粘弹性流体湍流减阻流动大涡数值模拟研究

发布时间：2018-04-29 09:18

本文选题：粘弹性流体 + 大涡数值模拟　；参考：《哈尔滨工业大学》2015年博士论文

【摘要】：将少量柔性长链高分子聚合物或某些表面活性剂加入水或者有机溶剂中可以明显减小湍流流动中的摩擦阻力,这种现象被称为添加剂湍流减阻效应。为了使这一现象更好地在实际工业系统中得到应用,亟需对其流动特性和减阻机理进行研究。由于实验方法不能获得粘弹性流体湍流减阻流动中的分子变形场和弹性应力场信息,因而大多采用数值模拟方法。大涡数值模拟(LES)因其在相同工况下计算量小于直接数值模拟(DNS)、可获得的信息量比雷诺平均数值模拟(RANS)多而备受关注。应用LES研究特定湍流流动时,关键在于所选用的亚格子模型是否适用于该种湍流流动,几乎所有已发表的亚格子模型只针对于牛顿流体湍流流动,而粘弹性流体湍流减阻流动LES研究刚刚起步,目前只有应用时间近似解卷积模型(TADM)尝试对粘弹性流体湍流减阻流动进行数值模拟。为了更好地应用LES研究粘弹性流体湍流减阻流动,需开发更多可靠、有效的亚格子模型。本文将从这一角度出发,结合粘弹性流体湍流减阻机理,构建针对粘弹性流体湍流减阻流动的新型亚格子模型,这对粘弹性流体湍流减阻流动LES研究有重要的理论意义和学术价值。从粘弹性流体湍流减阻机理的角度出发,基于反映粘弹性效应对湍流相干结构影响的思想,创新性地建立了耦合空间过滤和时间过滤的新型亚格子模型,命名为MCT (Mixed subgrid-scale model based on Coherent structures and Temporal approximate deconvolution)。选用粘弹性流体强迫各向同性湍流和槽道湍流作为验证对象,将低雷诺数(Re)下两种湍流流动的LES结果与对应的DNS结果进行比较,同时将较高Re下的LES槽道湍流结果与实验结果进行对比,结果表明本文所提出的MCT亚格子模型能够预测无壁面效应和有壁面效应的粘弹性流体湍流减阻流动,达到了本文的预期目的。在采用MCT和TADM亚格子模型数值模拟较高Re下的粘弹性流体强迫各向同性湍流时,发现TADM亚格子模型表现出过度耗散,而MCT亚格子模型并无此现象,表明MCT亚格子模型在计算较高Re下的湍流减阻流动方面具有优越性,这为LES研究高Re下粘弹性流体湍流减阻流动奠定了基础。采用MCT亚格子模型建立了较高Re下的粘弹性流体强迫各向同性湍流的LES数据库,并分析了其流动特性。结果表明粘弹性的存在明显改变了湍流的流动特性,涡量和拟涡能明显减小,小尺度涡结构数目明显减少,随着粘弹性效应的增强减阻率增大且粘弹性流体分子拉伸更为剧烈。同时,采用一维和二维小波变换进一步探究粘弹性效应对湍流多尺度特性的影响,发现湍流流动中的间歇性随着尺度的减小而增强,在粘弹性流体中不仅整个流动区域的涡结构以及间歇性受到抑制,而且局部涡结构对间歇性的贡献减弱。采用MCT亚格子模型对较高Re下的粘弹性流体槽道湍流进行数值模拟,建立了较高Re下的LES数据库,并结合槽道湍流实验数据库分析其流动特性及减阻机理。结果表明在粘弹性流体中平均速度场、湍流脉动强度、雷诺剪切应力、低速条带结构等均发生变化,且粘弹性流体中的过渡层有向主流区扩展的趋势。根据湍流流动阻力贡献、湍动能贡献、基于小波变换的标度律以及间歇性的结果,很好地解释了粘弹性流体湍流减阻机理。从整体角度出发,粘弹性流体中湍流贡献的减小程度比引入的粘弹性贡献大,从而使湍流流动的摩擦系数减小,进而产生减阻效果；从粘弹性效应对壁面法向不同区域的影响角度出发,粘弹性效应主要抑制了过渡层中从线性底层由湍流猝发事件引起的上抛运动而来的相干结构。
[Abstract]:A small amount of flexible long chain polymer or some surfactants can be added to the water or organic solvent to reduce the friction resistance in the turbulent flow. This phenomenon is called the additive turbulent drag reduction effect. In order to make this phenomenon better applied in the actual industrial system, it is urgent to study the flow characteristics and the mechanism of drag reduction. Because the experimental method can not obtain the information of the molecular deformation field and the elastic stress field in the viscoelastic fluid turbulent drag reduction flow, the numerical simulation method is mostly used. The large eddy numerical simulation (LES) can obtain more information than the Reynolds mean numerical simulation (RANS) because its calculation is less than the direct numerical simulation (DNS) under the same working condition. When LES is used to study specific turbulent flow, the key lies in whether the selected subgrid model is suitable for this kind of turbulent flow. Almost all published subgrid models are only for Newton fluid flow, and the viscous elastic fluid turbulent drag reduction flow LES is just starting. At present, only the application time is approximate. The product model (TADM) is used to simulate the turbulent drag flow in viscoelastic fluid. In order to better use LES to study the turbulent drag reduction flow in viscoelastic fluid, more reliable and effective subgrid models are needed. This paper will build a viscoelastic fluid turbulence drag reduction mechanism and build a viscoelastic fluid turbulence drag reduction mechanism from this angle. The new subgrid model of flow has important theoretical significance and academic value for the LES study of viscoelastic fluid turbulence drag reduction flow. From the angle of the mechanism of viscoelastic fluid turbulence drag reduction, based on the thought of the influence of viscoelastic effect on the coherent structure of turbulence, a new type of coupling space filtering and time filtering is innovatively established. The