实验流体力学

Phase-averaging waveforms of superstructures in outer layer of turbulent boundary layer

Bai Jianxia, Zheng Xiaobo, Jiang Nan

2016, 30(5): 1-8. doi: 10.11729/syltlx20160064

Abstract(202) HTML (111) PDF(17)

Abstract:
An experimental investigation of the turbulent boundary layer was conducted in a wind tunnel with IFA300 constant temperature anemometer. The simultaneous time series of the streamwise velocity component and the normal velocity component at different wall-normal positions in the turbulent boundary layer were finely measured by a double-sensor X hot-wire probe. The distributions of the turbulence kinetic energy along with the scale were obtained in the near-wall region and the outer region of the turbulent boundary layer through wavelet transform on different velocity components. In the outer region of the turbulent boundary layer, the maximum energy scale and the average burst period of the coherent structure gradually increased. Large-scale vortical structure appeared in the outer region of the turbulent boundary layer. The conditional phase-averaging waveforms of the ejection and sweep events at the maximum energy scale of the coherent structure were extracted by the conditional phase-averaging technique in the near-wall region and the outer region of the turbulent boundary layer. The waveforms of ejection and sweep events in the outer region of the turbulent boundary layer were different from the waveforms in the near-wall region. The amplitude of negative Reynolds shear stress was reduced. The conditional phase-averaging waveforms of Reynolds shear stress presented positive and negative fluctuations.

Deformation and breakup behaviors of a drop in ambient liquid under impact

Liao Bin, Zhang Guifu, Wang Luhai, Zhu Yujian, Yang Jiming

2016, 30(5): 9-16. doi: 10.11729/syltlx20160029

Abstract(303) HTML (174) PDF(6)

Abstract:
In this study, we carry out an experimental investigation of the behaviors as well as the mechanism of the liquid-liquid drop deformation and breakup process following an impact. With high speed photography, five distinct deformation and breakup modes are captured, for which the key factors that dominate the transition are quantitatively analyzed. The results show that similar deformation behaviors may occur for a proper combination of drop sizes, density ratios between drop and ambient fluid, interfacial tensions and free falling heights. Two non-dimensional parameters, i.e. Weber number (We) and Ohnesorge number (Oh), are calculated to estimate these effects. It is found that, similar deformation behaviors may have a strong correlation with the Weber number. After a further survey of the test range of present study (1 < We < 700, 0.001 < Oh < 0.005), it can be concluded that the deformation and breakup pattern is barely affected by the Ohnesorge number, whereas exhibits a strong dependence on the Weber number.

Experimental study of flow straightening and turbulence reduction characteristics for porosity honeycomb

Fu Cheng, Peng Qiang, Li Yi, Liao Daxiong, Lyu Jinlei, Zhu Bo

2016, 30(5): 17-22. doi: 10.11729/syltlx20150111

Abstract(586) HTML (283) PDF(84)

Abstract:
The honeycomb is an important device which is installed in wind tunnel settling chamber to improve the flow uniformity and decrease the flow angle as well as the turbulence intensity in the wind tunnel test section. For a regular honeycomb, in order to improve the flow straightening and turbulence reduction characteristics, a large length-diameter ratio must be used in the design of the honeycomb which results in the increase of total pressure loss of the honeycomb. A new type of honeycomb with a group of small holes on the wall of honeycomb cells is designed. This type of porosity honeycomb can achieve the exchange of the vortices and pressure between different cells to significantly improve the flow quality downstream the honeycomb. In the solid wall test section of the 0.55m×0.40m low noise aeronautic acoustic wind tunnel, the 2D hot wire is used to measure the turbulence intensity and velocity distribution downstream the porosity honeycomb and the regular one respectively with different flow velocities. The experimental results indicate that the honeycomb with 50% transverse porosity can reduce the turbulence intensity by 13.8% compared against the regular one. The velocity distribution downstream the honeycomb becomes smoother and the flow angle is also reduced. The design of the flow straightening and turbulence reduction system for the wind tunnel with the optimized porosity honeycomb can reduce the number of screens or the constriction ratio of the wind tunnel, and therefore, reduce the power consumption and the construction budget of wind tunnels.

