplets, which shows the potential of the ultrasonic vibration on the waterproof and anti-icing fields.

Experimental study on a bionic underwater detector

LIU Jiajun, LI Ziyan, REN Haigang, LIU Yingzheng, WEN Xin

2020, 34(4): 9-15. doi: 10.11729/syltlx20190102

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Abstract:
Underwater detectors are widely used in marine engineering equipment, which have important functions such as sensing underwater environment for ships and submarines. The currently applied sensors are faced with restrictions such as short detection distances, high power consumption, and insufficient signal-to-noise ratio, by which their large-scale applications are limited. For a long time, bionics research has provided a lot of innovative inspiration for the design of marine engineering equipment. Researchers found that the predation and hedging behavior of harbor seals depends on the perception of the underwater environment by the special structure of their vibrissa. In this paper, through the study of the vibrissa structure, a vibrissa-shaped underwater detector was designed and manufactured, and its excellent underwater target detection capability was verified by experiments conducted in a water tunnel. In the experiment, different working conditions such as angle, incoming flow velocity and detection target position were studied. The spectrum analysis of the signal collected by the detector was used to master the working rules of the vibrissa-shaped detector.

Measurement and correction of high frequency jet noise

ZHANG Junlong, LEI Hongsheng, ZHAO Yu, LI Jie

2020, 34(4): 16-21. doi: 10.11729/syltlx20190103

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Abstract:
In order to improve the accuracy of high frequency measurement of jet far field noise, the factors affecting the high frequency measurement of jet noise have been systematically studied based on the experimental method of the strict jet noise simulator test equipment. These factors contain: the directivity of microphone, the frequency response characteristics of microphone, the influence of microphone protective cover, the noise floor of microphone and the effect of atmospheric absorption, etc. The experimental comparison and quantitative analysis of each influence factor have been carried out. Recommendations and correction methods for high frequency jet noise measurement are given. Finally, the location of geometric far field, which is very important for the accurate measurement of the high frequency radiation characteristics of far field jet noise, is proposed based on experimental measurement results.

Active turbulence simulation study of wind loads on standard low-rise building

HU Shangyu, LI Qiusheng, ZHANG Ming

2020, 34(4): 22-29. doi: 10.11729/syltlx20190157

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Abstract:
This paper presents model scale tests for 1:50 geometrical scale laboratory modeling for the Texas Tech University (TTU) test building in a gust wind tunnel. This tunnel is able to add low frequency turbulence components for generation of large-scale turbulent flows by the conventional passive simulation technique or a technique combining active and passive turbulence generation devices. This paper mainly studies the effects of the turbulence integral length scale and turbulence intensity on wind-induced pressures on the flat roof of TTU full scale building. The results imply that the turbulence intensity has significant influence on the mean wind pressure coefficients at the leading edge of the flat roof under the separation bubble and in the corner zone for the conical vortex flow regime. Meanwhile, the turbulence intensity plays a predominant role in producing the fluctuation and peak pressure at the leading edge or in the corner area. On the other hand, the turbulence integral length scale has no noticeable influence on the mean wind pressure coefficients at the leading edge or in the corner zone. However, the turbulence integral scale length plays a slightly significant role in producing the fluctuation and peak pressure at the leading edge or in the corner zone.

Research on test technology of helicopter floating characteristics

JIANG Ting, JIAO Jun, TANG Binbin, WANG Guan, SUN Gang

2020, 34(4): 30-35. doi: 10.11729/syltlx20190078

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Abstract:
In order to ensure that the helicopter can provide sufficient floating time for people to evacuate after ditching on the water, the floating characteristics should be thoroughly and extensively studied in the design and development stage. In this paper, a helicopter model is taken as the research object, and the model test method is studied for the helicopter with emergency floatation bags. In this research, the stiffness of the emergency floatation bags, the experimental conditions and the wave direction are the key variables. The results show that the helicopter motion response under regular wave conditions shows a typical cyclical change feature. When the emergency floatation bags is made of flexible material, the structure can weaken the wave response of the helicopter to a certain extent, which is beneficial to the helicopter to maintain the stable floating on the wave surface. When the model floats freely on the water surface, the flexible material can also better assist the helicopter to avoid the most unfavorable wave direction- the horizontal wave that is easy to cause helicopter's overturning and turning over. The model test with no constraints called free floating state is more comprehensive and close to the actual situation. In the model test, the above characteristics should be noted to carry out the relevant test design.