subgrid model, named MCT (Mixed subgrid-scale model based on Coherent structures and Temporal approximate deconvolution). Using the viscoelastic fluid forced isotropic turbulence and channel turbulence as the verification object, the results of the two turbulent flow under the low Reynolds number are compared with the corresponding results. The results of LES channel turbulence under higher Re are compared with the experimental results. The results show that the proposed MCT subgrid model can predict the viscoelastic fluid turbulent drag reduction flow without wall surface effect and wall effect. The viscoelastic fluid under higher Re is simulated with MCT and TADM sublattice model. In forcing isotropic turbulence, it is found that the TADM subgrid model shows excessive dissipation, while the MCT subgrid model does not have this phenomenon. It shows that the MCT subgrid model has advantages in calculating the turbulent drag reduction flow under higher Re, which lays the foundation for the LES study of the viscous elastic fluid turbulent drag reduction flow under high Re. The construction of the MCT subgrid model is built. The LES database of viscoelastic fluid forced isotropic turbulence under high Re is established and its flow characteristics are analyzed. The results show that the existence of viscoelasticity obviously changes the flow characteristics of turbulence, the vorticity and quasi vorticity energy decrease obviously, the number of small scale vortex structure decreases obviously, and the viscoelastic flow increases with the increase of the viscoelastic effect and the viscoelastic flow is increased. At the same time, the effect of viscoelastic effect on the multiscale characteristics of turbulence is further investigated by using one and two dimensional wavelet transform. It is found that the intermittency in the turbulent flow is enhanced with the decrease of the scale. In the viscoelastic fluid, the vortex structure and the intermittency of the whole flow area are suppressed, and the local vorticity is also found. The MCT subgrid model is used to simulate the turbulent flow in the viscoelastic fluid channel under high Re, and the LES database under the higher Re is established. The flow characteristics and the drag reduction mechanism are analyzed with the experimental database of the channel turbulence. The results show that the mean velocity field, the turbulence pulsation intensity, and the thunder in the viscoelastic fluid The shear stress, the low velocity strip structure and so on all change, and the transition layer in the viscoelastic fluid has the tendency to expand to the main stream. According to the contribution of the turbulent flow resistance, the contribution of the turbulent kinetic energy, the scaling law of the wavelet transform and the intermittent results, the drag reduction mechanism of the viscoelastic fluid turbulence is well explained. From the whole point of view, the viscoelasticity of the viscoelastic fluid is explained. The reduction of turbulence contribution in the fluid is larger than the viscoelastic contribution introduced, which reduces the friction coefficient of the turbulent flow and then produces the effect of drag reduction. From the angle of the effect of the viscoelastic effect to the different regions of the wall, the viscoelastic effect mainly inhibits the upper layer of the transition layer caused by the turbulent burst event in the linear layer. A coherent structure that throws motion.

【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ021.1

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