实验流体力学

contents

2022, 36(2): 1-156.

Abstract(187) PDF(43)

Abstract:

2022, 36(2): 1-2.

Abstract(769) HTML (133) PDF(52)

Abstract:

Frontiers and developments of ultra-high time and space resolution magnetic nanometer temperature measurement

YI Wentong, ZHU Yiwen, LIU Wenzhong

2022, 36(2): 1-8. doi: 10.11729/syltlx20210107

Abstract(2576) HTML (236) PDF(80)

Abstract:
The frontier demand for remote and rapid temperature measurement under special conditions poses a challenge to the classical temperature sensing technology. The temperature measurement method based on the principle of magnetism has great potential in these fields. Magnetic nanoparticles have a significant and efficient temperature-to-magnetic field conversion effect and a nanosecond response time, which can realize remote, high-precision and rapid temperature measurement. This article reviews the current development status of the magnetic nanometer temperature measurement technology at home and abroad, including several different physical models and simulation analysis methods for magnetic nanometer temperature measurement, as well as the corresponding temperature information extraction method and measurement system design. The main principle of the remote temperature measurement method based on magnetic nanoparticles is to measure the magnetic susceptibility or magnetization signal, and to obtain temperature information through the Langevin equation magnetic model. At present, multiple prototype experiments have proved the feasibility of the magnetic nanometer temperature measurement method under remote or ultrafast constrained conditions. The magnetic nanothermometer provides a new measurement tool for temperature measurement under extreme conditions such as junction temperature measurement of high-power chips, transient temperature measurement, remote temperature measurement through metal, and temperature imaging downstream of the transition point in a super wind tunnel.

Progress on focused laser differential interferometry in measuring supersonic/hypersonic flow field

XIONG Youde, YU Tao, XUE Tao, WU Jie

2022, 36(2): 9-20. doi: 10.11729/syltlx20210126

Abstract(2485) HTML (505) PDF(122)

Abstract:
As a nonintrusive and high spatial/temporal resolution testing method, Focused Laser Differential Interferometry （FLDI） is very suitable for use in extreme experimental environment, such as hypersonic wind tunnel. Starting from the typical composition of the optical path, the principle of FLDI and the spatial filtering characteristics are introduced. Thereafter, a series of recent improvements based on typical FLDI is reviewed. Those improvements were implemented to meet different research needs. This is followed by applications and conclusions of FLDI in the field of hypersonic flow field measurement, including hypersonic freestream disturbance, hypersonic boundary layer transition, and supersonic jet noise. This review shows the potential of FLDI in supersonic and hypersonic flow field measurement, and it may provide reference for the follow-up improvement of FLDI testing technology and related precision measurement of hypersonic flow field.

Measurement of 3D airblast swirl atomization field at low temperature with off-axis holography

SONG Ge, ZHAO Yue, WANG Lei, LIU Tao, WU Yingchun, LIN Wenhui, WU Xuecheng

2022, 36(2): 21-29. doi: 10.11729/syltlx20210158

Abstract(2728) HTML (342) PDF(59)

Abstract:
A 25 kHz picosecond pulsed digital off-axis holography system is applied to measure the near-nozzle atomization characteristics of an airblast swirl atomizer under low oil temperature conditions. Experiments are carried out for the near-nozzle atomization process 30 mm down-stream from the nozzle exit under air pressure of 1 kPa and oil pressure of 0.03 MPa. The oil temperature varies from –40 ℃ to 28 ℃. It is found that the atomization field in the near-nozzle region contains non-spherical droplets. Typical dynamic processes of atomization such as the breakup of films and filaments are visualized with clear images. The sizes and three-dimensional（3D） positions of droplets ranging from 30 to 1500 μm are obtained through particle identifica-tion and locating, thereafter statistics on 3D distribution of Sauter Mean Diameters（SMD） is obtained. It is found that under conditions of oil pressure of 0.03 MPa and air pressure of 1 kPa, the droplet diameter is mainly distributed within 200 μm, and droplets with size range 30–40μm account for the largest proportion, above 15%. The 3D droplet size distribution is expressed as a cone where the central droplet size is larger than that on the edge; the decrease of oil temperature significantly deteriorates the atomization effect, which reduces the volume of the spray cone, and the density and the uniformity of atomized droplets; With oil temperature decreasing from 28 ℃ to –20 ℃, the central droplet diameter of the downstream cross-section increases from about 300 μm to over 450 μm and can be locally larger than 650 μm; with the oil temperature of –40 ℃, a large liquid core with multi-branched liquid films and filaments appears downstream of the nozzle, resulting in a longer breakup distance. The experimental results also demonstrate that the high-speed digital off-axis holography is a powerful tool for three-dimensional visualization and diagnostics of near-nozzle atomization under low oil temperatures conditions. This work can provide reference data for optimization of the nozzle structure design and the demonstration of the atomization model.