Effects of end condition and aspect ratio on aerodynamic properties of rectangular sectional model

WEN Qing, CHI Junhao, HUA Xugang, WANG Xiuyong, SUN Hongxin

2020, 34(4): 36-43. doi: 10.11729/syltlx20190088

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Abstract:
End condition and aspect ratio of the sectional model are two important influence factors on wind tunnel tests. To investigate the effects of end condition and aspect ratio on the characteristics of aerodynamic force of the sectional models, wind tunnel tests are performed on a stationary B/D=5 rectangular sectional model with various end conditions and aspect ratios. The results indicate that:(1) the aerodynamic force on the rectangular cylinder can be significantly influenced by end condition and aspect ratio; (2) when the length of the cylinder is twice larger than the interference length of end condition, the aerodynamic force distributes in spanwise as an isosceles trapezoid, however, when the length of the cylinder is twice smaller than the interference length of end condition, the aerodynamic force distributes in spanwise as an isosceles triangle. More attentions should be paid to the design on aspect ratio of model for wind tunnel tests. A suitable large aspect ratio of the model may lead to reliable results.

Application exploration of Tomo-PIV in the subsonic and transonic wind tunnel

LI Xiaohui, WANG Hongwei, ZHANG Miao, HUANG Zhan

2020, 34(4): 44-52. doi: 10.11729/syltlx20190061

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Abstract:
Tomographic Particle Image Velocimetry(Tomo-PIV) is a kind of instantaneous velocity measurement technology combined PIV and computer tomography (CT), which can quantitatively obtain the three-dimensional structure of the flow field. It is successfully applied in the subsonic, transonic and supersonic wind tunnel and the supercritical airfoil verification experiment about riblets drag reduction was carried out. The layout of body light and camera is designed based on the situation of FD-12 wind tunnel in China Academy of Aerospace Aerodynamics. The free flow field under the condition of Ma=0.6 is measured by solving the problem of the uniformity diffusion of tracer particle, and compared with the test results of PIV. The data of both accord well, verifying the accuracy of Tomo-PIV. Meanwhile, the three dimensional velocity field in the rear of trailing edge is measured by attaching the smooth film and symmetrical riblets film respectively which used supercritical airfoil OAT15a as the carrier. The results reveal that the Mach number of the measurement field increases after attaching the riblets film. It shows that the riblets film can reduce the friction resistance of the wing and has certain drag reduction effect.

The calibration and image post-processing method research of temperature-sensitive paint

LIU Xiang, XIONG Jian, MA Husheng, ZHOU Qiang, CHEN Liusheng, WANG Hongbiao, HUANG Hui, CHEN Zhi

2020, 34(4): 53-61. doi: 10.11729/syltlx20190054

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Abstract:
Temperature-sensitive Paint (TSP) calibration error is an important error source in the TSP experimental technique. Based on the domestic 615/EP TSP, the effects of some calibration technical details including the CCD exposure time, image average quantity, noise level, image definition and analog-to-digital (A/D) conversion digit on the calibration precision are studied. The research results show that, the image noise level has a significant effect on the calibration precision. In an image region of low noise level, the image definition and the image average quantity have less significant influence on the calibration precision. In order to obtain high-precision calibration results, the image gray should exceed 32% of the CCD full-well capacity for the Hamamatsu CCD camera, and the tunnel test and calibration test should have the same number of digits for the image A/D conversion.