Study of snow-load distribution on roof by wind tunnel test

Wang Weihua, Huang Hanjie

2016, 30(5): 23-28. doi: 10.11729/syltlx20160039

Abstract(152) HTML (78) PDF(6)

Abstract:
In order to predict the snow-load distribution on a roof, wind tunnel tests were conducted with fine quartz-sand to simulate snow particles. The snow coefficients of several typical roofs were obtained through snowfall simulation and redistribution tests, and were compared with the load design of building structures. The results show that, when the wind speed exceeds the threshold value, the efficient aerodynamic-rough-length increases as the wind speed increases; the snow coefficients of the stepped roof obtained in the tests are within the load specifications; for the single-span gable roof, the snow coefficients of the windward roof may exceed the load specifications, particularly for the 20° roof, while the 10° roof snow coefficients decrease with time and finally fall into the range of load specifications; for the double-span gable roof, in snowfall simulation tests, the largest coefficient lies on the first windward roof, the value of which is less than the load specifications, while in the simulation redistributions test, the largest coefficient lies on the first leeward roof, the value is approximately 1.5, larger than the load specifications.

Experimental and numerical study of oblique water exit in free surface penetration by a blunt body's supercavity

Shi Honghui, Chen Bo, Wang Yun

2016, 30(5): 29-35. doi: 10.11729/syltlx20150154

Abstract(218) HTML (130) PDF(11)

Abstract:
The supercavitation technique has important military applications such as underwater launched ballistic missiles, high-speed torpedo, airborne sea mine clearance system, etc. The relevant fundamental study was carried out in USSR and USA in 1960's and 1970's era respectively. Since the year of 2001, Chinese scientists have begun intensive and broad study on this subject and many innovative results have been achieved. This paper presents the research on the inclined water-exit of supercavitating vehicles through experimental, theoretical and numerical methods. It is found that when the supercavity is in touch with the free surface, a ventilated cavity is formed easily. When the underwater body jumps into air, it obtains a sudden increase in velocity. Using Ansys14.0 software and VOF multiphase flow model, the whole process from the cavity growth to its collapse in the inclined water-exit has been simulated. The computational results show that when the supercavitating body approaches the free surface, it piles up and the pressure at the lower part of the cavity is greater than that at the upper part of the cavity. However, after comparing with the experimental data, it is found that the cavitation model of Schnerr and Sauer used in the software is not suitable for the ventilation of the supercavity after its interaction with the free surface. This paper's work emphasizes the research strategy of conducting all of the experiment, theoretical analysis and numerical simulation for the problem.

Research on aerodynamic characteristics of transport aircraft with stall strips

Liu Yi, Zhao Xiaoxia, Ouyang Shaoxiu, Yuan Zhimin

2016, 30(5): 36-41. doi: 10.11729/syltlx20160012

Abstract(285) HTML (154) PDF(37)

Abstract:
In order to alleviate the violent roll motion during stall of a transport aircraft with landing flap configuration, the geometric parameters of stall strips are optimized and selected by numerical simulations and wind tunnel tests, and the aerodynamic force and flow field characteristics are studied. The height H and the install distance S from the leading edge are selected as design parameters for stall strips, and are evaluated by solving Reynolds Averaged Navior-Stokes (RANS) equations for the airfoil section of the landing flap configuration. The calculation indicates that smaller S value (installed closer to the upper surface) promotes earlier stall, while the larger H has similar but weaker effect. Separation bubble emerges after the stall strips when the angle of attack (AOA) of the airfoil becomes large, which grows larger and longer with increasing AOA. The bubble bursts eventually and causes the airfoil to stall earlier, leading to the rounded shape of the lift curve. The effect of the stall strips installed on the wing is studied by scaled model in wind tunnel tests, which shows that its spanwise length and arrangement have significant impact on the performance besides the cross section geometry. Four planform arrangements of stall strips are advanced and evaluated. Keeping the S parameter equal to 0, the stall trips installed at 40% half span with spanwise length of 2m change the abrupt stall of the lift curve to a flat roof type one, and eliminate the asymmetric roll moment after stall. The spanwise length of the stall strips is halved to form 2 new arrangements, which also ameliorate the stall of the lift curve and roll moment to some extent. The stall trips installed at 15% half span have no obvious effects on the stall characteristics. The suggested explanation is that the flow separation starts at about 40% half span of the wing at landing configuration, where the stall strips have the best performance.

