实验流体力学

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2022, 36(6)

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Abstract:

Experimental study of TRPIV for turbulent boundary layer of longitudinal concave curvature wall

WANG Xuan, FAN Ziye, CHEN Letian, TANG Zhanqi, JIANG Nan

2022, 36(6): 1-9. doi: 10.11729/syltlx20210084

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Turbulent boundary layers with curvature and pressure gradient are widely used in various engineering applications. A wall model with concave curvature in streamwise was used as the research object, and the instantaneous velocity fields at two different streamwise positions upstream and downstream were measured by TRPIV experiment with dual camera size field of view. Using experimental data from a smooth flat plate as a benchmark, the variation pattern of the turbulent boundary layer on a concave wall is investigated. By comparing the mean velocity profile and the Reynolds stress profile with those of the smooth flat plate, it was found that the mean velocity profile in the concave wall case gradually deviated from the conventional logarithmic law and that the turbulence intensity was weaker than that in the flat plate case. Conditional averaging is carried out under the condition of the prograde vortex identified by $ {\varLambda _{ci}} $ criterion. It is found that the peak of the positive fluctuations above the vortices in the concave wall varies oppositely to the scale range, while the negative fluctuations below the vortices is stronger than that of flat plate. The coherent structures were further extracted using a spatial two-point correlation method and the inclination of the coherent structures in the near-wall region to the wall was calculated using an ellipse fitting method. It was found that the scale range of the coherent structure on the concave wall gradually increases along the streamwise. The results show that when the turbulent boundary layer is subjected to a combination of concave curvature and favorable pressure gradient, the difference in turbulence intensity near the near-wall region and within the wake region increases, along with an increase in the intensity of vortices in the coherent structures within the boundary layer. With development downstream, the scale range of positive fluctuations above the prograde vortex on the buffer layer increases, but the peak value decreases, while the opposite phenomenon occurs on the upper side of logarithmic region. The trend of coherent structure migration to higher normal height in the curved wall turbulent boundary layer is weakened, and scale range of the coherent structures grows mainly towards the downstream side.

Experimental study of the effects of confinement on self-sustained oscillations and noise radiation in three-dimensional open cavities

WANG Chao, YUE Tingrui, WAN Zhenhua, SUN Dejun

2022, 36(6): 10-18. doi: 10.11729/syltlx20210050

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Flow past an open cavity has been studied extensively, while less attention has been paid to the effects of confinement due to side walls, which produces rich flow dynamics and noise characteristics. In this study, the effects of confinement on flow structures and noise radiation in three-dimensional rectangular cavities are investigated experimentally. The length and depth are fixed, and five ratios of width/length （W/L=0.1–0.5） are considered. The measurements are performed in an acoustic wind tunnel. The pressure oscillations are onset after the wind speed is greater than Ma 0.03. Once the wind speed is greater than or equal to Ma 0.20, the flow and noise radiation are dominated by the self-sustained oscillations corresponding to the third Rossiter’s mode. Furthermore, the present experiments show that the local pressure oscillations and noise radiation of this frequency can be weakened or even eliminated when W/L is equal to or less than 0.3 for the wind speeds of Ma 0.20 and Ma 0.25. The upstream OASPLs in the far field can be reduced by more than 3 dB when W/L decreases from 0.4 to 0.3 at Ma 0.20. By analyzing the surface pressure and TR-PIV（Time-Resolved Particle Image Velocimetry） results, it is found that the suppression of the tonal noise is closely related with the changes of the primary recirculation and some secondary vortical structures by decreasing W/L. In particular, the intensity of the primary recirculation is greatly weakened with strong confinement effects, and the feedback process is not strong enough to produce self-sustained oscillations.

