实验流体力学

Experimental research progress of hydrofoil tip vortex cavitation

LIU Yuwen, XU Lianghao, SONG Mingtai, GU Xiangnan, PENG Xiaoxing

2020, 34(5): 1-11. doi: 10.11729/syltlx20190083

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Abstract:
As a typical vortex cavitation phenomenon, Tip Vortex Cavitation (TVC) occurring on the propellers (or duct propellers) and hydro-turbines has been widely investigated in experiments. To figure out the research orientations and the key problems in the near future, experimental research progress and results about the three stages of TVC, that are inception, development and collapse, are summarized to provide reference for further studies. On the inception of TVC, analysis of vortex dynamics of the tip flow field is conducted to get preliminary acquaintance with mechanics of its inception under various phase transformation conditions of bubble nuclei, low pressure and action time of low pressure, based on vortex theoretical models and experimental measurement technology. On the development of TVC characteristiced by a sharp increase of the radiation noise, research on the morphology of TVC and characteristics of the corresponding radiation noise is carried out for a better understanding of the mechanism of the development and radiation noise of TVC, in combination of high-speed photography of the morphology and acoustic signal acquisition of TVC. The collapse of TVC is only mentioned briefly, because few attention is paid to this issue as it can hardly cause vibration and erosion on hull structures. An accurate prediction of the inception of TVC in practice is still an open question though some progress is made in the fundamental research of TVC. How to quantify the scale effect of the vital parameters, such as load, Reynolds number and water quality, and how to improve the incipient prediction formula of TVC are problems still to be solved.

Instability of an interface subjected to a perturbed shock: reflected shock effects

QUAN Tong, LIAO Shenfei, ZOU Liyong, QIU Hua

2020, 34(5): 12-19. doi: 10.11729/syltlx20200019

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Abstract:
The Richtmyer-Meshkov(RM) instability of a N₂/SF₆ interface subjected to a perturbed shock is investigated experimentally in a vertical shock tube. The perturbed shock is generated by a planar shock diffracting around a rigid cylinder and the initial uniform interface is formed by a membraneless method. Three different dimensionless distances η (the ratio of spacing from the cylinder to the interface over the cylinder diameter) are considered. Dynamic images of the interface evolution after the impact of the reflected shock are obtained using both schlieren and planar Mie scattering techniques. Our previous study (Zou, et al., 2017) indicated that, after the impingement of the incident shock, the interface evolves into a "Λ" shape structure with two interface steps at both sides and a cavity at the center. The results in present paper show that, due to the impingement of the reflected shock, the "Λ" shape structure interface first experiences a fast phase reversal and then the perturbation increases gradually. For η=2.0 case, the interface evolves into an overall bubble structure, while for η=3.3 and η=4.0 cases, a spike appears in the center of the interface besides the overall bubble. The mixing width is further measured from Mie scattering images and compared with the theoretical values. It is found that at the linear stage, the interface width can be predicted well by the linear model proposed by Meyer and Blewett, and at the nonlinear stage, the width can be reasonably estimated by the model proposed by Dimonte and Ramaprabhu. In particular, the distinction between the theoretical prediction and the experimental result is the lowest for the case of η=4.0.

PIV experimental study on fish swimming vortex structure

WANG Fujun, WANG Hongping, GAO Qi, WEI Runjie, LIU Yanpeng

2020, 34(5): 20-28. doi: 10.11729/syltlx20200039

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Abstract:
Fish escape and cruise have gradually become a research hotspot in the field of fish bionic propulsion hydrodynamics. Study of fish escape and cruise can provide a good theoretical foundation and guidance for the underwater vehicle propulsion technology. In the present work, planar PIV is used to measure the wake flow field of zebrafish swimming in the water, and different fish wake vortex patterns under different swimming states are further analyzed. Meanwhile, Tomo-PIV is also used to measure the three-dimensional wake flow field of the fish swimming, and the linked ring vortex wake pattern is obtained. All the results indicate that the fish swimming wake presents different flow structures and wake patterns under different swimming conditions, which is helpful to reveal the mechanism of fish swimming.

