Constitutive relation (
1) is a special case of the classical regularized Bingham model introduced by Papanastasiou in [
17]. We also notice that model (
1) can be included in the classification of incompressible fluids presented in [
18]. The sketch of the system is depicted in Fig.
1. The velocity field is of the form
\({\textbf{v}}^*=u^*{\textbf{e}}_{1}+v^*\textbf{e }_{2}+w^*{\textbf{e}}_{3}\). The governing equations are
$$\begin{aligned} {\left\{ \begin{array}{ll} \rho ^* {\displaystyle {\frac{D{{\textbf{v}}^*}}{Dt^*}}}=-\nabla p^*+\nabla \cdot {\textbf{S}}^*-\rho ^* g^*{\textbf{e}}_{z}, \\ \\ \nabla \cdot {\textbf{v}}^*=0, \end{array}\right. } \end{aligned}$$
(3)
where
\(\rho ^*\) is the constant density of the fluid and
\(g^*\) is the acceleration due to gravity. The upper plate, placed at
\(z^*=h^*\), rotates around the axis
\((0,a^*)\) with angular velocity
\(\Omega ^*\). The lower plate, placed at
\(z^*=-h^*\), rotates around
\((0,-a^*)\) with the same angular velocity. Assuming no-slip at the lower and upper plate we record the following boundary conditions
$$\begin{aligned} {\textbf{v}}^*\Big |_{z^*=-h^*}= & {} -\Omega ^*(a^*+y^*){\textbf{e}}_1+ \Omega ^*x^*{\textbf{e}}_2, \end{aligned}$$
(4)
$$\begin{aligned} {\textbf{v}}^*\Big |_{z^*=h^*}= & {} \Omega ^*(a^*-y^*){\textbf{e}}_1+ \Omega ^*x^*{\textbf{e}}_2. \end{aligned}$$
(5)
We rescale the system as follows
$$\begin{aligned} \begin{array}{l} x^*=a^*x,\quad y^*=a^*y,\quad z^*=h^* z,\quad t^*={\displaystyle {\frac{1}{ \Omega ^* }}}t,\quad u^*=\Omega ^* a^*u,\quad v^*=\Omega ^* a^*v, \\ \\ w^*=\Omega ^* h^*w,\quad p^*=\alpha _1^* p,\quad {\textbf{D}}^*=\Omega ^*{\textbf{D}}, \quad {\textbf{S}}^*=\alpha _1^*{\textbf{S}}, \quad {\dot{\gamma }}^*=\Omega ^*{\dot{\gamma }},\ \ \ \ \xi = \dfrac{h^*}{a^*}. \end{array} \end{aligned}$$
The non-dimensional constitutive equation becomes
$$\begin{aligned} {\textbf{S}}=\eta ({\dot{\gamma }}){\textbf{D}}, \ \ \ \ \ \ \ \ \ \ \eta ({\dot{\gamma }}) = \left[ \dfrac{1-e^{-\alpha {\dot{\gamma }}}}{{\dot{\gamma }}}\right] , \ \ \ \ \ \ \ \ \alpha = \alpha _2^*\Omega ^*, \end{aligned}$$
(6)
where
$$\begin{aligned} {\dot{\gamma }}= & {} \left\{ \dfrac{1}{2}\left[ \left( \dfrac{\partial u}{\partial x}\right) ^2+\left( \dfrac{\partial v}{\partial y}\right) ^2+\left( \dfrac{\partial w}{\partial z}\right) ^2\right] \right. \\{} & {} +\left. \dfrac{1}{4}\left[ \left( \dfrac{\partial u}{\partial y}+\dfrac{\partial u}{\partial y}\right) ^2+ \left( \dfrac{1}{\xi }\dfrac{\partial u}{\partial z}+\xi \dfrac{\partial w}{\partial x}\right) ^2+ \left( \dfrac{1}{\xi }\dfrac{\partial v}{\partial z}+\xi \dfrac{\partial w}{\partial y}\right) ^2\right] \right\} ^{1/2} \end{aligned}$$
The non-dimensional governing equations are
$$\begin{aligned} \left\{ \begin{array}{l} \dfrac{\partial u}{\partial x}+\dfrac{\partial v}{\partial y}+\dfrac{\partial w}{\partial z}=0,\\ \\ Re\dfrac{Du}{Dt} = -\dfrac{\partial p}{\partial x}+\dfrac{\partial }{\partial x}\big (\eta D_{11}\big )+\dfrac{\partial }{\partial y}\big (\eta D_{12}\big )+ \dfrac{1}{\xi }\dfrac{\partial }{\partial z}\big (\eta D_{13}\big ),\\ \\ Re\dfrac{Dv}{Dt} = -\dfrac{\partial p}{\partial y}+\dfrac{\partial }{\partial x}\big (\eta D_{12}\big )+\dfrac{\partial }{\partial y}\big (\eta D_{22}\big )+ \dfrac{1}{\xi }\dfrac{\partial }{\partial z}\big (\eta D_{23}\big ),\\ \\ \xi ^2 Re\dfrac{Dw}{Dt} = -\dfrac{\partial p}{\partial z}+\xi \dfrac{\partial }{\partial x}\big (\eta D_{13}\big )+\xi \dfrac{\partial }{\partial y}\big (\eta D_{23}\big )+ \dfrac{\partial }{\partial z}\big (\eta D_{33}\big )-\phi , \end{array} \right. \end{aligned}$$
