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On simulations of spinning processes with a stationary onedimensional upper convected Maxwell model
 Maike Lorenz^{1},
 Nicole Marheineke^{1}Email author and
 Raimund Wegener^{2}
https://doi.org/10.1186/2190598342
© Lorenz et al.; licensee Springer. 2014
 Received: 13 April 2013
 Accepted: 7 March 2014
 Published: 3 June 2014
Abstract
This work deals with the behavior of viscoelastic jets under gravitational forces described by an asymptotic upper convected Maxwell (UCM) model, system of partial differential equations. Considering fiber spinning, we show that the onedimensional model equations in general allow for the simulation of drawing processes with and without die swell effect. But, as the model is of hyperbolic type and the run of the characteristics crucially depend on the physical parameters, the existence regimes of the stationary solutions associated to certain boundary conditions turn out to be limited. We investigate the regimes for gravitational uniaxial and 2d spinning scenarios numerically.
Keywords
 nonlinear viscoelasticity
 UCM fluid
 fiber spinning
 die swell
 boundary value problem
 existence of solutions
1 Introduction
Fiber manufacturing is of special importance in communication, textile, automotive and building industry. Typical products are glass fibers, filters, insulating material, diapers or textile clothing. There are a variety of production processes, such a drawing with a takeup wheel, spunbond, meltblowing, rotational or electrospinning, [1–5]. In general, a fluid (polymeric liquid or glass melt) is emerged from a orifice, pulled and stretched by outer forces to form a long thin fiber. The fiber jets can be considered as dynamic slender bodies. Topic of this work are viscoelastic jets. When spun, they might show a die swell effect, i.e. the fluid flow swells after exiting the nozzle and forms out a diameter that is significantly larger than the nozzle diameter. For small extrusion velocities the onionshape rises directly at the nozzle, whereas for large velocities the swelling may happen away from the nozzle (delayed die swell), see [6, 7]. In industrial processes the forming of a die swell is undesirable since it changes the flow properties of the nonNewtonian fluid and consequently the quality of the resulting fabric. Hence, the understanding and prediction of this phenomenon is of special interest.
There exist a lot of viscoelastic models differing in the constitutive equation (e.g. Maxwell, Jeffrey, OldroyedB, Giesekus, PhanTien and Tanner models). We focus here on the (nonlinear) upper convected Maxwell (UCM) model. It is form invariant under translation and rotation, and obeys the principle of material frame indifference (i.e. the observed phenomena are independent of the inertial frame in which they are observed). Moreover, it is suitable for large deformation gradients. Additionally, and in contrast to more complex models such as the Giesekus or PhanThien and Tanner models, it is always evolutionary, i.e. stable with respect to short wave length perturbations. Limitations of the UCM model include the lack of multiple relaxation time scales as well as unbounded stress growth for extensional flow [8, 9]. In dimensionless form this single relaxation time occurs in the Weissenberg number We that is the product of a typical velocity gradient and a characteristic relaxation time.
Simulations of a Maxwell fluid as threedimensional free boundary value problem include the die swell effect [10, 11], but fail for large Weissenberg numbers, see large Welimit in [12]. Because of the slender jet geometry, these simulations require a high refinement and are computationally very expensive. Hence, asymptotic models have been derived, see e.g. [13–15]. The aim of this paper is the investigation of the onedimensional UCM model in [16] describing a dynamic curved jet by a timedependent arclength parameterized curve. We explore the applicability (solution regime) and the properties of the model, in particular whether it allows for a die swell.
As for the structure of this paper, we start with a brief introduction of the onedimensional UCM model [16], followed by a classification in Section 2. The characteristic type of the partial differential system turns out to be determined by two classifying functions that limit the existence regime of stationary solutions, Section 3. In Sections 4 and 5 we investigate the applicability of the UCM model to stationary gravitational uniaxial and 2d spinning scenarios, respectively. We present numerical simulations of the jet behavior for different parameter ranges.
