【作者】 刘静；

【导师】 李杰；

【作者基本信息】 西北工业大学 ， 流体力学， 2020， 博士

【摘要】 陷窝流动控制技术起源于其在高尔夫球中的应用。陷窝可以明显降低高尔夫球的飞行阻力,增加其飞行距离。从陷窝在高尔夫球飞行中的作用获得启发,研究者们探寻了陷窝流动机理及在分离控制、减阻以及强化传热等方面的应用。由于陷窝诱导旋涡结构复杂、缺乏旋涡强度定量分析方法、陷窝流动机理尚未理清以及应用研究影响因素多等原因,致使研究没有形成统一的定论。为澄清陷窝流动机理,定量陷窝诱导旋涡强度,并优选陷窝在控制分离、湍流减阻和强化传热应用中的最佳参数设计,本文首先提出了基于速度梯度张量特征值的旋涡强度定量分析方法,对比了陷窝诱导旋涡强度并分析了时均流场下的陷窝流动机理,然后在此基础上进行陷窝在控制层流分离、湍流减阻和强化传热中的有效性和参数优选研究,最后基于DES方法精细模拟并分析了陷窝诱导旋涡瞬态流动机理。主要研究内容及结论如下:1.针对旋涡强度无法定量分析的问题,提出了采用涡核速度和速度梯度张量特征值来定量分析旋涡的方法。通过用涡核处局部坐标系表示的速度矢量和速度梯度张量,得到了涡核的轴向速度、径向速度、旋转角速度、轴向速度变化率和径向速度变化率,并在此基础上简化出了用最大轴向速度、最大轴向速度变化率和最大旋转角速度综合表示的旋涡强度定量分析方法。此方法不受分离区压力和边界层集中涡量的影响,适用范围广,可以作为陷窝诱导旋涡强度对比的方法。2.采用本文发展的旋涡强度定量分析方法,进行旋涡结构、旋涡强度和旋涡整体发展过程分析,并进一步阐明时均流场下的陷窝诱导旋涡流动机理。不同深宽比陷窝诱导出不同的旋涡结构,随着深宽比的增加,陷窝内外旋涡强度均增加。陷窝内形成的低速回流区旋涡切断了边界层的连续发展,随后在再附线后生成了新的边界层,陷窝后尾涡脱落和湍流度增加强化了尾流区动量交换。陷窝流动机理特性分析为陷窝应用研究提供定量参考,并奠定机理基础。3.本文采用transition SST转捩模型封闭的RANS方法,针对存在层流分离的底部弯曲扩张管道简化模型,在分离线之前布置叉排多陷窝进行入口湍流度、陷窝位置、陷窝大小和深宽比的参数影响研究,得到了陷窝推迟层流分离的最佳参数设计,最大推迟分离幅度为33mm。研究发现陷窝能够提高流动湍流度,有效触发层流转捩,进而推迟流动分离。入口湍流度对陷窝推迟分离距离影响最大,陷窝大小和深宽比对推迟分离距离的影响次之,陷窝位置对推迟分离的距离影响最小。陷窝内旋涡强度和推迟分离幅度没有正相关关系。4.在平均来流速度为4m/s、15m/s和30m/s的充分发展入口边界条件下,变化陷窝表面直径和深宽比,进行平板上单陷窝湍流减阻参数优选研究,研究表明在来流速度从4m/s到30m/s范围内,经过参数优化后均给出了具有湍流减阻效果的陷窝设计。在来流速度为4m/s时,陷窝减阻幅度最大,陷窝区域的减阻幅度为30%,包括陷窝环区域的减阻幅度为1.8%。相对于等投影面积平板,内陷式陷窝的摩擦阻力减小,压差阻力增加,经过优选设计的陷窝,其摩阻的降低超过压差阻力的增加,达到陷窝减阻的效果。5.结合旋涡强度定量分析方法的支撑,进行充分发展入口边界条件下管道内陷窝强化对流传热优选分析。单陷窝强化对流传热效率优选研究表明,深宽比0.3陷窝诱导的旋涡强度最大,增加表面积最大,对流换热能力最强。多陷窝强化对流传热效率优选研究发现较大表面直径和较大深宽比的陷窝诱导旋涡强度更大,具有更高的对流换热强度和更大的流动阻力,而较小表面直径和较小深宽比的陷窝具有更高的综合换热系数。实际应用中应考虑阻力和强化换热的不同权重来选择不同的陷窝设计进行强化换热。6.本文基于DES精细数值模拟方法分析单陷窝内旋涡的脱落方式、陷窝外旋涡的演化过程、以及陷窝内外旋涡之间的作用方式。研究发现流场空间低压区位置和旋涡位置重合,旋涡的周期性脱落形成空间压力波的周期性变化。陷窝内旋涡的脱落和陷窝上侧边界层的脱落相互诱导融合,形成陷窝后尾涡系的龙卷风涡和发卡涡结构。尾涡系周期性发展,强化尾流区的动量和能量交换,为陷窝在触发层流转捩、强化对流传热等方面的成功应用提供机理参考。更多还原

