实验流体力学

2021, (5): 1-2.

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Abstract:

Application progress of LDV and PIV in measuring flow in centrifugal compressor

YU Xinghang, MA Hongwei, LIAO Xin

2021, 35(5): 1-18. doi: 10.11729/syltlx20200106

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Abstract:
The precise measurement of the centrifugal compressor flow field is very important to understand the internal flow characteristics. The traditional contact measurement technology has some defects, such as low spatial resolution, serious blockage effect, single measurement location and so on, which can not meet the measurement requirements of the modern advanced centrifugal compressor. Laser Doppler Velocimeter (LDV) and Particle Image Velocimeter (PIV) are two typical non-contact measurement technologies, which have the characteristics of high measurement accuracy, wide application range and non-contact measurement, showing great potential in the measurement of the internal flow field of the centrifugal compressor. By combing the application status of the LDV and PIV velocity measurement technology in measuring the internal flow field of the centrifugal compressor at home and abroad, the application progress of the LDV and PIV velocity measurement technology in measuring the internal flow field of the centrifugal compressor is introduced. Focusing on the experimental scheme, experimental details and technical difficulties, and combining with the future development trend of the measurement technology, the application of the LDV and PIV velocity measurement technology in the centrifugal compressor is discussed from the perspective of practical application. The application of internal flow field measurement is summarized and prospected.

An optimization algorithm for deriving the average flow velocity in a shallow microchannel through spatiotemporal concentration gradient

WU Sida, CHEN Kejie, ZENG Xiao, LI Yongjiang, QIN Kairong

2021, 35(5): 19-25. doi: 10.11729/syltlx20210075

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Abstract:
Precise measurement of the flow velocity in microfluidic channels plays an important role in the application of microfluidic chips for quantitative chemical analysis, sample preparation, drug synthesis, etc. In this study, based on the principle of mass transport in microchannels, an optimization algorithm is proposed to derive the average flow velocity within a shallow microchannel through the spatiotemporal concentration gradient. Firstly, based on the relationship between the flow field and the concentration field in the shallow microchannel governed by the Navier-Stokes equation and the Taylor-Aris dispersion equation, a direct inversion method and an optimization algorithm to derive the average flow velocity are demonstrated respectively. Secondly, the influence of the parameters of the spatiotemporal concentration signals (i.e. frequency, amplitude and diffusion coefficient) on the prediction accuracy of the average velocity has been analyzed using numerical simulation. Finally, experiments using fluorescent dye are carried out to verify the feasibility of the proposed method. Simulation results show that the correlation coefficient between the derived average velocity obtained by the optimization algorithm and the real velocity is one in the absence of noise interference, which indicates high calculation accuracy. In the case of noise interference, the accuracy of the optimization algorithm can be improved by increasing the frequency and amplitude of the dynamic concentration. And a low diffusion coefficient can also improve the accuracy. In the microfluidic experiments, a correlation coefficient between the inversion result of the optimization algorithm and the measurement of the flow sensor can be as high as 0.9814.

Experimental study on time-delay effect of unsteady hydrodynamics of the supercavitating vehicle in water tunnel

LIU Xiyan, YUAN Xulong, WANG Ying, LUO Kai, WANG Xinyu

2021, 35(5): 26-33. doi: 10.11729/syltlx20210039

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Abstract:
The cavity-delay-effect has a significant impact on the tail slapping induced by maneuvering, and it is an important mechanical characteristic of supercavitating vehicles, which must be considered in dynamic modeling. The experimental method of artificial ventilated supercavitation is used in the laboratory of Northwestern Polytechnical University’s high-speed water tunnel to carry out the continuous variable angle of attack experiment. The delay time is quantitatively analyzed by measuring the changes of attack angle and dynamic force with time. Varying delay times are obtained under different conditions, and the dimensionless time-delay coefficient is approximately linearly related to the water speed. The time-delay coefficient increases with the increase of the preset rudder angle. Among the oscillation frequency used in the experiment, the larger the oscillation frequency is, the smaller the time-delay coefficient is.