2 Asymptotic UCM model and its classification
for time $t\in {\mathbb{R}}^{+}$ and arclength parameter $s\in (0,L(t))$ with jet length L. The balance, constitutive and coupling equations describe the dynamics and behavior of a viscoelastic jet with crosssection A, momentumassociated velocity v, pressure p, stress component σ and jet curve γ. The intrinsic (convective) speed u is the Lagrange multiplier to the arclength constraint posed on γ. The model is characterized by the dimensionless parameters: Reynolds Re and Weissenberg We numbers that denote the ratio of the inertial and viscous forces and the ratio of the relaxation and process time, respectively. By alternating the outer forces f and the boundary conditions, the system (1) is applicable to various scenarios, e.g. growing jet with free end, jet spinning with takeup wheel, inflowoutflow problem of fixed length. But the setting up of a wellposed problem is often difficult, as it is determined crucially by the closure conditions and the considered parameter regime as we will discuss.
In this work we will refer to ${q}_{1}$ and ${q}_{2}$ as classifying functions.
The possible existence of different hyperbolic regimes depending on the physical parameters, such as Re, We and Fr, requires the careful consideration of the posed boundary conditions in view of the consistency and wellposedness of the problem (run of characteristics). This is also true in the viscous limit $We=0$. In many spinning processes the longtime behavior of a jet becomes stationary. Moreover, the behavior close to the spinning nozzle is dominated by stationary effects. Hence, in the viscous case the issue of existing solutions was investigated comprehensively for a stationary jet [21–25]. The stationary viscoelastic string model is now topic in this work.
3 Transition to stationarity
with $\mathrm{B}=Re/{Fr}^{2}$ in the gravitational setup.
holds true because of the jet dynamics due to the acting forces. The monotonicity allowed the analytical determination of limiting hyperplanes and the numerical exploration of the different solution regimes. For $We\ne 0$, the monotonicity of ${q}_{1}$ is kept. But the term ${q}_{2}$ leads to additional limitations which are even more difficult to predict since ${q}_{2}$ shows neither monotonicity nor other characteristic properties. In the uniaxial case (8) the term ${q}_{1}$ is not present such that we can exclusively focus on ${q}_{2}$.
In the following we study the applicability of the stationary asymptotic UCM model to the simulation of gravitational spinning processes. Thereby, we consider the uniaxial and the 2d setups.
Remark 1 For the numerical solution of the arising boundary value problems of ordinary differential equations we use a RungeKutta collocation method. The resulting systems of nonlinear equations are solved via Newton’s method.
4 Uniaxial spinning
4.1 Drawing processes and die swell
The system is supplemented by three boundary conditions that we specify later on.
Definition 1 Let $(u,b,a)$ be a continuously differentiable solution of the system (9) for arbitrary but fixed boundary conditions. We call ${s}^{\ast}\in (0,1)$ a point where a die swell occurs if ${\partial}_{s}u({s}^{\ast})=0$ and $u({s}^{\ast})$ is a local minimum.
Among all possible solutions we are only interested in drawing processes as defined below which we consider to be the physically relevant solutions in this scenario.
Definition 2 (Drawing process)
We call the solution of the system (9) a drawing process

without die swell if ${\partial}_{s}u(s)>0$ for all $s\in [0,1)$,

with die swell if there exists exactly one ${s}^{\ast}\in (0,1)$ with ${\partial}_{s}u({s}^{\ast})=0$, ${\partial}_{s}u(s)<0$ for all $0\le s<{s}^{\ast}$ and ${\partial}_{s}u(s)>0$ for all ${s}^{\ast}<s\le 1$.
With the following lemmata we can exclude the occurrence of a die swell for the boundary condition $b(1)=0$ that corresponds to a constant velocity end ${\partial}_{s}u(1)=0$ (e.g. via a takeup wheel).
Lemma 1 Let $(Re,We,Fr)$ be given with $\mathrm{B}\ne 0$ and suppose that $(u,b,a)$ is a continuously differentiable solution of the system (9) for arbitrary but fixed boundary conditions. Suppose that $a\ne 0$ for all $s\in [0,1]$. Then b can have at most one root on $[0,1]$.