Hypersonic wind tunnel drop-model test on cover ejection from cargo projectile at large angle of attack

Jiang Zenghui, Song Wei, Chen Nong, Jia Quyao

2016, 30(5): 42-48. doi: 10.11729/syltlx20160020

Abstract(299) HTML (154) PDF(13)

Abstract:
Hypersonic wind tunnel drop-model tests were conducted at Mach 5 to investigate the separation characteristics of cover ejection from cargo projectile at large angle of attack(25°). In the test, the whole separation process was observed. The motion trajectory and displacement curves of the covers in the separation process are analyzed. The comparison of the separation characteristics is made between the covers on the windward side and the leeward side. The motion trajectory of the covers is quite different between the windward side and the leeward side, but both motion trajectories of the covers on the two sides can be divided into two stages. For the covers on the windward side, the displacement of y orientation is dominant at stage Ⅰ, while x orientation becomes dominant at stage Ⅱ. For the covers on the leeward side, the displacements are approximately equivalent for x and y orientation at stage Ⅰ, while x orientation becomes dominant at stage Ⅱ.The x orientation displacements for covers on both sides can be evidently divided into two stages. At stage Ⅰ, the displacements change slowly, while those increase rapidly at stage Ⅱ. The y orientation displacements for covers on both sides have no obvious stage transition, but the y orientation velocity on the windward side is larger than that on the leeward side. Similar to x orientation displacements, the total displacements for covers on both sides can also be divided into two stages, and they also change slowly at stage Ⅰ, and then increase rapidly at stage Ⅱ.

Mechanism analysis and suppression of the outlet pressure rise of the large flux pressure reducing valve

Mu Wanhui, Li Haitao, Liu Weiqiang

2016, 30(5): 49-54. doi: 10.11729/syltlx20150142

Abstract(134) HTML (78) PDF(3)

Abstract:
The large flux pressure reducing valve (PRV) is a key part of the aircraft ground testing system. The problem of outlet pressure rise of the PRV is tackled in this paper by modeling analysis, control system design and experimental verification. First, the force analysis of the PRV kinetic components is conducted, which is utilized to establish the steady working model. It is observed from the expression that the sustaining decrease of the inlet pressure can cause the rise of the outlet pressure and the influence factors of the PRV outlet pressure is obtained. This problem can be solved by adjusting the control chamber pressure. Second, the control system framework is constructed based on Programmable Logic Controller (PLC). The outlet pressure feedback control is conducted by acquiring the outlet pressure signal via sensors and adjusting control chamber pressure using the booster jar with high-speed solenoid valves. At last, a series of long-term experiments of 30~60 seconds are designed to test the feasibility of the control system at a large mass flow rate of 16~27kg/s. Results show that this control system effectively stabilizes the outlet pressure and successfully solves the outlet pressure rise problem in the testing process.

Simultaneous OH and CH₂O PLIF imaging of flame structures

Zhu Jiajian, Zhao Guoyan, Long Tiehan, Sun Mingbo, Li Qing, Liang Jianhan

2016, 30(5): 55-60. doi: 10.11729/syltlx20160026

Abstract(747) HTML (353) PDF(40)

Abstract:
Simultaneous OH and CH₂O Planar Laser-Induced Fluorescence (PLIF) imaging plays an important role in studying flame structures and two-dimensional distribution of intermediate species in combustion. The OH distribution is used to represent the reaction zone structure of flames, whereas CH₂O is employed to show the pre-heating zone structure of flames. Flame structures of a CH₄/air partially premixed flame were investigated using simultaneous OH and CH₂O planar laser-induced fluorescence (PLIF) imaging. Experimental methods, including experimental system, optic alignment, synchronization scheme, excitation scan of OH A-X(1, 0), data collection and analysis, are discussed. Experimental results suggest that simultaneous OH and CH₂O PLIF imaging can show different instantaneous structures of reaction zones and preheating zones in the CH₄/air partially premixed flame. New separated preheating zones are locally generated due to the merge of the reaction zone between two adjacent areas.

