Experimental study on flow and heat transfer characteristics of synthetic jet driven by piezoelectric actuator

To investigate the flow and heat transfer characteristics of a synthetic jet driven by piezoelectric actuator, experimental investigation utilizing particle image veloci- metry, hot-wire anemometer and infrared camera was carried out. The results show that: (1) At the jet orifice exit, pairs of vortexes are generated, broken down and merged together periodically, forming a steady jet within a several slot width from distance near the orifice exit. And during the development, the synthetic jet spreads rapidly along the minor axis direction of the orifice. While along the major axis direction, the synthetic jet contracts firstly and then spreads slowly. (2) Exci- tation frequency forced on the actuator has a great effect on the synthetic jet flow field. There are two resonance frequencies at which the mean velocity and vorticity of the synthetic jet are maximized, especially at the higher resonance frequency. The resonance frequency values obtained by the experiment are lower than the theoretical values. (3) Similarly to the common jet impingement, the convective heat transfer coefficients at the target surface impinged by the synthetic jet also take on up-down tendency varying with the jet-to-surface spacing increment. But the jet-to-surface spacing ratio for optimum cooling achievement is greater and the cooling action region is wider than the former, indicating that the synthetic jet in- troduces a stronger entrainment and more vigorous penetration in the surrounding fluid.     

Experimental investigations on separation control and flow structure around a circular cylinder with synthetic jet

Circular cylinder separation control and flow structure influenced by the synthetic jet have been experimentally investigated in a water channel. The synthetic jet issues from a slot and ejects toward upstream from the front stagnation point of the cylinder. It has been found that, similar to the traditional synthetic jet which is positioned near the separation point or inside the separation region, the present synthetic jet arrangement constitutes an efficient way to control flow separation of the circular cylinder, but with a different control mechanism. The present synthetic jet leads to an upstream displacement of the front stagnation point and the formation of a vortex pair near both sides of the exit orifice. When Re _u based on the synthetic jet average exit orifice velocity is about lower than 43, a closed envelope forms in front of the windward side of the cylinder during the blowing cycle of synthetic jet, which acts as an apparent modification for the cylinder configuration. When Re _u is high enough, an open envelope forms upstream of the cylinder, and the flow around the cylinder becomes much energetic. Thus, regardless of Re _u, the present synthetic jet can improve separation for flow around a circular cylinder. With regard to the leeward side, as Re _u increases, the flow separation region behind the cylinder gradually disappears. The flow over cylinder may be fully attached when the open envelope forms upstream of the cylinder and Re _u is greater than 344. Then, the flow past the cylinder will converge near the back stagnation point of the cylinder, where a new vortex pair shedding periodically is generated due to the high shear layer. Recommended by Prof. DONG ZengNan, Member of Editorial Committee of Science in China, Series E: Technological Sciences Supported by the National Natural Science Foundation of China (Grant No. 10425207)     

同心环管圆孔射流流动特性的数值模拟

采用标准的k-ε模型及混合有限分析法对均匀横流条件下三维受限同心环管中单孔和双孔的横向紊动射流流动进行数值模拟.分别得到单圆孔不同流速比下的射流轨迹线与数学表达式、双圆孔不同间距下的速度场.结果显示：射流比越大,射流深度越深,射流越不容易发生弯曲;射流横断面上速度最大值的偏离程度随横流距离的增加而增大,但增大率越来越小;双孔射流孔间距影响射流轨迹线,由于前一个射流的遮挡作用,后面射流更不容易发生弯曲.     

