实验流体力学

Progress on acceleration algorithm of the computation for chemical reactions in turbulent combustion simulation

Liu Zaigang, Kong Wenjun

2019, 33(4): 1-10. doi: 10.11729/syltlx20180123

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Abstract:
To investigate the methods of accelerating the computation of chemical kinetics in turbulent combustion, the application of Dynamic Adaptive Chemistry (DAC) and exponential integrator with Krylov subspace approximation is discussed. In the large eddy simulation of a turbulent flame, using DAC can accelerate the computation of chemical kinetics. However, in the context of parallel combustion simulation, the loads on the different processors are extremely imbalanced, which limits its performance. For the exponential integrator with Krylov subspace approximation, the acceleration effect acts on each processor, which is beneficial to improve the global computational efficiency. Compared to that of the implicit scheme coupled with DAC and MTS, the accelerating performance of the exponential integrator with Krylov subspace approxi-mation under the same level of accuracy is more obvious.

Experimental and numerical investigations of flame acceleration after passing through a perforated plate in a confined space

Zhao Jianfu, Zhou Lei, Zhong Lijia, Wei Haiqiao

2019, 33(4): 11-20. doi: 10.11729/syltlx20190033

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Abstract:
Essentially, the engine knock or the super-knock is always accompanied by the interactions between the turbulent flame and the shock wave, with rapid chemical energy release. Thus, it is of great significance to investigate the interactions between the turbulent flame and shock waves which is the key to reveal the mechanism of the knock or super-knock. And the flame acceleration inducing pressure waves is the basic premise for the study of flame-shock interactions. Based on a self-designed constant volume chamber and 3-dimensional numerical simulation by Converge, the mechanism and impact factors of the flame acceleration after passing through the perforated plate are investigated. In addition, the influence of the initial pressure on the combustion phenomenon is discussed. According to the flame morphology and the flame tip velocity, the evolution of the flame acceleration is divided into three stages, which are the laminar flame stage, the jet flame stage and the turbulent flame stage. The flow field results show that there exists a strong jet flow at the perforated plate before the flame reaches, which drives the flame acceleration. However, after the flame passes through the perforated plate, the flow velocity downstream of the flame front decreases as it departs from the flame, which means the flow is driven by the flame. In addition, it is found that the turbulent flame velocity, pressure and pressure oscillation increase with the increase of the initial pressure.

Experimental study on temperature measurement of high pressure combustion based on filtered Rayleigh scattering technology

Yan Bo, Li Meng, Chen Li, Chen Shuang, Wu Yungang, Yang Furong, Mu Jinhe

2019, 33(4): 27-32. doi: 10.11729/syltlx20180168

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Abstract:
In order to explore the temperature measuring ability under the high-pressure condition and in a confined space, the filtered Rayleigh scattering technique is developed based on the iodine molecular ultrafine absorption. The filtered Rayleigh scattering temperature measuring apparatus, consisted of the seed laser, the Nd:YAG laser, the iodine molecule filter and the ICCD camera, is designed. And the iodine filter is used to remove the stray light interference from the soot and wall reflection. Moreover, this apparatus is applied on a high-pressure gas combustor (0.1~0.5MPa) to obtain the temperature distribution above the flat burner quantitatively. The results show that the relative uncertainty in the single-shot imaging is estimated to be about 15%. And a better than 10% agreement to the single point measurement is achieved by the thermocouple. Therefore, the filtered Rayleigh scattering technique is expected to be applied in the temperature measurement of the engine combustion.

Experimental study on the longitudinal pulse detonation in rotating detonation engine

Ma Hu, Xie Zongqi, Deng Li, Xue Sainan, Zhou Changsheng

2019, 33(4): 33-38, 64. doi: 10.11729/syltlx20190015

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Abstract:
The longitudinal pulse detonation phenomenon (LPD) in the annular combustor is experimentally studied, and the operation process of this mode is analyzed through the combination of the high frequency dynamic pressure measurement and the high speed imaging. The results show that for the hydrogen and air mixtures, the LPD occurs under the condition that the blockage ratio at the exit is larger than or equal to 0.6 and the air mass flux at the minimal cross-sectional area is greater than 200kg/(m²·s). The LPD in the combustor undergoes decoupling and re-initiation in each cycle, and the shock wave reflected from the exit develops into a detonation at the head of the combustor, accompanied by severe luminescence. The average propagation velocity of the detonation wave in each cycle is comparable to the sound speed of the combustion products, which leads to the usage of the linear acoustic theory to predict the operation frequency.