Proof For any root ${s}^{\ast}$ in b we find that ${\partial}_{s}b({s}^{\ast})=\mathrm{B}$. By continuity of b only one root can occur. □
Lemma 2 Let $(Re,We,Fr)$ and some $D\in \mathbb{R}\setminus \{0\}$ be given such that $a(0)=D$ or $a(1)=D$. Suppose that a continuously differentiable solution of the system (9) exists with $u(0)=1$, $b(1)=0$ and $a\ne 0$ for all $s\in [0,1]$. Then this cannot be a drawing process with die swell. Furthermore, this is only a drawing process if $D>0$.
Proof For a drawing process with die swell a root of ${\partial}_{s}u$ is required on $(0,1)$. This corresponds to a root in b at some ${s}^{\ast}\in (0,1)$. Due to the boundary condition and Lemma 1 this is not possible.
If $D<0$, also $a<0$ holds for all $s\in [0,1]$ and hence $b(1)=0$ enforces a minimum in u such that the velocity is monotonically decreasing on $[0,1]$ which is not a drawing process. □
Similarly, we can derive necessary conditions for a drawing process with die swell, considering the boundary condition $b(1)=We\mathrm{B}u(1)$ that corresponds to a stressfree end $\sigma (1)=0$.
Lemma 3 Let $(Re,We,Fr)$ and some $D\in \mathbb{R}\setminus \{0\}$ be given such that $a(0)=D$ or $a(1)=D$. Suppose that a continuously differentiable solution of the system (9) exists with $u(0)=1$, $b(1)=We\mathrm{B}u(1)$ and $a\ne 0$ for all $s\in [0,1]$. Then this can be a drawing process with die swell. Furthermore, this is only a drawing process if $D<0$.
Proof Since the sign of D determines the sign of a we distinguish two cases:
With regard to our investigations and the timedependent classification in Section 2, uniaxial drawing processes (DP) admit different sets of reasonable boundary conditions. First of all, the inflow velocity at the nozzle is prescribed by $u(0)=1$ due to the applied dimensionless scaling. Then, we can distinguish between four categories.
 (A): no die swell,$b(1)\ge 0\wedge a(0)>0$
 (B): no die swell,$b(0)<0\wedge a(0)<0$
 (C): DP without die swell if and only if $b(1)>0$,$b(0)>0\wedge a(1)>0$
 (D): DP with die swell if and only if $b(1)<0$.$b(0)>0\wedge a(0)<0$
In Category (A) the practically relevant drawing processes with constant velocity end (e.g. driven by a takeup wheel) are contained. Drawing processes with die swell are described by Category (D) if and only if $b(1)<0$. This implies a stressfree end.
4.2 Existence regime and jet dynamics
For the application, drawing processes of Categories (A) and (D) are of main interest. Therefore, we restrict here to the investigation of their solutions regimes. For Categories (B) and (C) see [26].
Category (A)
The parameter regime $(Re,We,Fr)$ where solutions exist essentially depend on the run of the nonmonotone function a (or respectively ${q}_{2}$). A root that characterizes a changeoftype of the corresponding timedependent equations limits the regime. Although the classifying function is not monotone a systematic determination of the existence regime in the highdimensional parameter space is possible because $a(1)\to 0$ for increasing Re. This substantiates our approach that follows the ideas of [22]. The aim is to find those parameters $(Re,We,Fr)$ for which a solution of the boundary value problem of Category (A) exists with $a(1)=0$. For the numerical treatment we impose the additional condition $a(1)=\delta $, $0<\delta \ll 1$ in favor of We.
Category (D)
For Category (D) we deal with initial value problems, whose classification of solutions is made by the sign of $b(1)$. The hyperplanes separating the drawing processes from the physically irrelevant solutions can be determined in the same spirit as above. For given Fr and Re find We such that a solution of the boundary value problem of Category (D) with $b(1)=0$ exists. Certainly, also the roles of We and Re can be interchanged, then the condition $a(1)=\delta $, $\delta \to 0$ need to be imposed instead.
