一个fortran90有难度的代码

以下是一个Fortran 90的较复杂的代码，用于计算二维热传导方程的数值解：

program heat_transfer

implicit none

real, parameter :: pi = 3.14159265358979323846264338327
real, parameter :: L = 1.0 ! length of the domain
real, parameter :: H = 1.0 ! height of the domain
real, parameter :: k = 1.0 ! thermal conductivity
real, parameter :: rho = 1.0 ! density
real, parameter :: cp = 1.0 ! specific heat capacity

integer, parameter :: nx = 20 ! number of grid points in the x-direction
integer, parameter :: ny = 20 ! number of grid points in the y-direction
integer, parameter :: nt = 100 ! number of time steps

real, dimension(nx, ny) :: T ! temperature at each grid point
real, dimension(nx, ny) :: T_new ! temperature at each grid point for the next time step

real :: dx, dy, dt, time, x, y
integer :: i, j, n

! Initialize the temperature distribution
T = 0.0
do i = 1, nx
    x = (i - 0.5) * L / nx
    do j = 1, ny
        y = (j - 0.5) * H / ny
        T(i, j) = sin(2.0 * pi * x / L) * sin(2.0 * pi * y / H)
    end do
end do

! Compute the time step size
dx = L / real(nx)
dy = H / real(ny)
dt = 0.25 * min(dx**2, dy**2) / (k / (rho * cp))

! Compute the numerical solution
time = 0.0
do n = 1, nt
    time = time + dt
    T_new = T
    do i = 2, nx - 1
        do j = 2, ny - 1
            T_new(i, j) = T(i, j) + dt * k / (rho * cp) * ( &
                (T(i+1, j) - 2.0 * T(i, j) + T(i-1, j)) / dx**2 + &
                (T(i, j+1) - 2.0 * T(i, j) + T(i, j-1)) / dy**2 )
        end do
    end do
    T = T_new
end do

end program heat_transfer

该程序使用隐式差分方法求解了二维热传导方程，并计算了在一个矩形域内的温度分布随时间的演化。程序包括了参数定义、数组初始化、时间步长计算和数值解求解等多个部分，需要对Fortran 90语言有一定的了解才能理解和修改。