Stokes drift is the difference between the mean velocity of fluid particles and the Eulerian mean velocity at fixed positions. It appears at second order because particles sample the oscillatory velocity at displaced positions.
Stokes drift refers to the phenomenon in fluid dynamics where particles or fluid elements move in a direction slightly different from that of the wave propagation itself due to the non-linear nature of surface waves. This effect is most commonly observed in ocean waves. In a simplified explanation, as a wave travels through the water, it creates circular or elliptical orbital motions of water particles.
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