MsFV Introductory Example

# Multiscale Finite Volume Pressure Solver

This example compares the multiscale finite volume method with a fine scale solution for a simple case of single phase incompressible flow with wells and boundary conditions on a Cartesian grid.

The Multiscale Finite Volume Method (MsFVM) is a multiscale method where instead of solving a full discretization for all fine cells, a series of smaller, local problems are solved with unit pressure to give a coarse pressure system. The resulting coarse pressure is then used to scale the local problems to find a fine scale pressure approximation.

The method is attractive because pressure updates are inexpensive once the pressure basis functions have been constructed and because it guarantees a conservative flow field if an additional set of basis functions are created.

## Load the modules required for the example

We will use streamlines and coarse grids.

mrstModule add coarsegrid streamlines msfvm


## We define a simple 2D Cartesian grid

The fine scale grid will consist of fine cells, with a coarse grid of coarse blocks.

nx = 50; ny = 50;

Nx = 3; Ny = 3;

% Instansiate the fine grid
G = cartGrid([nx, ny]);
G = computeGeometry(G);

% Generate coarse grid
p = partitionUI(G, [Nx, Ny]);
CG = generateCoarseGrid(G, p);
CG = coarsenGeometry(CG);


## Generate dual grid from the coarse grid using a logical algorithm

The dual grid is based on the coarse grid and has corners defined by the coarse centroids of the coarse grid. Here our grid is Cartesian which makes it easy to define a dual grid based on the logical indices.

We also visualize the dual grid. Cells corresponding to primal coarse centers are drawn in green, edge cells are drawn in orange and the primal coarse grid is drawn in red. Note that while the coarse grid is face centered, the dual coarse grid is cell centered, giving overlap between edges.

DG = partitionUIdual(CG, [Nx, Ny]);

clf;
plotDual(G, DG)
outlineCoarseGrid(G,p, 'red')
view(0,90), axis tight


## Set up permeability and fluid

We create a fluid object for one phase flow. The permeability is generated via a porosity distribution, which is then visualized.

fluid        = initSingleFluid('mu' ,    1*centi*poise     , ...
'rho', 1014*kilogram/meter^3);
% Permeability
poro = gaussianField(G.cartDims, [.4 .8], [11 3 3], 2.5);
K = poro.^3.*(1e-5)^2./(0.81*72*(1-poro).^2);
rock.perm    = K(:);

% Plot log10 of permeability
clf;
plotCellData(G, log10(rock.perm));
axis tight off;


## Add a simple Dirichlet boundary

Add a simple flow boundary condition based on pressure. This gives flow everywhere.

bc = [];
bc = pside   (bc, G, 'LEFT' , 1*barsa());
bc = pside   (bc, G, 'RIGHT', 0);


We are adding a producer / injector pair of wells to demonstrate their effect on the solver.

W = [];
cell1 = 13 + ny*13;
cell2 = 37 + ny*37;

W = addWell(W, G, rock, cell1,      ...
'Type', 'bhp' , 'Val', .0*barsa(), ...
'Comp_i', [0, 1]);

W = addWell(W, G, rock, cell2,      ...
'Type', 'bhp' , 'Val', 1*barsa(), ...
'Comp_i', [0, 1]);


## Solve the systems.

First we initiate a pressure system. This structure is always required, but without transport only the grid is relevant.

We also compute transmissibilities. The MS solver is based on the TPFA solver, and as such has many of the same arguments. Multiscale methods in general can be seen as approximations to a specific discretization, and will inherit any strengths and weaknesses from the parent method, for instance grid orientation effects.