Experimental study on flow separation control by flexible serrated trailing edge based on multi-scale coherent structure analysis

GONG Xu’an, ZHANG Xin, MA Xingyu, FAN Ziye, TANG Zhanqi, JIANG Nan

2022, 36(6): 19-27. doi: 10.11729/syltlx20210041

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This article reports our recent experimental study of airfoil flow separation control by flexible serrated trailing edge. The experiments were conducted in a straight-type wind tunnel and a hot-wire anemometer was used to measure the velocity profile downstream of the two-dimensional airfoil. Multi-scale coherent structures within the separated shear layers are analyzed both in the time and frequency domains. The results show that the separation bubble thickness decreases by almost 5% of the chord length, the flexible serrated trailing edge vibrates and deforms adaptively and absorbs nearly 20% of the trailing edge shear layer’s energy, perturbation transmits to the leading edge shear layer, and thus the power spectral density decreases significantly in the lower and larger bandwidth to reduce the noise. The coherent structures’ frequency and amplitude also decrease notably, breaking and inhibiting the large vortex package’s transmission obviously in the separation bubble.

Experimental study on the characteristics of swirl effervescent atomizer

ZHAO Fang, XU Bingbing, FU Cheng, WANG Yue, ZHANG Haiyang

2022, 36(6): 28-35. doi: 10.11729/syltlx20210026

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Compared with the conventional pressure atomization and pneumatic atomization, effervescent atomization has the advantages of high efficiency, economy and environmental protection, which has attracted the attention of various fields. In this paper, a series of experiments were carried out on a variable nozzle internal swirling effervescent atomizer, and the effects of the working parameters, hole structure and mesh number on the flow and spray characteristics were discussed. The results show that flow characteristics of the atomizer with different hole structures are basically the same. The change of the air-liquid ratio results in the change of the liquid mass flow rate under the same working pressure. Flow characteristics are not affected by the cutting screen, while the addition of the cutting screen induces 3%–7% attenuation of the spay mass flow rate of under the same working condition, and the smaller the cutting screen aperture is, the greater the reduction of the spray mass flow rate is. The distribution of spray particles presents a single peak structure, and the median mean diameter of the spray decreases with the increase of the working pressure or air-liquid ratio. Under the same spray energy consumption, the special-shaped hole structure is more helpful to improve the spray performance. The cutting screen is beneficial to the spray performance, but the choice of the mesh aperture should be made based on the atomizer structure, the working pressure and other comprehensive judgment; in addition, the addition of the cutting screen reduces the axial velocity of the spray mainstream to a great extent.

Experimental study on support interference of single support rod via force measurement and PIV

REN Fanfan, WEI Lihui, TAO Aihua, LI Yanpeng, YANG Lei, GAO Na, ZHANG Caicheng, JIANG Jiali, XU Xianghui, WANG Wenda

2022, 36(6): 36-46. doi: 10.11729/syltlx20210037

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In order to study and optimize the support interference characteristics for the single support rod in the FL–51 low-speed wind tunnel, the longitudinal and transverse two-step support interference test of the aircraft in cruise and uplift configuration is carried out for the three-dimensional support rods with the cross section of 24 ribbed, round and truncated airfoil at the wind speed of 70, 50 m/s and the support rod pre-deflection angle of 11° （the angle between the support rod and the axis of the model fuselage is 79°）. Based on the study of the support interference characteristics of the single support rod at the pre-deflection angle of 11°, the support interference characteristics wind tunnel test of 24 ribbed, round and truncated airfoil support rods was carried out at the pre-deflection angle of 30° and 60° （the angle between the support rod and the axis of the model fuselage is 60° and 30° respectively）. For further analyzing the characteristics of two-dimensional support rod flow around and wake characteristics with different sizes and cross section shapes, the wind tunnel PIV test is carried out. The results show that the longitudinal support interference of the airfoil support rod is the best, but it deteriorates in the transverse direction.With the increase of the maximum thickness, the wake vortex of the airfoil support rod has good consistency, and the wake vortex of the 24 ribbed and round cross section support rod varies with diameter. For wind tunnel tests that only conduct longitudinal tests or focus on longitudinal tests, airfoil cross section or truncated airfoil should be selected.

