VELOCITIES UNDER PERIODIC AND RANDOM WAVES

Using the linear approximation for predicting velocities under periodic water waves, relationships are established between the velocity spectrum and the surface elevation spectrum for three-dimensional waves. The two dimensional forms of these relationships are used for comparison with velocity and elevation spectra measured in a laboratory wave tank. The predicted distortion of the spectrum shape can be observed in the experimental results. Velocity measurements were recorded using a laser velocimeter employing a rotating diffraction grating for shifting the optical frequencies. This allows measurement without sign ambiguity. The random waves were generated using a servodriven paddle with a filtered white noise input.