linear_elasticity/modal_analysis.py

Description

Modal analysis of a linear elastic block in 2D or 3D.

The dimension of the problem is determined by the length of the vector in --dims option.

Optionally, a mesh file name can be given as a positional argument. In that case, the mesh generation options are ignored.

The default material properties correspond to aluminium in the following units:

  • length: m
  • mass: kg
  • stiffness / stress: Pa
  • density: kg / m^3

Examples

  • Run with the default arguments, show results (color = strain):

    python examples/linear_elasticity/modal_analysis.py --show
    
  • Fix bottom surface of the domain, show 9 eigen-shapes:

    python examples/linear_elasticity/modal_analysis.py -b cantilever -n 9 --show
    
  • Increase mesh resolution:

    python examples/linear_elasticity/modal_analysis.py -s 31,31 -n 9 --show
    
  • Use 3D domain:

    python examples/linear_elasticity/modal_analysis.py -d 1,1,1 -c 0,0,0 -s 8,8,8 --show
    
  • Change the eigenvalue problem solver to LOBPCG:

    python examples/linear_elasticity/modal_analysis.py --solver="eig.scipy_lobpcg,i_max:100,largest:False" --show
    

    See sfepy.solvers.eigen for available solvers.

source code

#!/usr/bin/env python
"""
Modal analysis of a linear elastic block in 2D or 3D.

The dimension of the problem is determined by the length of the vector
in ``--dims`` option.

Optionally, a mesh file name can be given as a positional argument. In that
case, the mesh generation options are ignored.

The default material properties correspond to aluminium in the following units:

- length: m
- mass: kg
- stiffness / stress: Pa
- density: kg / m^3

Examples
--------

- Run with the default arguments, show results (color = strain)::

    python examples/linear_elasticity/modal_analysis.py --show

- Fix bottom surface of the domain, show 9 eigen-shapes::

    python examples/linear_elasticity/modal_analysis.py -b cantilever -n 9 --show

- Increase mesh resolution::

    python examples/linear_elasticity/modal_analysis.py -s 31,31 -n 9 --show

- Use 3D domain::

    python examples/linear_elasticity/modal_analysis.py -d 1,1,1 -c 0,0,0 -s 8,8,8 --show

- Change the eigenvalue problem solver to LOBPCG::

    python examples/linear_elasticity/modal_analysis.py --solver="eig.scipy_lobpcg,i_max:100,largest:False" --show

  See :mod:`sfepy.solvers.eigen` for available solvers.
"""
from __future__ import absolute_import
import sys
import six
from six.moves import range
sys.path.append('.')
from argparse import ArgumentParser, RawDescriptionHelpFormatter

import numpy as nm
import scipy.sparse.linalg as sla

from sfepy.base.base import assert_, output, Struct
from sfepy.discrete import (FieldVariable, Material, Integral, Integrals,
                            Equation, Equations, Problem)
from sfepy.discrete.fem import Mesh, FEDomain, Field
from sfepy.terms import Term
from sfepy.discrete.conditions import Conditions, EssentialBC
from sfepy.mechanics.matcoefs import stiffness_from_youngpoisson
from sfepy.mesh.mesh_generators import gen_block_mesh
from sfepy.solvers import Solver

helps = {
    'dims' :
    'dimensions of the block [default: %(default)s]',
    'centre' :
    'centre of the block [default: %(default)s]',
    'shape' :
    'numbers of vertices along each axis [default: %(default)s]',
    'bc_kind' :
    'kind of Dirichlet boundary conditions on the bottom and top surfaces,'
    ' one of: free, cantilever, fixed [default: %(default)s]',
    'axis' :
    'the axis index of the block that the bottom and top surfaces are related'
    ' to [default: %(default)s]',
    'young' : "the Young's modulus [default: %(default)s]",
    'poisson' : "the Poisson's ratio [default: %(default)s]",
    'density' : "the material density [default: %(default)s]",
    'order' : 'displacement field approximation order [default: %(default)s]',
    'n_eigs' : 'the number of eigenvalues to compute [default: %(default)s]',
    'ignore' : 'if given, the number of eigenvalues to ignore (e.g. rigid'
    ' body modes); has precedence over the default setting determined by'
    ' --bc-kind [default: %(default)s]',
    'solver' : 'the eigenvalue problem solver to use. It should be given'
    ' as a comma-separated list: solver_kind,option0:value0,option1:value1,...'
    ' [default: %(default)s]',
    'show' : 'show the results figure',
}