(7)
where
$$\begin{aligned} Re = \left( \dfrac{\rho ^*\Omega ^{*^2}a^{*^2}}{\alpha _1^*}\right) , \ \ \ \ \ \ \ \ \xi =\dfrac{h^*}{a^*}, \ \ \ \ \ \ \ \phi = \left( \dfrac{\rho ^*g^*h^*}{\alpha _1^*}\right) , \end{aligned}$$
(8)
are the Reynolds number, the aspect ratio and the ratio between gravitational and viscous forces, respectively. The components of the traceless symmetric tensor
\({\textbf{D}}\) are
$$\begin{aligned} \begin{array}{l} D_{11}=\left( \dfrac{\partial u}{\partial x}\right) , \ \ \ \ \ \ \ D_{22}=\left( \dfrac{\partial v}{\partial y}\right) , \ \ \ \ \ \ \ D_{33}=\left( \dfrac{\partial w}{\partial z}\right) , \\ \\ D_{12}= \dfrac{1}{2}\left( \dfrac{\partial u}{\partial y}+\dfrac{\partial v}{\partial x}\right) , \ \ \ \ \ \ \ D_{13} = \dfrac{1}{2} \left( \dfrac{1}{\xi }\dfrac{\partial u}{\partial z}+\xi \dfrac{\partial w}{\partial x}\right) ,\\ \\ D_{23} = \dfrac{1}{2} \left( \dfrac{1}{\xi }\dfrac{\partial v}{\partial z}+\xi \dfrac{\partial w}{\partial y}\right) . \end{array} \end{aligned}$$
Rajagopal [
5] proved that the equations of motion for a simple fluid are compatible with the following ansatz for the velocity field:
$$\begin{aligned} u = -\big [y-g(z)\big ], \ \ \ \ \ \ \ \ \ \ \ \ v = \big [x-f(z)\big ], \ \ \ \ \ \ \ \ \ \ \ \ w = 0, \end{aligned}$$
(9)
where
f(
z) and
g(
z) are functions to be determined. It is straightforward to verify that the incompressibility constraint is automatically satisfied by (
9) and that
$$\begin{aligned} {\dot{\gamma }}= \dfrac{1}{2\xi }\sqrt{f^{\prime ^2}+g^{\prime ^2}}, \ \ \ \ \ \ ^{\prime }=\dfrac{d}{dz}. \end{aligned}$$
(10)
Because of (
9) the balance of linear momentum reduces to
$$\begin{aligned} \begin{array}{l} -Re \ v = -\dfrac{\partial p}{\partial x}+\dfrac{1}{\xi }\left[ \eta ({\dot{\gamma }})\dfrac{g^{\prime }}{2\xi }\right] ^{\prime },\\ \\ Re \ u = -\dfrac{\partial p}{\partial y}-\dfrac{1}{\xi }\left[ \eta ({\dot{\gamma }})\dfrac{f^{\prime }}{2\xi }\right] ^{\prime },\\ \\ 0 = -\dfrac{\partial p}{\partial z}-\phi . \end{array} \end{aligned}$$
(11)
Integrating the last equation of (
11) we find that
\(p=p_o(x,y)-\phi z\) with
\(p_o(x,y)\) unknown. Substituting
\(p_o(x,y)\) in (
11)
\(_{1,2}\) we find that
$$\begin{aligned} \dfrac{\partial p_o}{\partial x}-Re \ x=C_1, \ \ \ \ \ \ \ \dfrac{\partial p_o}{\partial y}-Re \ y=C_2, \end{aligned}$$
implying
$$\begin{aligned} p_o(x,y) = Re \left( \dfrac{x^2+y^2}{2}\right) +C_1 x+C_2 y+const \end{aligned}$$
(12)
The constants
\(C_1\),
\(C_2\) are nonzero only if an horizontal pressure gradient is applied. For the sake of simplicity, here we assume
\(C_1\),
\(C_2=0\). System (
11) reduces to
$$\begin{aligned} \left\{ \begin{array}{l} 2\xi ^2 Re f = \big [\eta ({\dot{\gamma }})g^{\prime }\big ]^{\prime },\\ \\ 2\xi ^2 Re g = -\big [\eta ({\dot{\gamma }})f^{\prime }\big ]^{\prime },\\ \end{array}\right. \end{aligned}$$
(13)
where
\(\eta ({\dot{\gamma }})\) and
\({\dot{\gamma }}\) are given in (
6) and (
10), respectively. Recalling (
4), (
5), the boundary conditions for
f,
g become
