Dong Myeong Shin

Transient and steady-state solutions of 2D viscoelastic nonisothermal simulation model of film casting process via finite element method

The various aspects of the nonlinear dynamics and stability of nonisothermal film casting process have been investigated solving a two-dimensional (2D) viscoelastic simulation model equipped with the Phan-Thien-Tanner (PTT) constitutive equation by employing a finite element method. This study represents an extension of the earlier report [Kim, Lee, Shin, Jung, and Hyun, J. Non-Newtonian Fluid Mech. 132, 53–60 (2005)] in that two important points are additionally addressed here on the subject: the nonisothermal nature of the film casting, and the differentiation of extension-thickening (strain hardening) and extension-thinning (strain softening) fluids in their different behavior in the film casting process. The PTT model, known for its robustness in portraying dynamics in the extensional deformation processes which include the film casting of this study along with film blowing and fiber spinning as well, renders the transient and steady state solutions of the dynamics in the 2D, viscoelastic, nonisothermal, film casting capable of explaining the effects of various process and material parameters of the system on the film dynamics of the process. Especially, the different behavior displayed by two polymer groups, i.e., the extension-thickening low density polyethylene (LDPE) type and the extension-thinning high density polyethylene (HDPE) type, in the film casting can be readily explained by the PTT equation-included simulation model. The three nonlinear phenomena commonly observed in film casting, i.e., draw resonance oscillation, edge bead, and neck-in, have been successfully delineated in this study using the simulation and experimental results.The various aspects of the nonlinear dynamics and stability of nonisothermal film casting process have been investigated solving a two-dimensional (2D) viscoelastic simulation model equipped with the Phan-Thien-Tanner (PTT) constitutive equation by employing a finite element method. This study represents an extension of the earlier report [Kim, Lee, Shin, Jung, and Hyun, J. Non-Newtonian Fluid Mech. 132, 53–60 (2005)] in that two important points are additionally addressed here on the subject: the nonisothermal nature of the film casting, and the differentiation of extension-thickening (strain hardening) and extension-thinning (strain softening) fluids in their different behavior in the film casting process. The PTT model, known for its robustness in portraying dynamics in the extensional deformation processes which include the film casting of this study along with film blowing and fiber spinning as well, renders the transient and steady state solutions of the dynamics in the 2D, viscoelastic, nonisotherm...

Multiplicity, bifurcation, stability and hysteresis in dynamic solutions of film blowing process

The complicated nonlinear dynamics in film blowing process has been investigated focusing on the multiplicity, bifurcation, and stability in the dynamic solutions of the system using both the theoretical model simulation and experiments. A number of interesting findings have been revealed about the dynamics of the process including a fundamentally different behavior of the system with a maximum of three steady states for the nonisothermal operations in contrast to the isothermal approximation where only two steady states were predicted. These differences have been identified as stemming from the fact that multiple values of a bubble radius at the freezeline height can give the same value of the air pressure inside the bubble depending upon the process conditions and the values of the bifurcation parameters of the system. The stability of the three steady states also displays many different patterns dictated by the process conditions, including the hysteresis in the bifurcation diagrams. These stability re...

Transient Solutions of Nonlinear Dynamics in Film Blowing Accompanied by Flow‐induced Crystallization

Using the newly developed real space vortex-lattice based theory of superconductivity, we study the maximum superconducting transition temperature (T c ) in the iron-based superconductors. We argue that the c-axis lattice constant plays a key role in raising the T c of the superconductors. It is found that all the reported FeAs superconductors can be divided into two basic classes (c/a ≈ 3 and c/a ≈ 5/2) depending on the lattice constants, where a is the Fe-Fe distance in the xy-plane and c is the Fe-Fe layer distance along the z-axis. Our results suggest that the former class has a maximum T c < 60 K, while the latter class has a lower T c ≤ 40 K. Our investigations further indicate that, in order to enhance T c in this family of compounds, new class of superconductors with a larger ratio of c/a should be synthesized. It is likely that their T c values could be raised into the liquid nitrogen range (77 K) and 100 K, supposing the new analogues with c/a ≈ 5 (approximately c > 13 Å, if a = 2.750 Å) and c/a ≈ 11 (c > 31 Å) can be experimentally achieved, respectively. For the new FeSe series, our mechanism predicts that their T c is impossible to exceed 30 K due to a relatively shorter c-axis lattice constant (c/a ≈ 2). Finally, based on the new experimental results (arXiv:0811.0094 and arXiv:0811.2205), the possible ways to raise the Tc of the iron-based superconductors into 70 K are also suggested.