Measurement and numerical simulation of flow field parameters of free flight spheres with flight velocity from 5 to 7 km/s in CO₂

LIAO Dongjun, LIU Sen, HUANG Jie, JIAN Hexiang, XIE Aimin, WANG Zonghao

2020, 34(5): 29-35. doi: 10.11729/syltlx20190055

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Abstract:
To investigate the nonequilibrium flow characteristics under Mars entry condition, shock standoff distances over free flight spheres with flight velocities from 5 to 7 km/s in CO₂ are measured in the ballistic range at Hypervelocity Aerodynamics Institute of China Aerodynamics Research and Development Center (HAI, CARDC). Test data are measured by the shadowgraph and compared with calculated results, based on which the temperature and species profiles of the test flow field are further calculated. Shock standoff distance is generally supposed to decrease monotonously as the free steam velocity increases. However, it is found through the present test results that, the shock standoff distances over spheres actually increase with the increase of the flight velocity from 5.5 to 7.0 km/s. Using Park's two-temperature model and a 5-species 6-reactions chemical reaction model can basically reproduce the measured shock standoff distances of the present test. It is shown from the test results that the flow field shortly after the shock over the spheres is mainly in theromchemical nonequilibrium. The specie CO starts to dissociate at the free stream velocity from 5.5 to 7.0 km/s, which is the possible cause of the increase of the shock standoff distances over spheres within this range of velocity.

Characteristic of 2-D supersonic inlet with auxiliary intake at low flight speed

FU Xiaogang, LI Junhao, XU Yanzhi

2020, 34(5): 36-43. doi: 10.11729/syltlx20190129

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Abstract:
Through the flight test, the internal flow field distribution and the inlet distortion of the 2-D supersonic inlet with the auxiliary intake door at low speed are investigated. The results show that mixing of the auxiliary intake flow and the main flow in the inlet would produce a low total pressure area in the downstream area of the auxiliary intake door, causing the distortion to increase and the total pressure recovery to decrease. Besides, the auxiliary intake flow rate and the resulting distortion increase with the increase of the engine speed, decrease with the increase of the flight Mach number, and there is no significant difference when the flight altitude changes. Meanwhile, the range of the low total pressure area and the distortion produced by the auxiliary intake flow increase when the sideslip angle increases to the left and right, and there is no obvious difference when the attack angle changes. In addition, by analyzing the correlation between the change of the low total pressure area at the outlet section of the inlet and the flow coefficient of each test point, the operating range corresponding to the opening and closing status of the auxiliary intake door is determined.

Experimental investigation of water droplet deformation and the effect on its drag characteristic

GUO Long, LIU Senyun, WANG Qiao, ZHAO Xianli, YI Xian

2020, 34(5): 44-49. doi: 10.11729/syltlx20200076

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Abstract:
In order to explore the dynamic behavior mechanism and essential feature for water droplets in the process of impact in aircraft icing, and improve the precision of numerical simulation of icing, an experimental investigation of the water droplet deformation and its effect on the drag coefficient in the acceleration was conducted by high-speed camera. Four typical forms of the droplet deformation were defined. The relationship between the deformation water and the dimensionless was also analyzed. The coefficient curve and calculation formula of the deformation droplet were also fitted. The results show that the droplet will distort under aerodynamic force, and the shape of the droplet changes from sphere, ellipsoid and saucer to disc. The aspect ratio of the water droplet linearly decreases with the increase of Weber number(We), and hyperbola decreases along with the increase of Bond number(Bo). The deformation of the water droplet will cause the increase of the drag coefficient, and the increase of the drag coefficient is consistent with the deformation process of the droplet.

Design and experimental investigation of ejector based on high temperature gas ejection

WANG Haifeng, XU Dachuan, ZHAO Fang, CHEN Zhiqiang, REN Zebin, SHI Yu

2020, 34(5): 50-56. doi: 10.11729/syltlx20190065

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Abstract:
Combining the gas thermodynamics with the isobaric ejector design theory, the calculation method of thermodynamic parameters of high temperature gas is presented, and the ejector design scheme based on the high temperature gas ejection is proposed. Through the docking experiment between the ejector and the gas generator, the working performance of zero injection and secondary flow are studied, and the influence of temperature change of ejected gas flow on the working performance are also studied. The experimental results show that the total inlet pressure can reach 3.89 kPa with the secondary flow at 360 g/s, and the variation of the ejector gas temperature within 100 K below the design point has no effect on the ejector performance, which verifies the reliability of performance calculation analysis and engineering design method of the supersonic ejector based on high temperature gas injection, and the relevant research results provide guidance suggestions for parameter optimization of the gas generator.