$$\begin{aligned} f(-1)=0, \ \ \ \ \ f(1)=0, \ \ \ \ \ g(-1)=-1, \ \ \ \ \ \ g(1)=1. \end{aligned}$$
(14)
After some manipulations, problem (
13) can be rewritten as
$$\begin{aligned} \left\{ \begin{array}{l} f^{\prime \prime }\big [2\overset{{\bullet }}{\mu }f^\prime g^\prime \big ]+g^{\prime \prime }\big [\mu +2\overset{{\bullet }}{\mu } g^{\prime ^2}\big ]-\varphi f =0,\\ \\ f^{\prime \prime }\big [\mu +2\overset{{\bullet }}{\mu } g^{\prime ^2}\big ]+g^{\prime \prime }\big [2\overset{{\bullet }}{\mu }f^\prime g^\prime \big ]+\varphi f =0,\\ \end{array}\right. \end{aligned}$$
(15)
where
\(\varphi =2\xi ^2Re\) and where we have re-defined the apparent viscosity in the following way:
$$\begin{aligned} \mu= & {} \left. \left[ \dfrac{1-e^{-\alpha \beta \sqrt{\theta }}}{\beta \sqrt{\theta }}\right] \right| _{\theta =f^{\prime ^2}+g^{\prime ^2}}=\eta ({\dot{\gamma }}), \ \ \ \ \beta =\dfrac{1}{2\xi }, \end{aligned}$$
(16)
$$\begin{aligned} \overset{{\bullet }}{\mu }= & {} \dfrac{d\mu }{d\theta }= \left. \left[ \dfrac{(1+\alpha \beta \sqrt{\theta })e^{-\alpha \beta \sqrt{\theta }}-1}{2\beta \theta \sqrt{\theta }}\right] \right| _{\theta =f^{\prime ^2}+g^{\prime ^2}}. \end{aligned}$$
(17)
On eliminating
\(g^{\prime \prime }\) from the first of (
15) and
\(f^{\prime \prime }\) from the second of (
15) we end up with:
$$\begin{aligned} \left\{ \begin{array}{l} f^{\prime \prime }\Big [\mu ^2+2\mu \overset{{\bullet }}{\mu }(f^{\prime ^2}+g^{\prime ^2})\Big ]+\varphi \Big [2\overset{{\bullet }}{\mu } g^\prime \big (ff^\prime + g g^\prime \big )+\mu g\Big ]=0,\\ \\ g^{\prime \prime }\Big [\mu ^2+2\mu \overset{{\bullet }}{\mu }(f^{\prime ^2}+g^{\prime ^2})\Big ]-\varphi \Big [2\overset{{\bullet }}{\mu } f^\prime \big (ff^\prime + g g^\prime \big )+\mu f\Big ]=0.\\ \end{array}\right. \end{aligned}$$
(18)
Our problem is thus reduced to solving second-order nonlinear BVP (
18) with BC (
14). We notice that the coefficient of the second derivatives in (
18) is always strictly positive so that the system does not degenerate. Indeed, one can show that the coefficient
$$\begin{aligned} \Gamma (\theta ):= \mu (\theta )+2\mu (\theta )\overset{{\bullet }}{\mu }(\theta ) \theta = \left[ \dfrac{\alpha e^{-2\alpha \beta \sqrt{\theta }}\big (e^{\alpha \beta \sqrt{\theta }}-1\big )}{\beta \sqrt{\theta }}\right]>0, \ \ \ \ \ \theta >0, \end{aligned}$$
(19)
is such that
$$\begin{aligned} \lim _{\theta \rightarrow 0^{+}}\Gamma (\theta )= \alpha ^2, \ \ \ \ \ \ \ \ \ \ \lim _{\theta \rightarrow \infty }\Gamma (\theta )= 0^{+}. \end{aligned}$$
(20)
Therefore, the system degenerates only if
\(\theta =\infty \), i.e., only if
\({\dot{\gamma }}=\infty \). We observe that the only nonzero components of the stress tensor are
$$\begin{aligned} S_{13}=S_{31}= \mu (\theta )\dfrac{g^\prime }{2\xi }, \ \ \ \ \ S_{23}=S_{32}= -\mu (\theta )\dfrac{f^\prime }{2\xi }, \ \ \ \ \ \ \ \theta = f^{\prime ^2}+g^{\prime ^2}, \end{aligned}$$
i.e., the shear stress components. The modulus of the stress becomes
$$\begin{aligned} \tau = \sqrt{\dfrac{1}{2}{\textbf{S}}\cdot {\textbf{S}}}=1-e^{-\alpha \beta \sqrt{f^{\prime ^2}+g^{\prime ^2}}}. \end{aligned}$$
(21)
Therefore the system degenerates only if the modulus of the stress becomes 1, which is impossible because the upper bound for the stress can be reached only if
\({\dot{\gamma }}=\infty \).