Problems for Chapter 11
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Problems for chapter 11
11.1 FDM for Elliptic problems in 2D
consider the poisson equation for the unknown function u(①, g):
u①① + ugg = 1,
with boundary conditions
u(O, g) = O, u(①, O) = O,
O < ① < 1,
u(1, g) = g,
u(①, 1) = ①,
O < g < 1,
O 三 ① 三 1,
O 三 g 三 1.
consider a uniform grid, choosing N = 5 for both the ① and the g direction, so that
h = 1/5, ①i = ih, (i = O, 1, . . . , 5), gj = jh (j = O, 1, . . . , 5).
Let ui,j 今 u(①i, gj) denote the approximate solutions. using inite diferent method, set up the system of linear equations for the unknown ui,j . (you don,t need to solve the system of linear equations.)
11.2 Heat equation
Let u(t, ①) satisfy the equation
ut(t, ①) = 4u①① (t, ①) + 1, O < ① < 1, t 持 O
with initial condition
u(O, ①) = O, O < ① < 1,
and boundary conditions
u(t, O) = O, u(t, 1) = O, t > O.
This equation describes the temperature in a rod. The rod initially has a temperature of Oc (zero degree celsius), and is then heated at a uniform rate 1. However, its two endpoints are kept at the temperature of Oc at all times.
The unknown function u(t, ①) describes the temperature in the rod at time t > O at the point ① e [O, 1].
(a). set up the forward-Euler method.
(b). set up the backward-Euler method. write out the tri-diagonal system one needs to solve at every time step.
11.3 Laplace Equation in 2D
Let u(①, g) satisies the equation and boundary conditions
u(①, O) = O, u(①, 1) = O, O < ① < 1
u(O, g) = sin(πg), u(1, g) = O, O < g < 1
write out the Finite Diference Method with uniform grid using h = O.25. Make sure to write out the system of linear equations for the unknowns.
11.4 Explicit and implicit methods for heat equation
Let u(t, ①) solve a heat equation with boundary and initial conditions
ut = 2u从从, O < ① < 4, t 持 O
u(t, O) = O, u(t, 4) = O, t 持 O (boundary conditions) u(O, ①) = ① (4 - ①), O < ① < 4 (initial condition)
(a). set up the forward-Euler time step, using uniform grid with grid sizes Δ①, Δt.
(b). show in detail that the forward-Euler method is conditionally stable. Make sure to identify the CFL condition.
2023-08-15