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二、 Finite Element Method

2.6 Derive the element matrix

2.6 Derive the element matrix

The variation formulation of Possin equation Z

on each element Ki the global coordinate x; y transfor into local 1; 2; 3 the linear La-grange interpolation of w in the local interpolant w = N1'1+N2'2+N3'3 =

the local element matrix isR

eir ; TZTZr ; jJj d d

Chapter 3

Numerical Method of Monge-Ampére Equation

We follow the Feng's method, that is adding a vanishing biharmonic term such that the fully non-linear Monge-Ampére equation become regular. The elliptic regularization Monge-Ampére equation:

42u + det D2u = f; in

u = gon @ (3.71)

4u = on @

where is a bound domain in the R2 with a smooth boundary @ , f is a given function

3.1 Linearization Regularization Monge-Ampére equation

the function of regularization Monge-Ampére equation

M A [u] = 42u + det D2u (3.72)

variation of MA [u] is

DuM A [u] = (Dyyu) (Dxx) + (Dxxu) (Dyy) 2 (Dxyu) (Dxy) 42

= O cof(D2u)O 42 (3.73)

the linearization regularization Monge-Ampére equation

42u +O cof(D2u)Ou = f (3.74)

36

3.3 Non-linear iteration 37

3.2 Variation formulation

the equivalent variational problem

Z

42u vdx +

Z O cof(D2u)Ou vdx =Z

f vdxin (3.75) the weak formulation of the biharmonic term R

42u vdxand 4u = the weak formulation of the fully non-linear term

Z

So the equivalent variational problem of equation (3:71) is Z

For non-linear problem, we usually use iterative method such as xed-point iteration, Newton's iteration etc.. Iteration method can be classi ed by the rate of convergence, q-quadratically, q-superlinearly, and q-linearly.

3.3 Non-linear iteration 38

3.3.1 Fixed-Point Iteration

Many non-linear equation are naturally formulated as xed-point problem

x = K (x) (3.79)

where K, the xed-point map may be non-linear. A solution ^x of (3:79) called a xed point of the map K. The xed-point iteration is given by

xn+1 = K (xn) (3.80)

This iterative method is also called non-linear Richardson iteration, Picard iteration, or the method of successive substitution.

3.3.2 Newton's method

The Newton's iteration is

xn+1 = xn F0(xn) 1F (xn) (3.81)

sometimes the F0(xn) 1is not easy to nd, then we can consider use approximate the term, such as chord method, Shamanskii method or secant method etc..

3.3.3 Non-linear iteration of regularization Monge-Ampére equation

The regularization Monge-Ampére equation

F [u] = f + 42u det D2u (3.82)

the DuF [u]is

DuF [u] = O cof(D2u)O + 42 (3.83)

3.3 Non-linear iteration 39

Newton's iteration of F

un+1 = un DuF [un] 1F [un] (3.84)

3.4 Basis function of BCIZ element 40

Fig. 3.12. BCIZ truangular element

3.4 Basis function of BCIZ element

To build the necessary technical tools, we shall derive and present a detailed study of the linearization of the elliptic regularization Monge-Ampere equation and its BCIZ nite el-ement approximation. Introduction to the BCIZ elel-ement, BCIZ elel-ement is conforming element, it can calculus the curvature easily, and its approximation is very well. But the basic BCIZ element has a problem, if the mesh is non-uniform mesh, then the numeri-cal result is lost the accuracy. So many people propose the revise BCIZ element such that numerical result has good approximation on non-uniform mesh.

BCIZ element:

Let P = P3. Let N = fN1; N2; :::; N9g

In the free-form design problem, we must to consider that the rst differential of the solution, so we choose BCIZ nite element approximation. It can easy the calculus the rst differential and curve of each element.

