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doc:user:integration:scheme:dynexpl

Explicit dynamic integration schemes

Description

The equilibrium equation between internal forces $F^{int}$, inertial forces $Ma$ (where $M$ is the diagonalized mass matrix and $a$ the acceleration) and external forces $F^{ext}$ :

$$Ma+F^{int}=F^{ext}$$

Central difference method

Relations between displacements $x$, velocities $v$ and accelerations $a$ are:

$$v(t^{n+1/2}) = v(t^{n-1/2}) + (t^{n+1}-t^n) a(t^n) $$
$$x(t^{n+1}) = x(t^n) + (t^{n+1}-t^n) v(t^{n+1/2}) $$

The equilibrium equation becomes :

$$a(t^{n+1}) = (F^{ext}(t^{n+1}) - F^{int}(t^{n+1}))/M $$

This scheme is conditionally stable (time step limited) and non dissipative.

Alpha-generalized scheme

Same relations as in the implicit alpha-generalized scheme, but with the parameter used to weight internal and external forces equal to 1, leading to :

$$(1-\alpha_M) a(t^{n+1}) + \alpha_M a(t^n) = \frac{F^{ext}(t^n) - F^{int}(t^n)}{M}$$

Relations between displacements $x$, velocities $v$ and accelerations $a$ are:

$$x(t^{n+1}) = x(t^n) + (t^{n+1}-t^n) v(t^n) + (t^{n+1}-t^n)^2 \left( (0.5-\beta)a(t^n) + \beta a(t^{n+1})\right) $$ $$v(t^{n+1}) = v(t^n) + (t^{n+1}-t^n) {(1-\gamma)a(t^n) + \gamma a(t^{n+1})} $$

Specific values leading to an optimal numerical dissipation are given as function of the spectral radius $\rho_\beta$ (MDR_ECHR) for a bifurcation frequency (a spectral radius equal to 1 leads to a conservative algorithm when a spectral radius lower than 1 leads to a dissipative one 1):

$$\alpha_M = (2\rho_\beta-1)/(1+\rho_\beta) $$
$$\gamma = 3/2 - \alpha_M $$
$$\beta = \frac{5-3\rho_\beta}{(1+\rho_\beta)^2 (2-\rho_\beta)}$$

Conditionally stable.

Tchamwa Scheme

Explicit algorithm where numerical dissipation is monitored by the parameter $\phi$.

Equilibrium computed with

$$a(t^{n+1}) = \frac{F^{ext}(t^{n+1}) - F^{int}(t^{n+1})}{M}$$

Relations between displacements $x$, velocities $v$ and accelerations $a$ are:

$$x(t^{n+1}) = x(t^n) + (t^{n+1}-t^n) v(t^n) + \phi (t^{n+1}-t^n)^2 a(t^n) $$
$$v(t^{n+1}) = v(t^n) + (t^{n+1}-t^n) a(t^n) $$

Stability guaranteed for $\phi \geq 1 $ and high frequencies killed over a single time step for \phi = 2$. the scheme is of :

  • second order for $\phi = 1$ (no numerical dissipation)
  • first order for $\phi > 1$ (numerical dissipation)

Relation between $\phi$ and spectral radius for the bifurcation $\rho_\beta$ (user parameter MDR_ECHR) is:

  • $$\phi = \frac{2(1- \rho_\beta^{1/2})}{(1-\rho_\beta)} \mbox{ if } \rho_\beta < 1 $$
  • $$\phi = 1 \mbox{ if } \rho_\beta = 1 $$

Input file

See dynamic implicit scheme for definition of density and initial velocities.

Old Metafor Version <= 2422

Choosing the algorithm

Scheme MDE_NDYN MDR_ECHR
Certered difference 1
Chung Hulbert 3 X
Tchamwa 6 X

(see Global Parameters [REMOVED])

New Metafor Version > 2422

Centered Difference

ti = CentralDifferenceTimeIntegration(metafor)
metafor.setTimeIntegration(ti)

Chung Hulbert

ti = ChExplicitTimeIntegration(metafor)
ti.setRhoB(_rhoB)
metafor.setTimeIntegration(ti)

The parameter _rhoB is the spectral radius at bifurcation point ([0, 1]). The default value is 0.8182.

Tchamwa

ti = TchamwaTimeIntegration(metafor)
ti.setRhoB(_rhoB)
metafor.setTimeIntegration(ti)

The parameter _rhoB is the spectral radius at bifurcation point ([0, 1]). The default value is 0.8182.

Other parameters : see Quasi-static integration schemes

1)
see real parameters
doc/user/integration/scheme/dynexpl.txt · Last modified: 2016/03/30 15:23 (external edit)