实验流体力学

Research progress on soot measurement by laser induced incandescence

Liu Fushui, Hua Yang, Wu Han, Gao Yongli, Wu Hao

2017, 31(1): 1-12. doi: 10.11729/syltlx20160104

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Abstract:
Laser induced incandescence is a non-contact optical diagnosis method. With this method, we can obtain the two-dimensional spatial distribution of instantaneous soot within the thin layer of incoming sheet laser. This method has become an important measurement technology of soot due to its high spatial and temporal resolution. This paper first introduces the development of LⅡ technology and basic theory. Then the technical methods of LⅡ and research progress at home and abroad are summarized detailedly from three aspects, that numerical simulation, qualitative and quantitative measurement. The LⅡ mathematical model mainly includes Melton model, Liu model and Michelsen model. These models can be used to predict the change rule of LⅡ signal and also lay a foundation for the test of soot particle size. To realize quantitative measurement, the calibration of LⅡ signal is necessary. This is also one of the difficulties in LⅡ measurement. There are mainly three methods for calibration, that are sampling technique, light extinction method (LEM) and 2-color laser induced incandescence (2C-LⅡ). The sampling technique is less used because it will disturb the combustion process and mix impurities. The LⅡ-LEM needs two laser systems and uses the measuring result of LEM to calibrate the LⅡ signal, so its operation is complicated. Nevertheless, 2C-LⅡ does not need other measurement technology and can realize online real-time calibration. Since this method is relatively simple, it develops rapidly and has achieved many significant results. Naturally, LⅡ technology still needs improvement, such as optimizing the incident laser wavelength and energy, controlling the uniformity of laser sheet, perfecting the LⅡ mathematical model, and extending the application in complex environment. Through summarizing the research achievements of LⅡ technology, this paper aims to emphasize the research status and the importance of this method in understanding the soot formation and oxidation mechanism, and providing some references for its future development.

Experimental study of the influence of the specific heat and pressure ratios on the hypersonic vehicle's nozzle plume

He Xuzhao, Qin Si, Zhou Kai, Le Jialing

2017, 31(1): 13-19. doi: 10.11729/syltlx20160084

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Abstract:
The experimental studies have been carried out to explore the influences of the Specific Heat Ratio (SHR) on the air-breathing hypersonic vehicle's nozzle plume at CARDC's 0.5m hypersonic wind tunnel. The SHR of the plume at the scramjet exit is simulated by CF₄+Air with SHR=1.25. The Laval nozzle is used in the experimental model to generate the supersonic nozzle plume and the flow field generated by the wind tunnel is used to simulate the model's external flow field. The differences of the pressure distributions and flow field structures are compared between the pure air and mixture gas plumes. Results show that in the mixture gas plume, the pressure distributions are higher than air plume in the core and interaction regions. The SHR is one of the key parameters for the study of air-breathing hypersonic vehicle's plume.

Breakup of a kerosene droplet at high Weber numbers

Kong Shangfeng, Feng Feng, Deng Hanyu

2017, 31(1): 20-25. doi: 10.11729/syltlx20160106

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Abstract:
In order to study the influence of the airstream velocity and droplet initial diameter on the secondary atomization process and the Rayleigh-Taylor wave, the experiment of recording the breakup process of a kerosene droplet at high Weber numbers was conducted, where the photographs were taken by a high speed camera.The analysis based on the Rayleigh-Taylor instability theory which includes viscosity and surface tension was done. The calculation was conducted in order to predict the wavelength of the most unstable Rayleigh-Taylor wave and breakup time, and the results were compared with the experimental data. The results indicate that the catastrophic breakup takes place when the Weber number is greater than 321. The airstream velocity and droplet initial diameter have great influence on the wavelength of the Rayleigh-Taylor wave with the maximum growth rate, the growth rate and the critical wavelength. The Rayleigh-Taylor instability theory which contains the viscosity and surface tension fits the experimental data well when being used to predict the wavelength of the most unstable Rayleigh-Taylor wave, the error less than 6%. Setting the value of M to be 8.9 can minimize the breakup time error.

