# Incompressible Navier–Stokes solver (MAC formulation)

We wish to approximate numerically the incompressible Navier–Stokes equations with phase change \displaystyle \partial_t\mathbf{u}+\nabla\cdot(\mathbf{u}\otimes\mathbf{u}) = -\nabla p + \nabla\cdot(\nu\nabla\mathbf{u}) \displaystyle \nabla\cdot\mathbf{u} = \dot{m} \left(\dfrac{1}{\rho_g} - \dfrac{1}{\rho_l}\right)\delta_\Gamma

We will use the generic time loop, a CFL-limited timestep and we will need to solve a Poisson problem.

#include "run.h"
#include "timestep.h"
#include "poisson.h"

The Markers-And-Cells (MAC) formulation was first described in the pioneering paper of Harlow and Welch, 1965. It relies on a face discretisation of the velocity components u.x and u.y, relative to the (centered) pressure p. This guarantees the consistency of the discrete gradient, divergence and Laplacian operators and leads to a stable (mode-free) integration.

scalar p[];
vector u[];
face vector uf[];

In the case of variable density, the user will need to define both the face and centered specific volume fields (\alpha and \alpha_c respectively) i.e. 1/\rho. If not specified by the user, these fields are set to one i.e. the density is unity.

Viscosity is set by defining the face dynamic viscosity \mu; default is zero.

The statistics for the (multigrid) solution of the pressure Poisson problems and implicit viscosity are stored in mgp, mgpf, mgu respectively.

(const) face vector mu = zerof, a = zerof, alpha = unityf;
(const) scalar rho = unity;

The volume expansion term is declared in evaporation.h.

extern scalar stefanflow;

## Helper functions

We define the function that performs the projection step with the volume expansion term due to the phase change.

trace
mgstats project_sf (face vector uf, scalar p,
(const) face vector alpha = unityf,
double dt = 1.,
int nrelax = 4)
{

We allocate a local scalar field and compute the divergence of \mathbf{u}_f. The divergence is scaled by dt so that the pressure has the correct dimension.

  scalar div[];
foreach() {
div[] = 0.;
foreach_dimension()
div[] += uf.x[1] - uf.x[];
div[] /= dt*Delta;
}

We add the volume expansion contribution.

  foreach()
div[] += stefanflow[]/dt;

We solve the Poisson problem. The tolerance (set with TOLERANCE) is the maximum relative change in volume of a cell (due to the divergence of the flow) during one timestep i.e. the non-dimensional quantity \displaystyle |\nabla\cdot\mathbf{u}_f|\Delta t Given the scaling of the divergence above, this gives

  mgstats mgp = poisson (p, div, alpha,
tolerance = TOLERANCE/sq(dt), nrelax = nrelax);

And compute \mathbf{u}_f^{n+1} using \mathbf{u}_f and p.

  foreach_face()

return mgp;
}

The only parameter is the viscosity coefficient \nu.y[-1,0]*nu.

The statistics for the (multigrid) solution of the Poisson problem are stored in mgp.

face vector nu[];
mgstats mgp;

#if EMBED
# define neumann_pressure(i) (alpha.n[i] ? a.n[i]*fm.n[i]/alpha.n[i] : \
a.n[i]*rho[]/(cm[] + SEPS))
#else
# define neumann_pressure(i) (a.n[i]*fm.n[i]/alpha.n[i])
#endif

p[right] = neumann (neumann_pressure(ghost));
p[left]  = neumann (- neumann_pressure(0));

double dtmax;

event init (i = 0)
{
trash ({uf});

We update fluid properties.

  event ("properties");

We set the initial timestep (this is useful only when restoring from a previous run).

  dtmax = DT;
event ("stability");
}

## Time integration

In a first step, we compute \mathbf{u}_* \displaystyle \frac{\mathbf{u}_* - \mathbf{u}_n}{dt} = \nabla\cdot\mathbf{S} with \mathbf{S} the symmetric tensor \displaystyle \mathbf{S} = - \mathbf{u}\otimes\mathbf{u} + \nu\nabla\mathbf{u} = \left(\begin{array}{cc} - u_x^2 + 2\nu\partial_xu_x & - u_xu_y + \nu(\partial_yu_x + \partial_xu_y)\\ \ldots & - u_y^2 + 2\nu\partial_yu_y \end{array}\right)

