射流卷吸边界层内相干结构的实验研究

在轴对称水射流实验台上，采用单帧长曝光图像法，测量了出口Reynolds数在1849~2509范围内的卷吸边界层内流场结构。发现在流向距离L=2D~3.5D，径向距离H=D~1.25D的区域内，流场中吞噬作用和侵蚀作用两种卷吸模式交替出现。分析得到，当Re>1915时，吞噬作用所占的比例随着Re的增大而增大，当Re>2311之后，Reynolds数对该结构发生概率的影响降低；通过快速傅里叶变换得到该相干结构发生的频率在10～19Hz之间；吞噬作用发生的同时在流场中观测到一些特殊涡结构。同时采用运动单帧长曝光图像法对射流流场进行拉格朗日法观测，发现射流流场中的涡结构一般存在于湍流与非湍流界面附近。     

Investigations on Emission Characteristics of a Liquid-Fueled Trapped Vortex Combustor

The combustion and emission formation process of liquid fuel in the trapped vortex combustor(TVC)are very complicated.A trapped vortex combustor with replaceable bluff-bodies was designed to investigate these processes.The bluff-body widths varied from 0.021 m to 0.036 m.Experimental tests were carried out.Liquid RP-3 aviation kerosene was used in the tests.Emissions were measured under atmospheric pressure.The combustion process was analyzed theoretically in the viewpoints of relative evaporation time,mixing time and reaction time.Numerical simulations were also conducted to help analyze the formation and depletion processes of pollutants in TVC.The results reveal that atomization was a critical factor for formation and depletion processes of pollutants.By controlling mixing speed of burned and unburned gases and thus fuel-air uniformity,turbulence intensity could also affect emission levels.In addition,residence time also affected the emissions by affecting combustion completeness and the time for NOx formation.All these factors were combined in a complicated way to affect combustion process and pollutant emissions.     

A simple model for deep dynamic stall conditions

The present study is focused on modeling of dynamic stall behavior of a pitching airfoil. The deep stall regime is in particular considered. A model is proposed, which has a low implementation and computational complexity but yet is able to deal with different types of dynamic stall conditions, including those characterized by multiple vortex shedding at the airfoil leading edge. The proposed model is appraised against an extensive data set of experimental (α,C_L) curves for NACA0012. The results of an existing widely used model, having comparable complexity, are also shown for comparison. The proposed model is able to well reproduce not only the classic curves of deep dynamic stall but also the curves characterized by lift oscillations at high angles of attack due to the shedding of multiple vortices. Furthermore, the model appears to be robust to variations of its parameters from the optimal values and of the airfoil geometry. Finally, the model is successfully implemented in a commercial CFD software and applied to the simulation of a vertical axis wind turbine within the actuator cylinder approach. The accuracy of the prediction of the turbine power coefficient in the whole rotation cycle is very good for the optimal working condition of the turbine, for which the model parameters were calibrated. Fairly good accuracy is also obtained in significantly different working conditions without any further calibration.     

A Minimalist Single-Layer Metasurface for Arbitrary and Full Control of Vector Vortex Beams

Vector vortex beams (VVBs) possess ubiquitous applications from particle trapping to quantum information. Recently, the bulky optical devices for generating VVBs have been miniaturized by using metasurfaces. Nevertheless, it is quite challenging for the metasurface-generated VVBs to possess arbitrary polarization and phase distributions. More critical is that the VVBs' annular intensity profiles demonstrated hitherto are dependent on topological charges and are hence not perfect, posing difficulties in spatially shared co-propagation of multiple vortex beams. Here, a single-layer metasurface to address all those aforementioned challenges in one go is proposed, which consists of two identical crystal-silicon nanoblocks with varying positions and rotation angles (i.e., four geometric parameters throughout). Those four geometric parameters are found to be adequate for independent and arbitrary control of the amplitude, phase, and polarization of light. Perfect VVBs with arbitrary polarization and phase distributions are successfully generated, and the constant intensity profiles independent of their topological charges and polarization orders are demonstrated. The proposed strategy casts a distinct perception that a minimalist design of just one single-layer metasurface can empower such robust and versatile control of VVBs. That provides promising opportunities for generating more complex vortex field for advanced applications in structural light, optical micromanipulation, and data communication.     

