High peaks of FEM solution

Following this question and user21 answered, I changed the BCs of the initial problem. The code following bellow.

<< NumericalDifferentialEquationAnalysis;
Needs["NDSolveFEM"];

G = Rationalize[6.894745 10^9]
E1 = Rationalize[26.25 G]; E2 = Rationalize[1.49 G]; G12 =
Rationalize[1.04 G]; nu12 = Rationalize[0.28]; nu21 = (E2*nu12)/E1;

t = Rationalize[0.0050 .0254];
Son = {{1/E1, -nu12/E1, 0}, {-nu21/E2, 1/E2, 0}, {0, 0, 1/G12}};

Qon = Inverse[Son];
Q11 = Qon[[1, 1]]; Q12 = Qon[[1, 2]]; Q22 = Qon[[2, 2]]; Q66 =
Qon[[3, 3]];

U1 = (3 Q11 + 3 Q22 + 2 Q12 + 4 Q66)/8; U2 = (Q11 - Q22)/
2; U3 = (Q11 + Q22 - 2 Q12 - 4 Q66)/
8; U4 = (Q11 + Q22 + 6 Q12 - 4 Q66)/
8; U5 = (Q11 + Q22 - 2 Q12 + 4 Q66)/8;

alpha = 45 (\[Pi]/180);
a = 1; b = 1; d = a Cos[alpha] + b Sin[alpha];
omega = Rectangle[{0, 0}, {a, b}];
mesh = ToElementMesh[omega, MaxCellMeasure -> 0.001];
u0 = Rationalize[0.01];
angle1 = 10; angle0 = 0;
angles = {{angle0, angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {angle0, angle1}};

num = Dimensions[angles][]; h = num*t; pos = Table[0, num + 1];
pos[] = -h/2;
For[i = 2, i <= num + 1, i++, pos[[i]] = pos[[i - 1]] + t];

\[Xi]A = {0, 0, 0, 0, 0};
\[Xi]B = {0, 0, 0, 0, 0};
\[Xi]D = {0, 0, 0, 0, 0};
For[i = 1, i <= num, i++, T0 = angles[[i, 1]];
T1 = angles[[i, 2]];
func[s_] :=
Simplify@((2/d) (T1 - T0) Sqrt[(s - d/2)^2] + T0) (\[Pi]/180);
theta[x_, y_] := alpha + func[x Cos[alpha] + y Sin[alpha]];
zA = pos[[i + 1]] - pos[[i]];
zB = pos[[i + 1]]^2 - pos[[i]]^2;
zD = pos[[i + 1]]^3 - pos[[i]]^3;
V1 = Cos[2 theta[x, y]]; V2 = Sin[2 theta[x, y]];
V3 = Cos[4 theta[x, y]]; V4 = Sin[4 theta[x, y]];
\[Xi]a = {1, V1, V2, V3, V4} zA;
\[Xi]b = {1, V1, V2, V3, V4} zB;
\[Xi]d = {1, V1, V2, V3, V4} zD;
\[Xi]A = \[Xi]A + \[Xi]a;
\[Xi]B = \[Xi]B + \[Xi]b;
\[Xi]D = \[Xi]D + \[Xi]d;];
mU = {{U1, U2, 0, U3, 0}, {U4, 0, 0, -U3, 0}, {U1, -U2, 0, U3, 0}, {0,
0, U2/2, 0, U3}, {0, 0, U2/2, 0, -U3}, {U5, 0, 0, -U3, 0}};

mA = mU.\[Xi]A; mB = (mU.\[Xi]B)/2; mD = (mU.\[Xi]D)/3;
A11[x_, y_] = mA[]; A12[x_, y_] = mA[]; A16[x_, y_] = mA[];
A22[x_, y_] = mA[]; A26[x_, y_] = mA[]; A66[x_, y_] = mA[];
D11[x_, y_] = mD[]; D12[x_, y_] = mD[]; D16[x_, y_] = mD[];
D22[x_, y_] = mD[]; D26[x_, y_] = mD[]; D66[x_, y_] = mD[];

Nx[x_, y_] =
A11[x, y] D[u[x, y], {x, 1}] + A12[x, y] D[v[x, y], {y, 1}] +
A16[x, y] (D[u[x, y], {y, 1}] + D[v[x, y], {x, 1}]);
Ny[x_, y_] =
A12[x, y] D[u[x, y], {x, 1}] + A22[x, y] D[v[x, y], {y, 1}] +
A26[x, y] (D[u[x, y], {y, 1}] + D[v[x, y], {x, 1}]);