The timestep for this iteration is controlled by the CFL condition (and the timing of upcoming events).

event set_dtmax (i++,last) dtmax = DT;

event stability (i++,last) {
dt = dtnext (timestep (uf, dtmax));
}

If we are using VOF or diffuse tracers, we need to advance them (to time t+\Delta t/2) here. Note that this assumes that tracer fields are defined at time t-\Delta t/2 i.e. are lagging the velocity/pressure fields by half a timestep.

event vof (i++,last);
event tracer_diffusion (i++,last);

The fluid properties such as specific volume (fields \alpha and \alpha_c) or dynamic viscosity (face field \mu_f) – at time t+\Delta t/2 – can be defined by overloading this event.

event properties (i++,last);

{

We update the dynamic viscosity.

  foreach_face()
nu.x[] = mu.x[]*alpha.x[];

We allocate a local symmetric tensor field. To be able to compute the divergence of the tensor at the face locations, we need to compute the diagonal components at the center of cells and the off-diagonal component at the vertices.

  scalar Sxx[], Syy[];
vertex scalar Sxy[];

We average the velocity components at the center to compute the diagonal components.

  foreach() {
Sxx[] = - sq(uf.x[] + uf.x[1,0])/4. + 2.*(nu.x[1,0]*uf.x[1,0] - nu.x[]*uf.x[])/Delta;
Syy[] = - sq(uf.y[] + uf.y[0,1])/4. + 2.*(nu.y[0,1]*uf.y[0,1] - nu.y[]*uf.y[])/Delta;
}

We average horizontally and vertically to compute the off-diagonal component at the vertices.

  foreach_vertex()
Sxy[] =
- (uf.x[] + uf.x[0,-1])*(uf.y[] + uf.y[-1,0])/4. +
(nu.x[]*uf.x[] - nu.x[0,-1]*uf.x[0,-1] + nu.y[]*uf.y[] - nu.y[-1,0]*uf.y[-1,0])/Delta;

Finally we compute \displaystyle \mathbf{u}_* = \mathbf{u}_n + dt\nabla\cdot\mathbf{S}

  foreach_face(x)
uf.x[] += dt*(Sxx[] - Sxx[-1,0] + Sxy[0,1] - Sxy[])/Delta;

foreach_face(y)
uf.y[] += dt*(Syy[] - Syy[0,-1] + Sxy[1,0] - Sxy[])/Delta;

We reset the acceleration field (if it is not a constant).

  if (!is_constant(a.x)) {
face vector af = a;
trash ({af});
foreach_face()
af.x[] = 0.;
}
}

event acceleration (i++,last)
{
foreach_face()
uf.x[] += dt*a.x[]*fm.x[];
}

### Projection

In a second step we compute \displaystyle \mathbf{u}_{n+1} = \mathbf{u}_* - \Delta t\nabla p with the condition \displaystyle \nabla\cdot\mathbf{u}_{n+1} = \dot{m} \left(\dfrac{1}{\rho_g} - \dfrac{1}{\rho_l}\right)\delta_\Gamma This gives the Poisson equation for the pressure \displaystyle \nabla\cdot(\nabla p) = \frac{\nabla\cdot\mathbf{u}_*}{\Delta t}

event projection (i++,last)
{
mgp = project_sf (uf, p, alpha, dt, mgp.nrelax);
}

### Acceleration term

The acceleration term \mathbf{a} needs careful treatment as many equilibrium solutions depend on exact balance between the acceleration term and the pressure gradient: for example Laplace’s balance for surface tension or hydrostatic pressure in the presence of gravity.

To ensure a consistent discretisation, the acceleration term is defined on faces as are pressure gradients and the centered combined acceleration and pressure gradient term \mathbf{g} is obtained by averaging.

The (provisionary) face velocity field at time t+\Delta t is obtained by interpolation from the centered velocity field. The acceleration term is added.

Some derived solvers need to hook themselves at the end of the timestep.

event end_timestep (i++, last) {

We reconstruct a colocated velocity using a linear interpolation just for visualization.

  foreach()
foreach_dimension()
u.x[] = 0.5*(uf.x[1] + uf.x[]);
}