高聚物喷孔上气泡的行为

研究了水中有机玻璃、硬聚乙烯喷咀上气泡生长的过程，探讨了气流率、喷孔形状对气泡形成的影响，并导出了气泡直径和气泡扩展直径的计算公式．关联式计算值与实验值相吻合．     

掺气对射流轴线速度衰减影响的试验研究(“研究生论坛”稿件)

通过物理模型试验,采用CQY—Z8a型针式掺气流速仪测量了掺气射流的轴线速度,对不同条件下的掺气射流轴线速度衰减情况进行了对比研究,分析了入射水流速度、掺气浓度以及射流水股厚度对射流轴线速度衰减的影响,获得了掺气射流轴线速度衰减的变化规律。结果表明：在本试验条件下,射流轴线速度呈线性衰减;入射水流速度对射流轴线速度衰减基本没有影响,表现为不同入射水流速度时,各衰减趋势基本一致;掺气浓度越大,射流轴线速度衰减越快;射流水股厚度越小,射流轴线速度衰减越快。因此提高射流水体的掺气浓度,尽可能分散入水水体,可以加快射流轴线速度衰减,有效减弱射流对下游河床的冲刷。     

合成射流物理参数对控制翼型流动分离的影响

应用计算流体力学(CFD)技术数值模拟了合成射流对NACA0015翼型流动控制的影响.合成射流施加的位置分别距离翼型前缘12%c、30%c和70%c（c为翼型的弦长）,研究分析在不同位置施加合成射流,控制流动分离的效果随攻角和射流偏角的变化趋势,对组合射流的位置、相位角和动量系数进行研究.以二维不可压非定常Reynolds-averaged Navier-Stokes（RANS）方程模拟非定常分离流动,采用SST湍流模型,压力修正采用压力隐式算子分裂（PISO）算法,时间积分采用隐式处理方法,空间离散采用二阶迎风格式.通过对数值模拟结果的分析表明：1）12%c、30%c和70%c等对应的射流情形,采用切向射流均优于法向射流的控制效果; 2）对于组合射流和单一射流,射流位置应靠近分离点或在分离点之前,才能达到流动控制的目的,射流位置越靠近分离点流动控制效果越佳;3）组合射流选择合适的相位角,可以增强流动控制的效果.     

受限空间内超声速混合层生长特性

为获得受限空间内激波作用下的超声速混合层生长规律,以支板喷射超燃冲压发动机典型流道为研究对象,开展了2.3Ma氢气射流与2.0Ma空气来流所形成的超声速混合层的生长特性研究.基于OpenFOAM计算平台,采用大涡模拟方法,数值研究了超声速混合层的流场结构和特征,流场结构和组分分布与实验结果吻合较好.通过超声速混合层组分浓度、厚度、可压缩效应及总压损失的分析,获得了超声速混合层的生长特性.研究结果表明:受限空间内超声速混合层的生长过程具有4个典型阶段,支板末端的膨胀波/激波结构会显著减低对流马赫数,从而降低混合层的可压缩性,促进混合层的生长;激波与混合层的相互作用能够增强局部湍流强度,获得涡量增益,加快混合层的生长速率,促进混合效率,但同时会引起较大的总压损失,降低发动机性能.发动机设计时要综合考虑波系结构与混合层相互作用带来的混合增强和总压损失,实现性能优化.     

错排射流冲击受限短通道内流场的实验研究

用五孔针对简化后的涡轮叶片错排射流冲击受限短通道流场进行了详细的测量,着重研究受限通道内各截面的流动规律,以及射流孔错排布置、出流孔、通道高度比和射流雷诺数对通道流动特性的影响。实验结果表明:通道高度比的变化会显著改变通道内的流场结构;在同一高度比下,射流雷诺数变化对流场结构的影响很小;出流孔的抽吸作用使附近气流加速并在通道内出流孔位置形成一局部低速区;受错排射流诱导及冲击靶面后壁面射流反卷通道内形成漩涡,使高速气流向通道侧壁甚至孔板面集中,该结果有助于深入了解错排射流受限通道内冲击冷却的内在机理。     