Unsteady surface pressure measurements of standard spinning missile model in supersonic flow

XIAO Heng, GU Yunsong, SUN Zhijun

2020, 34(4): 62-67. doi: 10.11729/syltlx20190100

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Abstract:
In order to obtain complex pressure changes of the spinning missile model, an embedded wireless pressure measurement system was designed, which can simultaneously acquire 8 pressure channels at a sampling frequency of 1 kHz. The absolute pressure measurement system has a range of 30 PSI and a static measurement error of less than 5/10 000. After connecting a 10 cm pressure measuring hose, the delay of the system is less than 1.16 ms, and the signal amplitude attenuation is less than 1%. By using the embedded wireless pressure measurement system, the model surface pressure test was carried out in a high-speed wind tunnel. The pressure in the critical area of the spinning missile model is measured and the dynamic characteristic of the surface pressure is obtained. The result shows that the unsteady pressure measurement technique proposed in this paper can synchronously measure the surface pressure of rotating model, which provides an effective unsteady surface pressure measurement method for wind tunnel test of rotating missile.

Research on the layout scheme for the axial compressor in the 0.6 m continuous transonic wind tunnel

CHEN Jiming, LEI Pengfei, LIAO Daxiong, ZHENG Juan, CONG Chenghua, WANG Yitian

2020, 34(4): 68-73. doi: 10.11729/syltlx20190034

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Abstract:
As the driving system of continuous transonic wind tunnel, compressor's matching design, which considered its operating performance and the general performance of wind tunnel, is one of the key technologies in wind tunnel design. With the development of large continuous transonic and supersonic wind tunnel, the compressor trends to have larger power, higher energy efficiency, wider range, higher precision and so on. The layout scheme of the axial compressor in the 0.6 m continuous transonic wind tunnel, which is a pilot wind tunnel for large transonic wind tunnel, is studied. The location and the scheme of the main compressor are analyzed from the aspects of aerodynamic performance, structural design and control. Also, the integrated design of the wind tunnel and compressor is important for the sake of efficiency. By performance test of compressor in the 0.6 m continuous transonic wind tunnel, while the compressor arranged between the first and the second corner, the feasibility of the layout scheme of external motors and two-end drive, the synchronous control of multi motors and the rectification technology of the compressor's internal flow path is validated. All the compressor's operating performance indicators can fulfill the design-technology requirements well. The studies provide technical support for the construction of large scale continuous transonic wind tunnel.

Technology research and test verification of distributed flow regulator for inlet test in wind tunnel

LI Fangji, ZHAO Qing, FAN Jianchao, JIA Shuang, RONG Xiangsen, GUO Min

2020, 34(4): 74-80. doi: 10.11729/syltlx20190022

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Abstract:
The downstream flow of inlet model can be translated into multiple regions by the distributed flow regulator which is composed of two sets of guide vanes. The mass flow is regulated by adjusting the relative position which changes the flow area between the two sets of guide vanes. Two simplified models of the distributed regulator and throttle cone are simulated by commercial software and improved in FL-24 wind tunnel. It is shown that the distributed regulator owns good flow and regulating capacities. Owing to the technology of distributed flow regulator, the size of the flow-meter can be decreased. The results from the distributed regulator are in good agreement with ones from the throttle cone when there is no surge emerging. Also it has little effect on the whole flow field, especially on the upper flow.

Research on test technology of dynamic force measurement of rotating missile in wind tunnel

DONG Jingang, JIN Jialin, LI Guangliang, QIN Yongming, MA Handong

2020, 34(4): 81-86. doi: 10.11729/syltlx20190119

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Abstract:
The missile encounters complex aerodynamic problems during the rotating flight. The rotary motion causes unsteady aerodynamic characteristics. The conventional static force measurement wind tunnel test cannot meet the requirements of the dynamic aerodynamic characteristics research, and the dynamic aerodynamics of missile model needs to be measured in the wind tunnel, the rotational motion of missile model performs similar simulations. In the 1.2 m-level supersonic wind tunnel, the active control technology of the rotary motion of the long slender missile model and the dynamic aerodynamic measurement technology corresponding to the rotary motion are studied. The established test technique was verified by the wind tunnel test using a rotating missile model with a slenderness ratio of 20. The experimental results show that the micro-drive system and the integrated design of the rotating component and the missile model can stably control the rotational speed of the missile model with large slenderness ratio. The established wind tunnel dynamic force test technology can obtain the dynamical data of the missile model in rotating motion, and the test data was reproducible with good precision.