Recent advances in background oriented Schlieren and its applications

XIONG Yuan

2022, 36(2): 30-48. doi: 10.11729/syltlx20210173

Abstract(3432) HTML (1864) PDF(475)

Abstract:
Background oriented Schlieren （BOS） has appeared near 2000 as a variant of the classical schlieren technique. It is a new non-intrusive optical diagnostic technique for measuring the refractive index of complex flows quantitatively. Compared to knife-edge and rainbow Schlieren, BOS is advantageous in terms of optical alignment, systematic calibration, and the dimension of the field of view. The principle and the essential system parameters of BOS are introduced in detail. An overview of recent advances in the BOS technology is presented according to the typical sequence of setting up a BOS system. Finally, recent applications of BOS in super/hypersonic flows, combustion, and plasma flow environments are also introduced.

A review on flow field velocimetry based on high-speed schlieren/shadowgraph systems

ZHU Haijun, WANG Qian, MEI Xiaohan, WU Yu, ZHAO Changying

2022, 36(2): 49-73. doi: 10.11729/syltlx20210110

Abstract(4323) HTML (1421) PDF(569)

Abstract:
The 2-Dimensional （2D） and 3-Dimensional （3D） velocimetry based on schlieren/shadowgraph methods are reviewed in this article. The main content includes the basic optical setups and principles of schlieren and shadowgraph systems, as well as the velocimetry algorithms. For 2D measurement, there are mainly two types of velocimetry algorithms: one is cross-correlation algorithm adopted by PIV, while the other is the optical flow method. The basic formulas, advantages and limitations are introduced comparatively. A recent developed schlieren motion algorithm can provide high accuracy and dense estimation, which is promising and applicable in a wide range of applications. The 3D reconstruction and particle tracking algorithms highly rely on the systems. In this review, three different setups are introduced, including tomographic shadowgraphy, two-view collimated light path shadowgraphy and two-view converging path shadowgraphy. The two-view systems are more concise in setup, requiring less equipment, which are advantageous for high-speed measurements. The 3D particle tracking algorithms of two-view systems are introduced, while the main focus is placed on the image space-based tracking algorithms and the spatial-temporal tracking methods. The latter introduces the temporal predictions into the stereo matching process. The particle reconstruction and tracking correctness in dense particle situations is improved significantly by using the strongly coupled spatial and temporal constraints for optimisation. Its performance is superior to several artificial intelligence methods. The progress of the velocimetry algorithms, together with the imaging advantages of short exposure and high-frequency framing rate, has promoted schlieren/shadowgraph from conventional flow visualization to advanced velocimetry techniques, which can play a role for experimental study in a wide range of complex turbulent and transient flow conditions.