Experimental and numerical investigations of the boundary layer parameters of an underwater flat plate model

Zhang Xuan, Shen Xue, Tian Yukui, Sun Hailang, Xie Hua, Zhang Nan

2017, 31(1): 26-31,46. doi: 10.11729/syltlx20160098

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Abstract:
On the basis of the turbulent boundary layer mean velocity profile over a zero pressure gradient flat plate measured in a water flume using Laser Doppler Velocimetry (LDV), this paper obtains the friction velocity and other boundary layer parameters by using the fit of velocity data to the full boundary layer profile from the wall to the top of the log region. Complementary to the measurements, numerical investigations of the flow field around the flat plate have also been performed. The wall shear stress extrapolated from the profile curve is compared with empirical formula results and CFD results. The data agreement shows that the fit of the LDV measured velocity data to the full boundary layer profile is of high precision for measuring the boundary parameters. Combined with simulation analysis, it can provide the ideal wall shear stress input for the calibration of MEMS wall shear stress sensors.

Study on combustion and flow characteristics in a rotating detonation combustor

Wang Yuhui, Le Jialing, Yang Yang, Tan Yu

2017, 31(1): 32-38. doi: 10.11729/syltlx20160119

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Abstract:
A rotating detonation combustor with the outer diameter100mm, the inner diameter 80mm and the axial length 117 mm for the detonation channel was designed. There is no exhaust nozzle attached to the combustor. Numerical and experimental studies were carried out to study combustion and flow characteristics under different equivalence ratio conditions. The air flows into the combustor through 60 orifices each with 2mm in diameter, and the hydrogen gas flows into the combustor through an annular channel with 2mm in width. The maximum total pressures of hydrogen and air can be 12 and 10.5MPa, respectively. When the equivalence ratio is greater than 2, deflagration occurs outside the combustor. When the equivalence ratio is close to 1, multiple counter-rotating detonation waves move in the combustor and the average detonation velocities are lower than 1000m/s. When the equivalence ratio is less than 0.58, only one detonation wave rotates. The detonation velocity is 1274m/s for the equivalence ratio 0.55. The rotating detonation engine without cooling ran for 17 seconds at the equivalence ratio 1 and apparent erosion wasn't found. Three detonation waves are co-rotating with velocities 1998m/s near the outer wall for the mass flow rate 400 g/s in the numerical study.

Research on accuracy assessment method of aerodynamic parameters identified from wind tunnel free-flight test data

Zhang Tianjiao, Wang Qing, He Kaifeng, Qian Weiqi

2017, 31(1): 39-46. doi: 10.11729/syltlx20160148

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Abstract:
The conventional Cramér-Rao lower bounds method is too optimistic to be a good quantitative assessment of the accuracy of aerodynamic parameters identified from the wind tunnel free-flight test data, considering the colored noise in the measurement data. This paper introduces a technique, that modified covariance matrix method, to process the residuals from a conventional maximum likelihood estimation to compute the accurate Cramér-Rao lower bounds for colored residuals. The modified accuracy assessment method is validated by Monte Carlo simulation and wind tunnel tests of pointed cone models with the semi-cone angle being 10°. The identified results indicate that the Cramér-Rao lower bounds calculated by the modified covariance matrix method are 3~5 times the quantity of the conventional. The modified results can be used as an accurate and impersonal assessment of the aerodynamic parameters estimated, which are consistent with the sample standard errors for the estimated parameters for colored residuals.

Mid-infrared absorption combustion diagnostics for an ADN based thruster

Zeng Hui, Li Fei, Yu Xilong, Chen Lianzhong, Yao Zhaopu, Zhang Wei

2017, 31(1): 47-53. doi: 10.11729/syltlx20160147

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Abstract:
ADN monopropellant green space propulsion is perceived as a focus of the space propulsion research worldwide. Experimental study is in urgent requirement for understanding the combustion process in the ADN based thruster and for quantitative evaluation and optimization of the combustion stability and the thruster performance. In this paper, experiments were conducted to measure the concentration of the key intermediate products (CO, N₂O) and the temperature of the combustion gas flow based on the mid-infrared quantum cascade laser absorption spectroscopy (QCLAS). Two main ignition modes of the 1 N ADN based thruster are studied: the steady-state firing and the pulse-mode firing over the injection pressure of 0.5~1.2MPa bar with catalytic bed length of 19 mm, corresponding to a current thruster prototype. It is found in the steady-state firing experiments that the whole process can be divided into the catalytic decomposition stage and the combustion stage, and the combustion kinetics mechanism of the monopropellant is experimentally demonstrated. Experiments for the pulse-mode firing show the variance of the measured multispecies concentration and temperature in consistence with the pulse trains, verifying the good performance of the thruster pulse-mode firing operation.