Orthogonal analysis of the influencing factors of gas-solid two-phase jet particles

SONG Xinhua, LIU Zhenfeng, LI Xiaojie, YAN Honghao

2020, 34(5): 57-64. doi: 10.11729/syltlx20190052

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Abstract:
Since the gas-solid two-phase particle injection technology is widely used, it is of great engineering significance to study its influencing factors. Firstly, to inverstigate two factors, pre-ssure and mass, experiments are arranged by the orthogonal experimental design principle. Then, based on the CFD-DEM model, the FLUENT software is used for numerical simulation, and the results are analyzed by range analysis and variance analysis. Finally, a set of jet experiments is designed to obtain the particle injection trajectory, and the photographs are processed by color histogram. The following conclusions are drawn: the influence of pressure is greater than that of mass by range analysis and variance analysis, and the mean value is the smallest when the pressure is 0.3 MPa and the mass is 2 g. When the pressure is 0.3 MPa and the mass is 2 g, the injection effect is optimized, and the shooting effect is the best, which is consistent with the results of numerical simulation.

Research on fall-evaluation method of towed decoy missile

ZHOU Jian, OU Ping, LIU Sen, ZHANG Jiang, WEI Wei, QIN Yongming

2020, 34(5): 65-69, 78. doi: 10.11729/syltlx20180163

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Abstract:
The wind tunnel towed test and force test were performed with the full scale model of the towed decoy missile to research the towed flight fall under real conditions. Firstly, the wind tunnel towed test was carried out on decoy missile models of different configurations, and it turns out that the head shape and the center of mass have a great influence on the final towed attitude. In addition, the static stability does not always increase with a forward movement of the center of mass by the combined effect of the strain of tether and the aerodynamic force. Then, by analyzing the stress of the decoy missile in stabilization, a new idea is recommended for the fall evaluation based on the wind tunnel force test, which can derive the fall formulas using the aerodynamic data gathered from the force test. Its feasibility is comfirmed by comparing with the towed test. Lastly, by deeply study the advantages and disadvantages of the two tests above, a detailed application method on the optimal design and fall evaluation of the towed decoy missile is proposed for engineering practice.

Control of side-by-side cylinders wake by a pair of tangential jets

YUAN Fangyang, CAO Yang, TU Chengxu, ZOU Heng

2020, 34(5): 70-78. doi: 10.11729/syltlx20190151

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Abstract:
The active control of two side-by-side cylinders wake with a Reynolds number of 4000 and a pitch ratio of 1.1 was studied experimentally by the Particle Image Velocimetry (PIV) technique. The flow has a bistable biased flow structure, and the biased gap flow leads to the asymmetry of the flow field. The vortex distribution and the lift spectrum of the cylinders wake were analyzed and it is found that the control effect of tangential jets on the wake can be divided into an invalid zone (θ≤20° or C_μ < 0.135), an incomplete zone (θ > 20° and C_μ > 0.135) and a complete zone (θ > 35° and C_μ > 0.304) according to the values of the jet angle and the jet momentum coefficient. In the incomplete zone, the induced tangential jets potential region can restrain the wake of side-by-side cylinders to a triangle region. The range of the vortex distribution decreases with the increase of the injection angle or the jet momentum coefficient, and the peak value of the lift force spectrum decreases gradually. In the complete zone, the wake of side-by-side cylinders is eliminated and no obvious vortex structure can be found in the time-averaged flow field, and the performance of tangential jets is at its best.

The compliance verification of thermodynamic flowfield in the large icing wind tunnel