Research on improvement measures of transverse heading of general aircraft based on spoiler

LIAN Zhenzeng, ZHANG Hui, YAN Wencheng, KONG Peng

2021, 35(5): 34-39. doi: 10.11729/syltlx20200066

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Abstract:
In order to adapt to the short take-off and landing requirement under the condition of strong side wind, the large general-purpose aircraft should still have good transverse heading stability at large side slip angle. In this paper, the transverse heading characteristics of a large general-purpose transport aircraft were studied, it is found that there is a turning phenomenon in the transverse heading heading test date of the aircraft. And the measures to improve the transverse heading stability of the general aircraft based on spoiler were studied by means of the wind tunnel test and theoretical analysis. The results show that the root cause of the turning phenomenon of the data is the local separation of the wing, that as the strong upwash caused by the fuselage with side slip angle greatly increases the local angle of the attack of the center wing on the windward side, local separation occurs when the local angle of attack of the center wing on the windward side exceeds the stall angle of attack of the model airfoil. The installation of a spoiler on the side of the fuselage interferes with the flow of the upwash air, and as a result the advance separation of the airflow on the upper surface of the wing is restrained, the turning phenomenon of the transverse heading characteristic curve is eliminated, and the lateral heading stability of the aircraft is improved.

Evolution prediction of HVI debris based on CVAE model

ZHOU Hao, LI Yi, ZHANG Hao, CHEN Hong, REN Leisheng

2021, 35(5): 40-46. doi: 10.11729/syltlx20200058

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Abstract:
Efficiently and accurately predicting the evolution of hypervelocity impact debris is crucial in the design of spacecraft protective structures. To this end a deep learning model is constructed. The model is based on the conditional variation auto encoder (CVAE) and massive smoothed particle hydrodynamics (SPH) simulations. The model includes four controllable labels, namely projectile velocity (3.00-8.00 km/s) radius (2.00-8.00 mm), target plate thickness (1.000-4.000 mm), and time instant (1.0-12.0 μs). The model uses 200 parameters to describe the debris mass distribution, resulting in an average error less than 0.6%. It takes less than 7 micro seconds to predict one debris mass distribution.

Experimental study on jet turning based on spiral flap

WANG Jun, LAI Qingren, KANG Hongming, ZHANG Liu, LI Binbin, ZHAO Lei, JIN Yi

2021, 35(5): 47-53. doi: 10.11729/syltlx20210036

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Abstract:
A new flow control flap, spiral flap, was designed by gradually increasing the curvature radius based on Archimedes helix theory. The influence of the key control parameters such as the initial radius and alignment radius of the spiral flap on the upper surface jet deflection is studied. The control effect of the spiral flap is compared with that of the traditional basic flap, and the control mechanism of the two is analyzed. The results show that the maximum average thrust deflection angle of the spiral flap is about 20°. Compared with the basic flap, the spiral flap has larger average thrust deflection angle and higher thrust efficiency at large drop pressure ratio, which indicates that changing the curvature profile can promote the flow adhesion of the jet and improve the performance of the upper surface blowing system.

PIV experimental study on vortex structures induced by free autorotation fall of a samaras

DONG Lin, WEN Guoan, LEI Ziwei, RINOSHIKA Akira

2021, 35(5): 54-60. doi: 10.11729/syltlx20200004

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Abstract:
Taking the typical maple samara blade as the research object, the flow field of the free rotation and falling process is measured by the particle image velocimetry, to study the evolution and the law of the spin flow structure, which has certain guiding significance for the design of the single-wing aircraft. By comparing the effects of different blade lengths, thicknesses, widths, falling angles, spin angular velocities, falling velocities, and different falling attitudes on the transition period and stability period, the shape and spatial characteristics of the induced blade spin are obtained. Combined with the results of characteristic parameter analysis, PIV flow field measurement experiments are carried out for the period of blade spin stability, and the rules of vortex generation and evolution in the process of falling are obtained. The experimental results show that the leading-edge vortices (conical structure along the spanwise direction) are generated at the front tip during the stable period, the trailing edge vortices in the reverse direction are generated at the back-edge, and the two vortices are coupled with each other. The strength of the leading-edge vortex is greater than that of the trailing-edge vortex, which leads to the angle of attack. In front of the leading edge and the tip of the leaf, a higher upward velocity region was observed, and a higher downward velocity region was observed near the rear edge and the root of the leaf, which resulted in an upward lifting force on the seed and a stable falling of the blade.ade.