Statistical properties of turbulent free jets issuing from rectangular nozzles with different aspect ratios

Ma Ziran, Xu Minyi, Luan Jian, Liu Xiaopeng, Zhao Feifei

2017, 31(1): 54-61. doi: 10.11729/syltlx20160116

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Abstract:
This paper reports the experimental investigation of the turbulence characteristics of the free air jets issuing from rectangular orifices with different aspect ratios (AR=1~15). The velocity was measured by a hot-wire anemometer with the exiting Reynolds number (Re) being 15,000 for all jets, based on the equivalent diameter of the rectangular orifices. This paper mainly analyzes the evolution of centerline average velocity, frequency spectrum, turbulence scale along the centerline of rectangular jets. The results show that the centerline mean velocity decays faster and the turbulence intensity grows higher as AR increases. With jet flows developing downstream (x/D_e>30), the turbulence energy spectrum, probability density function and turbulence length scales of rectangular jets approach to those of circular jets. The principle for this phenomenon is that the momentum from the jet always spreads into still surroundings in more efficient way.

Experimental study on cold separation flow in large expansion ratio nozzle

Liu Pei, Li Geng, Zhao Li

2017, 31(1): 62-66. doi: 10.11729/syltlx20160067

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Abstract:
In order to investigate flow characteristics of the separation flow, cold flow tests were conducted in a solid rocket motor nozzle with a high expansion ratio under the sea level condition. The influences of the inlet pressure on the pressure of test points upstream and downstream the separation location were revealed in the tests and pressure data were analyzed by means of FFT. The results show that the pressure increases for test points upstream the separation location but decreases for test points downstream the separation location. The pressure is steady and right below the ambient pressure for test points downstream and far away from the separation location. Pressure fluctuations become more evident after separation in comparison with that before separation, and the enhanced pressure fluctuations were mainly low-frequency fluctuations in the range of 50Hz. The research has important reference to the designing and testing of the nozzle with a high expansion ratio.

Preliminary application of planar two-photon LIF measurements of atom O in high-enthalpy flow field

Luo Jie, Jiang Gang, Wang Guolin, Ma Haojun, Liu Liping, Zhang Jun, Pan Dexian, Xing Yingli, Tang Fei

2017, 31(1): 67-72. doi: 10.11729/syltlx20160033

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Abstract:
The high-enthalpy ICP wind tunnel is one of the most reliable ground test facilities to test the thermal protection material for space vehicles. For good understanding the flow field, flow parameters need to be known. Due to high temperature nonequilibrium effects, the life of the thermal protection material can be directly influenced by the density of atom O. Therefore, the knowledge of the O density becomes important. The two-photon absorption laser-induced fluorescence (TALIF) technology is a good way to measure atom O. In this paper, the atom O fluorescent signal has been measured based on TALIF in a pure high-enthalpy flow field. To obtain information in a broader region, the laser beam is transformed into a plane with 80 mm width. A clear image of the object with a distance of 1.2 meters from the lens has been got when lens and ICCD parameters are properly set. After the analysis of fluorescent images, the experimental result shows that there is an obvious bow shock wave in front of the model in the supersonic flow field. The concentration of atom O exhibits a maximum value in the area of 30 to 50mm away from the head in the subsonic flow field, and decreases near the surface. These phenomena are consistent with expected result, so the method can be applied to flow parameters measurement in the future.

Numerical simulation of nano-particle following features for NPLS measurement technology used in hypersonic wind tunnel

Li Zhonghua, Li Zhihui, Jiang Xinyu, Wu Junlin

2017, 31(1): 73-79. doi: 10.11729/syltlx20160094

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Abstract:
A numerical simulation approach is presented to simulate the rarefied two phase flow by applying two-way coupling technique in DSMC method. The interaction between the rarefied gas and solid particles is dealt with in a decoupled way to compute the momentum and energy exchange between phases. A numerical study of nano-particle following features for NPLS measurement technique used in the hypersonic wind tunnel is carried out by employing the DSMC method suited to simulate the rarefied two phase flow. The 50nm TiO₂ particles in various rarefaction two phase flow cases are simulated. The results show that nano-particle following features are satisfactory in the low rarefaction flow. As the degree of the flow rarefaction rises, the following features decrease deteriorate the difference between particle and gas distributions increase, and therefore the flow structure can't be obtained from NPLS correctly.

