PSI - Issue 8

C. Groth et al. / Procedia Structural Integrity 8 (2018) 379–389

383

C. Groth et al. / Structural Integrity Procedia 00 (2017) 000–000

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Table 1. Common RBF with global and local support. Compactly supported RBF

Abbreviation

φ ( ζ )

Wendland C 0 Wendland C 2 Wendland C 4

(1 − ζ ) 2

C 0 C 2 C4

(1 − ζ ) 4 (4 ζ + 1) 2 + 6 ζ + 1) 3 ζ

(1 − ζ ) 6 ( 35

Globally supported RBF

Abbreviation

φ ( ζ )

r n , n odd r n log ( r ), n even

Polyharmonic spline

PHS

r 2 log ( r ) a 2 + ( r ) 2 1 √ a 2 + ( r ) 2 1 + ( r ) 2

Thin plate spline

TPS

Multiquadric biharmonics

MQB IMQB

Inverse multiquadric biharmonics

Quadric biharmonics

QB

1 1 + ( r ) 2 e − r 2

Inverse quadric biharmonics

IQB

Gaussian biharmonics

GS

A linear system of order equal to the number of points used (Buhmann (2000)) must be solved in order to find system coe ffi cients. Once the coe ffi cients are found, the displacement of a given node of the mesh, being it inside (interpolation) or outside the domain (extrapolation), can be calculated as the superimposition of the radial contribu tion of each source point. It is then possible to define at known points the displacement in the space and to retrieve the value at mesh nodes, obtaining a mesh deformation that leaves unaltered grid topology (Beckert and Wendland (2001)). The interpolation function is composed by the basis φ and by the polynomial term h with a degree that depends on the kind of the chosen basis. This latter contribution, in particular, is added to assure uniqueness of the problem and polynomial precision, allowing to recover exactly rigid body motions. If N is the total number of source points it follows:

N i = 1

γ i φ ( x − x k i ) + h ( x )

s ( x ) =

(11)

An interpolation exists if coe ffi cients and weights of the polynomial can be found such that the given value at source points can be retrieved exactly. At source points the polynomial contribution should be zero. It is then:

N i = 1

γ i p ( x k i ) = 0

s ( x k i ) = g i , 1 ≤ i ≤ N

and

(12)

for all the polynomials p of degree less or equal to polynomial h . A single interpolant exists if the basis is conditionally positive definite (Micchelli (1986)). If the degree is m ≤ 2 (Beckert and Wendland (2001)) a linear polynomial can be used:

in IR n

h ( x ) = β 1 + β 2 x 1 + β 3 x 2 + ... + β n + 1 x n

(13)