Figure 93. (a) Probability density functions of the drop size d, normalized by the arithmetic mean d for a number of jet velocities uj and for several conditions of forcing • as well as without forcing ◦. The solid curve represents the Gamma distribution with n = 5. (b) Semi-logarithmic representation of the size distributions shown in (a); for details see [278]. Figure 94. Two liquid jets, impacting at an angle 2θ, at small Weber number (We = 5). Figure 96 indeed shows that when two nearly (but not strictly) smooth jets merge, they give rise to drop distributions essentially independent of the merging angle, and well fitted by Gamma distributions with an order (n = 70), a large figure Figure 95. (a) Schematic of jet merging, and surface roughness generation. Corrugations expanded in Fourier modes: ξ = k
k cos(kx). (b) Dependence of the average drop size from the destabilization of the resulting jet on the impact angle. Solid line is d /d1 = √2/ cos θ. characteristic of ‘nearly smooth jets’, but somewhat smaller than the expected one. This latter difference is an indication of the residual fluctuations in the incoming jets (see figure 96(a) and (b)). According to this mechanism, the stronger the noise, the broader the distribution. Note, however, a recent observation that reports an opposite trend [299]. 6. Non-Newtonian effects The universal structure of pinch-off solutions owes everything to the fact that the Navier–Stokes equations retain the same structure, independent of the fluid. An important assumption underlying this statement is that the time scale of all relaxation processes is short compared with the time scale of the flow. Near pinch-off, in particular, this assumption is bound to be violated, since the time scale of the flow goes to zero. The most common case is that the fluid contains molecules of high molecular weight, whose relaxation time towards their equilibrium state is no longer negligible. In addition, if the polymers are flexible, they are able to store energy, resulting in elastic behaviour. Indeed, flexible polymers have the most dramatic effects, as concentrations as low as 10 ppm can completely alter the character of jet breakup [521]. Other examples of non-Newtonian behaviour whose effect on jet breakup has been studied are shear-thinning liquids [522], yield-stress fluids [523], liquid crystals [524], superfluid helium [525] and sand [526]. Surfactants, on the other hand, alter the surface tension, which is driving much of jet dynamics [527]. 6.1. Flexible polymers Many liquids of biological and industrial importance contain very long, flexible polymers. Figure 97 shows a jet of 66
รูป 93 (ก) ความหนาแน่นความน่าเป็นฟังก์ชันของ d ขนาดหล่น ตามปกติ โดยคณิต d จำนวน uj ตะกอนเจ็ท และเงื่อนไขต่าง ๆ ของ•การบังคับ และ ไม่บังคับ◦ เส้นโค้งทึบแสดงการแจกแจงแกมมากับ n = 5 (ข) กึ่งลอการิทึมแสดงการกระจายของขนาดที่แสดงใน (a); สำหรับรายละเอียดดู [278] รูป 94 Jets เหลวสอง ถึงที่ 2θ เป็นมุม ที่ขนาดเล็กแบ่งแยกเลข (เรา = 5) 96 รูปได้แสดงว่า เมื่อสองเกือบ (แต่ไม่เคร่งครัดไป) เรียบ jets เวียน พวกเขาก่อให้เกิดการปล่อยกระจายเป็นอิสระผสานมุม และหรูดี โดยการกระจายแกมมากับใบสั่งแน่นอน (n = 70), ตัวเลขขนาดใหญ่ 95 รูป (ก) แผนผังวงจรรุ่น jet ผสาน และพื้นผิวความหยาบ Corrugations ขยายในโหมดฟูรีเย: ξ = k k cos(kx). (b) Dependence of the average drop size from the destabilization of the resulting jet on the impact angle. Solid line is d /d1 = √2/ cos θ. characteristic of ‘nearly smooth jets’, but somewhat smaller than the expected one. This latter difference is an indication of the residual fluctuations in the incoming jets (see figure 96(a) and (b)). According to this mechanism, the stronger the noise, the broader the distribution. Note, however, a recent observation that reports an opposite trend [299]. 6. Non-Newtonian effects The universal structure of pinch-off solutions owes everything to the fact that the Navier–Stokes equations retain the same structure, independent of the fluid. An important assumption underlying this statement is that the time scale of all relaxation processes is short compared with the time scale of the flow. Near pinch-off, in particular, this assumption is bound to be violated, since the time scale of the flow goes to zero. The most common case is that the fluid contains molecules of high molecular weight, whose relaxation time towards their equilibrium state is no longer negligible. In addition, if the polymers are flexible, they are able to store energy, resulting in elastic behaviour. Indeed, flexible polymers have the most dramatic effects, as concentrations as low as 10 ppm can completely alter the character of jet breakup [521]. Other examples of non-Newtonian behaviour whose effect on jet breakup has been studied are shear-thinning liquids [522], yield-stress fluids [523], liquid crystals [524], superfluid helium [525] and sand [526]. Surfactants, on the other hand, alter the surface tension, which is driving much of jet dynamics [527]. 6.1. Flexible polymers Many liquids of biological and industrial importance contain very long, flexible polymers. Figure 97 shows a jet of 66
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