A Study on the Marked turbulent drag reduction mechanism in DNA-diluted Fluid

对DNA稀释液的显著减阻机理进行了研究。我们处理了均匀各向同性湍流和均匀剪切湍流，并利用了直接数值模拟方法产生的数值数据。我们首先对产生湍流能量级联的涡旋结构进行了识别。结果表明，伸展螺旋涡旋(Lundgren 1982，LSV)是牛顿流体中湍流产生的主要原因。阐明了LSV的发生、生长和灭绝。结果表明，除了前人研究的两种对称位形模式外，我们还提取了第三种非对称位形模式。通过差动旋转使螺旋片的螺旋旋转变得更紧，从而产生了更小尺度的速度起伏级联。基于o…进一步推测，DNA稀释液的显著减阻是由于DNA抗弯曲的刚性所致，我们考虑了非亲和力对高分子链对宏观应变的响应的影响。非仿射效应对聚合物应力的近似使用Johnson-Segalman本构方程(Johnson和Segalman 1977)。当非仿射效应最大时，湍流的产生确实达到了最大的减少。接下来，我们研究了湍流中的能谱特性。对构成惯性子区基态的柯尔莫戈罗夫-5/3能谱的修正，得到了由耗散率ε的涨落引起的代表非平衡态的-7/3能谱(Yoshizawa，1994)。我们将以ε的时间变化为条件的平均化应用于能谱系综中。结果表明，-7/3谱确实是作为谱中的一个隐藏元素而存在的。整个级联过程分为两个阶段，阶段1和阶段2。在阶段1中，与-5/3谱的偏差，即-7/3功率分量，在低和高波数范围内分别为正和负。随着-7/3功率分量符号的切换，包含在第一相低波数范围内的能量转移到第二相的高波数范围。当对整个能谱系综取平均值时，虽然-7/3谱只是暂时出现并被消除，但它起到了催化作用，启动了级联的产生。我们对粘弹性对这一分级谱的影响进行了评估。结果表明，粘弹性的引入抑制了第一相向第二相转变的发生，从而减少了向高波数范围的能量级联。当非亲和力最大时，这种抑制作用最强。较少

We studied on the mechanism of the remarkable drag reduction in the fluid which is diluted by DNA. We dealt with the homogeneous isotropic turbulence and the homogeneous shear turbulence, and utilized the numerical data which are generated by the direct numerical simulation method. We began with the identification of the vortical structure which is responsible for generation of turbulence energy cascade. It is shown that the stretched spiral vortex (Lundgren 1982, LSV) is primarily responsible for turbulence generation in the Newtonian fluid. The genesis, growth and annihilation of LSV are elucidated. We showed that aside from the two symmetric modes of configurations studied in previous works with regard to the voracity alignment along two spiral sheets and the vortex tube in the core region, a third asymmetric mode is extracted. The tightening of the spiral turns of the spiral sheets by the differential rotation produces a cascade of velocity fluctuations to smaller scale. Based on o … More ur conjecture that the remarkable drag reduction in the DNA diluted fluid is caused by the stiffness of DNA against its bending, we took into account of the effect of non-affinity into the response of the polymer chain against the macroscopic straining. The non-affine effect on the polymer stress was approximated using the Johnson-Segalman constitutive equation (Johnson and Segalman 1977). The largest reduction of turbulence generation was indeed achieved when the non-affine effect was largest. Next, we investigated on the properties of the energy spectrum in turbulent flows. The corrections to the Kolmogorov -5/3 energy spectrum which constitutes a base state in the inertial subrange yields the -7/3 spectrum which is induced by the fluctuation of the dissipation rate ε and represents a nonequilibrium state (Yoshizawa 1994). We applied the averaging conditioned on the temporal variations of ε to the ensemble of the energy spectra. It is shown that the -7/3 spectrum indeed resides as a hidden element of the spectrum. The entire cascade process is divided into the two phases, Phases 1 and 2. In Phase 1, the deviations from the -5/3 spectrum, i.e., the -7/3 power component, are positive and negative in the low- and high-wavenumber ranges, respectively. The energy contained in the low-wavenumber range in Phase 1 is transferred to the high-wavenumber range in Phase 2 with the occurrence of the switchover of the sign of the -7/3 power component. Although the -7/3 spectrum appears only transiently and is eliminated when averaging is applied to the whole ensemble of the energy spectra, it acts as a catalysis to initiate the generation of the cascade. We carried out the assessment on the effect of viscoelasticity on this hierarchical spectrum. It is shown that introduction of the viscoelasticity tends to suppress the occurrence of transition from Phase 1 to Phase 2. Subsequently, the energy cascade to the higher wave-number range is reduced. This suppression was strongest when the non-affinity was largest. Less