def main():
    parser = ArgumentParser(description=__doc__,
                            formatter_class=RawDescriptionHelpFormatter)
    parser.add_argument('--version', action='version', version='%(prog)s')
    parser.add_argument('-d', '--dims', metavar='dims',
                        action='store', dest='dims',
                        default='[1.0, 1.0]', help=helps['dims'])
    parser.add_argument('-c', '--centre', metavar='centre',
                        action='store', dest='centre',
                        default='[0.0, 0.0]', help=helps['centre'])
    parser.add_argument('-s', '--shape', metavar='shape',
                        action='store', dest='shape',
                        default='[11, 11]', help=helps['shape'])
    parser.add_argument('-b', '--bc-kind', metavar='kind',
                        action='store', dest='bc_kind',
                        choices=['free', 'cantilever', 'fixed'],
                        default='free', help=helps['bc_kind'])
    parser.add_argument('-a', '--axis', metavar='0, ..., dim, or -1',
                        type=int, action='store', dest='axis',
                        default=-1, help=helps['axis'])
    parser.add_argument('--young', metavar='float', type=float,
                        action='store', dest='young',
                        default=6.80e+10, help=helps['young'])
    parser.add_argument('--poisson', metavar='float', type=float,
                        action='store', dest='poisson',
                        default=0.36, help=helps['poisson'])
    parser.add_argument('--density', metavar='float', type=float,
                        action='store', dest='density',
                        default=2700.0, help=helps['density'])
    parser.add_argument('--order', metavar='int', type=int,
                        action='store', dest='order',
                        default=1, help=helps['order'])
    parser.add_argument('-n', '--n-eigs', metavar='int', type=int,
                        action='store', dest='n_eigs',
                        default=6, help=helps['n_eigs'])
    parser.add_argument('-i', '--ignore', metavar='int', type=int,
                        action='store', dest='ignore',
                        default=None, help=helps['ignore'])
    parser.add_argument('--solver', metavar='solver', action='store',
                        dest='solver',
                        default= \
                        "eig.scipy,method:'eigh',tol:1e-5,maxiter:1000",
                        help=helps['solver'])
    parser.add_argument('--show',
                        action="store_true", dest='show',
                        default=False, help=helps['show'])
    parser.add_argument('filename', nargs='?', default=None)
    options = parser.parse_args()

    aux = options.solver.split(',')
    kwargs = {}
    for option in aux[1:]:
        key, val = option.split(':')
        kwargs[key.strip()] = eval(val)
    eig_conf = Struct(name='evp', kind=aux[0], **kwargs)

    output('using values:')
    output("  Young's modulus:", options.young)
    output("  Poisson's ratio:", options.poisson)
    output('  density:', options.density)
    output('displacement field approximation order:', options.order)
    output('requested %d eigenvalues' % options.n_eigs)
    output('using eigenvalue problem solver:', eig_conf.kind)
    output.level += 1
    for key, val in six.iteritems(kwargs):
        output('%s: %r' % (key, val))
    output.level -= 1

    assert_((0.0 < options.poisson < 0.5),
            "Poisson's ratio must be in ]0, 0.5[!")
    assert_((0 < options.order),
            'displacement approximation order must be at least 1!')

    filename = options.filename
    if filename is not None:
        mesh = Mesh.from_file(filename)
        dim = mesh.dim
        dims = nm.diff(mesh.get_bounding_box(), axis=0)

    else:
        dims = nm.array(eval(options.dims), dtype=nm.float64)
        dim = len(dims)

        centre = nm.array(eval(options.centre), dtype=nm.float64)[:dim]
        shape = nm.array(eval(options.shape), dtype=nm.int32)[:dim]

        output('dimensions:', dims)
        output('centre:    ', centre)
        output('shape:     ', shape)

        mesh = gen_block_mesh(dims, shape, centre, name='mesh')

    output('axis:      ', options.axis)
    assert_((-dim <= options.axis < dim), 'invalid axis value!')

    eig_solver = Solver.any_from_conf(eig_conf)

    # Build the problem definition.
    domain = FEDomain('domain', mesh)

    bbox = domain.get_mesh_bounding_box()
    min_coor, max_coor = bbox[:, options.axis]
    eps = 1e-8 * (max_coor - min_coor)
    ax = 'xyz'[:dim][options.axis]

    omega = domain.create_region('Omega', 'all')
    bottom = domain.create_region('Bottom',
                                  'vertices in (%s < %.10f)'
                                  % (ax, min_coor + eps),
                                  'facet')
    bottom_top = domain.create_region('BottomTop',
                                      'r.Bottom +v vertices in (%s > %.10f)'
                                      % (ax, max_coor - eps),
                                      'facet')

    field = Field.from_args('fu', nm.float64, 'vector', omega,
                            approx_order=options.order)