The system is then solved three times: Once with a full TPFA solver, once with the multiscale sovler with conservative flow and lastly using the multiscale solver without conservative flow.

sol = initState(G, [], 0, [1, 0]);

T = computeTrans(G, rock);

% Solve TPFA reference solution.
solRef = incompTPFA(sol, G, T, fluid, 'wells', W, 'bc', bc);

% Solve multiscale pressure. Reconstruct conservative flow using flow basis
% functions.
solMSFV = solveMSFV_TPFA_Incomp(sol, G, CG, T, fluid,...
'Wells', W,...
'bc', bc,...
'Dual', DG,...
'Reconstruct', true,...
'Verbose', false);

solMSFV2 = solveMSFV_TPFA_Incomp(sol, G, CG, T, fluid,...
'Wells', W,...
'bc', bc,...
'Dual', DG,...
'Reconstruct', false,...
'Verbose', false);


## Plot reference solution

clf;
plotCellData(G, solRef.pressure);
axis tight off; colorbar;
set(gca, 'CLim', [0, max(solRef.pressure)]);
title('TPFA')


## Plot the multiscale solution

clf;
plotCellData(G, solMSFV.pressure);
axis tight off; colorbar;
title('MsFVM')
% Use same axis scaling for the multiscale solution as the TPFA solution
set(gca, 'CLim', [0, max(solRef.pressure)]);


## Plot error

Plot error scaled with local variation

reportError(solRef.pressure, solMSFV.pressure);
clf;
plotCellData(G, abs(solRef.pressure - solMSFV.pressure) ./ ...
abs(max(solRef.pressure - min(solRef.pressure))));
axis tight off; colorbar;

ERROR:
2: 0.02755890
Sup: 0.04438343
Minimum 0.00002045


## Decompose the solution

We observe that the error is large near the wells. This is not a coincidence: The MsFV solves bases its initial pressure solution of a coarse system, which does not actually include wells other than in a integral sense. We can decompose the solution to see this:

% Find the pressure basis
p_basis = solMSFV.msfvm.B*solMSFV.pressurecoarse;
p_corr = solMSFV.msfvm.Cr;
P = solMSFV.P';
% We need to permute this back to the original grid ordering using the
% permutation matrix...
p_basis = P*p_basis;
p_corr  = P*p_corr;


## Plot the basis solution

Note that the pressure has become low where the wells are! This happens because the system simultanously tries to create correct flux over the edges of the coarse grid as if the wells were there, but tries to find a pressure solution where the wells are not actually included. This leads to low pressure. However, a set of correction functions are constructed, which add inn fine scale behavior of the wells as well as boundary conditions:

clf;
plotCellData(G, p_basis)
title('Basis pressure');
outlineCoarseGrid(G,p, 'red')
plotDual(G, DG)


## Plot the correction functions

The actual pressure is the sum of these two solutions. Note that when applied as an iterative method, the correction functions also contain non-local effects, i.e. source terms trying to minimze the residual.

clf;
plotCellData(G, p_corr)
title('Correction pressure')
outlineCoarseGrid(G,p, 'red')
plotDual(G, DG)


## Plot streamlines for the reference

[i j] = ind2sub(G.cartDims, 1:G.cells.num);
% Start a streamline at all the boundaries, as well as in each well.
startpts = find(i == max(i) | i == min(i) | j == min(j) | j == max(j));
startpts = [startpts'; vertcat(W.cells)];

% Once a flux is plotted, reverse the flux to get more streamlines.
clf;
streamline(pollock(G, solRef, startpts));
tmp.flux = -solRef.flux;
streamline(pollock(G, tmp, startpts));
title('TPFA')
axis tight


## Plot streamlines for the multiscale solution

clf;
streamline(pollock(G, solMSFV, startpts));
tmp.flux = -solMSFV.flux;
streamline(pollock(G, tmp, startpts));
title('MsFVM')
axis tight


## Plot streamlines without the reconstructed flux

Note that the reconstructed flux is much closer to the reference.

clf;
streamline(pollock(G, solMSFV2, startpts));
tmp.flux = -solMSFV2.flux;
streamline(pollock(G, tmp, startpts));
title('MsFVM (No flux reconstruction)')
axis tight


Published October 2, 2012