实验流体力学

2021, (4): 1-2.

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Abstract:

POD analysis of the dynamic structures of a low swirl number precessing jet

FU Hao, HE Chuangxin, LIU Yingzheng

2021, 35(4): 1-9. doi: 10.11729/syltlx20210006

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Abstract:
The flow field of a low swirl number precessing jet at Reynolds number Re = 4.5×10⁴ is measured using particle image velocimetry （PIV） and the dynamics of the large-scale flow structures are examined further using the proper orthogonal decomposition（POD） analysis. The spatial modes obtained by POD and the fluctuating velocity field obtained by POD reconstruction at three swirl numbers, i.e., S = 0, 0.26 and 0.41, are compared and analyzed. The POD results show that the precession induces an alternating flow, switching between outflow from one side of the chamber along the chamber wall and inflow from another side. When the precession occurs, the vortex structures in the upstream shear layers have not broken down completely. They will develop downstream until approaching the starting point of the precession and then deflect with the mainstream. However, the large-scale structures in the downstream shear layers are completely destroyed. As the swirl number increases, the region affected by the precession moves upstream, and the orderly vortex structures in the shear layers break down.

Experimental investigation on flow mechanism driving heat transfer enhancement in a channel with circular pin fins

DUAN Jingtian, ZHANG Ke, XU Jin, LEI Jiang, WU Junmei

2021, 35(4): 10-18. doi: 10.11729/syltlx20200134

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Circular pin fins are commonly used in the internal cooling channel of the turbine blade. This paper mainly investigated experimentally the flow mechanism driving heat transfer enhancement in a wide channel with staggered circular pin fins. The flow field in the mid-plane of the channel was measured using Particle Image Velocimetry (PIV). Nusselt number distributions on the endwall was obtained by means of Thermochromic Liquid Crystal (TLC) in the same geometry under the same Reynolds number (1.0×10⁴ or 2.0×10⁴). Results indicate that downstream of circular pins the distribution of v_rms is similar to that of Nu. However, when the flow is developed, smaller scale fluctuation increases, and the distributions of turbulent kinetic energy (K_t) and Nu are more uniform. The heat transfer enhancement and cross-stream velocity fluctuation are decreased when Re increases. It is concluded that intense lateral velocity fluctuation induced by vortex shedding is the main flow mechanism driving local heat transfer enhancement. Small scale fluctuation makes local heat transfer uniform.

Wind tunnel experimental study on the wind load interference effect of solar panel arrays

MA Wenyong, MA Chengcheng, WANG Caiyu, HAN Xiaole, GAO Fei

2021, 35(4): 19-25. doi: 10.11729/syltlx20200127

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Wind load is one of the main concerns in the design of the solar panel. The wind-induced interference effect is significant in the solar panel array, which needs further investigation. Pressures on the rigid solar panel models were measured in a wind tunnel to study the interference effect of solar panels on their pressure coefficients. By changing the inclination angle, wind direction and the number of solar panels, the interference effect on the wind load on the panels is studied. The results show that the most unfavorable wind load occurs when the panel is orientated to the wind. The upstream panels obviously shield the downstream ones and reduce their wind load. This shielding effect becomes more significant with the increase of the inclination angle. The wind load on the downstream panel tends to be constant when it locates downstream three or more panel rows. Based on the above interference effect, the pressure coefficients on the solar panel arrays are recommended and compared with those in standards, which provides a reference for the wind-resistant design of the solar panel supporting system.

Wind field characteristics on a bridge site under complex mountain terrain

SHEN Guohui, ZHANG Shuaiguang, YU Shice

2021, 35(4): 26-33. doi: 10.11729/syltlx20200020

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In order to obtain wind field characteristics on a bridge site on a complex mountain terrain, small-scale topographic wind-tunnel model tests are employed. The variation characteristics of the mean wind speed, wind attack angle, turbulent intensity and turbulence integral scale with respect to the wind azimuth and measuring point position are analyzed. The changes of the wind speed spectrum of typical measuring points on the complex mountain terrain are also studied. The results show that the mean wind speeds of all the measuring points on the bridge site are less than that of the gradient height. When wind blows along the valley, significant positive attack angles are produced due to the wind climbing effect, with the maximum value reaching +35.3°. The along-wind and crosswind turbulent intensities in this wind direction reach their minimum values as low as nearly 10%, and become larger in other wind directions. The turbulence integral scale in this direction is much larger than that in other directions, and increases with the increasing height of the measuring point. The wind speed power spectrum in the direction has significant differences compared with the incoming wind speed spectrum, of which the energy in the high frequency section increases significantly and the feature of the single peak diminishes.