基于人工神经网络的磨料水射流切削工艺建模

磨料水射流（AWJ）切割工艺已经被遍及世界的许多车间所采用，其优点广为人知。为了进行精密加工，如精密切割、铣削、钻孔和磨削等，必须精确预测AWJ的侵蚀深度。文章基于人工神经网络（ANN）对AWJ切割工艺进行建模。模型采用三层结构，输入变量有水射流压力、水喷嘴直径、磨料粒子粒度（直径）、磨料流量和切割头进给速度。输出量为AWJ的切割深度。样本数据在JJ-Ⅰ水射流切割机床上实验获取，A3钢样板作为切割试件。采用改进的BP算法和样本数据对建立的人工神经网络进行训练。训练好的网络以一定精度建立了AWJ切割工艺中各参数之间的映射关系。所建模型可以精确预测AWJ的切深。将该模型集成到AWJ切割机床的计算机数控器中，可以实现AWJ精密加工。     

填料塔液体分布器设计中一些问题的探讨──穿孔射流研究

提出了填料塔液体分布器设计中孔口流速、流量系数和安装高度的选取和确定方法。孔口流速可在穿孔射流的层流和湍流区选取；流量系数应通过实验测定；安装高度原则上与射流的连续段长度相当。文章提供了部分供设计选用的数据和关联式。     

准三维矩形交叉射流的数值试验研究

交叉射流是一种较复杂的湍流组合直流射流。本文运用作者所提出的计算机辅助数值试验(CAT)方法,对冷态炉内一次风、二次风两股射流以一定的角度喷射而形成三维交叉射流流场进行了研究。通过对不同高宽比的矩形喷嘴、喷射角、一二次风速度比工况下的数值试验,得到描述交叉射流空气动力特性的两个经验公式。计算结果与现有的实验数据吻合较好。     

Experimental and numerical study on the flapping motion of submerged turbulent plane jet

A submerged,vertical turbulent plane water jet impinging onto a free surface will be self-excited into a flapping oscillation when the jet velocity,leaving the jet orifice,exceeds a critical value.The flapping phenomenon was verified simultaneously in this paper by laser Doppler velocimeter measurement and numerical analyses with volume of fluid approach coupled with a large eddy simulation turbulent model.The general agreement of mean velocities between numerical predictions and experimental results in self-similar region is good for two cases:Reynolds numbers 2090 and 2970,which correspond to the stable impinging jet and flapping jet.Results show that the flapping jet is a new flow pattern for submerged turbulent plane jets with characteristic flapping frequency,and that the decay of the mean velocity along the jet centerline is considerably faster than that of the stable impinging state.     

A new type of leak-floor flip bucket

Flip buckets are commonly used to discharge flow away from a hydraulic structure into the downstream to dissipate energy.A new leak-floor flip bucket is presented,making the ski-jump water jet a typical long-narrow nappe.Based on the model experiments and numerical simulation,the flow pattern,formation process and mechanism of the leak-floor flip bucket are studied.The results show that cross section flow shape develops from the"Y-type"to"|-type",and this is because the natural pressure difference is generated when water flows through the leak-floor area and moves transversely from both sides to the center.Different from the slit-type flip bucket with sidewall contraction,the leak-floor flit bucket makes the water jet narrow and long without high pressure on the side walls of the flip bucket.Under the same jet length condition,the maximum sidewall pressure of the slit-type is 4.67 times that of the leak-floor flip bucket.The effects of flow discharge on the jet length are less significant for the leak-floor bucket than for the slit-type bucket.     

单微射流作动器定向控制主流数值模拟

采用Beam—Warming近似因式分解法求解二雄、粘性、非定常、可压N—S方程,对单微射流作动器定向控制主流的合成流场进行数值模拟。计算结果揭示了主流在单微射流旋涡对的影响下会产生偏转,并初步探讨了其产生偏转的原因。分析了主流的偏转程度与主流和微射流作动器之间距离的关系。     

毛细作用气液分离过程的理论分析

针对分液冷凝换热器的毛细作用气液分离流动过程,建立了稳定流动情况下的一维数学模型.模型综合考虑了静压、重力、毛细压力、切应力的影响 ;考察了在稳定状态下孔径、孔数以及其组合方式对液膜高度的影响,同时还考察了入口干度和流量对液膜高度的影响;发现孔径和孔数对液膜高度影响基本相同,都是随着数值的增大而液膜高度减小.孔径为1.3 mm、孔数为8时液膜高度都分别为零,气液分离失效,但是干度和流量的对液膜高度影响趋势却相反.在干度为0.5、流量为0.058 kg/s时液膜高度也都分别为零,气液分离失效.     