Nxy[x_, y_] =
A16[x, y] D[u[x, y], {x, 1}] + A26[x, y] D[v[x, y], {y, 1}] +
A66[x, y] (D[u[x, y], {y, 1}] + D[v[x, y], {x, 1}]);

PDEs = {D[Nx[x, y], {x, 1}] + D[Nxy[x, y], {y, 1}],
D[Ny[x, y], {y, 1}] + D[Nxy[x, y], {x, 1}]};

gammaD = {DirichletCondition[{v[x, y] == u0}, y == 0],
DirichletCondition[{v[x, y] == -u0}, y == b]};
omega = Rectangle[{0, 0}, {a, b}];

mesh = ToElementMesh[omega, MaxCellMeasure -> 0.001];

PDEs = FullSimplify[PDEs];

{U, V} = NDSolveValue[{PDEs == {0, 0}, gammaD,
DirichletCondition[u[x, y] == 0, x == a/2]}, {u,
v}, {x, y} \[Element] mesh];

Plotting it the solution for U[x,y], it's possible to see two peaks. Differentiation of x (D[U[x, y], {x, 1}]) potentiates these peaks as can be seen bellow These are not expected results. Would anyone know the reason of these peaks? I've been tried to refine the mesh (~ 10000 quad elements) and these peaks keep happening. Am I losing some important thing here?

Don't use exact number since it turns symbolic calculation and probably force LinearSolve to get exact solution. Just put for instance alpha = 45.001 (\[Pi]/180); and then you get desirable result (I deleted line of code you repeat twice with omega = Rectangle[{0, 0}, {a, b}]; mesh = ToElementMesh[omega, MaxCellMeasure -> 0.001];)

Needs["NDSolveFEM`"];

G = 6.894745 10^9;

E1 = 26.25 G; E2 = 1.49 G; G12 =
1.04 G; nu12 = 0.28; nu21 = (E2*nu12)/E1;

t = 0.0050 .0254;
Son = {{1/E1, -nu12/E1, 0}, {-nu21/E2, 1/E2, 0}, {0, 0, 1/G12}}; Qon =
Inverse[Son];

Q11 = Qon[[1, 1]]; Q12 = Qon[[1, 2]]; Q22 = Qon[[2, 2]]; Q66 =
Qon[[3, 3]];

U1 = (3 Q11 + 3 Q22 + 2 Q12 + 4 Q66)/8; U2 = (Q11 - Q22)/
2; U3 = (Q11 + Q22 - 2 Q12 - 4 Q66)/
8; U4 = (Q11 + Q22 + 6 Q12 - 4 Q66)/
8; U5 = (Q11 + Q22 - 2 Q12 + 4 Q66)/8;

alpha = 45.001 (\[Pi]/180);
a = 1; b = 1; d = a Cos[alpha] + b Sin[alpha];
omega = Rectangle[{0, 0}, {a, b}];
mesh = ToElementMesh[omega, MaxCellMeasure -> 0.001,
PrecisionGoal -> 5, AccuracyGoal -> 5];
u0 = 0.01;

angle1 = 10; angle0 = 0;
angles = {{angle0, angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {angle0,
angle1}, {-angle0, -angle1}, {angle0, angle1}};

num = Dimensions[angles][]; h = num*t; pos = Table[0, num + 1];
pos[] = -h/2;
For[i = 2, i <= num + 1, i++, pos[[i]] = pos[[i - 1]] + t];

\[Xi]A = {0, 0, 0, 0, 0};
\[Xi]B = {0, 0, 0, 0, 0};
\[Xi]D = {0, 0, 0, 0, 0};

