This paper concerns laboratory wavemaking in shallow and in
termediate water
conditions. A comparison is made between two wave generatio
n techniques, a first
based on controlling the wavemaker displacement, and a seco
nd based on controlling
the wavemaker force. Nonlinear wave generation in position
control is well under-
stood, and many laboratories rely on established second-or
der or Stream-function
inputs. In deep water, using flap-type wavemakers, a force-c
ontrol approach based
on a linear demand signal was recently shown to offer benefits in
terms of wave
quality. The shallow water operation of such force-control
strategies is less certain,
which motivates the present study.
To investigate the influence of the water depth on this type of
control, a range of
generation scenarios is considered, including small ampli
tude and large amplitude
regular waves. Adopting both supporting calculations and e
xperimental evidence,
the work demonstrates that first-order force-based wave gen
eration in shallow water
suffers from similar limitations as first-order position cont
rol. This principally con-
cerns the contamination of the testing area due to unwanted f
ree waves, where the
present focus is placed on the superharmonic range.
The main advance of the work lies in the solutions it offers to ov
ercome this free
wave contamination. A number of nonlinear wave solutions up
on which force-based
generation can be based are discussed, and a suitable method
ology is proposed and
validated for each case. The developed methodology allows f
or high quality wave
generation, whilst maintaining the benefit of active wave ab
sorption. The work is
timely in the sense that is responds to two recent developmen
ts. First, the majority
of wavemaking facilities commissioned over the past two dec
ades are computer con-
trolled, and active absorption has become commonplace. The
work presented offers
solutions highly relevant to such installations. Second, d
evelopments particularly
in offshore wind, have seen many new structures placed in relat
ively shallow-water
depth. It is essential that the model testing of such structu
res adequately accounts
for the issues and solutions presented herein.