    u = FieldVariable('u', 'unknown', field)
    v = FieldVariable('v', 'test', field, primary_var_name='u')

    mtx_d = stiffness_from_youngpoisson(dim, options.young, options.poisson)

    m = Material('m', D=mtx_d, rho=options.density)

    integral = Integral('i', order=2*options.order)

    t1 = Term.new('dw_lin_elastic(m.D, v, u)', integral, omega, m=m, v=v, u=u)
    t2 = Term.new('dw_volume_dot(m.rho, v, u)', integral, omega, m=m, v=v, u=u)
    eq1 = Equation('stiffness', t1)
    eq2 = Equation('mass', t2)
    lhs_eqs = Equations([eq1, eq2])

    pb = Problem('modal', equations=lhs_eqs)

    if options.bc_kind == 'free':
        pb.time_update()
        n_rbm = dim * (dim + 1) // 2

    elif options.bc_kind == 'cantilever':
        fixed = EssentialBC('Fixed', bottom, {'u.all' : 0.0})
        pb.time_update(ebcs=Conditions([fixed]))
        n_rbm = 0

    elif options.bc_kind == 'fixed':
        fixed = EssentialBC('Fixed', bottom_top, {'u.all' : 0.0})
        pb.time_update(ebcs=Conditions([fixed]))
        n_rbm = 0

    else:
        raise ValueError('unsupported BC kind! (%s)' % options.bc_kind)

    if options.ignore is not None:
        n_rbm = options.ignore

    pb.update_materials()

    # Assemble stiffness and mass matrices.
    mtx_k = eq1.evaluate(mode='weak', dw_mode='matrix', asm_obj=pb.mtx_a)
    mtx_m = mtx_k.copy()
    mtx_m.data[:] = 0.0
    mtx_m = eq2.evaluate(mode='weak', dw_mode='matrix', asm_obj=mtx_m)

    try:
        eigs, svecs = eig_solver(mtx_k, mtx_m, options.n_eigs + n_rbm,
                                 eigenvectors=True)

    except sla.ArpackNoConvergence as ee:
        eigs = ee.eigenvalues
        svecs = ee.eigenvectors
        output('only %d eigenvalues converged!' % len(eigs))

    output('%d eigenvalues converged (%d ignored as rigid body modes)' %
           (len(eigs), n_rbm))

    eigs = eigs[n_rbm:]
    svecs = svecs[:, n_rbm:]

    omegas = nm.sqrt(eigs)
    freqs = omegas / (2 * nm.pi)

    output('number |         eigenvalue |  angular frequency '
           '|          frequency')
    for ii, eig in enumerate(eigs):
        output('%6d | %17.12e | %17.12e | %17.12e'
               % (ii + 1, eig, omegas[ii], freqs[ii]))

    # Make full eigenvectors (add DOFs fixed by boundary conditions).
    variables = pb.get_variables()

    vecs = nm.empty((variables.di.ptr[-1], svecs.shape[1]),
                    dtype=nm.float64)
    for ii in range(svecs.shape[1]):
        vecs[:, ii] = variables.make_full_vec(svecs[:, ii])

    # Save the eigenvectors.
    out = {}
    state = pb.create_state()
    for ii in range(eigs.shape[0]):
        state.set_full(vecs[:, ii])
        aux = state.create_output_dict()
        strain = pb.evaluate('ev_cauchy_strain.i.Omega(u)',
                             integrals=Integrals([integral]),
                             mode='el_avg', verbose=False)
        out['u%03d' % ii] = aux.popitem()[1]
        out['strain%03d' % ii] = Struct(mode='cell', data=strain)

    pb.save_state('eigenshapes.vtk', out=out)
    pb.save_regions_as_groups('regions')

    if len(eigs) and options.show:
        # Show the solution. If the approximation order is greater than 1, the
        # extra DOFs are simply thrown away.
        from sfepy.postprocess.viewer import Viewer
        from sfepy.postprocess.domain_specific import DomainSpecificPlot

        scaling = 0.05 * dims.max() / nm.abs(vecs).max()

        ds = {}
        for ii in range(eigs.shape[0]):
            pd = DomainSpecificPlot('plot_displacements',
                                    ['rel_scaling=%s' % scaling,
                                     'color_kind="tensors"',
                                     'color_name="strain%03d"' % ii])
            ds['u%03d' % ii] = pd

        view = Viewer('eigenshapes.vtk')
        view(domain_specific=ds, only_names=sorted(ds.keys()),
             is_scalar_bar=False, is_wireframe=True)

if __name__ == '__main__':
    main()