Experimental study on morphology and flow structure of liquid cone in flow focusing

KANG Peng, GUO Jianfeng, MU Kai, SI Ting

2022, 36(2): 74-81. doi: 10.11729/syltlx20210159

Abstract(2416) HTML (225) PDF(56)

Abstract:
Flow focusing is one of capillary flow techniques that can produce monodisperse droplets, particles and capsules at micro-/nano-scales. In flow focusing, the formation of a stable liquid cone upstream of the small orifice is a prerequisite for further jet generation and efficient preparation of microdroplets. In this work, dimensional analysis is first used to analyze the effects of the focused phase flow rate, driving gas pressure difference, and the distance between the capillary and the focusing orifice on the stability of the liquid cone. Based on the aspirating flow focusing experimental platform, the morphology and stability of the liquid cone are observed, verifying the theoretical analysis. In addition, the stable parameter range of the liquid cone is obtained by adjusting the main process parameters. Moreover, the flow field is visualized by adding tracer particles into the fluid of the focused phase, and the high-speed photography is employed to capture the flow field images, which are analyzed quantitatively to explore the structure of the recirculation zone inside the liquid cone. It is found that the generation of the recirculation cell is closely related to the tangential velocity distribution on both sides of the cone interface, and the size of the recirculation cell is affected significantly by the driving gas pressure difference, the flow rate of focused phase, and the geometric parameters.

TPIV study for near-field tip vortex from an elliptical hydrofoil

ZHAO Hang, SHE Wenxuan, GAO Qi, SHAO Xueming

2022, 36(2): 82-91. doi: 10.11729/syltlx20210108

Abstract(2348) HTML (301) PDF(82)

Abstract:
Tip vortex cavitation （TVC） is a common type of cavitation in hydraulic machinery and marine propulsion. Since TVC inception is highly relevant to the vortical flow around the blade tip of turbines and propellers, it is essential to give more insights into the flow field of the tip vortex to reveal the inherent relationship between flow properties and TVC. Measurement for the tip vortex from an elliptical hydrofoil has been conducted in a high-speed cavitation tunnel utilizing tomographic particle image velocimetry （TPIV） with high time-resolution. The results show that the wandering motion of the tip vortex is noticeable in the near field. The time-averaging process without taking into account wandering motion can bring extra errors into the time-averaged flow field. Therefore, it is necessary to filter out the wandering motion for the quantitative analysis on vortex characteristics. The tip vortex is under roll-up process and can be greatly affected by the shear layer from the hydrofoil, which contributes to the asymmetric circumferential velocity distribution and a high-axial-velocity area between the shear layer sheet and the vortex core. The tip vortex contains the most of the turbulence energy within its core and the turbulence energy is dominated by the vertical and spanwise velocity fluctuations which are considered as the main source of the fluctuating pressure in the core center combining with previous researches.

Recent advances in hypersonic fast pressure-sensitive paint measurement technology

PENG Di, LI Yongzeng, LIU Xu, JIAO Lingrui, LIU Yingzheng

2022, 36(2): 92-101. doi: 10.11729/syltlx20210122

Abstract(2473) HTML (307) PDF(143)

Abstract:
Complex flow phenomena such as transition, separation and shock-wave–boundary-layer interaction commonly exist in hypersonic flows, which lead to fast-changing and complicated surface pressure distributions. Pressure-sensitive paint（PSP） is a highly desired fine measurement technology in hypersonic aerodynamic testing, which has shown clear advantages such as non-intrusive, high spatial resolution and full-field measurement. In recent years, due to the enhancement of PSP’s response time and the development of measurement methods, the application of this technology has been expanded from regular low-speed/high-speed wind tunnel testing to the hypersonic field. Meanwhile, there are breakthroughs in high-speed moving model testing techniques. This paper first introduces the most recent research advances in the fast PSP measurement technology, and then discusses the challenges and countermeasures of PSP measurement based on application examples in two typical hypersonic wind tunnels and one relatively special free-flight ballistic range facility. Finally, the conclusion and outlook are given.

