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

我们研究了DNA稀释液显著减阻的机理。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。我们首先确定了产生湍流能量级联的涡旋结构。研究表明，在牛顿流体中，被拉伸的螺旋涡（Lundgren 1982， LSV）是产生湍流的主要原因。阐明了lsvv的发生、生长和湮灭。我们发现，除了先前研究的关于沿两个螺旋片和核心区域涡管的贪婪度排列的两种对称构型模式外，还提取了第三种不对称模式。螺旋片的螺旋转动由于差速旋转而收紧，从而产生较小尺度的速度波动级联。基于我们的推测，DNA稀释液中显著的阻力减少是由DNA对其弯曲的刚度引起的，我们考虑了非亲和性对聚合物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 and Segalman 1977）。当非仿射效应最大时，湍流产生的减少确实最大。其次，我们研究了湍流中能谱的性质。对构成惯性子范围内基态的Kolmogorov -5/3能谱进行修正，得到由耗散率ε波动引起的非平衡态-7/3能谱（Yoshizawa 1994）。我们将ε随时间变化的平均方法应用于能谱系综。结果表明-7/3谱确实作为谱的一个隐藏元素存在。整个级联过程分为两个阶段，阶段1和阶段2。在相位1中，与-5/3频谱的偏差，即-7/3功率分量，在低波数范围内为正，在高波数范围内为负。随着-7/3功率分量符号的切换，相位1低波数范围内的能量转移到相位2高波数范围。虽然-7/3谱只是短暂出现的，当对整个能谱集合进行平均时，它被消除了，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层次谱的影响进行了评估。结果表明，粘弹性的引入抑制了第1相向第2相转变的发生。随后，能量级联到更高的波数范围被减少。当非亲和性最大时，这种抑制作用最强。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