The Research “On the Merging & Splitting Processes in the Lobe-and-cleft Structure at a Gravity Current Head” Selected as the Cover of the Journal of Fluid Mechanics

Multiple pairs of tooth-like vortex structures form for the gravity currents propagating on a non-slip boundary. There is a cleft between two adjacent tooth-like vortices. The merging and splitting processes in the lobe-and-cleft structure remain a mystery for near a century. With the help of direct numerical simulations, the mystery regarding the splitting and merging process in the lobe-and-cleft structure at a gravity current head is successfully solved. The research was recognized by contemporary scientists and has been selected as the cover of the Journal of Fluid Mechanics.

Gravity currents, also known as density currents, are driven by a density difference and occur ubiquitously in the ocean, atmosphere, and other man-made environments. A number of factors that are likely to cause variations in the density of fluid include temperature differentials, dissolved materials, and suspended sediments. The erosion and accumulation of bed material by gravity currents have great influence on the changes of the topography. The understanding of vortex structure in the gravity current head holds the key to explaining erosion and accumulation. Therefore, revealing the underlying mechanism of the merging and splitting processes of the lobe-and-cleft structure is a highlight in this study.

Our research shows that a cleft is formed between a pair of tooth-like vortices, and a tooth-like vortex structure has two counter-rotating vortices as its legs. For the merging of two clefts, it is observed that three tooth-like vortices merge into two tooth-like vortices. As the two clefts approach each other, the middle lobe bounded by the two clefts shrinks in size. The middle tooth-like vortex breaks up into two legs and reconnects with the left and right neighboring tooth-like vortices. These two reconnected vortical structures recede as the gravity current propagates forward and the two unbroken tooth-like vortices remain after the merging process.

For the splitting process, a new born vortex is created by the parent vortex of opposite orientation, which can be either the left part or the right part of the existing tooth-like vortex inside a lobe. This new born vortex then induces the other vortex of opposite orientation and a pair of counter-rotating streamwise vortices are positioned on the left- and right-hand sides of the developing new cleft.

Financial supports from the Ministry of Science and Technology (MOST) and National Taiwan University (NTU) are greatly acknowledged.