PINN解偏微分方程实例1

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1. PINN简介

PINN是一种利用神经网络求解偏微分方程的方法，其计算流程图如下图所示，这里以偏微分方程(1)为例。
$\begin{align} \frac{\partial u}{\partial t}+u \frac{\partial u}{\partial x}=v\frac{\partial^2 u}{\partial x^2} \end{align}$
神经网络输入位置x,y,z和时间t的值，预测偏微分方程解u在这个时空条件下的数值解。
pinn,深度学习解PDE,算法,深度学习,pytorch
上图中可以看出，PINN的损失函数包含两部分内容，一部分是来源于训练数据误差，另一部分来源于偏微分方程误差，可以记作(2)式。
$\begin{align} \mathcal{l} = w_{data}\mathcal{l}_{data}+w_{PDE}\mathcal{l}_{PDE} \end{align}$
其中
$\begin{align} \begin{aligned} \mathcal{l}_{data} &= \frac{1}{N_{data}}\sum_{i=1}^{N_{data}} (u(x_i,t_i)-u_i)^2 \\ \mathcal{l}_{PDE} &= \frac{1}{N_{data}}\sum_{j=1}^{N_{PDE}} \left( \frac{\partial u}{\partial t}+u \frac{\partial u}{\partial x}-v\frac{\partial^2 u}{\partial x^2} \right)^2|_{(x_j,t_j)} \end{aligned} \end{align}$

2. 偏微分方程实例

考虑偏微分方程如下：
$\begin{align} \begin{aligned} \frac{\partial^2 u}{\partial x^2} - \frac{\partial^4 u}{\partial y^4} = (2-x^2)e^{-y} \end{aligned} \end{align}$
考虑以下边界条件，
$\begin{align} \begin{aligned} u_{yy}(x,0) &= x^2 \\ u_{yy}(x,1) &= \frac{x^2}{e} \\ u(x,0) &= x^2 \\ u(x,1) &= \frac{x^2}{e} \\ u(0,y) &= 0 \\ u(1,y) &= e^{-y} \\ \end{aligned} \end{align}$
以上偏微分方程真解为 $u(x,y)=x^2 e^{-y}$ ,在区域 $[0,1]\times[0,1]$ 上随机采样配置点和数据点，其中配置点用来构造PDE损失函数 $\mathcal{l}_1,\mathcal{l}_2,\cdots,\mathcal{l}_7$ ，数据点用来构造数据损失函数 $\mathcal{l}_8$ .
$\begin{align} \begin{aligned} \mathcal{l}_1 &= \frac{1}{N_1}\sum_{(x_i,y_i)\in\Omega} (\hat{u}_{xx}(x_i,y_i;\theta) - \hat{u}_{yyyy}(x_i,y_i;\theta) - (2-x_i^2)e^{-y_i})^2 \\ \mathcal{l}_2 &= \frac{1}{N_2}\sum_{(x_i,y_i)\in[0,1]\times\{0\}} (\hat{u}_{yy}(x_i,y_i;\theta) - x_i^2)^2 \\ \mathcal{l}_3 &= \frac{1}{N_3}\sum_{(x_i,y_i)\in[0,1]\times\{1\}} (\hat{u}_{yy}(x_i,y_i;\theta) - \frac{x_i^2}{e})^2 \\ \mathcal{l}_4 &= \frac{1}{N_4}\sum_{(x_i,y_i)\in[0,1]\times\{0\}} (\hat{u}(x_i,y_i;\theta) - x_i^2)^2 \\ \mathcal{l}_5 &= \frac{1}{N_5}\sum_{(x_i,y_i)\in[0,1]\times\{1\}} (\hat{u}(x_i,y_i;\theta) - \frac{x_i^2}{e})^2 \\ \mathcal{l}_6 &= \frac{1}{N_6}\sum_{(x_i,y_i)\in\{0\}\times [0,1]}(\hat{u}(x_i,y_i;\theta) - 0)^2 \\ \mathcal{l}_7 &= \frac{1}{N_7}\sum_{(x_i,y_i)\in\{1\}\times [0,1]}(\hat{u}(x_i,y_i;\theta) - e^{-y_i})^2 \\ \mathcal{l}_8 &= \frac{1}{N_{8}}\sum_{i=1}^{N_{8}} (\hat{u}(x_i,y_i;\theta)-u_i)^2 \end{aligned} \end{align}$

3. 基于pytorch实现代码

"""
A scratch for PINN solving the following PDE
u_xx-u_yyyy=(2-x^2)*exp(-y)
Author: ST
Date: 2023/2/26
"""
import torch
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

epochs = 10000    # 训练代数
h = 100    # 画图网格密度
N = 1000    # 内点配置点数
N1 = 100    # 边界点配置点数
N2 = 1000    # PDE数据点

def setup_seed(seed):
    torch.manual_seed(seed)
    torch.cuda.manual_seed_all(seed)
    torch.backends.cudnn.deterministic = True