Design and property analysis on insulation structure of cryogenic wind tunnel

SONG Yuanjia, LIAO Daxiong, CHEN Wanhua, LAI Huan, HOU Yu

2021, 35(5): 61-67. doi: 10.11729/syltlx20200137

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Abstract:
The cryogenic wind tunnel is the best way to achieve flight Reynolds number testing in engineering application, with great strategy importance in the national defense field. The insulation structure has become one of the key technologies for building large cryogenic wind tunnels. On this account, design and property analysis on insulation structure is investigated with the help of numerical simulation and experiment. Firstly, considering the wind tunnel working condition, design and material selection for the insulation structure is carried out. Then, the finite element modeling of the insulation structure is built. On this basis, the property of the insulation structure is numerically studied based on the actual worst working condition of the wind tunnel. The insulation characteristics and stress/displacement distribution of the insulation structure are analyzed. Finally, the experimental platform able to simulate the working condition of cryogenic wind tunnel is designed and constructed to carry out the temperature and stress/strain measurement experiment for the insulation structure under cryogenic alternating pressure working condition. The obtained results show that the designed insulation structure satisfies the demands of the cryogenic wind tunnel in service, which solves the problem of insulation structure designation. Additionally, the designed experimental platform solves the problem of property evaluation of the insulation structure, which paves the way for building the cryogenic wind tunnel of our country in the future.

Wind tunnel test for aerodynamics of wing-in-ground craft flying near smooth/wavy surface

GAO Lihua, HUANG Longtai, FU Hao, WANG Kunlun, HUANG Yong

2021, 35(5): 68-74. doi: 10.11729/syltlx20200077

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Abstract:
In order to investigate the aerodynamics of the wing-in-ground craft flying near wavy surface the dedicated wavy ground floor and moving wavy belt are devised to simulate the real water wave with the height of one meter. The wavy ground floor is capable of varying its vertical and horizontal position. The moving wavy belt which contains two waves on the belt is suitable for the wind tunnel test of the wing-in-ground craft flying over all the phases of the wave. The wind tunnel test has been carried out which simulates the wing-in-ground craft flying in the sky, near smooth surface and wavy surface by means of flat ground floor, wavy ground floor and moving wavy belt, respectively. The wind tunnel test results show that propellers and the floor have a coupling effect on the taking-off and landing aerodynamics of the wing-in-ground craft, instead of simple superposition. What is more, the aerodynamic loads of the wing-in-ground craft fluctuate dramatically over the various phases of the wave.

Propeller slipstream effect on pitching static margin of airplane and optimization of horizontal tail

YAN Wencheng, JIN Hua, JIANG Yubiao, LIAN Zhenzeng, ZHANG Hui

2021, 35(5): 75-80. doi: 10.11729/syltlx20200075

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Abstract:
Propeller slipstream has an important effect on the airplane's aerodynamic characteristics. The wind tunnel test indicates that under the condition of large pulling force, the airplane under study has longitudinal static instability at small angle of attack which affects the safety of aviation seriously. The test research on slipstream shows that the longitudinal static instability at small angle of attack is mainly caused by the horizontal tail doing the traverse progress in the slipstream, which induces the horizontal tail's efficiency to change obviously. With the layout characteristics under consideration, it is advised to reduce the height of the horizontal tail to reduce the power's adverse effects. The test proves that reducing the height of the horizontal tail can improve the longitudinal static margin at small angle of attack obviously, and enlarging the range of the longitudinal static stability.

