The picture is from the experiments conducted in a wave flume on a two-layer fluid. The motion of a surface wave has led to generation of a three-dimensional nearly-standing wave pattern at the interface through a three-wave resonant interaction mechanism. The pattern is due to combination of two nearly-subharmonic oblique interfacial waves and the corresponding reflected interfacial waves from the flume side walls. The original interfacial waves have nearly the same kinematic properties but opposite propagation directions. The upper layer is fresh water, and the lower layer is saline water.

References:

1. Jamali, M. (1998) "Surface wave interaction with oblique internal waves". Ph.D. Thesis, The University of British Columbia, Vancouver, Canada.

2. Jamali, M., Lawrence, G., and Quick, M. (2000) "Generation of oblique internal waves by a surface wave", Proc. 5^th Int. Symp. Stratified Flows, p. 779.

*Sherif University of Technology, Tehran