移动床中颗粒运动的微观分析

通过实验和离散元模型考察了移动床中颗粒的运动，在微观尺度分析了粒径分布和移动床出口大
小对颗粒运动、床层结构等的影响.结果发现:颗粒的粒径分布对颗粒垂直速度分布有很大影响，不同粒
径的混合有利于颗粒的流动；对于同一粒径分布G2/5与DO呈线性关系，移动床出口直径对系统的出料速
率有很大影响；对不同粒径混合系统，移动床的空隙率和颗粒配位数分布曲线均只有一个峰，而对等粒
径系统的分布曲线均有两个峰；在移动床出口上方都存在拱形的应力链，出口两侧颗粒运动的死区内颗
粒间的应力很大，中心流动区域应力较小，在移动床的上部颗粒间的应力最小；移动床出口处颗粒与移
动床底部壁面的应力很大，随着床层深度的增加，颗粒与移动床侧壁面的应力增加，但当床层深度达到
一定值后颗粒与移动床侧壁面的应力不变；颗粒与底部壁面的应力要大于与侧壁面的应力.     

水垫塘冲击射流附壁区的水力特性

采用粒子成像测速技术(PIV)获得了水垫塘冲击射流全域瞬时精细流场,并在此基础上,对附壁射流区的流速分布规律和水流紊动特征等水力特性进行了系统研究.结果表明:在附壁射流区,射流横向断面的流速、紊动强度分布呈自相似分布规律,具有自由射流的特征;在射流纵向,流速随着流程增加而线性减小,射流半扩展厚度和紊动强度随流程增加而线性增大;并提出了描述附壁射流区射流横向断面流速、紊动强度分布规律的方程,同时得到了附壁射流区射流纵向上流速、半扩展厚度和紊动强度沿流程变化的计算公式.     

雷诺数对气固两相圆湍射流影响的实验研究

为了探求气固两相圆湍流射流的特性，研究了不同雷诺数对气固两相圆湍射流的影响.在水平圆喷口近场处使用了粒子图像测速技术(PIV)，对雷诺数分别为6 230、10 010时的气固两相湍流射流进行测试.采用3种粒径的玻璃微珠作为示踪粒子.获得了距喷口出口截面轴向距离为3倍喷口内径处的示踪粒子速度矢量分布.分析比较了不同雷诺数下气相拟序结构对于颗粒扩散的影响.结果表明，随着雷诺数的增大，大颗粒受气相流场的作用更大，颗粒沿轴向的运动刚性加强，颗粒跟随主气流前进的趋势更加明显     

合纤长丝热气流变形中加热对气压与流速的影响分析

对合纤长丝热气流变形中加热对空气压强与流速的影响进行了分析．指出加热时发生变化的流动空气参数不仅是温度、压强、速度，马赫数Ｍａ、密度、总压和嫡等参数也发生了变化．在重点对气流压强与速度的变化进行分析的基础上，进一步指出对空气加热的３个限制因素是：纤维对气流温度升高的承受能力、避免气流发生壅塞和气流的热阻状态．     

气液边缘位置控制装置气动检测器的结构参数优化

气液边缘位置控制装置广泛用于纺织、印刷、包装等制造领域,它的气动检测器的结构特性极大影响该装置的工作性能。本文在初步研究检测器喷口流的动状态基础上,采用正交试验和方差分析方法,优选气动检测器的结构参数,并对具有优选结构检测器的气液边缘位置控制装置进行整机动态试验,试验结果表明:检测器中固定节流器和主喷口的结构尺寸对控制气液伺服阀的膜盒中的气压变化影响最显著,采用优选结构检测器的装置的动态响应性能得到了改善。