Investigation of roll damping derivative of aircraft by wind tunnel tests

Liang Bin, Zhao Junbo, Fu Zengliang, Gao Qing, Zhang Shiyu

2016, 30(5): 61-66. doi: 10.11729/syltlx20150158

Abstract(155) HTML (95) PDF(14)

Abstract:
Roll damping derivative experimental techniques for air vehicles are investigated in wind tunnel of CAAA. The free oscillation dynamic derivative technique based on the integrated roll flexible hinge, the free oscillation dynamic derivative technique based on the combined roll flexible hinge, and the free rolling experimental technique based on the air bearing are studied respectively for different models. The mechanical damping characteristics, frequencies and anti-load capacity are the key points for the development of experimental technique. The study show that the test data have good regularity, and the test vibration frequency ranges are met when the proper technique is adopted. Thus the appropriate test technique should be chosen according to the test requirements for different vehicle shapes.

Experimental study on flowrate measurement of air-water two phase flow by double-cone flowmeter

An Yali, Xu Zhipeng, Liu Tiejun, Xie Dailiang

2016, 30(5): 67-73. doi: 10.11729/syltlx20160044

Abstract(153) HTML (93) PDF(5)

Abstract:
The double-cone flowmeters with equivalent diameter ratio 0.424 and 0.586 are designed. Experimental study on the parameter measurement is carried out on the multi-phase flow facility. The measurement model is established for the gas volume fraction through analysis of characteristic values obtained from time serial signals of the differential pressure measured by the meters. On the basis of the gas volume fraction model, the flowrate measurement models are established in terms of linear relationship between the Lockhart-Martinelli constant and the quasi-ration of the gas flowrate. A series of experiments are conducted on the facility of the air-water two phase flow. The experimental results show that the gas volume fraction measurement model provides good performance in the given measurement range with the error within 5%. The measurement errors of the total flowrate and liquid flowrate are within 6% by using the modified Murdock model. The measurement error of the gas flowrate is relatively larger possibly due to the small gas mass fraction.

Digital signal process of low turbulence intensity based on EMD

Zhu Bo, Peng Qiang, Tang Gengsheng

2016, 30(5): 74-79. doi: 10.11729/syltlx20150148

Abstract(151) HTML (125) PDF(7)

Abstract:
Hot wire anemometer is used in three typical low speed wind tunnels to measure the flow field turbulence intensity, including a low turbulence wind tunnel, a general closed wind tunnel and an open jet wind tunnel. To deal with the disturbance to the turbulence signal caused by noise, an EMD(Empirical Mode Decomposition) self-adaptive filter and HHT(Hilbert-Huang Transform) time-frequency analysis methodology are introduced in turbulence intensity data processing. The EMD method is compared with several other turbulence intensity processing methods, including BPF(Band Pass Filter), ENC(Electronic Noise Correction) and HPIA(High Pass Inertia Attenuation Filter). Using the EMD filter, the turbulence intensity in the low turbulence wind tunnel test section is measured and found to be less than 0.05% in the flow speed range of 30~100m/s. Using HHT method, we complete the time-frequency analysis of the flow fluctuation signal, and the low frequency flow flucturation signal is found predominatnt in the open jet wind tunnel. The designed EMD self-adaptive filter is effective at controlling the influence of the noise on the hot wire output signal, and therefore it is a practical method for low turbulence intensity signal processing.

Research on pressure measuring device installation at AIP of inlet

Zhang Xiaofei, Jiang Jian, Tian Xiaoping, Fu Xiaogang

2016, 30(5): 80-87. doi: 10.11729/syltlx20150147

Abstract(148) HTML (123) PDF(7)

Abstract:
To discuss the transition part deformation of installing test equipment at AIP during inlet/engine compatibility flight tests, an evaluation index of transition part radial deformation is developed and applied in engineering calculation. Moreover, influences of perforating, installing static pressure base and total pressure test rake on the radial size of the transition part are compared and analyzed. Accordingly, it is indicated that installing pressure test device at AIP may force the radial size to deviate from the designed value and standard circle; welding static pressure test base may introduce obvious deformation while riveting may introduce less deformation relatively; restricting on both sides of the total pressure test rake may not seriously influence the radial size whereas effectively restrain its vibration amplitude. The optimized installation scheme of the pressure measuring device has been adopted in engineering.

2016 Vol. 30, No. 5