实验流体力学

Numerical simulation of thermochemical non-equilibrium flow field in arc-jet tunnel

Fu Yang'aoxiao, Dong Weizhong, Ding Mingsong, Liu Qingzong, Gao Tiesuo, Jiang Tao

2019, 33(3): 1-12. doi: 10.11729/syltlx20180138

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Abstract:
Due to the thermochemical non-equilibrium effects and the freezing of species mass fractions and vibration energy, it is difficult to determine the flight conditions based on the arc-jet tunnel test data by extrapolation. In consideration of this problem and based on the idea of the integrated numerical simulation of the nozzle/test section/test model flow field, the numerical simulation of FD-15 arc-jet tunnel test under the typical operating condition is conducted by solving three dimensional Navier-Stokes equations of the thermochemical non-equilibrium flow. Based on the simulation result, the comparison between the numerical simulation and the tunnel test result is presented, and the problem of extrapolating the tunnel test data to flight as well as the influence of the reservoir pressure on extrapolation are discussed. The result shows:(1) the inflow in the test section has a high level of dissociation, and thus the thermochemical non-equilibrium effect is severe. (2) The tunnel test heat flux result is in between the full catalytic heat flux and non-catalytic heat flux of the integrated numerical simulation, which is reasonable and indicates the validity of the computation method and program. (3)The surface pressure and the heat transfer can be influenced by the installation position of the test model. The surface pressure and the heat transfer flux decrease when the distance from the test model to the nozzle exit increases. (4)When the reservoir pressure is low, extrapolation of the tunnel test heat flux data to the flight conditions by binary scaling (keeping total enthalpy and ρ_∞L the same) is invalid, and the tunnel test heat flux data also shows discrepancies in extrapolation to flight conditions by partial simulation (keeping total enthalpy and stagnation pressure the same), especially under non-catalytic condition. (5)When the reservoir pressure increases, discrepancies in extrapolation of the tunnel test data are significantly reduced with both binary scaling and partial simulation methods.

Study on the influence of catalytic effect on the aerothermal environment by the flow-heat transfer coupling numerical analysis

Wang Guolin, Zhou Yinjia, Jin Hua, Meng Songhe

2019, 33(3): 13-19. doi: 10.11729/syltlx20180159

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Abstract:
In view of the high-temperature gas effect in the hypersonic flight, the wall catalytic reaction can significantly increase the aerodynamic thermal load. For the analysis and prediction of the aerodynamic thermal environment and structural thermal response, the influence of the catalytic reaction should be fully considered. In this paper, the simplified atomic recombination catalytic model and the finite-rate catalytic reaction model are embedded in the ultra-high-speed-flow heat-transfer coupling analysis model to establish a ultra-high-speed flow/catalytic reaction/heat transfer multi-field coupling analysis model. Among them, the surface catalytic coefficient of the ZrB₂-SiC ultra-high temperature ceramic material is obtained as a function of the temperature through the catalytic experiment of the high-frequency plasma wind tunnel. The coupled calculation and the uncoupled calculation, and the simplified atomic recombination catalytic model and the finite-rate catalytic reaction model are compared. It is found that the total heat flow of the wall depends on the surface catalytic properties of the material. For the thermal response of materials with higher thermal conductivity, the coupled heat transfer analysis can effectively avoid the uncoupled calculation zone. The finite-rate catalytic reaction model can improve the calculation accuracy to avoid over-estimation. On this basis, the intrinsic relationship between the catalytic reaction and the wall heat transfer is revealed by the coupled heat transfer analysis. It is proved that the surface catalytic effect should be considered in the heat transfer analysis to improve the thermal response accuracy of the structure to promote the design capabilities of the thermal protection system.

Analysis of surface recombination effect in arc-jet aero-heating test

Miao Wenbo, Shi Ketian, Ou Dongbin, Cao Zhanwei, Ai Bangcheng

2019, 33(3): 20-24. doi: 10.11729/syltlx20180177

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Abstract:
There exists evident surface recombination of atoms in the high enthalpy aero-heating test. When the catalysis of TPM (Thermal Protection Material) is low, the calibration of aero-heating should consider surface recombination; otherwise it would cause under-estimation of TPM. Based on the flat aero-heating test, the analysis method of surface recombination effects in the arc-jet flow was developed by comparison of the structure heat transfer simulation and test data. This method can evaluate surface recombination effects on special TPM in arc-jet tests, and give support on modification and improvement of test projects. The analysis results show that, for a kind of TPM, the surface recombination effect makes the real aero-heating to be only about 85 percent of the calibrated heat flux. Therefore, the surface recombination effect should be considered when the copper sensor is used to calibrate the aero-heating, and the flow condition should be enhanced to ensure effective assessment.