3.4 Basis function of BCIZ element 41

The visible degree of freedom of the the element collected in v are

vT = w1 x1 y1 w2 x2 y2 w3 x3 y3 (3.85)

where the xand xis consider rotation, that is different from u, under Cartesian coordinate u = w; uy = x; ux = y:

'1 = 21(3 2 1) + 2 1 2 3 '2 = 21(y12 2+ y13 3) 1

2(y12+ y13) 1 2 3 '3 = 21(x12 2+ x13 3) + 1

2(x12+ x13) 1 2 3 '4 = 22(3 2 2) + 2 1 2 3

'5 = 22(y23 3+ y21 1) 1

2(y23+ y21) 1 2 3 '6 = 22(x23 3+ x21 1) + 1

2(x23+ x21) 1 2 3 '7 = 23(3 2 3) + 2 1 2 3

'8 = 23(y31 1+ y32 2) 1

2(y31+ y32) 1 2 3 '9 = 21(x31 1+ x32 2) + 1

2(x31+ x32) 1 2 3 (3.86) where i are triangular coordinate.

3.4 Basis function of BCIZ element 42

Let

= '1 '2 '3 '4 '5 '6 '7 '8 '9 (3.87)

and

w ( 1; 2; 3) = v (3.88)

Derive the element matrix Derive the element matrix of the variation equation (3:78) with BCIZ element

3.4.1 The linearization of non-linear term and element matrix

Change coordinate from the global coordinate to the local coordinate, the relationship of Ox;yandO 1; 2; 3 is

the w use BCIZ element to approximation, w = v

2

3.4 Basis function of BCIZ element 43

so theOv = ZBv; then the element matrix of R

(cof (D2u)Ou) Ovdx is

3.4 Basis function of BCIZ element 44

The second derivative of a function w ( 1; 2; 3)with respect to x or y from (2:59) and application of the chain rule:

@2w which matrix form is

2

3.4 Basis function of BCIZ element 45

the w use BCIZ element to approximation, w = v 2

3.4 Basis function of BCIZ element 46

In this thesis, we will follow this algorithm 1. Given a initial u0, and tolerance T

2. Fixed

3. Newton's iteration if kun+1 unk T then it converge, if not the iteration is diverge

4. if h2 where h is mesh size then out the algorithm

5. let = =c where c is a constant, then go to 2

Chapter 4 Numerical Study

The numerical result will be given, These are three part of this chapter: Poisson equation ,biharmonic equation and Monge-Ampére equation.

4.1 Poisson Equation

Poisson Equation:

4u = f in uj@ = g

where f and g are obtained from a given analytical solution u. We use Linear element and BCIZ element to approximate the Poisson equation. We compute the Poisson equation on different mesh size. Our calculation domain is [0; 1] [0; 1] :The boudary condition are Dirchlet type.

47

4.1 Poisson Equation 48

4.1.1 Example:

The analytical solution u = ex+y; f = 2ex+y and g = ex+y: we use linear element to approximate the Poisson equation in this case.

Computed solution uh Error

h ku0 uhk1 ku0 uhkL2 ku0 uhkH1

1=10 5.97E-03 8.28E-04 2.04E-01 1=20 1.98E-03 2.21E-04 1.02E-01 1=40 6.28E-04 5.68E-05 5.10E-02 1=80 1.91E-04 1.44E-05 2.55E-02 1=160 5.65E-05 3.61E-06 1.28E-02

Table 1. Change of ku uhk w.r.t. h

The convergence rate of L2 norm is second order and H1 norm is rst order. This result is same as error estimates of the biharmonic equation using BCIZ element approxi-mation.

4.1 Poisson Equation 49

4.1.2 Example:

The analytical solution u = sin (2 x) sin (2 y) ; f = 8 2sin (2 x) sin (2 y)and g = 0:

we use BCIZ element to approximate the Poisson equation in this case.

Computed solution uh Error

h ku0 uhk1 ku0 uhkL2 ku0 uhkH1

2 3 1.23E-3 8.03E-4 4.37E-2

2 4 1.67E-4 7.54E-5 4.19E-3

2 5 1.32E-5 7.24E-6 5.32E-4

2 6 8.83E-7 7.85E-7 1.18E-4

Table 2. Change of ku uhk w.r.t. h

The convergence rate of L2 norm is third order and H1 norm is second order. This result is same as error estimates of the biharmonic equation using BCIZ element approxi-mation.

