View source for Bottom Mounted Cylinder

= Introduction =

The bottom mounted cylinder is one of the simplest scattering problems in water waves. It is 
the basis for many more complicated scattering problems in which it is desired to keep
the scatterers as simple as possible. The theory can be found in many books. The derivation
here is related to [[Cylindrical Eigenfunction Expansion]]. We begin with the [[Frequency Domain Problem]].

= Theory =
A cylinder of radius <math>a</math> (which we will suppose is at the centre
of a cylindrical coordinate system) extends from the free surface to the 
bottom of the fluid

The problem is Laplace's equation through out the fluid,
<center>
<math> \nabla^2\phi = 0, \quad r>a, </math>
</center>
the linearised free surface condition 
<center>
<math>\frac{\partial \phi}{\partial z} - \alpha \phi = 0, \quad r>a, \quad z =0 </math>
</center>
and the non flow conditions through the bottom boundary at <math>z=d</math>
<center>
<math> \frac{\partial \phi}{\partial z} = 0, \quad z = -h, </math>
</center>
and the cylinder
<center>
<math> \frac{\partial \phi}{\partial r} = 0, \quad r = a. </math>
</center>

= Removing the depth dependence =

We can [[Removing the Depth Dependence| remove the depth dependence]] and the equation reduces to the [[Helmholtz's Equation]] 
in a two dimensional domain (i.e. there is no dependence on <math>z</math>
<center>
<math> \nabla^2\phi - k^2 0, \quad r>a, </math>
</center>
<center>
<math> \frac{\partial \phi}{\partial r} = 0, \quad r = a. </math>
</center>
where <math>k</math> is the positive imaginary root of the [[Dispersion Relation for a Free Surface]]
<center>
<math>
\alpha + k \tan k h = 0.\,
</math>
</center>
We can now separate
variables by writing Laplace's equation in cylindrical polar coordinates.
<center>
<math> \frac{1}{r} \frac{\partial}{\partial r} \left( r \frac{\partial
\phi}{\partial r} \right) + \frac{1}{r^2} \frac{\partial^2
\phi}{\partial \theta^2} - k^2 = 0, \quad r>a,
 </math>
</center>
The separate solution is 
<center>
<math>
\phi (r,\theta) =  \sum_{\nu = -
\infty}^{\infty} \left[ D_{\nu} I_\nu (k r) + E_{\nu} K_\nu (k
r) \right] \mathrm{e}^{\mathrm{i} \nu \theta}, 
</math>
</center>
(this follows exactly as for [[Cylindrical Eigenfunction Expansion]]). Note that in many
cases a symmetry arguement is used to express the complex exponentials in terms
of sine and cosines. This follows from a symmetry in the incident potential.

= Boundary condition at <math>r = a</math> =

The boundary condition at <math>r = a</math> is now substituted into the above
equation to give
<center>
<math> D_{\nu} I^{\prime}_\nu (k a) + E_{\nu} K^{\prime}_\nu (k a) = 0 </math>
</center>
so that 
<center>
<math> E_{\nu}   = -\frac{D_{\nu} I^{\prime}_\nu (k a)}{K^{\prime}_\nu (k a)}</math>
</center>
and we often assume that the incident wave is a plane wave travelling in the positive <math>x</math>
direction, i.e.
<center>
<math>
\phi^{i} (r,\theta) = e^{kx} =  \sum_{\nu = -
\infty}^{\infty} I_\nu (k r) \mathrm{e}^{\mathrm{i} \nu \theta}, 
</math>
</center>
We therefore have the total potential is
<center>
<math>
\phi (r,\theta) =  \sum_{\nu = -
\infty}^{\infty} \left[ I_\nu (k r) - \frac{ I^{\prime}_\nu (k a)}{K^{\prime}_\nu (k a)} K_\nu (k
r) \right] \mathrm{e}^{\mathrm{i} \nu \theta}, 
</math>
</center>
[[Category:Problems with Cylindrical Symmetry]]