Parameter optimization for image-based measurement of bed-load transport in pressurized closed channel flow

Miao Wei, Cao Liekai, Chen Qigang, Li Danxun

2017, 31(1): 80-86. doi: 10.11729/syltlx20160057

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Abstract:
Image-based measurement of bed-load transport involves a set of parameters, e.g. sample size, sampling duration, sampling area, and the time interval between two frames in an image pair. These parameters are important to guarantee reliable quantification of bed-load motion which is temporally intermittent and spatially stochastic. We conducted experiments in a closed channel and investigated the influence of parameter selection on the measurement of probability in motion, moving speed and transport rate. Under the experimental conditions, it can be revealed that the statistical average results are convergent only when the sample size is no less than 5000, sampling duration is no shorter than 100s and the sampling area is more than 400 times the square of grain size. Meanwhile, the velocity and sediment flux decrease with increased time interval, but the probability increases almost linearly with the time interval. The present findings provide useful information to facilitate parameter optimization for image-based measurement of bed-load transport.

Effects of low Reynolds number on performance of 0.6m continuous wind tunnel

Xiong Bo, Cheng Song, Luo Xinfu, Zhou Enmin

2017, 31(1): 87-92,99. doi: 10.11729/syltlx20160079

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Abstract:
In the wind tunnel tests, the method of decreasing stagnation pressure is usually adopted to expand the simulation scope of Reynolds number. Compared with the normal pressure test, in the low Reynolds number situation, the accuracy of the test data is directly affected by whether or not the change of the flow field is obviously and whether or not the quality of the flow field satisfies the index. In order to study the influence of Reynolds number on the 0.6m continuous wind tunnel performance, the relevant tests were carried out in the 0.6m continuous wind tunnel. Based on arrangement and analysis of the tests data, the results were given. It shows that: (1) The Reynolds number has obvious effect on the performance of the compressor, stagnation pressure control precision, Mach number control precision, the flow uniformity etc. When Re_c<5×10⁵(c=0.1√A is the cross-sectional area of the test section),the effect of Reynolds number on the performance of wind tunnel is obvious. The smaller the Reynolds number is, the greater the effect is. (2) The 0.6m continuous wind tunnel can accuratly manifest the influence law of the effects of Reynolds number on the flow field performance of the wind tunnel and the force test data. Therefore, it is a good test platform for the capability research on aircraft, airfoil, engine, etc., at low Reynolds number.

Development of hot-wire probe calibration wind tunnel based on compressible fluid

Ma Husheng, Liu Huilong, Qin Tianchao, Du Wei, Shi Peijie, Ren Siyuan

2017, 31(1): 93-99. doi: 10.11729/syltlx20160108

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Abstract:
The output of the hot-wire is related not only to the fluctuation velocity, but also to the fluid temperature and density when the hot-wire technology is used to measure the turbulence intensity in a compressible fluid. Therefore, it is necessary to establish a calibration facility with similar characteristics to a high speed compressible fluid so that the accurate calibration of the hot-wire probe can be done before its usage. In this paper, the aerodynamic overall scheme, structural design and development of measurement and control system of the hot-wire probe calibration wind tunnel are introduced and illustrated based on subsonic and transonic compressible fluids. According to the results of the wind tunnel flow field calibration tests, the maximum deviation of the Mach number in the model area is 0.002; the dynamic pressure can be adjusted from 0.5 to 1.7 times of the normal dynamic pressure; the variation range of temperature and density is wide; the uniformity of the flow field are good. All the requirements for the hot-wire probe calibration are satisfied.

Study on the support interference of vane suspension support system in high speed wind tunnels

Li Qiang, Liu Dawei, Chen Dehua

2017, 31(1): 100-108. doi: 10.11729/syltlx20160053

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Abstract:
Experiments with and without fake vanes are conducted using Ty-154 standard model, and the interference magnitudes are acquired. Chimera grids are used in numerical investigation. Through the work of experiments and CFD, the interference mechanisms are studied, the influence of model's breakage caused by the vanes is investigated, and the sting and belly blade support methods are also brought to compare with the vanes. The numerical results coincide well with experiments, proving the reliability of the study method. Due to the vanes' interference, the C_D increase about 0.0005 and a small head-up pitching moment occurs while C_L changes little. The vane support shows apparent advantages over sting and belly blade support in support interference characteristics. Under the Ma>0.9 condition, the vanes' interference increases and becomes unpredictable. The influence of the crack between the front vane and the fuselage as well as the shaft of back vane is small.

2017 Vol. 31, No. 1