4.2 Biharmonic Equation 50

4.2 Biharmonic Equation

Biharmonic Equation:

42u = f in uj@ = g Ou nj@ = h

where f, g and h are obtained from a given analytical solution u. We use BCIZ element to approximate the Biharmonic equation. We compute the Biharmonic equation on different mesh size. Our calculation domain is [0; 1] [0; 1] : The boudary condition are Dirichlet type and Neumann type. Because of the Biharmonic equation is fourth order equation, so the approximation of linear element maybe not have a high accuracy.

4.2 Biharmonic Equation 51

4.2.1 Example:

The analytical solution u = x cos(x)ey; f = 0,g = x cos(x)eyandOu = cos(x)ey x sin(x)ey x cos(x)ey :

Computed solution uh Error

h ku0 uhk1 ku0 uhkL2 ku0 uhkH2

2 2 0.002971694 0.001206162 0.499530706 2 3 0.00065049 0.000273676 0.242128734 2 4 0.000155176 6.32838E-05 0.119339924 2 5 3.78495E-05 1.51706E-05 0.059270777 2 6 9.32505E-06 3.71417E-06 0.02954046 2 7 2.31532E-06 9.1924E-07 0.014747201

Table 3. Change of ku uhk w.r.t. h

The convergence rate of L2 norm is second order and H2 norm is rst order. This result is same as error estimates of the biharmonic equation using BCIZ element approxi-mation.

4.2 Biharmonic Equation 52

4.2.2 Example:

The analytical solution u = (cos(2 x) 1) (y2 2y3 + y4) ; f = 0,g = x cos(x)ey and h = 0:

Computed solution uh Error

h ku0 uhk1 ku0 uhkL2 ku0 uhkH2

2 2 0.01171965 0.004154116 0.645238157 2 3 0.004269365 0.001568274 0.307797116 2 4 0.001169401 0.000445123 0.150571686 2 5 0.000302368 0.000116688 0.074556331 2 6 7.67083E-05 2.97733E-05 0.037111559 2 7 1.93091E-05 7.5141E-06 0.018516197

Table 4. Change of ku uhk w.r.t. h

The convergence rate of L2 norm is second order and H2 norm is rst order. This result is same as error estimates of the biharmonic equation using BCIZ element approxi-mation.

4.3 Monge-Ampére 53

4.3 Monge-Ampére

Regularization Monge-Ampére Equation:

42u + det D2u = f; in u = gon @

4u = on @

where f and g are obtained from a given analytical solution u. We use BCIZ element to approximate the Monge-Ampére equation. We compute the Poisson equation on different parameter with xed mesh size h. The boudary condition are Dirchlet type. In this section, we provide several 2-D numerical experiments of BCIZ element. And the initial condition is given by zero. The start from 1 to h2:

4.3 Monge-Ampére 54

4.3.1 Example:

This test, we calculus ku0 uhk for xed mesh size h = 2 8;while varying in order to approximate ku0 u k : We use BCIZ element and set to solve problem (3:71) with the analytical solution u = x4+ y2 ; f = 24x2and g = x4+ y2;Our calculation domain is [0; 1] [0; 1] :

solution error

ku0 uhk1 ku0 uhkL2 ku0 uhkH2 iter

1 3.05E-1 1.61E-1 5.32 6

2 2 2.30E-1 1.21E-1 4.67 10

2 4 1.13E-1 5.71E-2 3.63 10

2 6 4.23E-2 1.90E-2 2.71 8

2 8 1.45E-2 5.67E-3 1.99 8

2 10 4.50E-3 1.60E-3 1.44 8

2 12 1.29E-3 4.33E-4 1.03 9

2 14 3.48E-4 1.13E-4 7.33E-1 10

Table 5. Change of ku0 uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 55

ku0 uhkL2 ku0 uhkH2

p4

1 0.160842924 5.319207667 2 2 0.482036757 6.609013597 2 4 0.913779843 7.268095576 2 6 1.218354047 7.670044224 2 8 1.450567862 7.955838131 2 10 1.637202829 8.133261803 2 12 1.772707203 8.235401948 2 14 1.85193153 8.290262971 Table 6. Change of ku0 uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 56

