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d !
2
2
" K ! = 0 (10.4.16)
dz 2
whose solution can be written,

! = AcoshK(z + D) + BsinhK(z + D) (10.4.17)

and the application o f the boundary condition at z=-D implies that B=0. Note that to this

point Laplace’s equation and the lower boundary condition have yielded only a constraint
on the spatial structure of the motion but very little about its dynamics. For that we need

to consider (10. 4. 10, a, b). Eliminating η between the equations yields the boundary

condition in terms only of ! ,

! " !" 1 !p
2
+ g + a = 0 (10.4.18)
!t 2 !z # !t
If the pressure field were time independent it would not force a nontrivial velocity

potential. In that case the full solution would be ! =0 and η would hydrostatically

p
balance the applied pressure, i.e. ! = " a , the so-called inverted barometer. In our
g#

case , though the pressure is a function of time and a non trivial wave solution is forced.

Substituting (10.4.14), (10.4.15) and (10.4.17) into (10.4. 18) yields,

P
#
2
A !" coshKD + gK sinhKD% = i" ' (10.4.19)
$
&
so that

!i"P / #
A = ,
{
coshKD " ! gK tanhKD}
2
$ (10.4.20)
(!i"P / #)e i(kx+ly!"t)
% = Re coshK(z + D)
coshKD " ! gK tanhKD}
{
2
or ,

Chapter 10 24