Development of low-noise drag-type vertical wind turbines

In this study, the aerodynamic noise characteristics of Savonius wind turbines were investigated using hybrid computational aero-acoustics techniques, and low-noise designs were proposed based on the understanding of the noise generation mechanism. First, the flow field around the turbine was analyzed in detail by solving three-dimensional unsteady incompressible Reynolds-averaged Navier–Stokes equations using computational fluid dynamics techniques. Then, the aerodynamic noise radiating from the wind turbine was predicted using the Ffowcs Williams and Hawkings equation with the obtained flow field information. Two distinct harmonic noise components—the blade passing frequency (BPF) and harmonics with a fundamental frequency that is much higher than the BPF—were identified in the predicted noise spectrum. The origin of the higher harmonic components was found to be related to vortex shedding from the rotating turbine. Based on this finding, the proposed low-noise design for Savonius wind turbines uses S-shaped blades. S-shaped blades were found to reduce the noise levels of Savonius wind turbines by up to 2.7 dB.     

内置涡流发生器的管内过冷沸腾与强化换热的模拟

采用欧拉双流体模型结合RPI沸腾模型对高压管内过冷沸腾进行三维非稳态模拟，考察了涡流发生器对管内过冷沸腾的影响，模拟了高压下光管内的过冷沸腾、层流下内置涡流发生器的换热管内的流动和湍流下内置涡流发生器的换热管内的过冷沸腾。结果表明，内置涡流发生器的换热管在层流状态下换热能力明显提升，过冷沸腾时管内换热能力有一定提升，且壁面附近的气泡由于扰流作用被大量卷入锥形片内，降低了壁面附近产生气膜的可能性，延迟了过冷沸腾起始点的位置。     

90°圆截面弯管内稠油流动特性分析

采用计算流体力学三维层流模型模拟，研究了温度50~75℃、雷诺数Re=300~800、弯管内径D=50.7~131.7 mm、弯径比B=0.75~3.0条件下稠油在90°弯管内的阻力特性，分析了弯管局域阻力系数波动的机理。结果表明，随温度升高、入口雷诺数下降、弯管直径增加，局域阻力系数提高；在弯管0~15°范围内阻力下降，原因是弯管内形成双纵向涡，75°到弯管后0.5D范围内阻力下降，原因是弯管内形成4个纵向涡；弯管的弯径比对局域流动阻力影响很大，B=0.75时相邻截面最大落差达B=3.0时的28.35倍，但管道进出口阻力仅为1.68倍，原因是弯径比B≤1.0时，弯管后1.0D范围内侧形成了局域低压区，对应位置出现流向涡旋，同时弯管后0.5D截面稠油剪切速率达到峰值。     

喷气涡流纺喷嘴喷孔参数的分析

喷气涡流纺是利用喷嘴内高速气流的旋转对纤维加捻成纱的设备,为了分析喷嘴的结构参数对成纱质量的影响,选择喷嘴的喷孔直径d为0.5 mm,分别在不同喷孔数量n(5,6)及不同喷孔角度θ(60°,65°,70°,75°,80°)下,在新型涡流纺纱机上对粘胶原料进行纺纱实验。实验结果得到了喷孔最优结构参数:喷孔孔径×数量φ0.5×5;喷孔角度70°。     

考虑流固耦合的Spar型浮式风力机涡激运动特性研究

对弹性支撑单圆柱在均匀流作用下的涡激运动特性进行数值模拟,捕捉到"锁定区"、"拍"、"频率变换"等现象。柱体周围流场采用Fluent求解,将4阶Runge-Kutta方法代码写入用户自定义函数(UDF)求解运动微分方程,运用动网格技术更新流场,实现圆柱与流场的非线性耦合作用。发现随着折合速度的增大,涡激运动响应可分为锁定前支、锁定区、锁定后支3个阶段,在进入锁定区前(折合速度Ur=3)横向运动响应发生拍现象,当跨过锁定区后(折合速度Ur=10)发生频率变换现象。结果表明,横向涡激运动有较大范围的频率锁定现象,频率解锁前后圆柱涡激运动轨迹由"右8字"形变换为"左8字"形。     

泰勒涡流流动及强化传热数值研究

泰勒涡流为叠加于剪切流之上的二次流，其具有强化传热作用，在航空、水处理、制药工程和化工等领域都具有很大的应用价值。运用Fluent软件，建立长径比Γ = 30的模型并对同轴套管间的流态演变和传热特性进行了数值模拟。模拟结果显示了环隙内流体流态随着内筒转速增加的演变过程，表明在存在径向温差的情况下，涡流的存在强化了传热效率。对不同转速下的强化传热效果进行了对比分析，并确定了最佳状态点。