4.3.2 Example:

This test, we calculus ku0 uhk for xed mesh size h = 2 8;while varying in order to approximate ku0 u k : We use BCIZ element and set to solve problem (3:71) with the analytical solution u = 20x6 + y6 ; f = 18000x4y4 and g = 20x6+ y6;Our calculation domain is [0; 1] [0; 1] :

solution error

ku0 uhk1 ku0 uhkL2 ku0 uhkH2 iter

4 6.43 2.88 177.28 5

1 5.22 2.17 167.40 9

2 2 3.31 1.19 148.99 10

2 4 1.79 6.20E-1 125.31 10

2 6 8.72E-1 2.57E-1 101.97 10

2 8 4.02E-1 9.17E-2 82.10 10

2 10 1.80E-1 3.22E-2 65.25 11

2 12 7.93E-2 1.11E-2 51.21 13

2 14 3.45E-2 3.74E-3 39.77 20

Table 7. Change of ku uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 57

ku0 uhkL2 ku0 uhkH2

p4

4 0.720510991 125.3567907 1 2.165121991 167.3973101 2 2 4.758836554 210.7002103 2 4 9.926086663 250.6286892 2 6 16.4590354 288.4238466 2 8 23.47587569 328.3874835 2 10 32.93384144 369.1038869 2 12 45.33800541 409.6430813 2 14 61.24535142 449.9969415 Table 8. Change of ku uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 58

4.3.3 Example:

This test, we calculus ku0 uhk for xed mesh size h = 2 8;while varying in order to approximate ku0 u k : We use BCIZ element and set to solve problem (3:71) with the analytical solution u = ex2+y22 ; f = (1 + x2+ y2) ex2+y2 and g = ex2+y22 ;Our calculation domain is [0; 1] [0; 1] :

solution error

ku0 uhk1 ku0 uhkL2 ku0 uhkH2 iter

1 1.78E-1 1.01E-1 3.03 29

2 2 1.41E-1 8.17E-2 2.72 48

2 4 7.14E-2 4.45E-2 2.13 38

2 6 2.26E-2 1.56E-2 1.57 9

2 8 6.30E-3 4.52E-3 1.13 8

2 10 1.81E-3 1.22E-3 8.00E-1 8

2 12 5.00E-4 3.16E-4 5.67E-1 8

2 14 1.33E-4 8.00E-5 4.01E-1 9

Table 9. Change of ku uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 59

ku0 uhkL2 ku0 uhkH2

p4

1 0.100518486 3.026105949 2 2 0.32688765 3.840967403 2 4 0.71238757 4.259311736 2 6 0.99970787 4.434163657 2 8 1.157804247 4.506118399 2 10 1.24700059 4.527808719 2 12 1.294427725 4.534697517 2 14 1.309904805 4.53757113

Table 10. Change of ku uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 60

4.3.4 Example:

This test, we calculus ku0 uhk for xed mesh size h = 2 8;while varying in order to approximate ku0 u k : We use BCIZ element and set to solve problem (3:71) with the analytical solution u = 2p2(x2+y2)34

3 ; f = p 1

x2+y2 and g = 2p2(x2+y2)34

3 ;Our calculation domain is [0; 1] [0; 1] :Where f has a singular point at (0; 0) :

solution error

ku0 uhk1 ku0 uhkL2 ku0 uhkH2 iter

1 1.41E-1 7.89E-2 2.16 5

2 2 1.23E-1 6.93E-2 2.01 19

2 4 7.59E-2 4.48E-2 1.64 9

2 6 2.78E-2 1.81E-2 1.22 10

2 8 8.01E-3 5.57E-3 8.95E-1 8

2 10 2.12E-3 1.55E-3 6.51E-1 8

2 12 5.57E-4 4.08E-4 4.68E-1 9

2 14 1.44E-4 1.04E-4 3.34E-1 11

Table 11. Change of ku uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 61

ku0 uhkL2 ku0 uhkH2

p4

1 0.078906702 2.164987734 2 2 0.277330363 2.843363264 2 4 0.717285822 3.275360039 2 6 1.15590166 3.442870121 2 8 1.425652095 3.57845558 2 10 1.583580447 3.680240848 2 12 1.671089711 3.747674023 2 14 1.709884392 3.773972909 Table 12. Change of ku uhk w.r.t. (h = 2 8)

4.3 Monge-Ampére 62

4.3.5 Example:

This test, we calculus ku0 uhk for xed mesh size h = 2 8;while varying in order to approximate ku0 u k : We use BCIZ element and set to solve problem (3:71) with the analytical solution u =p

x2+ y2; f = 0 if (x; y) 6= (0; 0)

? if (x; y) = (0; 0) and g =p

x2+ y2;Our calculation domain is [ 1; 1] [ 1; 1] :Where f has a singular point at (0; 0), and our guess the value of f at (0; 0) is 3 :

solution error

ku0 uhk1 ku0 uhkL2 ku0 uhkH2 iter

1 8.17E-1 6.00E-1 8.65 5

2 2 5.51E-1 3.72E-1 8.82 9

2 4 2.48E-1 1.42E-1 9.27 10

2 6 9.77E-2 5.31E-2 9.72 11

2 8 4.151E-2 2.56E-2 10.09 14

2 10 2.08E-2 1.49E-2 10.32 19

2 12 9.28E-3 6.92E-3 10.51 28

2 13 3.59E-3 1.95E-3 10.66 21

Table 13. Change of ku uhk w.r.t. (h = 1=127)

4.3 Monge-Ampére 63

ku0 uhkL2 ku0 uhkH2

p4

1 0.599947604 8.652542689 2 2 1.48793695 12.4786026 2 4 2.278636795 18.53711229 2 6 3.398612852 27.50569621 2 8 6.546636298 40.36922514 2 10 15.30464406 58.38278827 2 12 28.35360433 84.08190819 2 13 16.00805235 101.4092367

Table 14. Change of ku uhk w.r.t. (h = 1=127)

4.3 Monge-Ampére 64

Compared with Feng's and Oberman's result:

case1: u = ex2+y22 ; f = (1 + x2 + y2) ex2+y2

Ours Feng's

ku uhkL2 ku uhkL2

2 1 0.093580805 0:5 0.038717 2 2 0.081721912 0:25 0.040988 2 3 0.064370429 0:1 0.032218 2 4 0.044524223 0:05 0.022259 2 6 0.015620435 0:0125 0.007817 2 9 0.002361013 0:0025 0.001864 2 11 0.00062253 0:0005 0.000405

Ours Oberman's

N iter N M1 M2

128 69 121 31965 1205

4.3 Monge-Ampére 65

case 2: u = p

x2+ y2; f = 0 if (x; y) 6= (0; 0)

? if (x; y) = (0; 0)

Ours Feng's

ku uhkL2 ku uhkL2

2 1 0.504058 2 2 0.371984 2 3 0.239381

2 4 0.142415 none

2 6 0.053103 2 9 0.014946 2 11 0.001954

Ours Oberman's

N iter N M1 M2

128 117 121 36396 10486

Under same accuracy, iteration number is less than other group. And the case have singularty can be achieved with high ef cient computing.

Chapter 5 Conclusion

1. The error of ku uhkL2 of the Monge-Ampére is O ( ) from test cases. The error of ku uhkH2 of the Monge-Ampére is O (p4

)from test cases.

2. In numerical simulation of the elliptic regularization Monge-Ampére, the h2; where h is mesh size.

3. In the singular case, we can shift the grid point such that the singular point locate in a element.

66

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