Measurement and validation of nitrogen radiative intensity in shock tube

Lyu Junming, Li Fei, Lin Xin, Cheng Xiaoli, Yu Xilong, Yu Jijun

2019, 33(3): 25-30, 111. doi: 10.11729/syltlx20180156

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Abstract:
The radiative energy emitted and absorbed by high temperature gas in the shock layer must be considered in the thermal protection system design of hypervelocity vehicles. Efficient evaluation methods are needed to predict the radiative heat flux. Absolute radiance measurement in ground facilities is an important way to understand the physics of the high enthalpy flow and to improve the numerical models. Radiance calibration techniques have been developed in a combustion-driven shock tube. High resolution spectral radiative intensities have been measured in rich N₂ environment to validate the numerical models. Detailed radiance spectral structures have been acquired at shock velocity 5.70 and 6.20km/s. It is found that the non-equilibrium process behind the shock affects the gas radiation remarkably. Numerical simulations under corresponding experimental conditions have been conducted using an in-house built code solving Navier-Stokes equations with chemical reaction models and radiation models. The results show that computational results agree well with experimental data.

Recent research progress on motion characteristics and flow mechanism of shock train in an isolator with background waves

Xu Kejing, Chang Juntao, Li Nan, Bao Wen, Yu Daren

2019, 33(3): 31-42. doi: 10.11729/syltlx20180196

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Abstract:
The present paper aims to provide a summary report on recent research progress about motion characteristics and flow mechanism of a shock train in a hypersonic inlet-isolator with complex background waves to help the researchers working on hypersonic inlet-isolator easily with their further work. It covers shock train motion characteristics, mechanism of a shock train jumps, and the method for the model of a shock train jumps with complex background waves. At first, the investigations for the motion characteristics of shock train with fixed or variable background waves are described, which point out that the boundary layer separation caused by the alternating favorable or adverse pressure gradient in the isolator is the physical mechanism of the shock train motion characteristics. Followed, the triggering mechanism and condition of shock train jumps in an isolator with background waves are discussed. At last, based on the understanding of the motion characteristics and jump mechanism, the method for the mathematical model of the shock train motion in an isolator with background waves is given to provide a reference for the control of the shock train leading edge.

Overview on integrated design of inward-turning inlet with aircraft forebody

Qiao Wenyou, Yu Anyuan

2019, 33(3): 43-59. doi: 10.11729/syltlx20190028

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Abstract:
The integrated design of aircraft forebody/hypersonic inward-turning inlet has become a hot spot in the research of airbreathing hypersonic propulsion system. This paper mainly analyzes the design method of hypersonic inward-turning inlet and its integration with aircraft forebody from the perspective of aerodynamic design. For the design method of the inward-turning inlet, it mainly includes direct streamline-tracing method, the osculating method with uniform incoming flow and the inverse design method based on the forebody non-uniform flow-field. The integrated design method based on the inward-turning inlet mainly includes two types:the independent intake mode facing the incoming flow and the pre-compression intake mode with the forebody. Combined with the design method of the inward-turning inlet, the design methods of these two types are analyzed in details. According to the analysis, the design method of the inward-turning inlet based on the uniform forebody flow-field has been further developed, but it is necessary to develop the design method under non-uniform incoming flow to enhance the flexibility of the integrated design. With the in-depth development of the inward-turning inlet design method, the integrated design method is bound to be further developed.