# 设置随机数种子
setup_seed(888888)

# Domain and Sampling
def interior(n=N):
    # 内点
    x = torch.rand(n, 1)
    y = torch.rand(n, 1)
    cond = (2 - x ** 2) * torch.exp(-y)
    return x.requires_grad_(True), y.requires_grad_(True), cond


def down_yy(n=N1):
    # 边界 u_yy(x,0)=x^2
    x = torch.rand(n, 1)
    y = torch.zeros_like(x)
    cond = x ** 2
    return x.requires_grad_(True), y.requires_grad_(True), cond


def up_yy(n=N1):
    # 边界 u_yy(x,1)=x^2/e
    x = torch.rand(n, 1)
    y = torch.ones_like(x)
    cond = x ** 2 / torch.e
    return x.requires_grad_(True), y.requires_grad_(True), cond


def down(n=N1):
    # 边界 u(x,0)=x^2
    x = torch.rand(n, 1)
    y = torch.zeros_like(x)
    cond = x ** 2
    return x.requires_grad_(True), y.requires_grad_(True), cond


def up(n=N1):
    # 边界 u(x,1)=x^2/e
    x = torch.rand(n, 1)
    y = torch.ones_like(x)
    cond = x ** 2 / torch.e
    return x.requires_grad_(True), y.requires_grad_(True), cond


def left(n=N1):
    # 边界 u(0,y)=0
    y = torch.rand(n, 1)
    x = torch.zeros_like(y)
    cond = torch.zeros_like(x)
    return x.requires_grad_(True), y.requires_grad_(True), cond


def right(n=N1):
    # 边界 u(1,y)=e^(-y)
    y = torch.rand(n, 1)
    x = torch.ones_like(y)
    cond = torch.exp(-y)
    return x.requires_grad_(True), y.requires_grad_(True), cond

def data_interior(n=N2):
    # 内点
    x = torch.rand(n, 1)
    y = torch.rand(n, 1)
    cond = (x ** 2) * torch.exp(-y)
    return x.requires_grad_(True), y.requires_grad_(True), cond


# Neural Network
class MLP(torch.nn.Module):
    def __init__(self):
        super(MLP, self).__init__()
        self.net = torch.nn.Sequential(
            torch.nn.Linear(2, 32),
            torch.nn.Tanh(),
            torch.nn.Linear(32, 32),
            torch.nn.Tanh(),
            torch.nn.Linear(32, 32),
            torch.nn.Tanh(),
            torch.nn.Linear(32, 32),
            torch.nn.Tanh(),
            torch.nn.Linear(32, 1)
        )

    def forward(self, x):
        return self.net(x)


# Loss
loss = torch.nn.MSELoss()


def gradients(u, x, order=1):
    if order == 1:
        return torch.autograd.grad(u, x, grad_outputs=torch.ones_like(u),
                                   create_graph=True,
                                   only_inputs=True, )[0]
    else:
        return gradients(gradients(u, x), x, order=order - 1)

# 以下7个损失是PDE损失
def l_interior(u):
    # 损失函数L1
    x, y, cond = interior()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(gradients(uxy, x, 2) - gradients(uxy, y, 4), cond)


def l_down_yy(u):
    # 损失函数L2
    x, y, cond = down_yy()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(gradients(uxy, y, 2), cond)


def l_up_yy(u):
    # 损失函数L3
    x, y, cond = up_yy()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(gradients(uxy, y, 2), cond)


def l_down(u):
    # 损失函数L4
    x, y, cond = down()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(uxy, cond)


def l_up(u):
    # 损失函数L5
    x, y, cond = up()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(uxy, cond)


def l_left(u):
    # 损失函数L6
    x, y, cond = left()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(uxy, cond)


def l_right(u):
    # 损失函数L7
    x, y, cond = right()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(uxy, cond)

# 构造数据损失
def l_data(u):
    # 损失函数L8
    x, y, cond = data_interior()
    uxy = u(torch.cat([x, y], dim=1))
    return loss(uxy, cond)


# Training

u = MLP()
opt = torch.optim.Adam(params=u.parameters())

for i in range(epochs):
    opt.zero_grad()
    l = l_interior(u) \
        + l_up_yy(u) \
        + l_down_yy(u) \
        + l_up(u) \
        + l_down(u) \
        + l_left(u) \
        + l_right(u) \
        + l_data(u)
    l.backward()
    opt.step()
    if i % 100 == 0:
        print(i)

# Inference
xc = torch.linspace(0, 1, h)
xm, ym = torch.meshgrid(xc, xc)
xx = xm.reshape(-1, 1)
yy = ym.reshape(-1, 1)
xy = torch.cat([xx, yy], dim=1)
u_pred = u(xy)
u_real = xx * xx * torch.exp(-yy)
u_error = torch.abs(u_pred-u_real)
u_pred_fig = u_pred.reshape(h,h)
u_real_fig = u_real.reshape(h,h)
u_error_fig = u_error.reshape(h,h)
print("Max abs error is: ", float(torch.max(torch.abs(u_pred - xx * xx * torch.exp(-yy)))))
# 仅有PDE损失    Max abs error:  0.004852950572967529
# 带有数据点损失  Max abs error:  0.0018916130065917969

# 作PINN数值解图
fig = plt.figure()
ax = Axes3D(fig)
ax.plot_surface(xm.detach().numpy(), ym.detach().numpy(), u_pred_fig.detach().numpy())
ax.text2D(0.5, 0.9, "PINN", transform=ax.transAxes)
plt.show()
fig.savefig("PINN solve.png")

# 作真解图
fig = plt.figure()
ax = Axes3D(fig)
ax.plot_surface(xm.detach().numpy(), ym.detach().numpy(), u_real_fig.detach().numpy())
ax.text2D(0.5, 0.9, "real solve", transform=ax.transAxes)
plt.show()
fig.savefig("real solve.png")

# 误差图
fig = plt.figure()
ax = Axes3D(fig)
ax.plot_surface(xm.detach().numpy(), ym.detach().numpy(), u_error_fig.detach().numpy())
ax.text2D(0.5, 0.9, "abs error", transform=ax.transAxes)
plt.show()
fig.savefig("abs error.png")