Experimental study on fluctuating thermal loads in swept shock wave/turbulent boundary-layer interactions

An experimental research program providing new knowledge of the fluctuating thermal loads in swept shock wave/turbulent boundary-layer interactions is described. An equilibrium turbulent boundary-layer on a flat plate is subjected to an impingement by the swept planar shock waves generated by a sharp fin. Two different fin angles at a freestream mach number of 4.0 produce relatively strong interactions. For each different interaction case the fluctuations of surface temperature in the interactions are obtained by miniature thin-film-resistance sensors connected to an anemometer. The distributions of properties of the surface temperature fluctuation, such as their rms levels and power spectra are obtained for each interaction. From the present measurement, the fluctuations of thermal loads near the reattachment of the separated flow under the λ-shock structure contains considerable high-frequency components, which is contrary to the previously observed unsteady phenomena of surface pressure in the same types of interactions.     

Experimental study on the non-equilibrium condensation of a liquid film after shock wave in a vertical diaphragmless shock tube

In this study, the physical characteristics of ethanol vapor behind the incident and reflected shock waves in a vertical diaphragmless shock tube are measured. To verify the excellent properties of the vertical diaphragmless shock tube, the experimental data are compared with the theoretical Rankine-Hugoniot curves. Combined with the shock wave visualization imaging, it is verified that the designed shock tube can be effectively used for investigating the phase-change heat transfer behind the shock waves. According to measurements based on a He–Ne laser beam optical interference system, the growth rate of the liquid film behind the reflected shock wave (V_fr) is smaller than that behind the incident shock wave (V_fi). The experimental value of condensation parameter is approximately 0.025, which is in good agreement with the theoretical value determined using the molecular dynamics approach. When the Mach number of the incident shock wave (M_i) is employed as a factor influencing liquid film growth, it is observed that when all the other parameters remain constant, the larger the M_i, the faster the growth of condensed liquid film. The vertical diaphragmless shock tube facilitates strong reproducibility of results under the same conditions, which can increase the accuracy of experiments such as phase-change heat transfer.     

状态转换时流动带电的实验研究

该文通过实验的方法,对液压系统中状态转换时的流动带电进行实验研究,得出在系统状态转换的过渡过程中,同产生压力冲击一样,会产生流动带电的冲击,其峰值是定常状态的160%,峰值的大小是与其状态转换时的阀开启速度和定常流速有关的结论。     

Flow structure and flow-induced noise in an axisymmetric cavity with lids

Direct numerical simulations of incompressible turbulent flow through an axisymmetric cavity with or without lids were performed at Re_t,in = 186 to examine the hydrodynamic effects of the lids on the flow-induced noise. The strength of the recirculation in the downstream region was weakened by the installation of the lids. Comparison of the acoustic sources of the Lighthill equation indicated that the lid in the downstream region attenuated the flow-induced noise substantially. Frequency spectra and spatio-temporal correlations of pressure fluctuations revealed the most energetic mode and the convective nature of the flow over the cavity. It was evident from a detailed investigation of the instantaneous flow fields that the introduction of lids into the cavity significantly weakened the interaction between the separated shear layer and the trailing edge of the cavity. The present results clearly showed that the installation of lids is an effective means of reducing flow-induced noise.

     

Experimental study of wave gas dispersers in liquid

A new type of device for dispersing gas in liquid, a wave disperser, is developed and studied. Its functioning is based on the utilization of pressure pulses generated by waves propagating from a generator of hydrodynamic vibrations. The purpose of the work is to reduce the gas bubble size and the energy losses to dispersion compared to the known disperser types. This is important for many chemical technologies, systems of biological purification of wastewater, and chlorination and ozone treatment of tap water.     

扇形区域洒水机射流元件试验研究

射流元件有许多优点,可应用于旋转式洒水机.旋转式洒水机需要在射流端在不完全堵塞出口通道后能切换到另一端的射流元件,以实现旋转装置在一定角度往复摆动,故有必要研究该新型射流元件.新型的射流元件进行了试验研究,在改变喷嘴深宽比、中板、分流劈条件下,进行了旋转切换研究.当第三组射流元件在喷嘴宽度较大、分流劈曲率较大且喷嘴宽度与分流劈宽度相匹配时,射流元件在一定角度往复式旋转.     

Experimental study of droplet splashing phenomena on hydrophobic micro-and micro/nano-textured surfaces

Journal of Mechanical Science and Technology - Droplet splashing phenomena are observed experimentally on the well-designed hydrophobic micro and micro-/nano-textured surfaces. The critical Weber...     

Experimental and numerical study of heat transfer downstream of an axisymmetric abrupt expansion and in a cavity of a circular tube

This research is an experimental and numerical investigation of heat transfer and fluid flow characteristics in separated, recirculated and reattached regions created by an axisymmetric abrupt expansion and by an abrupt expansion followed by an abrupt contraction (called a “cavity”) in a circular tube at a uniform wall temperature. The flow just upstream of the expansion was unheated and proved to be fully-developed at the entrance to the heated cavity region. Local heat transfer coefficients were measured using a balance-type isothermal heat flux gage. Measurements were made at a small-to-large tube diameter ratio of d/D = 0.4 and downstream Reynolds numbers ranging from Re_D = 4,300 to 44,500. Generally, the maximum Nusselt numbers downstream of an axisymmetric abrupt expansion at a uniform wall temperature occur between 9 and 12 step heights from the expansion step. Numerical simulation has been carried out by a two-equation turbulence model and its results such as mean velocity profiles and local Nusselt numbers are in good agreement with experimental results.     

Experimental study on rate dependence of macroscopic domain and stress hysteresis in NiTi shape memory alloy strips

NiTi polycrystalline shape memory alloys, when stretched, can deform through the formation and growth of localized macroscopic martensite domains. In this paper, we study the effects of stretching rate on the stress-induced domains and stress hysteresis in NiTi strips. Synchronized measurements of the nominal stress–strain curve, macroscopic domain pattern and the associated temperature field were conducted in the strain rate range of 10⁻⁴–10⁻¹/s. It was found that the nominal stress–strain curve changed from the near-isothermal plateau-type with distinct stress drops at the very low strain rate to the near-adiabatic smooth hardening-type in the high strain-rate region. The corresponding deformation mode changed from the nucleation propagation mode with a few parallelepiped martensite domains to the near-homogeneous multiple-nucleation mode with many fine alternating austenite–martensite stripes. The number of the domains (domain spacing) increased (decreased) monotonically with the strain rate and followed a power law scaling, while the stress hysteresis (or material damping capacity) changed non-monotonically with the strain rate, reaching a peak at strain rate of 2.0×10⁻³/s. We show that, though the rate dependence of both pattern and hysteresis originates from the transfer of the released/absorbed heat and the thermo-mechanical coupling, the domain spacing in the test of static air is mainly controlled by heat conduction and the hysteresis change is mainly controlled by the heat convection with the ambient.     

Experimental and numerical study on jet properties and penetration of double-layered shaped charge

Journal of Mechanical Science and Technology - Double-layered shaped charge (DLSC) is a special type of shaped charge which has been recently attracted to enhance the penetration performance. The...     

Experimental study on the gas jet characteristics of a diesel-piloted direct-injection natural gas engine

Journal of Mechanical Science and Technology - The natural gas (NG) jet characteristics of NG/diesel dual-fuel injection under different NG injection pressures and dual-fuel injection intervals...     

Experimental study on synthetic jet array for aerodynamic drag reduction of a simplified car

This paper describes an experimental parametric study of synthetic jet array actuation to reduce the aerodynamic drag of a threedimensional simplified car. By using two configurations of an Ahmed body with 25° and 35° slant angles, we performed wind tunnel tests under different conditions of synthetic jet array. Several parameters, namely, jet location, jet direction, jet momentum coefficient, jet driving frequency, and number and position of activated jets within the actuator array, were considered. The total aerodynamic drag coefficients were compared, and the rear wake flows were studied by using the data obtained from rear surface pressure distribution and flow visualization tests. Results of the parametric study show that the aerodynamic drag exhibits different behavior depending on the location of the jet for each slanted model. Jet direction, jet momentum coefficient, and jet driving frequency affect only the amount of change in the aerodynamic drag. The distribution of the activated jets also affects jet efficiency.     

半主动控制浮筏隔振系统抗冲击特性的试验研究

针对水下舰艇内部设备的抗冲击问题,对安装开关可控阻尼器的半主动控制浮筏隔振系统的抗冲击特性进行了试验研究,搭建了半主动控制浮筏隔振系统的试验装置,在试验装置的基础上施加冲击激励,测试可控阻尼器分别处于高、低阻尼状态下的基础和机组的加速度响应,并通过对比分析了可控阻尼器处于不同阻尼状态时,半主动控制浮筏隔振系统抗冲击特性的差异。试验结果表明:半主动控制浮筏隔振系统具有良好的抗冲击特性;可控阻尼器处于低阻尼状态时,机组加速度响应的最大峰值较小;可控阻尼器处于高阻尼状态时,机组加速度响应的衰减较快;通过改变可控阻尼器的阻尼状态,可以进一步优化其抗冲击特性。     

Experimental investigation on dilute gas-solid multiphase jet in crossflow

In the current study, an experimental setup was built to investigate the gas flow and particle distribution in a normal jet crossflow to a main flow in a confined test section. The experiments were conducted under two test conditions: with Re_c/Re_jet of 7.9×10⁴/3.1×10⁴ and 7.0×10⁴/1.8×10⁴. Four classes of particles were used in both tests. The planar gas flow field and particle distribution on the symmetric cross-section were measured by a DPIV system. Mean fluid velocity results and transient flow visualization images were used to analyze the jet influence on the gas flow field. The analysis of the time-average particle concentration reveal that the jet control method may set a gas barrier in the flow field, which the tiny particles are able travel around, large particles are able travel through, and only 10-micronscale particles could be successfully blocked. The results show that the wall jet control method can be applied in inertia particle separator.     

Investigation of the structure of a wave gear with an external deformation wave generator

Design diagrams of wave gears with external deformation wave generators based on a parallel crank mechanism are considered. Wave gears can be used to develop new drives with a high-quality rating in terms of efficiency, kinematic accuracy, torsional stiffness, and interesting layout solutions. In order to obtain the best possible ratings of these gears, it is necessary to determine if there are redundant constraints in the design and to reduce (or eliminate) them. To do this, one of the modern structural analysis methods, the graph method, is used. It makes it possible to find circuits with redundant constraints and reasonably eliminate them. The simplicity and illustrativeness of the method are shown.     

Experimental and numerical study of piston thermal management using piston cooling jet

This research investigates the effects of piston cooling jet (PCJ) on the temperature and heat transfer of a piston. A numerical model was developed by using the computational fluid dynamic approach in which the fluid and solid domains of the piston were coupled in a three-dimensional space. Two-phase flow of oil and air was also simulated. This method was used to analyze the effects of oil velocity and piston position on the heat transfer coefficient at the bottom of the piston as the new outcomes of this study. For the experiment, combustion heat flux on the piston was simulated in a test rig, and numerical results were validated. The results showed a linear relation between the oil jet velocity and the average of heat transfer coefficient at the bottom of the piston, and a periodic correlation between the piston’s vertical position and the average of heat transfer coefficient. The average of the piston crown temperature could be reduced to about 70 K by using the PCJ system, but this cooling method could create 50 K temperature gradient in the piston.     

Experimental investigation on the hysteresis behavior of the wire rope isolators

Vibration isolation has been widely applied to filter the external excitation energy and impact forces in building structures and equipment. Wire rope isolator (WRI), a kind of isolator for vibration and shock isolation, shows a better performance in attenuating these forces. WRIs are able to deviate these external forces through their mechanical configuration and high-energy dissipative capabilities. The application of WRI demands knowledge of its behavior and the relation between various geometrical properties and input force. The present work investigates the influence of geometrical parameters, such as wire rope diameter, number of coils, and displacement amplitude on the hysteresis behavior of WRI under quasi-static loading in both vertical and horizontal directions. The hysteresis behavior of different WRIs was evaluated using the calculated parameters from hysteresis force-displacement curves: energy loss ratio (ELR), and effective stiffness. The study indicates that the geometric properties significantly influence the effective stiffness than the energy loss ratio. It is observed that, increased displacement amplitude results in decreased ELR and hence damping capabilities. The study also confirms that the wire rope isolator possesses a good ability in damping through its stiffness and high-energy dissipation capability.     

焊接结构虚拟疲劳试验技术研究

针对焊接结构的疲劳问题,提出虚拟疲劳试验技术.基于有限元数值仿真技术及焊接接头S-N曲线数据库,在计算机中模拟物理样机的疲劳试验过程,预测焊接结构的疲劳寿命,使产品在设计阶段开展抗疲劳设计成为可能.虚拟疲劳试验在铁路车辆焊接摇枕上的应用验证了该技术的可行性.     

Experimental and numerical investigation of the wake structure and aerodynamic loss of trailing edge jet

The influence of the velocity ratio (VR) between the jet and main flow on the wake structure and aerodynamic loss of the trailing edge jet is studied using particle image velocimetry and numerical simulations. Three different velocity ratios, namely, VR = 0.5, 1.0, 1.5, are chosen for this comparative study. The Reynolds number (Re _h ) based on the slot height (h) and the mainstream velocity (U₀) are 3380. Results show that the influence of jet on wake structure is significant such that the wake region shrinks and the turbulent kinetic energy is enhanced as the velocity ratio increases. The distribution area of strong vorticity is enlarged with increasing velocity ratio. By using proper orthogonal decomposition and fast Fourier transfer analysis, the variation of velocity ratio demonstrates significant impact on vortex shedding and turbulent kinetic energy. The aerodynamic loss coefficient is nearly constant between VR = 0.5 and 1.0, but increases by 3.25 % as the velocity ratio increases from 1.0 to 1.5.