GUO Xiangdong, ZHANG Pingtao, ZHAO Xianli, YANG Shengke, LIN Wei

2020, 34(5): 79-88. doi: 10.11729/syltlx20190113

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Abstract:
The compliance of thermodynamic flowfield in the large icing wind tunnel is the precondition for the airworthiness application of the large icing wind tunnel. In order to verify the compliance of the thermodynamic flowfield in the CARDC icing wind tunnel, a verification method is established firstly, and then a verification test is conducted for the main test section. The influences of the test section total temperature, velocity and the nozzle dry air jet flow on the spatial uniformity and temporal stability of thermodynamic flowfield are examined, and the correction relationship of the test section total temperature is obtained. Finally, the thermodynamic flowfield operating envelop is built. Results show that decreased test section total temperature and increased test section velocity could reduce the spatial uniformity of the thermodynamic flowfield in the test section for the actual refrigeration system of the CARDC icing wind tunnel, but have no obvious effects on the temporal stability of the thermodynamic flowfield. The nozzle dry air jet flow could increase the test section total temperature, while it has no significant influence on the spatial uniformity and temporal stability of the thermodynamic flowfield. The quality of the thermodynamic flowfield of the CARDC icing wind tunnel in the main test section almost meets the requirement of SAE ARP 5905-2003 under the main test condition.

The structural design for 0.3 m cryogenic continuous transonic wind tunnel

LAI Huan, CHEN Wanhua, SUN Dewen, NIE Xutao, ZHU Changjiang

2020, 34(5): 89-96. doi: 10.11729/syltlx20190156

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Abstract:
During the process of cryogenic wind tunnel operation, low temperature and a wide range of temperature variation lead to strong thermal stress and deformation of structure circuit, which may decreases the aerodynamic performance and safety of cryogenic wind tunnel. In order to control the thermal deformation and decrease thermal structure stress, several technological approaches have been applied in the 0.3 m cryogenic wind tunnel structure design, including selecting most reasonable cryogenic materials, active heat transfer in plenum chamber, stress and deformation releasing design and thermal stress calculation by using Finite Element Method (FEM). Calculation shows the ultimate thermal stress appeared on plenum chamber pressure shell, extending to 110 MPa after the 7200 s cooling down process of 0.3 m cryogenic wind tunnel, with a 110 K and 250 K strucure temperature on corner vanes and shell flange respectively. The predicted structure safety factor is greater than 1.8. The ultimate thermal deformation is appeared on the fourth corner shell when the circuit temperature cooling down to 90 K, contracting to 29 mm in aero axes direction and 12 mm in crossleg axes direction, respectively. The subsequent wind tunnel test shows that the simulation results of FEM are consistent with the measure results Calibration indicates that the structural design of 0.3 m cryogenic wind tunnel is reliable.

Research on heat flux calculation and correction methods of the slug calorimeter

ZHU Xinxin, WANG Hui, YANG Kai, ZHU Tao, YANG Qingtao, LIU Jinbo

2020, 34(5): 97-102, 108. doi: 10.11729/syltlx20190134

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Abstract:
Heat flux calculation and correction methods of slug calorimeter were studied in order to reduce measurement error of heat flux. Based on the energy conservation principle and the heat transfer theory, the heat transfer model and calculation method of slug calorimeter with insulating sleeve were built. By analyzing the simulation model of slug calorimeter, the method of extracting the temperature rise rate and the main factors affecting the calculation error were given. Finally the direct comparison calibration correction method and the numerical calculation correction method based on heat flux calibration were proposed. The simulation and test results show that the two methods can greatly reduce the heat flux measurement error. The measurement error of the direct comparison calibration correction method is relatively smaller but the method requires that the heat flux calibration system can cover the heat flux measurement range. While the numerical calculation correction method based on heat flux calibration has lower requirement on the heat flux calibration system and thus has a wilder application field.

Experimental research of special perforated wall test section for transonic wind tunnel

LIU Qin, ZHANG Guobiao, ZHANG Zhifeng, ZHANG Cheng

2020, 34(5): 103-108. doi: 10.11729/syltlx20190073

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Abstract:
The perforated test section of the 2.4 m×2.4 m injection driven transonic wind tunnel is redesigned so as to study the aerodynamic force of advanced aircraft. A scaled experimental research of the special perforated wall test section is conducted in the 0.24 m×0.20 m transonic wind tunnel (the pilot wind tunnel of the 2.4 m×2.4 m transonic wind tunnel). The experimental study has demonstrated that the project is feasible, and the flow quality of the flow field in the special perforated wall test section reaches the required specification. Influences of the diffuse angle in the test section, the open width of the mainstream ejected slot and the porosity of the perforated wall are also investigated. The results have been used to design the perforated test section of the 2.4 m×2.4 m transonic wind tunnel. The calibration parameters for the future perforated test section flow field calibration in this tunnel are also provided.

2020 Vol. 34, No. 5