【作者】 王哲；

【导师】 杨渐志；

【作者基本信息】 合肥工业大学 ， 供热、供燃气、通风及空调工程， 2022， 硕士

【摘要】 近年来,随着我国经济的快速发展,越来越多的环境问题引起人们的广泛关注。与此同时,随着生活水平的提高,人们也不断追求高质量的生活环境,特别是室内环境。根据美国环境总局调查结果表明,室内颗粒污染物浓度约为室外的2.5倍。因此,有效控制室内污染物浓度是改善室内空气质量的重要研究方向。室内空气中悬浮的污染物颗粒流动除了受到空气流速影响外,往往还受到其附近壁面的影响。开展近壁面颗粒绕流及其升阻力特性研究对帮助认识室内颗粒污染物运动具有重要意义。同时,实际环境中颗粒污染物并非圆球状,而形状非球形会增加颗粒绕流的复杂性并深刻改变其升阻力特性。本文采用直接数值模拟方法,对近壁面椭球形颗粒绕流尾迹和升阻力特性进行研究,主要内容包括:1)首先,对大空间均匀来流中不同雷诺数Re(=300、500、700和1000)和攻角α(=0°-90°)对椭球尾迹演化的影响开展研究,共捕捉到八种不同尾迹模态,其中稳态非对称模态、多周期且具有低频调制的非对称双侧涡旋脱落模态与多周期且具有低频调制的非对称单侧涡旋脱落模态首次被观察到。演化研究发现在较低的攻角下,狭长的钝体可以延迟非稳态分岔的触发。最后,对尾迹模态当中低频现象的物理机理进行解释。2)其次,对大空间均匀剪切来流中不同雷诺数Re(=480、600、700和750)和剪切率K(=0-0.1)对倾斜椭球(α=45°)尾迹演化、升阻力特性以及斯特劳哈尔数的影响开展研究。捕捉到五种不同尾迹模态,其中,多周期且分离区规则旋转的涡旋脱落模态首次被观察到。研究发现,剪切率的增加总体会抑制尾迹演化,在雷诺数Re=480时,在较高的剪切率下,尾迹甚至从非稳态转变为稳态,这可能是由于前缘雷诺数显著减小。随着剪切率增加,时均升力系数和阻力系数以及斯特劳哈尔数均增大。除了均匀来流,随着雷诺数不断增大,时均升力系数和阻力系数以及斯特劳哈尔数同样增大。3)最后,对近壁面均匀剪切来流中雷诺数Re=1000工况下,不同攻角α(=0°-90°)和壁面间距L(=L/D_e=1,3,6)对椭球尾迹演化和升阻力特性的影响开展研究。捕捉到一种新的尾迹模态,即周期性非对称非稳定单侧涡旋脱落模态。研究发现,攻角的增加会促进尾迹演化发展,与大空间均匀流比,涡旋脱发生的临界攻角明显增大。分析了攻角和壁面间距对时均升力和阻力系数的影响,随着壁面间距的减少,升力系数的峰值发生在比较大的攻角,最后根据当前的模拟结果,提出了升力和阻力系数的修正关系式。更多还原

【Abstract】 In recent years,with the rapid development of China’s economy,increasing environmental problems have attracted extensive attention.At the same time,with the improvement of living standards,people continue to pursue high-quality living environment,especially indoor environment.According to the investigation results of the US environmental administration,the concentration of indoor particulate pollutants is about 2.5 times that of outdoor pollutants.Therefore,effective control of indoor pollutant concentration is an important research direction to improve indoor air quality.The flow of suspended pollutant particles in indoor air is often affected not only by the air velocity,but also by the wall near it.The research on the characteristics of particle flow near wall and its lift and drag is of great significance to help understand the movement of indoor particulate pollutants.And in the actual environment,the shape of particle pollutants is not spherical,which will increase the complexity of particle flow and profoundly change its lift and drag characteristics.In this paper,the wake,lift and drag characteristics of ellipsoidal particles near a wall are studied by using the direct numerical simulation method.The main work is summarized as follows:1)First,in the uniform flow,the effect of Reynolds number(Re=300,500,700,and 1000)and incidence angle(α=0°-90°)on the wake evolution is considered.Eight flow regimes are identified,of which three are new modes and first reported,i.e.,steady asymmetric flow regime,multi-periodic asymmetric flow regime with double-sided hairpin vortex shedding and low frequency modulation,and multi-periodic asymmetric flow regime with single-sided hairpin vortex shedding and low frequency modulation.Moreover,it can be found that the elongated body can delay the onset of unsteadiness for small incidence angles at the same condition.And finally,the physical mechanisms of the low-frequency phenomena observed at different wake modes are explored.2)Then,in the uniform shear flow,the effect of Reynolds number Re(=480,600,700,and 750)and shear rate K(=0-0.1)on the wake evolution and the variation of forces coefficients and Strouhal number is considered.Five flow regimes are identified,of which one is new wake mode and first reported,i.e.,multi-periodic shedding of the hairpin vortex with a regular rotation of separation region mode.In general,the wake transition is suppressed with increasing the shear rate.At high shear rate,the flow even changes from the unsteady to the steady at Re=480,which can be attributed to the obvious decrease of the leading-edge Reynolds number.The time-averaged drag and lift coefficients,and the Strouhal number are increased with increasing the shear rate.Except for K=0,time-averaged drag and lift coefficients and the Strouhal number are also increased with increasing the Reynolds number.3)Finally,in the linear shear flow close to a wall,at Re=1000,the effect of incidence angle(α=0°-90°)and distance from the wall L(=L/D_e=1,3,6)on the wake evolution and the variation of forces coefficients is considered.the increase of the incidence angle advances the wake transition,but the wake transition occurs at high incidence angle comparing with that in an unbounded uniform flow.Because of the shear freestream and the presence of the nearby wall,one new wake mode is observed,i.e.,periodic asymmetric flow regime with unstable single-side vortex shedding(PAUS).Finally,the effect ofαand L on time-averaged drag and lift coefficient are analyzed and simple correct correlations predicting the variation of (?)_d and (?)_l are proposed.更多还原