Experimental study on pipe flow transition of XG solution and drag reduction characteristics with different mass fractions of NaCl

XU Shengxuan, ZHAO Wenbin, LI Mingyi, LIN Yuying, LI Changfeng

2021, 35(4): 34-40. doi: 10.11729/syltlx20200041

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Abstract:
The Reynolds experiment was conducted to study the transition characteristics of Xanthan Gum (XG) solution (mass fraction: (0-400)×10^–6) in a horizontal smooth circular pipe flow. The influence of different mass fractions of the salt (NaCl) environment on the drag reduction characteristics of XG solution was also investigated experimentally. The relationships of the drag reduction efficiency of XG solution (mass fraction: (100-300)×10^–6) with the flow Reynolds number (Re) and the shearing time in different NaCl mass fractions (0、250×10^–6、500×10^–6和1000×10^–6) were measured. Characterized by the generalized Reynolds number, the flow transition lower critical Reynolds number is delayed in an approximately linear mode with the increase of XG mass fraction, while the value of the upper critical Reynolds number rapidly drops to about 3000. The saturated drag reduction efficiency of XG solution decreases significantly with the dissolution of NaCl under the high Reynolds number condition. Nevertheless, a better drag reduction effect appears under the low Reynolds number condition. The shearing resistance of XG solution shows mass fraction effect, while the addition of an appropriate amount of salt can slightly improve the shearing resistance.

Improvement and evaluation of thermal flow-field quality in CARDC icing wind tunnel

GUO Xiangdong, ZHANG Pingtao, ZHANG Ke, GUO Qiling, GUO Long

2021, 35(4): 41-51. doi: 10.11729/syltlx20200118

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The compliance of the thermal flow field quality of the large icing wind tunnel is the foundation of its airworthiness application. In order to understand the effects of upgrading of the refrigeration system on the thermal flow field quality in the CARDC icing wind tunnel, a comprehensive verification test is carried out for the main test section. Then, the thermal flow field qualities, both at the exit of the heat exchanger and in the test section, are evaluated. Finally, the correction relationship of the airflow total temperature and the thermal flow field operating envelop are achieved. Results show that the thermal flow field qualities, both at the exit of the heat exchanger and in the test section, are better than the quality index given in SAE ARP5905, under the main test conditions. Compared with the test results in 2019, the spatial uniformity in the model area of the test section is greatly enhanced. Particularly, non-uniform temperature peak points exceeding the standard in the model area are eliminated under the conditions of high airspeed and low total temperature. Finally, the upgrading of the refrigeration system in 2020 greatly extends the thermal flow field operating envelop, so that the temperature simulation capability of the CARDC icing wind tunnel is enhanced significantly.

An ice wind tunnel test study on the scaling law of a rotating cone

ZHANG Lifen, GE Xin, ZHANG Fei, LIU Zhenxia, MA Dong, LYU Weijin

2021, 35(4): 52-59. doi: 10.11729/syltlx20200166

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In order to verify the scaling law of a rotating cone, a study on a rotating cone icing was carried out, and the icing scaling law of the rotating cone was derived. Taking a cone with a bottom diameter of 100 mm and cone angle of 80° as the full size model, and a cone with a bottom diameter of 50 mm and cone angle of 80° as the scaled model, the ice wind tunnel tests were carried out under different conditions. The ice shape on the surface of the cone was obtained by image method and processed in a dimensionless manner. Then the ice shape of the full size model was compared with that of the scaled model. The results show that, for glaze ice the ice shape of the front half of the cone on the full size and scaled models is in good agreement, but it is not in good agreement at the back half of the cone, and for rime ice the ice shape is in fairly good agreement on the full size and scaled models. The reasons are discussed, and suggestions are put forward for improvement.

