Abstract: The evolution of coherent structures in a 2-D time-developing mixing layer of the Oldroyd-B fluids are examined numerically. The results indicate that adding polymer to the Newtonian fluids will cause stronger vorticity diffusion, accompanied with weaker fundamental and subharmonical perturbations,less intense vortex roll-up and pairing and slower rotational motion of pairing vortices. When Weissenberg number is up to 10, the vorticity gradients are enhanced and even the vorticity with opposite sign to the mean vorticity appears at the region where there is a build - up of polymer first normal stress difference. The vorticity with opposite sign results in the absence of the usual quasi - steady state of the primary vortex, and disappears due to the collapse of polymer normal stress difference if there exist pairing modes. The time when the consecutive rollers are completely coalesced into one is delayed in the viscoelatic mixing layer compared to the Newtonian one of the same total viscosity.

摘要： <正>混合层是一种在化工过程中经常见到的基本流场,对该流场拟序结构的研究具有理论和实际意义.自70年代以来,混合层中的拟序结构得到了深入和广泛的研究,然而对于加入聚合物而产生了粘弹性的混合层流场,国内外的研究工作还不多.实验方面,Hibberd发现聚合物的加入使流场中小尺度结构减少,拟序结构更加明显;Riediger发现拟序结构具有更长的生存期,动量厚度的增长受到抑制.理论方面,最新的进展是Azaiez和Homsy(简称AH)用Oldroyd-B模型求解了流场,但是在求解时,发现在Weissenberg数为5时数值解发散,并用有限拉伸率下聚合物法向应力会趋于无穷来解释这一现象.因而对流场没作仔细的研究.但事实上,数值解的发散并不表示此时应力已出现奇点,而是由于应力迅速增长的性质导致数值计算出现不稳定,正如计算激波时还没有真正到达间断,数值解就会由于振荡而发散.因此,可以采用适当的滤波措施来稳定计算而得到合理的近似解.