Analyses and solvers

Analyses and solvers

After a global assembly for each problem is ready, they must somehow put together, usually construct a linear system $\boldsymbol{A}\boldsymbol{x} = \boldsymbol{b}$ , solve system of equations and potentially update solution back to problems / elements.

For this task we have Analysis. Just like problems are "containers" for a set of some elements, analyses are containers for a set of problems, performing some spesific task. This task usually (but not always) involves solving big systems of equations.

For example, typical nonlinear quasistatic analysis is

1. initialize problems (if needed)
2. assemble problems
3. combine assemblies to construct a linear system $\boldsymbol{A}\boldsymbol{x} = \boldsymbol{b}$
4. solve linear system, as effectively as possible
5. update solution back to problems or elements
6. postprocess fields
7. write requested results to file
8. check convergence, if not converged, go back to 2.

Implementing solver for LinearSystem

When models gets big enough, step 4 is dominating in a solution process. For that reason we have abstract type AbstractLinearSystemSolver which can be subclassed to construct own solution strategy. This strategy can be, for example to use Julia's build-in solvers, MUMPS, iterative solvers and so on. And to make this as standard as possible, we have LinearSystem containing all the relevant matrices, i.e.

$\begin{bmatrix} \boldsymbol{K} & \boldsymbol{C}_{1}\\ \boldsymbol{C}_{2} & \boldsymbol{D} \end{bmatrix} \begin{bmatrix} \boldsymbol{u}\\ \boldsymbol{\lambda} \end{bmatrix} = \begin{bmatrix} \boldsymbol{f}\\ \boldsymbol{g} \end{bmatrix}.$

First setup linear system, we take matrices introduced in the context of [Problems]:

ls = LinearSystem(5)
ls.K = sparse([  4.0  -1.0  -2.0  -1.0   0.0
-1.0   7.0  -4.0  -2.0   0.0
-2.0  -4.0  10.0  -1.0  -3.0
-1.0  -2.0  -1.0   4.0   0.0
0.0   0.0  -3.0   0.0   3.0])
ls.C1 = sparse([ 1.0  0.0  0.0  0.0  0.0
0.0  0.0  0.0  0.0  0.0
0.0  0.0  0.0  0.0  0.0
0.0  0.0  0.0  1.0  0.0
0.0  0.0  0.0  0.0  0.0])
ls.C2 = ls.C1
ls.f = sparsevec([33.0, 33.0, 165.0, 33.0, 132.0])
nothing # hide

For example, a simple solver for small problems would be to use lufact from UMFPACK:

type LUSolver <: AbstractLinearSystemSolver
# may contain some solver-spesific settings
end

"""
solve!(ls::LinearSystem, solver::LUSolver)

Solve linear system using LU factorization. If final system has zero rows,
add 1 to diagonal to make matrix non-singular.
"""
function solve!(ls::LinearSystem, solver::LUSolver)

println("Solving system using LUSolver")

A = [ls.K ls.C1; ls.C2 ls.D]
b = [ls.f; ls.g]

# add 1.0 to diagonal for any zero rows in system
p = ones(2*ls.dim)
p[unique(rowvals(A))] = 0.0
A += spdiagm(p)

# solve A*x = b using LU factorization and update solution vectors
x = lufact(A) \ full(b)
ls.u = x[1:ls.dim]
ls.la = x[ls.dim+1:end]

return nothing
end

To test it:

julia> solver = LUSolver()
ERROR: UndefVarError: LUSolver not defined

julia> solve!(ls, solver)
ERROR: UndefVarError: solve! not defined

julia> full(ls.u)
ERROR: UndefVarError: ls not defined

julia> full(ls.la)
ERROR: UndefVarError: ls not defined

We try to be clever with solvers, because solution of linear system is taking the most of the time in analysis for big models. It's possible to build a list of solvers which are then run in order, so if first one cannot solve the system, then we can try the next one and so on. For example, direct solvers are very robust but they need quite lot of memory for factorization. If computer / cluster has enough of memory, we can prefer direct solver over iterative ones. If stiffness matrix is symmetric, and problem is positive definite, Cholesky decomposition is preferred over LU decomposition and so on. Before starting the actual solution, one can study matrix structure using can_solve:

type FakeSolver <: AbstractLinearSystemSolver
end

function can_solve(ls::LinearSystem, solver::FakeSolver)
if issymmetric(ls.K)
return (true, "OK")
else
return (false, "stiffness matrix not symmetric")
end
end
julia> solvers = [FakeSolver(), LUSolver()]
ERROR: UndefVarError: FakeSolver not defined

julia> ls.K[1,2] += 1
ERROR: UndefVarError: ls not defined

julia> solve!(ls, solvers)
ERROR: UndefVarError: solve! not defined

Implementing new analyses

Like said, Analysis is a type containing all problems and is performing some sort of operation with them. Typically this can be a solution of static or dynamic equilibrium, eigenvalue analysis, linear perturbation analysis or doing some postprocessing for already done analysis, like fatigue analysis, calculating critical stresses and so on.

In industrial computing, a bigger workflow typically contains several different analyses. Compared to some commerfial FEM softwares, e.g. ABAQUS, Analysis can be consided as STEP, but in a more general sense. Because Julia is a real programming language, one can then create, for example, an optimization loop, combining several different analyses to optimize model against some design goal.

There's not so much rules about how to define new analysis. It must be a subtype of AbstractAnalysis and implement run!-function. As a minimal working example, implementation for static analysis would then be:

type Static <: AbstractAnalysis
time :: Float64
end

function Static()
return Static(0.0)
end

function run!(analysis::Analysis{Static})

time = analysis.properties.time
problems = get_problems(analysis)

# assemble matrices for all problems
N = 0 # size of resulting matrix
for problem in problems
assemble!(problem, time)
N = max(N, size(problem.assembly.K, 2))
end

# create new LinearSystem and add assemblies to that
ls = LinearSystem(N)

for problem in problems
ls.K += sparse(problem.assembly.K, N, N)
ls.f += sparse(problem.assembly.f, N, 1)
ls.C1 += sparse(problem.assembly.C1, N, N)
ls.C2 += sparse(problem.assembly.C2, N, N)
ls.D += sparse(problem.assembly.D, N, N)
ls.g += sparse(problem.assembly.g, N, 1)
end

# solve linear system using e.g. LUSolver
solve!(ls, LUSolver())

# update solution back to problems
u = Dict(i=>j for (i,j) in enumerate(ls.u))
for problem in problems
is_field_problem(problem) || continue
field_name = get_unknown_field_name(problem)
elements = get_elements(problem)
update!(elements, field_name, time => u)
end
end

To put everything together, typical solution process of time-intependent variational problem looks the following:

X = Dict(1 => [0.0, 0.0],
2 => [1.0, 0.0],
3 => [1.0, 1.0],
4 => [0.0, 1.0],
5 => [2.0, 1.0])

el1 = Element(Quad4, [1, 2, 3, 4])
el2 = Element(Tri3, [3, 2, 5])
el3 = Element(Seg2, [3, 5])
elements = [el1, el2, el3]
update!(elements, "geometry", X)
update!(elements, "thermal conductivity", 6.0)
update!(el3, "heat flux", 264.0)
update!(el1, "heat source", 132.0)
problem = Problem(Heat, "test problem", 1)

bel1 = Element(Seg2, [1, 4])
update!(bel1, "geometry", X)
update!(bel1, "temperature 1", 0.0)
bc = Problem(DirichletBC, "fixed", 1, "temperature")

analysis = Analysis(Static, "static analysis of test problem")
run!(analysis)
julia> el1("temperature", 0.0)
ERROR: UndefVarError: el3 not defined