5 Gravitational 2d spinning
In the gravitational 2d spinning setup the fluid is extruded from the nozzle perpendicularly to the gravitational direction. In the absence of further outer forces the jet moves in a plane (e.g. ${\mathbf{e}}_{\mathbf{1}}$${\mathbf{e}}_{\mathbf{2}}$plane) and its tangent τ can be described by a single angle α. Without loss of generality we consider $\mathbf{f}=\mathrm{B}{\mathbf{e}}_{\mathbf{2}}$ and $\mathit{\tau}(\alpha )=cos\alpha {\mathbf{e}}_{\mathbf{1}}+sin\alpha {\mathbf{e}}_{\mathbf{2}}$ with $\alpha (0)=0$ in Equations (7). At the nozzle $s=0$, the jet position and velocity are prescribed, i.e. $\mathit{\gamma}(0)={\mathit{\gamma}}_{\mathbf{0}}$ and $u(0)=1$. Moreover, we impose $\sigma (1)+We\mathrm{B}u(1)sin\alpha (1)=0$ for a end with constant velocity (${\partial}_{s}u(1)=0$), similarly as in [22] for the viscous jet. The last boundary condition we choose is ${q}_{2}(0)=D$, $D\ne 0$.
5.1 Existence regime
The gravitational 2d UCM model is similar to the gravitational 2d viscous model [22] with respect to ${q}_{1}$ (monotonically increasing) and similar to the uniaxial UCM model of Category (A) with respect to ${q}_{2}$ (${q}_{2}(1)\to 0$). Hence the solution regime can be systematically investigated by imposing these two additional conditions ${q}_{1}(0)=0$ and ${q}_{2}(1)=0$ instead of two dimensional parameters. Numerically, we use ${q}_{1}(0)={q}_{2}(1)=\delta $, $\delta \ll 1$ for the search, and apply a continuationcollocation strategy to keep track of grid points and function evaluations used. This procedure that was developed for the viscous case is faster and more robust than direct calculations of the next solution (see [22] for details).
5.2 Influence of parameters on jet behavior
The jet behavior (in terms of curve γ, angle α, velocity u, pressure p and stress component σ) is influenced by the parameters Re, We and Fr. In the following, we consider a jet with $D=1$, other boundary values $D>0$ yield similar results. We have a drawing process with monotonically increasing velocity, no die swell forms out in agreement with the uniaxial investigations for Category (A).
Small Re imply a almost constant velocity. For increasing Re, the maximal velocity $u(1)$ grows larger. Also the stress profile rises, but its maximum is attained inside the interval $[0,1]$. The pressure tends to zero. As the Froude number is inversely proportional to gravity, the jet is more curved and accelerated for smaller Fr, implying higher velocity and stress. The effect is much more significant than for increasing Re. In contrast to the influence of the Reynolds number, the pressure attains lower values for lower Fr. The influence of We is comparable to the one of Re. For $We\to 0$ we observe the rising of a boundary layer in the pressure as ${q}_{2}(0)=D=1$ is chosen instead of the viscous limit ${q}_{2}=3$. For the remaining variables the viscous solution and the UCM solution for $We=0.1$ coincide well.
6 Conclusion and outlook
The onedimensional upper convected Maxwell model [16] allows for the simulation of viscoelastic fiber spinning, in particular die swell effects can be reproduced. The stationary model for an uniaxial straight jet driven by gravity admits four different sets of boundary conditions: the occurrence of a die swell is analytically excluded for a constant velocity end, whereas it may arise for a stress free end. It remains an open question whether respective boundary conditions for a die swell can be motivated physically or whether further effects need to be considered in order to achieve numerical predictions that are comparable with experimental data.
The stationary asymptotic UCM model is a string model whose applicability turns out to be restricted to certain parameter ranges because of occurring singularities (singularly perturbed model). In contrast to the viscous case, this holds already true for the uniaxial UCM string. For other spinning setups the investigation of the solution regime requires the determination of the roots of  not only one, but  two classifying functions in a highdimensional parameter space. A respective numerical search strategy is proposed and applied to the gravitational uniaxial and 2d spinning scenarios in this work. Its extension to industrial spinning processes is difficult and not productive when aiming for the simulation of the whole parameter regime. Instead of this, alternative asymptotic models that overcome the limitation should be asked for.
Declarations
Acknowledgements
This work has been supported by the Center for Mathematical and Computational Modeling ${(\mathrm{CM})}^{2}$, Kaiserslautern (RhinelandPalatinate research initiative).
Authors’ Affiliations
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