Unformatted text preview: the form x(
y( 3
3
) = X X xij N 2 (
)
yij i j
i=1 j =1 ) (5.4.4) where Ni2j ( ), i j = 1 2 3, is given by (5.4.3).
This transformation produces an element in the (x y)plane having curved (quadratic)
edges as shown in Figure 5.4.3. An isoparametric approximation would be biquadratic 5.4. Coordinate Transformations
y 19
η
3
11
00 3 11
00
11
00
6
11
00
11
00 11
00
11
00
1 11
00 5 11
00
11
00 11
002
11
00 11
00
11
00 11
00
6
11
00 5
11
00
11
00 11
00
11
00 11
00
11
00 4 x 1 11
00 ξ
11
00 4 2 Figure 5.4.4: Quadratic mapping of a triangle having one curved side.
and have the form of (5.4.3). The interior node (3,3) is awkward and can be eliminated
by using a secondorder serendipity (cf. Problems 4.3.1) or hierarchical transformation
(cf. Section 4.4).
Example 5.4.3. The biquadratic transformation described in Example 5.4.2 is useful
for discretizing domains having curved boundaries. With a similar goal, we describe a
transformation for creating triangular elements having one curved and two straight sides
(Figure 5.4.4). Let us approximate the curved boundary by a quadratic polynomial and
map the element onto a canonical right triangle by the quadratic transformation x(
y( 6
) = X xi N 2 (
)
yi i
i=1 ) (5.4.5a) where the quadratic Lagrange shape functions are (cf. Problem 4.2.1) Nj2 = 2 j (
N42 = 4 j ; 1=2) N52 = 4 12 j=1 2 3
23 N62 = 4 (5.4.5b)
31 (5.4.5c) and
1 =1; ; 2 = 3 =: (5.4.6) Equations (5.4.5) and (5.4.6) describe a general quadratic transformation. We have a
more restricted situation with x4 = (x1 + x2 )=2 y4 = (y1 + y2)=2 x6 = (x1 + x3 )=2 y6 = (y1 + y3)=2: 20 Mesh Generation and Assembly This simpli es the transformation (5.4.5a) to x(
y( ) = x1 N 2 + x2 N 2 + x3 N 2 + x5 N 2
^
^
^
^
)
y1 1
y2 2
y3 3
y5 5 (5.4.7a) where, upon use of (5.4.5) and (5.4.6),
^
N12 = N12 + (N42 + N62 )=2 = 1 = 1 ; ; (5.4.7b) ^
N22 = N22 + N42 =2 = (1 ; 2 ) (5.4.7c) ^
N32 = N32 + N62 =2 = (1 ; 2 ) (5.4.7d) ^
N52 = N52 = 4 : (5.4.7e) From these results, we see that the mappings on edges 12 ( = 0) and 13 ( = 0) are
linear and are, respectively, given by x =...
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This document was uploaded on 03/16/2014 for the course CSCI 6860 at Rensselaer Polytechnic Institute.
 Spring '14
 JosephE.Flaherty

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