A schlieren motion estimation method for water flow velocimetry

HUANG Tianli, WANG Qian

2021, 35(5): 81-89. doi: 10.11729/syltlx20200155

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Abstract:
In the present study, a schlieren motion estimation algorithm is proposed for two-dimensional seedless water flow measurement. A physical constraint is derived combining the schlieren intensity and continuity equations. The space smoothness constraint adopts the second-order div-curl regularizer. Based on the two constraints, the global cost function is derived and minimized to resolve the velocity field using the variation method. As an example, a buoyant plume in the water tank is tested using a Z-type schlieren imaging system. The sample images are calculated using a correlation algorithm in PIVlab, an optimized optical flow algorithm, and the newly proposed schlieren motion estimation algorithm. The results show that the new algorithm can resolve more details of the flow field with higher spatial resolution, while the velocity gradient is consistent with the continuity characteristics. It also features with good robustness without obvious outlier mistakes. The schlieren setup is simple and cost effective in setup. The proposed algorithm has shown great potentials for velocity measurement in more complex configurations.

Stage separation trajectory simulation test technique in high speed wind tunnel

QIAN Fengxue, GUO Peng, GAO Peng, LIU Qi, WANG Yuanjing, YI Guoqing

2021, 35(5): 90-98. doi: 10.11729/syltlx20200125

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Abstract:
Stage separation trajectory capturing test technique in the FL-23 wind tunnel was developed for investigating the stage separation dynamics of future multi-stage launch vehicle systems. By making use of the tunnel's upper and lower attack angle support mechanism and modification of servo-motors, transmission mechanism and control modules, a upper rig with two degrees of freedom of alpha, x, a lower rig with three degrees of freedom of alpha, x and y, and a coupling motion control system were set up. For a two stage vehicle, the two stage models and their force balances could be mounted on the upper and lower rigs respectively, and the stage separation trajectory could be simulated in the wind tunnel under control of the stage separation test computer, with coordinated motion of the angle of attack of the fore stage model, the angle of attack of the aft stage model, and the relative position in x and y directions of the two stage models. Conclusions could be obtained from verification experiments and client experiments, namely, the upper driving mechanism of the stage separation test system could pitch from –15° to 15°, and move from 0 to 200 mm in the x direction. The lower driving mechanism could pitch from –11° to 49°, and move from 0 to 680 mm in the x direction and from 0 to 507 mm in y direction. And the system could be used to conduct the tests for force measurement, model injection experiment, grid force measurement and capture trajectory simulation.

Experimental research and analysis of gust generator for low speed wind tunnel

QU Xiaoli, LIU Qin, ZHU Bo, NIE Xutao, WANG Chao

2021, 35(5): 99-105. doi: 10.11729/syltlx20200114

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Abstract:
The gust generator is the key equipment for the wind tunnel test in response to the gust. According to the operating characteristics of the oscillating vanes gust generator, a simplified steady vortex theory is used to deduce the calculation formula of the Y-direction wind speed in the downstream flow field of the gust generator. With the experimental researches done in the 0.55 m × 0.4 m low-speed wind tunnel (a pilot acoustic wind tunnel), the influences on the extreme wind speed of the gust flow field were systematically studied by changing the design parameters of the gust generator (vane chord length, number, spacing) and operating parameters (vane swing amplitude, swing frequency, incoming flow velocity). The research shows that the simplified formula derived in this paper can explain the changing mechanism of the Y-direction wind speed in the downstream flow field after the design and operating parameters of the gust generator are changed. The gust flow field generated by the gust generator can be simply analyzed during the design of the gust generator. To increase the Y-speed extreme value of the downstream flow field of the gust generator, increasing the number of vanes has an advantage over increasing the vanes chord length. Before the vanes stall, the larger Y-speed extreme value can be obtained by increasing the vanes swing speed rather than by increasing the vanes swing frequency. The spacing of the vanes for the gust generator should not be too small, otherwise the equivalent lift coefficient of the vanes would decrease. When the vanes spacing is 1.2 times the chord length, the maximum Y-direction wind speed can be obtained. This research can provide reference for the design of gust generators in other wind tunnels.