Supersonic combustion sensing by the passive endoscopic flame sensor

LI Zhongpeng, ZHOU Ruixu, MENG Fanzhao, CHEN Chi, LI Tuo, LIAN Huan

2022, 36(2): 102-114. doi: 10.11729/syltlx20220004

Abstract(2355) HTML (191) PDF(42)

Abstract:
The improvement of ramjet performance in the future wide-area flight envelope requires advanced combustion organization strategies and engine system control technology. Among them, high-dynamic frequency response sensors and actuators are one of the key technologies of the high-performance control system FADEC. Stable and reliable, simple structure, non-intrusive, low energy consumption, and light weight are the basic requirements of the ramjet online sensors. This paper develops an optical fiber flame sensor based on passive flame chemiluminescence for optical diagnosis, andpreliminarily verifies the value of the optical fiber flame sensor's data for sensing the combustion process. Based on the direct-connected supersonic combustion test bed of the Institute of Mechanics, Chinese Academy of Sciences, the flight conditions with a total incoming flow temperature of 1475 K, a total incoming pressure of 1.68 MPa and an incoming Mach number of 5.6 are simulated. Using a newly developed endoscopic fiber optic flame sensor, the combustion heat release rate characterized by CH* and the local equivalence ratio characterized by C₂*/CH* were measured under different equivalence ratios and momentum-to-flux ratios. The analysis results show that the endoscopic optical fiber sensor can sense the temporal and spatial evolution characteristics of the heat release rate of the combustion chamber; The endoscopic fiber optic sensor can sense the combustion oscillation characteristics in the frequency domain, and experiments show that there may be spanwise thermoacoustic oscillations in the combustion process; The C₂*/CH* optical signal of the endoscope optical fiber sensor can perceive the temporal and spatial evolution characteristics of the local equivalence ratio, and can be combined with the CH* optical signal to study the correlation between the mixed field and the combustion field; The statistical characteristics of the local flame centroid position characterized the shear-layer flame stabilization mode and jet wake flame stabilization mode.

Measurement and visualization of surface friction beneath jet flow over planar surface using liquid crystal coatings

ZHAO Jisong, ZHANG Jinming, WANG Boqiao

2022, 36(2): 115-121. doi: 10.11729/syltlx20210106

Abstract(2126) HTML (150) PDF(48)

Abstract:
The ability of Shear Sensitive Liquid Crystal （SSLC） coatings to measure and visualize surface friction fields is studied. Liquid crystal coatings are used to measure surface friction beneath the subsonic jet flow over a planar surface at various jet velocities, and to visualize unsteady surface friction fields of supersonic jet flow with shock cells. Experimental study shows that in terms of quantitative measurement, a single liquid crystal coating is able to measure surface friction rapidly with a high resolution and a wide range, and is also suitable for studying the variation of surface friction with the jet flow velocity. In the aspect of flow visualization, liquid crystal coatings provide another way for visualizing the flow field structures such as diamond shock cells of the supersonic jet flow and the unsteady characteristics of the shock cells.

Experimental study on instability and transition over hypersonic boundary layer on a straight cone

LIU Shicheng, JIANG Yinglei, Dong Hao

2022, 36(2): 122-130. doi: 10.11729/syltlx20210136

Abstract(2289) HTML (211) PDF(50)

Abstract:
The instability and transition of a cone model with a nose radius of 2 mm and a half cone angle of 7° were studied using infrared thermography and a high-frequency pressure sensor （PCB） at Ma = 6. The experimental results show the presence of a stationary crossflow streaks, low and high frequency unstable waves in the three-dimensional boundary layer of the straight cone in the flow field of a conventional hypersonic wind tunnel. Under nonzero angle of attack, low-frequency （15−50 kHz） and high-frequency （210−340 kHz） unstable waves exist at the same time, and the boundary layer transition is more likely to occur. With the increase of the angle of attack, the streaks become clearer, the transition front of the model moves toward the windward side, and the amplitude of low-frequency and high-frequency unstable waves increases and the band ranges enlarge. The amplitude of the unstable waves increases and the frequency band widens, and the low-frequency unstable waves appear earlier than the high-frequency unstable waves. At the same angle of attack, with the increase of unit Reynolds number, high-frequency and low-frequency unstable waves and stationary crossflow streaks appear earlier and grow to saturation, accompanied by the increase of the amplitude and bandwidth of the unstable waves. In addition, the low frequency unstable waves appear earlier than the high-frequency unstable waves.

