Laboratory investigations of nonlinear surface wave transformation in a field of two-dimensionally inhomogeneous currents

The variability of various-amplitude surface waves in a field of inhomogeneous currents was investigated in the laboratory tank of the Institute of Applied Physics, Russian Academy of Sciences. Inhomogeneous currents were produced by streamlining of submerged spheres of diameters 0.15 m and 0.3 m moving with velocity 0.4 m/s. The spheres were submerged at depths of 0.3 m, 0.45 m, 0.6 m and 0.9 m. Surface waves of frequencies 1.7 Hz and 2 Hz and amplitudes 2 mm, 6 mm and 10 mm were generated by a wavemaker at the end of the tank from which a sphere started its motion and were absorbed by a wave absorber at the other end of the tank. The surface deviation was recorded by acoustic sensors at distances 8 m from the wavemaker. On the assumption of smooth variation of the surface wave amplitude the spatial two-dimensional distribution of the surface wave amplitude was reconstructed. Experiments have demonstrated that the variability of 2 Hz surface waves whose group velocity is close to the velocity of sphere motion is more essential than that of surface waves of the frequency 1.7 Hz. Arrangement of regions of the surface wave amplitude increase and decrease greatly depends on the surface wave frequency and is practically independent of the sphere sizes and the depth of its submersion. At a large depth of the small sphere motion cases are recorded when surface wave anomalies caused by an inhomogeneous current are absent for small amplitudes of surface waves but occur at an increased amplitude of surface waves generated by a wavemaker. Comparison of experimental data to the results of theoretical calculations have shown that they agree well as far as the arrangement of regions of the surface wave amplitude increase and decrease are concerned. At the same time the variation values of the surface wave amplitude in theoretical calculations is, as a rule, smaller than that recorded in the experiment