The lateral radiation of heat from a bar of length L insulted at both ends can be modelled with the linear partial differential equation (PDE) du 8²u - au for 0 0, (1) Ət əx² with the conditions ux (0, t): = ux (L, t) = 0 and u(x,0) = f(x), where L, a, DER are positive constants. 1. Seek a separation solution of the form u(x, t) = X(x)T(t) to show T' Ο X" + = = k₁ DT D X (2) where k denotes the separation constant. 2. Use equation (2) to derive two ordinary differential equations (ODES), one in space x and one in time t. 3. Determine the boundary conditions for the ODE that depends on x. 4. Find the non-trivial solutions of X(x) = Xn(x) and corresponding values of the separation constant k = kn, for n = 0, 1, 2, 3.... = -p², k = 0, k = p² (p = 0) to Hint: you may need to consider each of the three cases k=-1 find all the non-trivial solutions 5. Find the solutions T(t) = Tn(t), for n = 0, 1, 2, 3 .... 6. Use the superposition principle for linear PDEs to write down the solution for u that satisfies the boundary conditions uz (0, t) = ux (L, t) = 0. = D

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Separation of variables
The lateral radiation of heat from a bar of length L insulted at both ends can be modelled with
the linear partial differential equation (PDE)
Ju
² u
= D.
- au for 0<x<L, t> 0,
(1)
Ət მე2
with the conditions
ux(0, t) = ux (L, t) = 0 and u(x,0) = f(x),
where L, a, DER are positive constants.
1. Seek a separation solution of the form u(x, t) = X(x)T(t) to show
T' a X"
+ = = k,
(2)
DT D X
where k denotes the separation constant.
2. Use equation (2) to derive two ordinary differential equations (ODEs), one in space x and
one in time t.
3. Determine the boundary conditions for the ODE that depends on x.
4. Find the non-trivial solutions of X(x) = Xn(x) and corresponding values of the separation
constant k = kn, for n = 0, 1, 2, 3....
Hint: you may need to consider each of the three cases k=-p², k = 0, k = p² (p = 0) to
find all the non-trivial solutions
5. Find the solutions T(t) = Tn(t), for n = 0, 1, 2, 3 ....
6. Use the superposition principle for linear PDEs to write down the solution for u that satisfies
the boundary conditions ur (0, t) = u(L, t) = 0.
7. Determine expressions for the unknown coefficients in your solution for u that satisfies the
initial condition u(x,0) = f(x).
8. Find the solution when f(x) = 1 + cos πx, for values of L = 1, a = 1 and D = 1.
9. In a single figure, plot the solution found in Question 8 at times t = 0, 0.25, 0.5, 2. Briefly
describe the behaviour as t increases. What is the constant value temperature of the
environment that surrounds the bar?
Transcribed Image Text:Separation of variables The lateral radiation of heat from a bar of length L insulted at both ends can be modelled with the linear partial differential equation (PDE) Ju ² u = D. - au for 0<x<L, t> 0, (1) Ət მე2 with the conditions ux(0, t) = ux (L, t) = 0 and u(x,0) = f(x), where L, a, DER are positive constants. 1. Seek a separation solution of the form u(x, t) = X(x)T(t) to show T' a X" + = = k, (2) DT D X where k denotes the separation constant. 2. Use equation (2) to derive two ordinary differential equations (ODEs), one in space x and one in time t. 3. Determine the boundary conditions for the ODE that depends on x. 4. Find the non-trivial solutions of X(x) = Xn(x) and corresponding values of the separation constant k = kn, for n = 0, 1, 2, 3.... Hint: you may need to consider each of the three cases k=-p², k = 0, k = p² (p = 0) to find all the non-trivial solutions 5. Find the solutions T(t) = Tn(t), for n = 0, 1, 2, 3 .... 6. Use the superposition principle for linear PDEs to write down the solution for u that satisfies the boundary conditions ur (0, t) = u(L, t) = 0. 7. Determine expressions for the unknown coefficients in your solution for u that satisfies the initial condition u(x,0) = f(x). 8. Find the solution when f(x) = 1 + cos πx, for values of L = 1, a = 1 and D = 1. 9. In a single figure, plot the solution found in Question 8 at times t = 0, 0.25, 0.5, 2. Briefly describe the behaviour as t increases. What is the constant value temperature of the environment that surrounds the bar?
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