Influence of nozzle wall temperature on plate test in arc-heated wind tunnel

LONG Yongsheng, YUAN Jie, ZHAO Shunhong, YANG Bin, ZHU Xinxin

2022, 36(6): 47-53. doi: 10.11729/syltlx20210055

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In order to study the influence of the nozzle’s wall temperature on the plate test in the arc-heated wind tunnel, a thermal insulation semi-elliptical nozzle was designed. A plate test was adopted in the arc-heated wind tunnel with the semi-elliptical nozzle. The bottom of the nozzle wall was connected with the plate model. The flow extended to the surface of the model during the test. The surface heat flux and equilibrium temperature of the model under the conditions of the hot nozzle wall and the cold nozzle wall were measured and analyzed, respectively. The results show that the enthalpy of the nozzle ranged from 1.00 MJ/kg to 2.55 MJ/kg. Compared with the condition of the cold nozzle wall, the surface heat flux of the hot nozzle wall increased by 4.7% to 15.0%, and the equilibrium temperature increased by up to 4.24%. In the thermal protection test, the influence of the nozzle wall temperature on the results of the plate test should be considered, and it is necessary to increase the enthalpy of the flow.

Research on the static aero-elastic sensitivity of stiffness of flexible wing

CHEN Kai, LIU Xiaoyan, CHENG Pan, MAO Kun

2022, 36(6): 54-60. doi: 10.11729/syltlx20210013

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In order to ensure that the aerodynamic performance of high aspect ratio wing in cruise flight can meet the design targets, it is necessary to carry out the jig shape design in the wing design stage. The wing stiffness has a significant effect on the aerodynamic loads, which is one of the important factors affecting the aeroelastic characteristics. Robustness analysis of jig shape design with various stiffness properties was conducted based on CFD/CSD method. The aerodynamic characteristics evaluation model was established with the stiffness value as the independent variable. The sensitivity analysis of the stiffness was carried out with the wing twist angle and lift efficiency as constraints. The research results, providing support for the static aeroelastic evaluation of wing stiffness in the engineering design, show that the vertical bending stiffness and twist stiffness are the main characteristics that affect the wing twist angle and lift efficiency. The Jig shape can be frozen with the increasing stiffness ratio less than 1.1. And linear relationship between elastic aerodynamic derivatives and stiffness ratio is established.

Dynamic boundary layer transition wind tunnel test of blunt cone

ZHANG Shiyu, ZHAO Junbo, FU Zengliang, ZHOU Ping, ZHOU Jiajian, LIANG Bin

2022, 36(6): 61-66. doi: 10.11729/syltlx20210120

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The time delay effects of the boundary layer transition movement on the surface compared to aircraft pitching oscillation should produce serious unstable aerodynamic moments, which probably decrease the aerodynamic stabilities in the pitch direction, even leading to unpredicted pitching divergence in the reentry flight of the blunt cone. A boundary layer dynamic transition test technology of the blunt cone in the hypersonic wind tunnel is improved, using a gas bearing as support of the test model in order to realize free rotation of the test model in the longitudinal direction with extremely low damping moment. In addition, an infrared thermo-graphic system is used to observe and estimate the boundary layer transition distribution on the model, to study the dynamic coupling phenomenon and interaction between boundary layer transition and longitudinal rotation of the model. A series of tests are completed using the blunt cone model of which the half cone angle is 9 degree, to study dynamic coupling effects between boundary layer transition and longitudinal rotation. As a conclusion, an unsteady oscillation phenomenon of angle of attack was observed when dynamic boundary layer transition occurred, and at the same time, the delay phenomenon and delay time of boundary layer transition to angle of attack rotation were identified.

Study on deep learning-based anomaly detection method for wind tunnel balance force data

ZHANG Jing, SUN Wenju, NI Wenbin, WEI Wei, ZHANG Jiang, YANG Wubing, LI Qingyong

2022, 36(6): 67-73. doi: 10.11729/syltlx20210061

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Anomaly detection for wind tunnel balance force data is beneficial to analyze anomaly reasons, improve test schema and troubleshoot equipment problems. To solve the high time cost and low-efficiency problems of the manual detection method, a deep learning-based anomaly detection method is proposed. To solve the problem of no abnormal data, we summarize the most common abnormal types in the wind tunnel test. For the problem that the dimensions of data in different experiments are different, a standardization scheme based on statistical characteristics is proposed. Finally, a deep learning model is utilized to learn abnormal features and detect abnormal data. Experimental results show that our deep learning-based anomaly detection method can achieve 81.7% accuracy and 72.6% recall, and has a good detection performance for isolated jump points and multipoint anomalies.

