Systematic experiments focusing on the evolution of wave trains with initially sidebands on uniform currents are carried out in a physical wave-current flume (65m long, 2m wide and 1.8m deep). Wave trains with different initial steepness and perturbed sidebands are generated to propagate on currents with velocity in the range −0.1 < U/c < 0.1 (U is the current velocity, c is the phase velocity of the carrier wave in quiescent water). The experimental results demonstrate that following currents can suppress the modulational instability and reduce the onset criterion. However, opposing currents have the contrary effect. In addition, the estimated experimental initial growth rates of the sidebands are close to the result predicted by the current modified cubic nonlinear Schrödinger equation of Gerber. Furthermore, the development of instability of waves on opposing currents tends to occur in a continuous frequency band rather than in some discrete frequencies.

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