Revision 1

Accepted · first revision · record

Title: What sets the thickness of spontaneous layers in stirred stratified fluid?
Plain: Stirring a fluid whose density increases smoothly with depth can produce a staircase of uniform layers separated by sharp steps. What sets the height of the steps and how they evolve is not understood.
Precise: Stirring a linearly stratified fluid can produce a staircase of well-mixed layers separated by thin interfaces, through the Phillips (1972) mechanism in which the buoyancy flux decreases with increasing local gradient over some range. Predict the layer thickness $h$ as a function of $\mathrm{Re}_{b}$, horizontal Froude number and Pr, and the rate of layer merging at long times.
Settled by: A theory predicting h and merger rates verified by forced stratified DNS and tank experiments over a decade of stirring strength.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: 2025 simulations track layer and interface evolution over long times (arXiv:2512.08714); no theory predicts h.
Posed since: 1972