Optimal design of steady enthalpy probe and test verification

ZHU Xinxin, LONG Yongsheng, SHI Youan, YANG Qingtao, ZHOU Ping, ZHAO Shunhong

2020, 34(4): 87-93. doi: 10.11729/syltlx20190062

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Abstract:
For high precision test requirements of the total enthalpy in the ground thermal shielding test of hypersonic vehicles, a kind of steady enthalpy probe with heat shield was developed. The enthalpy probe test system was designed based on the energy balance principle. And the influence of the sampling tube and the cooling water on the enthalpy value was analyzed by FEM. Then the heat insulating structure, water mass flow and gas mass flow were designed and optimized. Finally, the verification test was implemented in the arc heater flow field. The test results show that the design of the enthalpy probe parameter is reasonable, the enthalpy probe structure has a good insulating performance, the repeatability accuracy of the enthalpy probe is better than 2.6%, and the deviation of measurement values of the enthalpy probe and the Fay-Riddell method is less than 13.0%.

Study on extraction method of liquid jet trajectory in low-speed air crossflow based on image processing

LAN Tian, KONG Lingzhen, CHEN Jiaqing, WANG Kuisheng

2020, 34(4): 94-101. doi: 10.11729/syltlx20190089

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Abstract:
In order to study the trajectory characteristics and influencing factors of liquid jets injected into low-speed air crossflow under different working conditions, the liquid jet breakup image is attained by the high speed camera, and the extraction method of trajectory characteristics of liquid jets injected into air crossflow is proposed, combined with image processing technology. Firstly, histogram equalization is used to enhance the gray-scale original image. Secondly, the optimal histogram entropy method (KSW entropy method) and traditional genetic algorithm are employed to achieve threshold segmentation. Finally, the Sobel operator and convex hull algorithm are served for extracting the jet edge contour, and obtaining the data point set of the jet trajectory. The results of trajectory extraction and fitting of typical jet breakup modes under different working conditions show that the proposed method can effectively extract the characteristics of the liquid jet fracture trajectory, and adapt to the track extraction of different liquid jet breakup modes under the action of low speed air crossflow. Besides, the empirical formula of jet breakup obtained by nonlinear fitting can predict the jet trajectory well.

A study on chaotic characteristics and short-term prediction of pressure difference fluctuation signal of gas-liquid two-phase flow in small channel

PAN Hui, LI Haiguang, WU Xuan

2020, 34(4): 102-108. doi: 10.11729/syltlx20190077

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Abstract:
Experimental research on the pressure difference fluctuation signal was conducted in the channel of a horizontal circular pipe with a diameter of 3.0 mm, using air and water as the working medium. According to the pressure difference fluctuation signal diagram and the flow pattern diagram taken by the high-speed camera, the chaotic dynamic analysis of the pressure difference fluctuation signal was carried out by using the phase space reconstruction and Lyapunov index discrimination methods. Then the Volterra adaptive short-term prediction of the pressure difference fluctuation signal was carried out. The experimental results show that the attractor diagram obtained by the chaos analysis can show the flow characteristics more accurately. The Volterra adaptive prediction model has the relative errors of 1.86%, 0.71%, 3.90% and 2.49% for the prediction of the pressure difference time series of the annular flow, the layered flow, the intermittent flow and the slug flow respectively, which can effectively make short-term prediction of pressure difference time series of the gas-liquid two-phase flow in the pipeline.

2020 Vol. 34, No. 4