# The Burstedde variable viscosity benchmark *This section was contributed by Iris van Zelst.* This benchmark is intended to test solvers for variable viscosity Stokes problems. It begins with postulating a smooth exact polynomial solution to the Stokes equation for a unit cube, first proposed by {cite:t}DohrmannBochev2004 and also described by {cite:t}burstedde:etal:2013: {math} :label: eq:burstedde-velocity {\mathbf u} &= \left( \begin{array}{c} x+x^2+xy+x^3y \\ y + xy + y^2 + x^2 y^2\\ -2z - 3xz - 3yz - 5x^2 yz \end{array} \right)  {math} :label: eq:burstedde-pressure p = xyz + x^3 y^3z - \frac{5}{32}.  It is then trivial to verify that the velocity field is divergence-free. The constant $-\frac{5}{32}$ has been added to the expression of $p$ to ensure that the volume pressure normalization of can be used in this benchmark (in other words, to ensure that the exact pressure has mean value zero and, consequently, can easily be compared with the numerically computed pressure). Following {cite:t}burstedde:etal:2013, the viscosity $\mu$ is given by the smoothly varying function {math} :label: eq:burstedde-mu \mu = \exp\left\{1 - \beta\left[x (1-x) + y(1-y) + z(1-z)\right]\right\}.  The maximum of this function is $\mu = e$, for example at $(x,y,z)=(0,0,0)$, and the minimum of this function is $\mu = \exp \Big( 1-\frac{3\beta}{4}\Big)$ at $(x,y,z) = (0.5,0.5,0.5)$. The viscosity ratio $\mu^\ast$ is then given by {math} \mu^\ast = \frac{\exp\Big(1-\frac{3\beta}{4}\Big)}{\exp(1)} = \exp\Big(\frac{-3\beta}{4}\Big).  Hence, by varying $\beta$ between 1 and 20, a difference of up to 7 orders of magnitude viscosity is obtained. $\beta$ will be one of the parameters that can be selected in the input file that accompanies this benchmark. The corresponding body force of the Stokes equation can then be computed by inserting this solution into the momentum equation, {math} :label: eq:burstedde-momentum {\nabla} p - \nabla \cdot (2 \mu {\epsilon(\mathbf u)}) = \rho \mathbf g.  Using equations {math:numref}eq:burstedde-velocity, {math:numref}eq:burstedde-pressure and {math:numref}eq:burstedde-mu in the momentum equation {math:numref}eq:burstedde-momentum, the following expression for the body force $\rho\mathbf g$ can be found: {math} \begin{gathered} {\rho\mathbf g} = \left( \begin{array}{c} yz+3x^2y^3z\\ xz +3x^3y^2z \\ xy+x^3y^3 \end{array} \right) -\mu \left( \begin{array}{c} 2+6xy \\ 2 + 2x^2 + 2y^2 \\ -10yz \end{array} \right) \\ + (1-2x)\beta \mu \left( \begin{array}{c} 2+4x+2y+6x^2y \\ x+y+2xy^2+x^3 \\ -3z -10xyz \end{array} \right) +(1-2y)\beta \mu \left( \begin{array}{c} x+y+2xy^2+x^3 \\ 2+2x+4y+4x^2y \\ -3z-5x^2z \\ \end{array} \right) \\ +(1-2z)\beta \mu \left( \begin{array}{c} -3z -10xyz \\ -3z-5x^2z \\ -4-6x-6y-10x^2y \end{array} \right)\end{gathered}  Assuming $\rho = 1$, the above expression translates into an expression for the gravity vector $\mathbf g$. This expression for the gravity (even though it is completely unphysical), has consequently been incorporated into the BursteddeGravity gravity model that is described in the benchmarks/burstedde/burstedde.cc file that accompanies this benchmark. We will use the input file benchmarks/burstedde/burstedde.prm as input, which is very similar to the input file benchmarks/inclusion/adaptive.prm discussed above in {ref}sec:benchmarks:inclusion. The major changes for the 3D polynomial Stokes benchmark are listed below: {literalinclude} burstedde.prm  The boundary conditions that are used are simply the velocities from equation {math:numref}eq:burstedde-velocity prescribed on each boundary. The viscosity parameter in the input file is $\beta$. Furthermore, in order to compute the velocity and pressure $L_1$ and $L_2$ norm, the postprocessor BursteddePostprocessor is used. Please note that the linear solver tolerance is set to a very small value (deviating from the default value), in order to ensure that the solver can solve the system accurately enough to make sure that the iteration error is smaller than the discretization error. Expected analytical solutions at two locations are summarised in {numref}tab:burstedde-table and can be deduced from equations {math:numref}eq:burstedde-velocity and {math:numref}eq:burstedde-pressure. {numref}fig:burstedde-benchmark shows that the analytical solution is indeed retrieved by the model. {figure-md} fig:burstedde-benchmark Burstedde benchmark: Results for the 3D polynomial Stokes benchmark, obtained with a resolution of $16\times 16$ elements, with $\beta = 10$.  {table} Analytical solutions :name: tab:burstedde-table | Quantity | $\mathbf{r} = (0,0,0)$ | $\mathbf{r} = (1,1,1)$ | |:---------------|:----------------------:|:----------------------:| | $p$ | $-0.15625$ | $1.84375$ | | $\mathbf{u}$ | $(0,0,0)$ | $(4,4,-13)$ | | $\|\mathbf{u}\|$ | $0$ | $14.177$ |  The convergence of the numerical error of this benchmark has been analysed by playing with the mesh refinement level in the input file, and results can be found in {numref}fig:burstedde:errors. The velocity shows cubic error convergence, while the pressure shows quadratic convergence in the $L_1$ and $L_2$ norms, as one would hope for using $Q_2$ elements for the velocity and $Q_1$ elements for the pressure. {figure-md} fig:burstedde:errors Burstedde benchmark: Error convergence for the 3D polynomial Stokes benchmark.