Evanescent Wave Reduction Using a Segmented Wavemaker in a Two Dimensional Wave Tank

The concept of a segmented wavemaker, in a two dimensional tank, has been investigated analytically to see if it can reduce the effect of parasitic evanescent waves in a wave tank. Evanescent waves can contaminate test areas in tanks leading to unreliable results, but are typically avoided by establishing the test area two to three times the water depth away from the wavemaker. This space requirement can be quite restrictive in terms of the necessary tank size and, with the increasing interest in off-shore renewable energy, many technology developers may not be able to afford a workspace large enough to accommodate a long tank. Previously, flexible wavemakers have been designed to tackle the problem and, in some cases have proven very effective in eliminating evanescent waves at the wavemaker’s “tuned” frequency. However, flexible wavemakers have been shown to have little benefit in modeling panchromatic seas. Discussed here is the linear potential theory of a segmented wavemaker designed to reduce the evanescent waves in a tank over a large frequency range. Each segment in the segmented wavemaker is programmed with an individual stroke, allowing the system to best approximate the horizontal displacement of the fluid over the depth of the water in a naturally occurring wave. A comparison of the influence of evanescent waves created by segmented wavemakers, piston and hinged paddle wavemakers, on the free surface elevation is presented.