Research on icing wind tunnel test technology of helicopter rotor model

YUAN Honggang, HUANG Mingqi, PENG Xianmin, ZHANG Guichuan, LIU Qinglin

2021, 35(4): 60-66. doi: 10.11729/syltlx20200121

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In order to meet the needs of the wind tunnel test of helicopter rotor icing in China, China Aerodynamics Research and Development Center has developed a rotor model icing test technology. A helicopter rotor model icing test system was developed in a large multi-function icing wind tunnel. The test method and data acquisition and processing method of the rotor model in the icing wind tunnel were developed. The process of the rotor model icing test was standardized, and thus the research of the rotor model icing test could be carried out safely and reliably. Through the first icing wind tunnel test of the helicopter rotor model in China, the icing characteristics of the rotor model under typical working conditions were studied, and the variation characteristics of aerodynamic load and vibration load of the rotor model during the icing process were obtained. The test results show that: with the increase of icing time, the rotor thrust decreases sharply, and the power increases sharply. With the ice shedding, growth, re-shedding and regrowth on the blade surface, the rotor performance fluctuates greatly. Before the ice shedding on the surface of the rotor blade, the test data have good repeatability and reliability.

Wind tunnel test of blade number effect on icing distribution of vertical axis wind turbine

SHEN He, WEI Bo, JIANG Yu, GUO Wenfeng, LI Yan

2021, 35(4): 67-72. doi: 10.11729/syltlx20200094

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Icing on the surface of wind turbine blades can reduce the turbine’s aerodynamic characteristics and power generation efficiency. In this paper, the effect of the number of blades on the icing distribution on the blade surface of a vertical axis wind turbine is studied. A natural low temperature icing wind tunnel experiment system was built to test the icing distribution of three groups of blades (1, 2, 4) under three tip speed ratios (0.2, 0.6, 1.0). The results show that the number of blades has a significant effect on the distribution of icing on the blade surface of the vertical axis wind turbine under different tip speed ratios, and the icing distribution on the inner and outer surfaces of the blade is asymmetric. With the increase of the number of blades, the thickness of the ice layer on the surface of the blades increases first and then decreases. In the 10% area at the leading edge of the chord, the increase of the thickness of the ice layer is significantly greater than that at other parts of the blade. With the increase of the tip speed ratio, the thickness of the ice layer on the leading edge of the blade is significantly larger than that at other parts of the blade, and the thickness difference of the ice layer between the inner and outer surface on the leading edge of the blade increases gradually. The research results can provide a reference for the anti-icing and de-icing technology of wind turbine blades.

Research on the influence of the piezoelectric ceramics layout on de-icing effect

BAO Mingxin, MIAO Bo, ZHU Chunling

2021, 35(4): 73-82. doi: 10.11729/syltlx20200091

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The piezoelectric de-icing method is studied by simulation and experiment on a composite plate. The effect of piezoelectric ceramic layout (number, spacing) on the de-icing effect under the same energy consumption is studied by finite element simulation. The de-icing experiments are done under cold environment condition. The results show that when the piezoelectric ceramics with larger dimension are divided into smaller pieces according to a certain layout, the de-icing effect becomes better than that with large size single piezoelectric ceramic. As the spacing of the piezoelectric ceramics decreases, the deicing effect is further enhanced. The experimental results show that the de-icing time can be greatly shortened by increasing the input power of actuators. After dividing the large piezoelectric ceramics into four smaller ones, the de-icing effect is enhanced with the same energy consumption. The de-icing time is further shortened after the spacing of the piezoelectric ceramics is reduced.