Study on the influence of cold spot effect on the thermal measurement characteristics of circular foil heat flow sensor in hypersonic convection environment

Li Yu, Zhu Guangsheng, Nie Chunsheng, Tan Meijing, Chen Weihua, Cao Zhanwei

2019, 33(4): 39-44. doi: 10.11729/syltlx20180110

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Abstract:
When a circular foil heat flux sensor is used to measure aerodynamic heating in a hypersonic convection environment, the surface temperature of the sensor is often lower than the surface temperature of the measured object. This surface temperature discontinuity would affect the flow of the boundary layer, which could distort the heat flux measurement result. For a blunt-head plate model, the numerical simulation method was used to study the formation mechanism of the "cold-spot effect" and the influence of the local low temperature of the sensor surface on the surface heat flux under hypersonic flow conditions. The results show that:the higher H_w/H_re is, which represents the ratio of the surface enthalpy of the measured object to the recovery enthalpy of the flow, the more obvious the "cold-spot effect" is; the lower T_w2/T_w1 is, which represents the ratio of the sensor surface temperature to the measured surface temperature, the more obvious the "cold-spot effect" is; the flow Reynolds number Re has less effect on the cold spot effect. The deviation of measurement results in response to the "cold spot effect" is analyzed under the condition of Mach 18 flow. The results show that the "cold spot effect" can make the measurement result 1.25 times higher, which reproduces the difference between the heat flow prediction results and the test results.

The experimental investigation on full-scale dynamic separation for an inlet shroud

Zhu Guoxiang, Wang Lei, Yuan Chaokai, Wang Chun

2019, 33(4): 45-51. doi: 10.11729/syltlx20180176

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Abstract:
The inlet shroud is widely used in high-speed vehicles since it can avoid unstart phenomenon under the low Mach number flight condition. As the separation process directly determines the safety of the flight vehicle, it is necessary to testify the separation process in the ground testing. In this paper, the advantage of the JF-12 shock wind tunnel is utilized fully to develop the high-speed separation testing technique. The similarity criterion which is suitable for the high-speed separation is deduced for three testing purposes. The observation and identification methods are developed for the high speed separation trace. A set of the high-precision sequence control system is built. Some protection measures are adopted to avoid the risk from the high speed shroud. Using these techniques, a typical 2-dimensional forebody/inlet with the shroud was used for the separation testing in JF-12. The separation trace under the freestream condition of dynamic pressure 100kPa is obtained successfully by the testing freestream of dynamic pressure 50kPa.

Experimental study on the aero-heating characteristics of waverider in the high enthalpy shock wave tunnel

Wang Xiaopeng, Zhang Chen'an, Zhai Jian, Wang Famin, Ye Zhengyin

2019, 33(4): 52-57. doi: 10.11729/syltlx20190080

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Abstract:
Long-range aerodynamic heating is a key scientific problem. In this paper, the cone-derived waverider is designed firstly and then the characteristics of the heat flux distribution along the leading edge and on the lower surface of the waverider are studied by means of the high enthalpy wind tunnel test and the high temperature chemical non-equilibrium aerodynamic heating numerical calculation. Results show that:The high heat flux mainly concentrates on the small region around the stagnation point and the change of angle of attack (0°~6°) does not have an obvious effect on the high heat flux region at the leading edge, but it induces a significant increase of the heat flux on the lower surface. In addition, the change of the sideslip angle (0°~4°) has a significant effect on the heat flux at the leading edge, so it is not recommended to fly with a large sideslip angle.

Free flight experiment on boundary layer transition of 7° half angle cone at Mach 6

Wang Zonghao, Huang Jie, Shi Anhua, Song Qiang, Liao Dongjun, Liu Sen

2019, 33(4): 58-64. doi: 10.11729/syltlx20180185

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Abstract:
A series of boundary layer transition measurements of the 7° half angle cone at Mach 6 in the Aero-physic Range were conducted. The models are made of aluminum alloy with black oxidation or low thermal conductivity coating. The base diameter of the cone is 33mm, and the radius of the nose tip is among 0.27~2.50mm. The parameter covers Mach number 4.89~6.63, Unit Reynolds number 4.8×10⁷/m~5.2×10⁷/m, and the total angle of attack 0.8°~5.8°. The boundary layer shadowgraph and surface inferred images which reflect the transition line configuration were obtained, and the transition Reynolds number was calculated to be 2.4×10⁶~5.6×10⁶. The results indicate that:the transition Reynolds number decreases by enlarging the angle of attack, certain nose tip scale would delay the transition.

Review on the development of the free-piston high enthalpy impulse wind tunnel and its testing capacities

Chen Xing, Shen Junmou, Bi Zhixian, Ma Handong

2019, 33(4): 65-80. doi: 10.11729/syltlx20180169

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Abstract:
The free-piston high enthalpy impulse wind tunnel is one of the main ground testing facilities for the study of the high enthalpy flow, which is able to simulate the hypervelocity flow and is mainly categorized into two types as the high enthalpy shock wind tunnel and the high enthalpy expansion tube wind tunnel. After decades of development, the free-piston high enthalpy impulse wind tunnel can be used not only to study the complex aerodynamics thermodynamics and aero-optic of the aircraft under the hypervelocity free flow, but also to carry out research on technologies such as the free flight, the scramjet and the electromagnetic radiation measurement. The development process is summarized, focusing on the three stages of the basic theoretical research stage, the early construction stage and the practical development stage, in order to provide reference for the development of the large-scale free-piston high enthalpy impulse wind tunnel and its testing capacities.