Measurement of wall-shear stress via micro-particle tracking velocimetry

XU Dechen, ZHANG Yue, LIU Xinle, LI Wenfeng

2022, 36(2): 131-138. doi: 10.11729/syltlx20210156

Abstract(2296) HTML (275) PDF(82)

Abstract:
Micro-Particle Tracking Velocimetry （µ-PTV） was used to measure the flow field near the wall with high spatial resolution. By analyzing the velocity distribution of the viscous sublayer, the wall-shear stress can be analyzed by one-time linear regression. The wall-shear stress of the turbulent boundary layer at different Reynolds numbers based on momentum loss thickness was measured and the flow structure of reversal flow events was obtained at Re_θ=1200. The results show that the micro-particle tracking velocimetry technology can accurately measure the wall-shear stress and the measurement error of wall-shear stress is below 2% for Re_θ=1634–4070. Furthermore, the probability of reversal flow is extreme low, i.e., about 0.05% at Re_θ=1200. The measured spatial scale of reversal flow structures is around 8×30 wall units, and therefore the measurement of reversal flow events requires high spatial resolution of the measurement technology. The results show that the reversal flow events occur with the appearance of strong spanwise vortices near the wall.es near the wall.

Investigation of unburned/preheated area characteristics of a premixed flame under transverse acoustic excitation based on acetone and CH₂O PLIF technology

YAN Bo, SUN Yongchao, ZHU Jiajian, WU Ge, WAN Minggang, TIAN Yifu, CHEN Shuang, SUN Mingbo

2022, 36(2): 139-145. doi: 10.11729/syltlx20210111

Abstract(2285) HTML (232) PDF(35)

Abstract:
The thermoacoustic instability is one of the most difficult problems in the development of aerospace propulsion systems. The research on the thermoacoustic instability mechanism of premixed flame can be useful to realize and solve the thermoacoustic oscillation problems of the practical engines. The characteristics of the premixed flame excited by acoustic wave are investigated by the simultaneous acetone/CH₂O planar laser-induced fluorescence（PLIF） technique, and the variation of unburnt zone and preheating zone is acquired. It is shown that the effect of the acoustic wave on the flame is more obvious at lower frequency and higher Sound Pressure Level （SPL）. As the frequency decreases and SPL increases, the change of the flow structure gradually aggravates. It is also found that the morphology of unburned zone/preheating zone changes periodically with the acoustic wave phase. The edge of acetone PLIF images is extracted, and the dependence of the lifting height, diffusion area of the premixed gas on the acoustic wave frequency and the SPL is obtained. The evolution process of the flame morphology under typical acoustic frequency and pressure level is acquired by the simultaneous acetone/CH₂O PLIF images. The phenomenon and mechanism of combustion flameout under typical acoustic condition are analyzed.

Multi-parameter measurement of aerodynamic load via flexible sensing skin

GUO Dongliang, HOU Chao, ZHU Chen, XIONG Wennan, CHEN Shuang, XU Xiaobin, YANG Hua, HUANG Yong, an

2022, 36(2): 146-154. doi: 10.11729/syltlx20210115

Abstract(2735) HTML (381) PDF(152)

Abstract:
Aerodynamic characteristics of aircraft are critical for the design and security evaluation of aircraft structures. Wind tunnel test is the most effective experimental method, which is faced with the problems of structure damage and fewer parameters measurement. Herein, we propose a conformal measurement technique based on flexible sensing skin integrated with variety of ultra-thin flexible sensors, that are attached on surface of aircrafts consistently by kirigami assembly strategy. The sensing skin can simultaneously obtain multiple aerodynamic parameters like static pressure, pulsating pressure, temperature and wall shear force, without changing the surface morphology of the structure. Experiments on NACA0012 wing and aircraft tail under variable wind speed and variable angle of attack are carried out in the direct wind tunnel, jet platform and FL–9 wind tunnel. The characteristics of parameters collected in the wind tunnel are analyzed, which proves the availability of the system, paving a way for simultaneous measurement of various aerodynamic characteristics by flexible sensing skin in wind tunnel experiments.

2022, 36(2): 155-156.

Abstract(230) HTML (81) PDF(26)

Abstract:

2022 Vol. 36, No. 2