Measurement and analysis of motion characteristics of vapor clouds induced by aluminum-aluminum hypervelocity impact

DU Xuefei, SHI Anhua, MA Zhaoxia, HUANG Jie, LIU Sen

2021, 35(4): 83-91. doi: 10.11729/syltlx20200071

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A sequenced imaging method for obtaining the velocity of the impact-induced vapor shock wave is designed according to the generating mechanisms and radiation characteristics of vapor clouds in hypervelocity impacts. Tests were conducted for 2A12 aluminum plates being impacted by aluminum spheres with a diameter of 4.5 mm and velocity of 6 km/s. Sequenced images of the movement of the impact-induced vapor shock wave were obtained. The expanding radius, the velocity of the vapor shock wave, the total energy of the vapor clouds and the distribution of parameters in the flow field behind the shock wave, etc., were quantitatively analyzed. It is revealed from the results that the location information of the impact vapor shock wave at different times can be obtained properly by the designed measurement method, providing data for analyzing the motion characteristics of the vapor cloud. The measured expansion of the vapor wave radius with time is consistent with fitting results of the Taylor model, proving that the Taylor model theory can be used for studies related to hypervelocity impact vaporization.

The study on transient heat flux measurement based on hybrid heat transfer modes

WANG Hui, WU Dong, ZHU Xinxin, ZHU Tao, YANG Kai, CHENG Guanghui

2021, 35(4): 92-97. doi: 10.11729/syltlx20210030

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The transient heat flux is an important parameter simulated in the high temperature flowfield of the arc heater test. With respect to the harsh aerodynamic heating environment with high-level heat flux and intensive erosion, a modified transient heat flux measurement method is firstly proposed based on the hybrid heat transfer modes of heat capacity and one-dimensional semi-infinite heat transfer. Then, a novel structure of the transient heat flux sensor is designed and fabricated for the hybrid method. The sensor calibration is evaluated on the calibration system, and it is found that it has a good linear sensitivity and the proposed measurement method is favorable to its rapid-response. Finally, the on-site measurement results show that this type of heat flux sensor has a good robust and dynamical response, which can be applied under the high-level heat flux environment of the arc heater test.

A pitching and plunging dynamic test equipment of rotor blade airfoils in the FL-11 wind tunnel

KANG Hongming, TANG Ling, KONG Peng, LI Guoqiang, ZHANG Weiguo

2021, 35(4): 98-105. doi: 10.11729/syltlx20200130

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In order to fully study the dynamic stall of the helicopter rotor, a pitching and plunging dynamic test device is developed in the FL-11 wind tunnel. Firstly, the paper introduces the design of its mechanical structure, and the statics and model analysis of the mechanical frame and the moving base is conducted through the finite element analysis. Then, the pitching and plunging oscillatory motion of the device is achieved by the multi-motor synchronous driving control, and the control precision is guaranteed through the motion control system based on SIMOTION D and S120. Finally, the synchronous control technology based on the locked gain compensation algorithm of the gantry axis is used to improve the synchronous precision of the motor, and the stepless adjustment of the amplitude and frequency is realized by the electronic CAM technology. The results show that the device can implement the pitching motion and the plunging motion separately or in conjunction, the maximum amplitude of the pitching oscillation is 15°, the highest frequency is 5 Hz, the maximum amplitude of the plunging oscillation is 130 mm, the highest frequency is 5 Hz, and the angle and displacement accuracy is 3′ and 1 mm respectively. The device has been successfully applied to the aerodynamic characteristics wind tunnel test of CRA309 airfoil, which provides a test platform for in-depth study of dynamic stall characteristics.

Development of small size Schmidt-Boelter heat flux sensor

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

2021, 35(4): 106-111. doi: 10.11729/syltlx20200065

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A kind of small size Schmidt-Boelter Gage was successfully developed for measuring heat flux in conventional hypersonic wind tunnel test. The simulation model of the Schmidt-Boelter Gage was built. The structure and size of the sensor were optimized based on the simulation model. According to the optimized results, the sensor with the size of Φ 3×10 mm was made and tested in the arc lamp heat flux calibration system. The results show that the size and performance of the developed sensor are comparable to the foreign sensor level. The sensitivity is greater than 30 μV·m²/kW, and the response time is about 50 ms.

10 kHz OH-PLIF imaging measurement of a scramjet combustor

CHE Qingfeng, QI Xinhua, TU Xiaobo, HAO Fuyue, CHEN Guozhu, ZHANG Shunping, CHEN Shuang

2021, 35(4): 112-112. doi: 10.11729/syltlx20210088

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2021 Vol. 35, No. 4