In the laboratory we examine the exchange flow through a long rectangular channel that separates a saltwater reservoir from a freshwater reservoir. Within the channel fresh and saltwater layers flow in opposite directions, and wave-like instabilities form on the interface between them. This configuration is modeled numerically using Direct Numerical Simulation (DNS). The important difference between the two approaches is that in the laboratory experiments there is a spatial variation in the mean flow, whereas in the DNS there is a temporal variation. A meaningful comparison is possible since these variations are gradual. Two important results arise from this comparison: wave formation occurs when the spacing between adjacent instabilities increases; and wave merging occurs due to vortex pairing as well as "ejections". These ejections arise from the mutual advection of oppositely signed vortex couples that transport fluid from one layer far into the other - a potentially very important mixing process. We had not fully appreciated either of these results before we embarked on a serious comparison of the numerical and physical models.
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