The wind tunnel test of the active flow control on the flying wing model based on the plasma synthetic jet

Sun Jian, Niu Zhongguo, Liu Rubing, Lin Qi

2019, 33(4): 81-88. doi: 10.11729/syltlx20190041

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Abstract:
To explore the effects and mechanisms of plasma synthetic jet flow control of the 3D model, a wing layout model with medium aspect ratio decorated with plasma synthetic jets on the leading edge is studied by means of low speed wind tunnel tests. The effects of the aerodynamic force and the aerodynamic moment on the airfoil model are investigated by measuring the force of the six component balance and the different distribution positions of the synthetic jet of the plasma. The flow field distribution on the surface of the model measured by PIV(Particle Image Velocimetry) is used to study the mechanism of the plasma jet flow control. Test results show that the unilateral arrangement of the plasma synthetic jet actuator can effectively improve the aerodynamic characteristics of the flying wing model, and can produce an additional roll moment with the variation of the roll moment coefficient reaching 0.009; The lateral stability of the flying wing model can be significantly enhanced by using the plasma synthetic efflux on the left and right side of the flying wing model, and the range of the rolling torque coefficient fluctuation decreases by 66.7%. Along the string, the closer the position of the plasma jet to the leading edg is, the better the control effect is, and the control effect of the exciter at the leading edge is the best. The more the plasma synthesized jet flows along the exhibition are arranged, the more obvious the improvement of the lift characteristics of the model is, and the uniform arrangement is the best. The flow control mechanism of the plasma synthetic jet actuator is different before and after the stall angle of attack. Under the small angle of attack, the synthesis of the plasma jet advances the transition, and near the stall angle of attack, the plasma synthetic jet accelerates the separation of the flow and reduces the separation-zone thick-ness.

Free flight static and dynamic aerodynamic characteristics for re-entry capsule at transonic speed

Song Wei, Ai Bangcheng, Jiang Zenghui, Lu Wei

2019, 33(4): 89-94. doi: 10.11729/syltlx20180083

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Abstract:
This paper studies the motion and aerodynamic characteristics of the re-entry capsule with stable fins at transonic speed by the wind-tunnel free-flight test in which the motion freedom is not limited. When the wind tunnel test model flights freely by the observational window, the high speed camera captures the motion image immediately by a single light path, and then the model motion trajectory and attitude angle can be acquired by image acquisition software. Finally, the pitching static and dynamic derivatives coefficient are derived by the parameter differential method from the recorded angular motion of the model of more than two cycles with the linear and nonlinear motion equations. The result shows that the static derivative coefficient C_mα obtained by the linear and nonlinear aerodynamic parameter identification is less than zero and the variation is not significant. From the results of nonlinear aerodynamic parameters identification, the value of static derivatives coefficient of the re-entry capsule is mainly determined by the linear term C_mα0, and the nonlinear term C_mα²α² is relatively smaller. The nonlinearity of the static derivatives coefficient is weak, which can be approximated by the linear aerodynamic model. The dynamic derivative coefficient of the re-entry capsule is nonlinear in the range of experimental angles of attack. It is determined by the linear term

in the range of small angle of attack, and dominated by the nonlinear term

in large angle of attack.

Blister formation phenomenon for droplet impact under different liquid pool depths

Cao Gang, Yu Sixiao, Yan Tingjian, Dong Qiqi, Huang Xiao, Hu Haibao

2019, 33(4): 95-99. doi: 10.11729/syltlx20190016

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Abstract:
Droplet impact in the liquid pool is an ubiquitous phenomenon in nature and it has essential scientific research value. Via the high-speed camera technology, the extraordinary blister formation phenomenon on the liquid pool surface is studied when the droplets fall from various heights of 3m to 15m to the liquid pool of different depths. The effects of the depth of the liquid pool and the We number of droplets on the blister formation are given. It is shown that a droplet can form a blister at the center of the impact when it hits the shallow pool. However, when it hits the deep pool, it forms an annular blister, which then develops into one or two blisters, and the blister formation position is not at the location of the impact. In addition, under the influence of factors such as the droplet impact velocity, the liquid pool depth and the secondary droplet drop, the blister formation phenomenon for the droplet impact presents complex probability distri-bution characteristics.

Image processing algorithm for particle trajectory image and reconstruction study on flow field

Wu Fan, Zhou Wu, Cai Xiaoshu

2019, 33(4): 100-107. doi: 10.11729/syltlx20180070

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Abstract:
The aim of the present study is to develop an efficient image processing approach and flow field reconstruction method to obtain the particles velocities distributions from a single frame and single exposure image (SFSEI). The image processing approach contains de-noising, laplacian sharpening, adaptive threshold segmentation, close operation, removing of small particles and skeletonizing; the velocity calculating algorithm contains velocity calculating, ambiguity estimating, tangent vector calculating. The error produced by these methods is calculated. The flow field reconstruction approach is based on RBF (Radial Basis Function) interpolation and optimized by windward interpolation, the deviation of the reconstruction method is also calculated. A processing example using jet flow image demonstrates that the algorithm is practicable.

2019 Vol. 33, No. 4