【Abstract】 Dimple flow control technology is originated from its application in golf ball.Dimple can obviously reduce the drag of golf ball and increase its flight distance.Why does golf ball with dimples on surface fly farther? What is the mechanism? Can dimple be used in other fields? Inspired by the role of the dimple in the aerodynamic control of golf balls,researchers began to explore the mechanism of dimple flow and it’s application in separation control,drag reduction and heat transfer enhancement.Because of the complexity of vortex structure,lacking of vortex strength quantitative analysis method,unclear flow mechanism and too many influencing factors in application research,the conclusion of the dimple study has not formed a unified conclusion.A vortex strength quantitative analysis method based on the eigenvalues of velocity gradient tensor is proposed.Using this method,the vortex strength induced by dimple is compared and the flow mechanism is analyzed.Dimple’s effectiveness in controlling separation,reducing turbulent drag and enhancing heat transfer is verified.Based on the DES simulation method,the detail of the vortex evolution induced by the dimple is further analyzed.The main research contents and conclusions are as follows:1.A method is proposed to analyze the vortex quantitatively by using the velocity and eigenvalues of velocity gradient tensor in vortex core.The axial velocity,radial velocity,angular velocity,axial change rate and radial change rate of the vortex core are obtained by using the velocity vector and velocity gradient tensor expressed in the local coordinate system of the vortex core.Besides,the quantitative analysis elements of vortex strength are simplified to the combination of the maximum axial velocity,the maximum axial change rate and the maximum angular velocity of rotation.This method is not affected by the pressure in the separation zone,nor by the vorticity concentration in the boundary layer.It can be used in a wide range of applications and can be used as a method to compare the strength of the vortex induced by the dimple in this thesis.2.The quantitative analysis methods are used to analyze the vortex structure and overall development process of the vortex induced by different depth to width ratio dimple,and the mechanism analysis of the vortex induced by the dimple is carried out.The results show that different depth to width ratio induces different vortex structures.With the increase of the ratio of depth to width,the strength of vortex in the dimple increases,the strength of longitudinal vortex outside the dimple increases,and the drag increases.The flow mechanism of the dimple is concluded as: the low velocity re-circulation zone formed in dimple,the regeneration of the boundary layer on the upper side of the dimple,the momentum exchange in the wake area strengthened by the vortex shedding after the dimple,and the turbulence in the wake area strengthened by the dimple,etc.3.With the RANS method closed by transition SST transition model,the bottom curved expansion channel which laminar flow separation existed is solved.The parameters of inlet turbulence,position,size and depth to width ratio of the dimple were studied by arranging staggered dimples in front of the separation line to explore which factor has the greatest impact on the delayed separation amplitude of the dimple.The results show that the dimples all have the effect of trigger laminar flow transition and delay separation.The inlet turbulence has the greatest influence on the delayed separation distance,the second is the ratio of dimple size and depth to width ratio,and the least is the position of dimple.There is no positive correlation between the strength of vortex in the dimple and the extent of delayed separation.4.Based on the RANS method closed by k-ω SST two equation turbulence model,under the fully developed entrance boundary conditions with an average inflow velocity of 4m/s,15 m/s and 30 m/s,the optimization of turbulent drag reduc tion for a single dimple on a flat plate is studied.The results show that,compared with the equal area plate,the friction drag of the dimple decreases and the pressure difference drag increases.After the optimum design of the dimple,the reduction of the friction drag exceeds the increase of the pressure difference drag to achieve the effect of the dimple drag reduction.In the range of flow velocity from 4 m/s to 30 m/s,the dimple has the effect of drag reduction.At 4 m/s,the drag reduction range of the dimple area is as high as 30%,and that of the area containing the dimple ring is as high as 1.8%.When the flow velocity is fixed,the smaller the surface diameter is,the larger the upper limit value of drag reduction depth to width ratio is.When t he surface diameter is fixed: the larger the inflow velocity is,the smaller the depth to width ratio of the drag reduction dimple is,and the smal er the drag reduction magnitude is.5.Based on the RANS equations closed by k-ω SST Turbulence model,the numerical simulation of the heat transfer enhancement in the fully developed inlet channel with dimple is carried out.The optimization of convective heat transfer efficiency with single dimple shows: the vortex induced by 0.3 dimple has the largest strength,the largest increase of surface area and the strongest convective heat transfer.The optimization of the convective heat transfer efficiency with multiple dimples shows: the dimple with larger surface diameter and larger depth to width ratio has higher intensity of convective heat transfer and larger flow drag,and the dimple with smaller surface diameter and smaller depth to width ratio has higher comprehensive heat transfer coefficient.In practical application,different weight of drag and enhanced heat transfer should be considered to choose different dimple designs for enhanced heat transfer.6.In view of the lack of analysis on the correlation between the vortex in the dimple and the wake vortex outside the dimple,this paper uses DES method to analyze the shedding mode of the vortex in the single dimple,the evolution process of the vortex outside the dimple,and the interaction mode between the vortex inside and outside the dimple.It is found that the wake structure captured by DES method can be used for vortex evolution analysis.The position of the low pressure region coincides with that of the vortex,and the periodic shedding of the vortex forms the periodic change of the space pressure wave.The vortex shedding in the dimple and the boundary layer shedding on the upper side of the dimple induce and merge with each other,forming the tornado vortex and hairpin vortex structure of the wake vortex system behind the dimple.The periodic development of the wake vortex system strengthens the momentum and energy exchange in the wake region,which provides a theoretical foundation for the application of the dimple in the triggering laminar flow transition and the enhancement of convective heat transfer.更多还原