Three-dimensional liquid film flow measurement based on digital image projection technology

SONG Huazhen, ZHAO Huanyu, ZHU Chengxiang, WANG Zhengzhi, TIAN Wei, LI Haixing, ZHU Chunling

2021, 35(5): 106-114. doi: 10.11729/syltlx20200031

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Abstract:
Quantitative measurement of fluid film flow is a necessary means to analyze the heat transfer process of the icing phase change. Digital Image Projection (DIP) based on image processing enables non-invasive quantitative measurement of fluid films. Firstly, the basic principles, cross-correlation algorithm and calibration method of the DIP technology are introduced. Based on this, the DIP system and the flat water film flow test bench are designed and built. Subsequently, the overall error of the DIP system was tested and was found to be within 5%, indicating the reliability and accuracy of the system. A series of water film flow experiments were carried out on a flat water film flow test bench. The DIP system was used to restore the three-dimensional full picture of the flat water film flow. Finally, the relationship between the average water film thickness as well as the dimensionless water film thickness and the water film Reynolds number is obtained by fitting the measurement results, and is compared with the theoretical derivation and literature experimental results. Results show the overall trend is consistent.

Experimental study on structural characteristics of separation flow induced by 3D wedge in hypersonic laminar flow by oil visualization

WANG Junqi, CHEN Zheng, NI Zhaoyong, GAN Caijun, LI Lang

2021, 35(5): 115-120. doi: 10.11729/syltlx20180026

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Abstract:
Experimental investigation of hypersonic laminar separation flow over a 3D wedge is carried out in the hypersonic wind tunnel by utilizing oil flow visualization technique. Two test models of rectangular/ triangular flat plate are used. The 3D wedges which have different compression angles are mounted on the afterbody of models. The experimental result shows that the forebody shape has a great influence on the structures of hypersonic laminar separation induced by the 3D wedge. For the triangular plate model, the flow in the upstream of the wedge has obvious crossflow effect, and the separation structure is completely different from that of the rectangular plate model. At different attack angles, the shapes of the separation lines are also very different. The result shows very significant 3D effect of the local laminar separation which is induced by the triangular forebody and the 3D wedge.

Design and application of launch device for free-flight test in hypersonic wind tunnel

HE Chao, XIE Fei, XU Xiaobing, CHEN Lei

2021, 35(5): 121-127. doi: 10.11729/syltlx20200038

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Abstract:
The Free-flight test launch technology is one of the key technologies in the free-flight test. Hypervelocity Aerodynamics Institute, CARDC, researched and developed the model launch device for the conventional hypersonic wind tunnel, and conducted a series of free-flight tests in the hypersonic wind tunnel with Mach number being 6. The free-flight test results indicate that: the launch device worked well, and the attitude of the model is stable. According to the different wind tunnel specific circumstances, the size of the launch device can be reformed, which can be applied to different hypersonic wind tunnels. Launch pressure and launch velocity are important parameters, which should be considered at the structure design stages. The model fixed component also needs the adaptability design according to the different model shapes.

Development of internal strain gauge balance with variable cross-section axial force flex beam based on Finite Element Analysis

SHI Yujie, PENG Chao, MI Peng, ZHANG Huangwei

2021, 35(5): 128-133. doi: 10.11729/syltlx20200109

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Abstract:
When the axial section of internal strain gauge balance is applied large load, especially large moment load, the maximum stress of its flex beam often becomes the main factor limiting the maximum load capacity of the balance. This paper introduces the development of a large moment internal strain gauge balance. The flex beam and measuring beam of the axial section are optimized and improved by using finite element simulation analysis. The shape of the flex beam is improved from traditional constant cross section to variable cross section. The thickness of the middle part of the flex beam is reduced, and the thickness of both ends is increased. The maximum stress on the flex beam is reduced on the basis of maintaining the same axial stiffness of the balance. The axial force measuring beam adopts variable section structure, which reduces the strain gradient on the measuring beam. The finite element analysis shows that the stress distribution on the variable-section flex beam is more uniform than that of the traditional flex beam, and the maximum stress at the root of the variable-section flex beam is reduced by more than 20%. The strain gradient on the measuring beam with variable cross section is reduced by 79% compared with that on the beam with constant cross section. The calibration results of the balance are in good agreement with the finite element analysis results, and the wind tunnel force test also shows that the balance has good stability.

2021 Vol. 35, No. 5