I am trying to solve a specific heat diffusion problem with a semi-infinite, isolated rod, where the initial temperature distribution is $10^\circ$ C for $0 < x < 1$, and $0$ for $x > 1$. The problem is thus of the form
$$ \frac{\partial u}{\partial t} – \frac{\partial^2u}{\partial x^2} = 0 $$
$$ \frac{\partial u}{\partial x}(0,t) = 0 $$
$$ u(x,0) = \begin{cases} 10 & \mathrm{for} \:\: 0 < x < 1 \\
0 & \mathrm{for} \:\: x > 1 \end{cases}. $$
One-sided Laplace transform in $t$ and rearranging yields
$$ \frac{\partial^2 \bar{u}}{\partial x^2}(x,s) – s\bar{u}(x,s) = -u(x,0) = \begin{cases} -10 & \mathrm{for} \:\: 0 < x < 1 \\
0 & \mathrm{for} \:\: x > 1 \end{cases}.$$
I'm not sure how to approach this, but I've tried solving for the first region,
$$ \frac{\partial^2 \bar{u}}{\partial x^2} – s\bar{u} = -10 \Rightarrow $$
$$ \bar{u} = \bar{u}_h + \bar{u}_p = a(s)e^{\sqrt{s}x} + b(s)e^{-\sqrt{s}x} + \frac{10}{s} $$
and then applying the boundary conditions,
$$ u \:\: \mathrm{bounded} \Rightarrow a(s) = 0$$
$$ \frac{\partial u}{\partial x}(0,t) = 0 \Rightarrow \frac{\partial \bar{u}}{\partial x}(0,s) = 0 \Rightarrow -\sqrt{s}b(s) = 0 \Rightarrow b(s) = 0. $$
But this means that
$$ \bar{u}(x,s) = \frac{10}{s} \Rightarrow u(x,t) = 10H(t) \:\:\:\: (0 < x < 1) $$
which does not seem to make much sense since this means that the temperature remains constant in the first region and one would expect it to decrease as $t \rightarrow \infty$. Maybe some kind soul could point out what I'm doing wrong and how to best approach this problem?
(A similar problem with a piecewise I.C. was posted here, but the solution is attempted using the Fourier transform, which doesn't seem to work.)
Best Answer
The Laplace transform approach also works, but your conclusion $a(s) = 0$ is wrong, since $\mathrm{e}^{\sqrt{s} x}$ is bounded for $x < 1$ anyway. The boundedness of $u$ can only be used to discard the term with the positive exponent in the region where $x > 1$. The general solution then reads $$\bar{u}(x,s) = \begin{cases} a(s) \mathrm{e}^{\sqrt{s} x} + b(s) \mathrm{e}^{-\sqrt{s} x} + \frac{10}{s} &, \, x<1 \\ c(s) \mathrm{e}^{-\sqrt{s} x} & , \, x>1 \end{cases} \, .$$ From the boundary conditions $\partial_1 \bar{u}(0,s) = 0$, $\bar{u}(1^+,s) = \bar{u}(1^-,s)$, $\partial_1 \bar{u}(1^+,s) = \partial_1 \bar{u}(1^-,s)$ we obtain $$ a(s) = b(s) = - \frac{\mathrm{e}^{-\sqrt{s}}}{\mathrm{e}^{\sqrt{s}}-\mathrm{e}^{-\sqrt{s}}} c(s) = - 5\frac{\mathrm{e}^{-\sqrt{s}}}{s} \, ,$$ so \begin{align} \bar{u}(x,s) &= \frac{10}{s} \begin{cases} 1 - \frac{1}{2} \left[\mathrm{e}^{\sqrt{s} (x-1)} + \mathrm{e}^{-\sqrt{s} (x+1)}\right] &, \, x \leq 1 \\ \frac{1}{2} \left[\mathrm{e}^{-\sqrt{s} (x-1)} - \mathrm{e}^{-\sqrt{s} (x+1)}\right] & , \, x>1 \end{cases} \\ &= \frac{10}{s} \left[\operatorname{H}(1-x) - \frac{1}{2} \mathrm{e}^{-\sqrt{s} (x+1)} + \frac{\operatorname{sgn}(x-1)}{2} \mathrm{e}^{-\sqrt{s} \lvert x-1\rvert}\right]. \end{align} Finally, since $$ \operatorname{\mathcal{L}}^{-1} \left(s \mapsto \frac{\mathrm{e}^{-a \sqrt{s}}}{s}\right) (t) = \operatorname{erfc}\left(\frac{a}{2 \sqrt{t}}\right) = 1 - \operatorname{erf}\left(\frac{a}{2 \sqrt{t}}\right) $$ holds for $a, t \geq 0$, we recover the result you derived using the Fourier transform: \begin{align} u(x,t) &= 5 \left[\operatorname{erf}\left(\frac{x+1}{2 \sqrt{t}}\right) - \operatorname{erf}\left(\frac{x-1}{2 \sqrt{t}}\right)\right] + 10 \left[\operatorname{H}(1-x) - \frac{1 - \operatorname{sgn}(x-1)}{2}\right] \\ &= 5 \left[\operatorname{erf}\left(\frac{x+1}{2 \sqrt{t}}\right) - \operatorname{erf}\left(\frac{x-1}{2 \sqrt{t}}\right)\right] . \end{align}