For[i = 1, i <= num, i++, T0 = angles[[i, 1]];
T1 = angles[[i, 2]];
func[s_] :=
Simplify@((2.0/d) (T1 - T0) Sqrt[(s - d/2)^2] + T0) (\[Pi]/180);
theta[x_, y_] := alpha + func[x Cos[alpha] + y Sin[alpha]];
zA = pos[[i + 1]] - pos[[i]]; zB = pos[[i + 1]]^2 - pos[[i]]^2;
zD = pos[[i + 1]]^3 - pos[[i]]^3;
V1 = Cos[2 theta[x, y]]; V2 = Sin[2 theta[x, y]];
V3 = Cos[4 theta[x, y]]; V4 = Sin[4 theta[x, y]];
\[Xi]a = {1, V1, V2, V3, V4} zA;
\[Xi]b = {1, V1, V2, V3, V4} zB;
\[Xi]d = {1, V1, V2, V3, V4} zD;
\[Xi]A = \[Xi]A + \[Xi]a;
\[Xi]B = \[Xi]B + \[Xi]b;
\[Xi]D = \[Xi]D + \[Xi]d;];

mU = {{U1, U2, 0, U3, 0}, {U4, 0, 0, -U3, 0}, {U1, -U2, 0, U3, 0}, {0,
0, U2/2, 0, U3}, {0, 0, U2/2, 0, -U3}, {U5, 0, 0, -U3, 0}};

mA = mU.\[Xi]A; mB = (mU.\[Xi]B)/2; mD = (mU.\[Xi]D)/3;

A11[x_, y_] = mA[]; A12[x_, y_] = mA[]; A16[x_, y_] = mA[];
A22[x_, y_] = mA[]; A26[x_, y_] = mA[]; A66[x_, y_] = mA[];
D11[x_, y_] = mD[]; D12[x_, y_] = mD[]; D16[x_, y_] = mD[];
D22[x_, y_] = mD[]; D26[x_, y_] = mD[]; D66[x_, y_] = mD[];

Nx[x_, y_] =
A11[x, y] D[u[x, y], {x, 1}] + A12[x, y] D[v[x, y], {y, 1}] +
A16[x, y] (D[u[x, y], {y, 1}] + D[v[x, y], {x, 1}]);

Ny[x_, y_] =
A12[x, y] D[u[x, y], {x, 1}] + A22[x, y] D[v[x, y], {y, 1}] +
A26[x, y] (D[u[x, y], {y, 1}] + D[v[x, y], {x, 1}]);

Nxy[x_, y_] =
A16[x, y] D[u[x, y], {x, 1}] + A26[x, y] D[v[x, y], {y, 1}] +
A66[x, y] (D[u[x, y], {y, 1}] + D[v[x, y], {x, 1}]);

PDEs = {D[Nx[x, y], {x, 1}] + D[Nxy[x, y], {y, 1}],
D[Ny[x, y], {y, 1}] + D[Nxy[x, y], {x, 1}]};

gammaD = {DirichletCondition[{v[x, y] == u0, u[x, y] == 0}, y == 0],DirichletCondition[{v[x, y] == -u0, u[x, y] == 0}, y == b]};

{U, V} = NDSolveValue[{PDEs == {0, 0}, gammaD,
Dirichlet

Condition[u[x, y] == 0, x == a/2]}, {u,
v}, {x, y} \[Element] mesh];

Visualisation

{DensityPlot[U[x, y], {x, y} \[Element] mesh,
ColorFunction -> "Rainbow", PlotLegends -> Automatic],
DensityPlot[V[x, y], {x, y} \[Element] mesh,
ColorFunction -> "Rainbow", PlotLegends -> Automatic]} If we change boundary condition as gammaD = {DirichletCondition[{v[x, y] == u0}, y == 0], DirichletCondition[{v[x, y] == -u0}, y == b]};, then the solution looks as follows • Alex, you used gammaD = {DirichletCondition[{v[x, y] == u0, u[x, y] == 0}, y == 0], DirichletCondition[{v[x, y] == -u0, u[x, y] == 0}, y == b]};, instead gammaD = {DirichletCondition[{v[x, y] == u0}, y == 0], DirichletCondition[{v[x, y] == -u0, y == b]};. My plots was created using the last one gammaD, as it's in my question. Did you see? Apr 16 '20 at 20:56
• @DiegoMagela Ok, I used a new gammaD and now can see small pick due to boundary condition DirichletCondition[u[x, y] == 0, x == a/2] Apr 16 '20 at 22:43
• @DiegoMagela Yes, it seems like this BC is wrong in your problem. I don't understand how we can physically manage this BC. Apr 17 '20 at 11:27
• @DiegoMagela For alpha=0 there is symmetrical case, so BC is valid, but for $alpha \ne 0$ this BC is not true. Apr 17 '20 at 13:27
• @user21 It is not first time I am getting this kind of error due to mixing of numerical and symbolic calculations. It is better to make all symbolic calculations separately and use final result with 'NDSolve[]'. Apr 17 '20 at 13:33