A preliminary study of segmented arc heater used for high pressure operation

YANG Hong, YAO Feng, ZHU Chao, ZHU Tao, CHEN Dejiang

2022, 36(6): 74-82. doi: 10.11729/syltlx20220060

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The segmented arc heater is an ideal thermal protection test equipment for having a wide range of enthalpy. In order to broaden the simulation range of pressure, the analysis method of the segmented arc heater internal flow is developed. The prediction results match the high pressure operation test results. To solve the problems in the high pressure test, some technical measures have been taken the pressure and heat resistance segments are developed cooling is strengthen and the electrode is shortened. The internal protective measures are studied to reduce surface oxidation and heat loss of segments. The operational mode is explored to solve the string arc problem in the high pressure operation test. The operation pressure of the test exceeds 10 MPa, the operation current exceeds 5000 A on a single electrode, which improves the testing ability of the segmented arc heater.

Research on fluid-thermal coupling simulation of water-cooled calorimeter and experimental analysis

ZHU Xinxin, LI Zeyu, ZHAO Wenfeng, WANG Hui, YANG Kai, YANG Qingtao

2022, 36(6): 83-88. doi: 10.11729/syltlx20210011

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A kind of spherical water-cooled calorimeter including the ball crown calorific body and the heat shield is developed. The fluid-thermal coupling model of the calorific body and test water is established. The water temperature distribution characteristics in the waterway and the influence of water temperature on the heat flux measurement result are analyzed based on the model and heat flux calibration test. The results show that the closer the water in the waterway is to the heated surface, the higher the water temperature is and the greater the radial temperature gradient is. The smaller the water mass flow rate is, the greater the radial and axial temperature gradients are. So the thermocouple should be kept away from the heated surface and closer to the central axis of the waterway when the water-cooled calorimeter is designed. And the water-cooled calorimeter should be calibrated and the appropriate water mass flow rate range needs to be determined before use. Finally, the test results show that the spherical water-cooled calorimeter can be used to measure the stagnation point heat flux accurately in the long-term arc-heated wind tunnel test with multiple heat flux states.

Heat flux measurement of small scale gap corner at high Mach numbers

CHEN Suyu, DING Tao, KONG Rongzong, TIAN Runyu, LIU Jichun, GONG Hongming

2022, 36(6): 89-96. doi: 10.11729/syltlx20210063

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To investigate the aero-heating environment of distributed insulation tiles on re-entry flight vehicles, integrated coaxial thermocouples of only 0.3 mm in diameter are utilized to measure the heat flux at high Mach numbers. Intense interacted flow may prevail in interested regions such as the gap corner with small curvature radius. This makes it difficult to measure the heat flux. The curvature radius of the gap corner, height difference between insulation tiles, gap width, boundary layer state and Mach number are investigated to determine the influence on the aero-heating environment. Temporal signals are analyzed to obtain fluctuation characteristics of the transient heat flux. Results show that the inverse step leads to obvious heat flux rise. Difference in boundary layer state means notable discrepancy in the heat flux distribution over the gap corner. Higher Mach number induces less fluctuated heat signals and moderate heat flux. Negative heat flux phenomena emerges under some conditions. Results are useful to TPS design of insulation tiles, and increase the knowledge of the mechanism of the complex flow induced by gaps and steps.

Experiments on 2500 K high temperature stable combustion environment of hydrocarbon fuel via CARS

WANG Chaozong, BAI Bing, QI Xinhua, CHEN Shuang

2022, 36(6): 97-98. doi: 10.11729/syltlx20220126

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2022 Vol. 36, No. 6