- #1
Dennis_P
- 2
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I got the following PDE.
[tex]
{\frac {\partial }{\partial t}}u \left( y,t \right) = \nu \cdot {\frac {\partial
^{2}}{\partial {y}^{2}}}u \left( y,t \right)[/tex]
With boundary conditions:
[tex]y=0: u(0,t)=0[/tex]
[tex]y=h: u(h,t)=U_0 \cdot cos(\omega \cdot t)[/tex]
Now I need to show by using substitution that,
[tex]\frac{u}{U_0} = \left( A\cos \left( \omega\,t \right) +B\sin \left( \omega\,t
\right) \right) \sinh \left( {\frac {\alpha\,y}{h}} \right) \cos
\left( {\frac {\alpha\,y}{h}} \right) + \left( C\cos \left( \omega\,t
\right) + \left( D \right) \sin \left( \omega\,t \right) \right)
\sin \left( {\frac {\alpha\,y}{h}} \right) \cosh \left( {\frac {\alpha
\,y}{h}} \right)[/tex]
, is a solution of the above equation.
Here:
[tex]\alpha = \sqrt{\frac{\Omega}{2}}[/tex] where [tex]\Omega = \frac{\omega \cdot h^2}{\nu}[/tex]
Next to that I need to find the expressions to calculate the coefficients for a given [tex]\alpha[/tex].
Attempt at a solution:
Starting with the hint I substitute the expression back into the PDE.
[tex] \left( -A\sin \left( \omega\,t \right) \omega+B\cos \left( \omega\,t
\right) \omega \right) \sinh \left( {\frac {\alpha\,y}{h}} \right)
\cos \left( {\frac {\alpha\,y}{h}} \right) + \left( -C\sin \left(
\omega\,t \right) \omega+ \left( D \right) \cos \left( \omega\,t
\right) \omega \right) \sin \left( {\frac {\alpha\,y}{h}} \right)
\cosh \left( {\frac {\alpha\,y}{h}} \right)[/tex]
=
[tex]-2\, \left( A\cos \left(
\omega\,t \right) +B\sin \left( \omega\,t \right) \right) \cosh
\left( {\frac {\alpha\,y}{h}} \right) {\alpha}^{2}\sin \left( {\frac
{\alpha\,y}{h}} \right) {h}^{-2}+2\, \left( C\cos \left( \omega\,t
\right) + \left( D \right) \sin \left( \omega\,t \right) \right)
\cos \left( {\frac {\alpha\,y}{h}} \right) {\alpha}^{2}\sinh \left( {
\frac {\alpha\,y}{h}} \right) {h}^{-2}
[/tex]
This gives me that A = D and B = C. Now using this information I continue with the BC's. The first BC is automatically satisfied by the sine and hyperbolic sine term. Next I continue with the second BC.
[tex]\left( A\cos \left( \omega\,t \right) +B\sin \left( \omega\,t
\right) \right) \sinh \left( {\frac {\alpha\,y}{h}} \right) \cos
\left( {\frac {\alpha\,y}{h}} \right) + \left( B\cos \left( \omega\,t
\right) + \left( A \right) \sin \left( \omega\,t \right) \right)
\sin \left( {\frac {\alpha\,y}{h}} \right) \cosh \left( {\frac {\alpha
\,y}{h}} \right) =\cos \left( \omega\,t \right)[/tex]
Here I don't see how to continue. Maple gives a rather lengthy solution. Doing it by hand it seems I miss information, but my professor said this should be enough. I have been looking at this problem for a while and maybe I am overlooking things. So I need some assistance from someone with a fresh look.
[tex]
{\frac {\partial }{\partial t}}u \left( y,t \right) = \nu \cdot {\frac {\partial
^{2}}{\partial {y}^{2}}}u \left( y,t \right)[/tex]
With boundary conditions:
[tex]y=0: u(0,t)=0[/tex]
[tex]y=h: u(h,t)=U_0 \cdot cos(\omega \cdot t)[/tex]
Now I need to show by using substitution that,
[tex]\frac{u}{U_0} = \left( A\cos \left( \omega\,t \right) +B\sin \left( \omega\,t
\right) \right) \sinh \left( {\frac {\alpha\,y}{h}} \right) \cos
\left( {\frac {\alpha\,y}{h}} \right) + \left( C\cos \left( \omega\,t
\right) + \left( D \right) \sin \left( \omega\,t \right) \right)
\sin \left( {\frac {\alpha\,y}{h}} \right) \cosh \left( {\frac {\alpha
\,y}{h}} \right)[/tex]
, is a solution of the above equation.
Here:
[tex]\alpha = \sqrt{\frac{\Omega}{2}}[/tex] where [tex]\Omega = \frac{\omega \cdot h^2}{\nu}[/tex]
Next to that I need to find the expressions to calculate the coefficients for a given [tex]\alpha[/tex].
Attempt at a solution:
Starting with the hint I substitute the expression back into the PDE.
[tex] \left( -A\sin \left( \omega\,t \right) \omega+B\cos \left( \omega\,t
\right) \omega \right) \sinh \left( {\frac {\alpha\,y}{h}} \right)
\cos \left( {\frac {\alpha\,y}{h}} \right) + \left( -C\sin \left(
\omega\,t \right) \omega+ \left( D \right) \cos \left( \omega\,t
\right) \omega \right) \sin \left( {\frac {\alpha\,y}{h}} \right)
\cosh \left( {\frac {\alpha\,y}{h}} \right)[/tex]
=
[tex]-2\, \left( A\cos \left(
\omega\,t \right) +B\sin \left( \omega\,t \right) \right) \cosh
\left( {\frac {\alpha\,y}{h}} \right) {\alpha}^{2}\sin \left( {\frac
{\alpha\,y}{h}} \right) {h}^{-2}+2\, \left( C\cos \left( \omega\,t
\right) + \left( D \right) \sin \left( \omega\,t \right) \right)
\cos \left( {\frac {\alpha\,y}{h}} \right) {\alpha}^{2}\sinh \left( {
\frac {\alpha\,y}{h}} \right) {h}^{-2}
[/tex]
This gives me that A = D and B = C. Now using this information I continue with the BC's. The first BC is automatically satisfied by the sine and hyperbolic sine term. Next I continue with the second BC.
[tex]\left( A\cos \left( \omega\,t \right) +B\sin \left( \omega\,t
\right) \right) \sinh \left( {\frac {\alpha\,y}{h}} \right) \cos
\left( {\frac {\alpha\,y}{h}} \right) + \left( B\cos \left( \omega\,t
\right) + \left( A \right) \sin \left( \omega\,t \right) \right)
\sin \left( {\frac {\alpha\,y}{h}} \right) \cosh \left( {\frac {\alpha
\,y}{h}} \right) =\cos \left( \omega\,t \right)[/tex]
Here I don't see how to continue. Maple gives a rather lengthy solution. Doing it by hand it seems I miss information, but my professor said this should be enough. I have been looking at this problem for a while and maybe I am overlooking things. So I need some assistance from someone with a fresh look.
Last edited: