The transformed boundary-value problem for the unknown height h above the flat bed is
Equation (24)
for h even and periodic of period in the q-variable. Since in our approach, depending on the context, it is advantageous to use either the moving frame or the conformal coordinates, let us record for further use the formulas
Equation (25)
and
Equation (26)
where
Equation (27)
and
Equation (28)
2.2. Basic properties of the velocity field
Let us now derive some basic properties of the velocity field beneath a Stokes wave with a non-flat free surface. In the fluid region
beneath the free surface and above the flat bed , delimited laterally by the crest line x = 0 and the trough line , (9) and (11) ensure that v is a harmonic function. Since for , from (13) and (19) we infer that for . Since by (12), the strong maximum principle (see [10]) yields that in the interior of . By anti-symmetry, in the fluid region
obtained by reflection of in the crest line, while v = 0 on the crest and trough lines. These considerations provide us with the sign of the vertical fluid velocity component in the moving frame (x, y) in which the flow is steady, and, due to (7)–(8), also when one takes into account variations in time in the physical frame (X, Y) through which the waves are moving; see figure 3.