Effect of the inlet internal compression shock waves on restart characteristics of the hypersonic inlets

Jia Yinan, Zhang Qifan, Tong Xiaotong, Yue Lianjie, Zhang Xinyu

2019, 33(3): 60-67. doi: 10.11729/syltlx20190031

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Abstract:
The specific hypersonic inlet models have been tested at Ma=4.0 wind tunnel to enrich the understanding of the effect of expansion waves on the inlet shoulder and the different compression ways on the inlet restart characteristics. The cowl angle and the thickness of the boundary layer have been regarded as the key influence factors. Results show that the expansion waves originated from the shoulder accelerate the local flow velocity and decrease the static pressure, which promotes the separation to move downstream. Thus, the inlet restart capability can be enhanced. And the multiple noncoalesced cowl shock waves can also improve the two-dimensional inlet restart capability. Due to the obvious three-dimensional structure of the separation induced by the swept shock, the inlet restart performance of sidewall-compression inlet differs from that of the cowl compression inlet. For the un-restart inlet caused by aerodynamic throat choke, the sidewall-compression can enhance the inlet restart capability effectively compared to the cowl-compression.

Study of flow field characteristics of an over-under TBCC exhaust system during mode transition process

Wang Feng, Xu Jinglei, Wang Yangsheng

2019, 33(3): 68-75. doi: 10.11729/syltlx20190037

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Abstract:
To study the flow characteristics and aerodynamic performance of an over-under TBCC exhaust system during the mode transition process, the unsteady numerical simulation of the mode transition process is completed by using dynamic grid method. Moreover, a series of cold flow wind tunnel tests of the TBCC exhaust system are carried out under several working conditions during the mode transition process, and the results are compared with the numerical simulation results. The results show that the wave structure of the TBCC exhaust flow field is extremely complex during the mode transition process, and the shock generated at the trailing edge of the splitter plate has some effects on the aerodynamic performance of the exhaust system. The axial thrust coefficient keeps above 0.9, yet the lift varies greatly during the process. The static pressure distribution on the walls and schlieren images obtained by wind tunnel experiments are consistent well with the numerical simulation results, which proves the accuracy of the numerical simulation results in this paper.

Experimental study on hysteresis phenomenon of hypersonic inlet caused by variations of angle of attack

Xu Shangcheng, Wang Yi, Su Dan, Fan Xiaoqiang, Wang Zhenguo

2019, 33(3): 76-82. doi: 10.11729/syltlx20190010

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Abstract:
Hysteresis phenomenon occurs in the process of hypersonic inlet start, which has an important influence on the working range of the scramjet. In this paper, a hypersonic inlet is introduced, and the inlet hysteresis caused by variations of angle of attack is studied by experiment and numerical simulation. The experiment is conducted in a free-jet wind tunnel LF-220 of NUDT (National University of Defense Technology) at Ma=5.0. The regenerative electric heater is used to heat the high-pressure airflow. The total pressure in the settling chamber is 1.59MPa, and the static temperature in the test section is 91.67K. The model is constituted of the first part of forebody, the second part of forebody, the cowl and the lampstand. The pressure sensors are used to measure the wall pressure at a frequency of 100Hz. The dynamic process from unstart to start with the change of angle of attack is captured in the experiment, and the self-starting/-unstarting performances are obtained. The results show that a significant hysteresis occurs in hypersonic inlets with the change of angle of attack. In the experiment, the self-starting angle of attack is -1.3°, while the self-unstarting angle of attack is larger than 10°. In the study of the self-starting/-unstarting process, it is found that the angle of attack and the large-scale separation zone alternately dominate the mass flow.

On the maximum spreading of liquid droplets impacting on soft surfaces

Yang Lei, Yang Xianglong, Wang Fujun

2019, 33(3): 83-89. doi: 10.11729/syltlx20180086

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Abstract:
With the method of the high-speed camera and image recognition, the spreading procedure of the liquid droplet impacting on the surface of Polydimethylsiloxane (PDMS) with different thickness and different modulus is obtained. The variation curves of between the spread factor with time are also plotted. Compared with the total energy of the system, the viscous energy dissipation caused by the compression deformation of the PDMS substrate is too small to affect the spreading procedure. In the case of lower impact velocity, the viscoelastic energy dissipation caused by the wetting ridge, which is formed on the surface of PDMS, is the major component of the total energy dissipation of the system. It increases with the decrease of the modulus of flexible materials. For this reason, the spread factor shows a decrease trend with the decrement of the modulus of PDMS. When the impact velocity increases, the viscous energy dissipation becomes the major component of the total energy dissipation and the spread factor remains unchanged with the change of the modulus of the flexible material.

TRPIV experimental study of the effect of superhydrophobic surface on the coherent structure of turbulent boundary layer

Liu Tiefeng, Wang Xinwei, Tang Zhanqi, Jiang Nan

2019, 33(3): 90-96. doi: 10.11729/syltlx20180101

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Abstract:
The existence of coherent structures in the turbulent boundary layer contributes greatly to the skin friction. The investigation of the influence of the superhydrophobic wall on the coherent structure is of great significance in revealing the drag reduction mechanism. The instantaneous velocity vector fields of turbulent boundary layers over hydrophilic and superhydrophobic surfaces were measured using time-resolved Particle Image Velocimetry (TRPIV) at the free-coming velocity of 0.165m/s. The mean velocity profiles and turbulence intensity profiles of the two types of surfaces were compared. And the drag reduction rate of 5.39% was obtained. The coherent structure is extracted by the two-point correlation function. By contrast, it is found that the superhydrophobic surface can effectively reduce the streamwise scale of the coherent structure. And the λ_ci criterion is employed to identify the hairpin vortex head as the conditional event. The linear stochastic estimation of the fluctuating velocity field around the conditional event is performed. The results show that the superhydrophobic surface can effectively weaken the strength of the single hairpin vortex head at the center of the conditional event, and can affect the surrounding hairpin vortex package structures. At the same time, the fluctuating velocity of the flow under the vortex package in the near wall region is weakened as a whole, and thus the skin friction is effectively reduced.

Investigation of wake flow characteristics for tandem hydraulic rotors using time-resolved PIV

Ji Yanguang, Kang Can, Zhang Yongchao

2019, 33(3): 97-105. doi: 10.11729/syltlx20180092

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Abstract:
To study the influence of the downstream rotor on the wake of the upstream rotor, the time-resolved particle image velocimetry (TR-PIV) was used to measure the flow between two hydraulic Bach rotors. For different flow velocities, the influence of the setting angle of the downstream rotor on the upstream rotor wake was considered, and wake characteristics associated with various kinds of rotor boundaries were compared and analyzed. The results indicate that as the upstream velocity increases, the velocity recovery zone extends towards the upstream rotor. As the setting angle of the downstream rotor is smaller than 108°, velocity decreases as the setting angle increases. Such a tendency is overturned as the setting angle exceeds 108°. The positions of the vortex cores in the wake are shifted up and down with the variation in the setting angle, and the vortices are stretched and flattened at certain setting angles. Meanwhile, streamlines are deflected with respect to the main flow, which is significantly different from the situation without the downstream rotor. At some setting angles, with the increase of the upstream flow velocity, high-vorticity regions gradually develop in the streamwise direction and towards the wake center. Meanwhile, the number of sparsely distributed small-scale vortices increases continuously. Large-scale vortex structures in the wake are involved in the first three orders of proper orthogonal decomposition (POD) modes, while the high-order POD modes are featured by small-scale flowstructures.

Correlation analysis of large low speed wind tunnel test on CHN-T1 calibration model

Zhang Hui, Fan Litao

2019, 33(3): 106-111. doi: 10.11729/syltlx20180046

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Abstract:
China Aerodynamics Research and Development Center (CARDC) established a uniform large aspect ratio transport aircraft calibration model system for high and low speed wind tunnels to fulfill the data quality requirements of aircraft type development and CFD validation and confirmation. To acquire reliable wind tunnel data, CARDC tested the first self-developed transport calibration model CHN-T1 (1:6.4, 4.667m wing span) in FL-13 wind tunnel and DNW-LLF wind tunnel at chord Reynolds number from 1.4 to 2.5 million in FL-13 wind tunnel and from 1.4 to 3.2 million in DNW-LLF wind tunnel for the same configuration. CHN-T1 model was mounted on a TG1801A six-component strain-gauge internal main balance connected to the large angle of attack (AOA) support mechanism in FL-13 wind tunnel and on a W616 internal main balance connected to a sting support in DNW-LLF wind tunnel. Both of the force and moment data were obtained in both facilities. Tunnel to tunnel variations including repeatability, aerodynamic characteristics and Reynolds effect have been assessed. Results comparison shows that slop of lift curve (C_Lα) varies very little and drag coefficient (C_D) is of a difference of 0.0004 around the design point with Ma=0.78 and lift coefficient (C_L)=0.5. Test data indicates good agreement. Reynolds number effect on CHN-T1 calibration model aerodynamic characteristics follows the expected trends.

2019 Vol. 33, No. 3