{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"e48847f88d6ff1deed7c1624cb649db0068e773e974e19e21cc7a3f45858a9f0","created":"2026-10-03T07:18:04Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011","e63ee327747bcfad2d5d8e990f17a84cadc6fa47142aa9b3f0c612a01fa2862d"],"salt":"54d1833f83b79f9c3a1689bedc5334663f7f2a94f1721969ad084fa7daa067fd","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"fluid.rb-convection.transition-onset","kind":"well-posed","literature_status":"contested","n":"1","parents":[{"note":"","parent_revision":"e63ee327747bcfad2d5d8e990f17a84cadc6fa47142aa9b3f0c612a01fa2862d","relation":"special_case"}],"plain":"If the steep heat-transport regime exists, it starts at very different heating strengths in different experiments. What fixes the starting point is unknown.","posed_since":"","precise":"Assuming a transition exists, determine how the onset Rayleigh number $Ra*$ depends on Pr, aspect ratio $\\Gamma$, plate and sidewall conditions and residual perturbations, and whether onset corresponds to a universal critical shear Reynolds number $Re_{s}$ of the plate boundary layers (Grossmann-Lohse estimate $Re_{s} \\sim 420$, approximately). Answer: a predictive criterion for $Ra*$, or a demonstration that the onset is a subcritical, perturbation-dependent transition with no single $Ra*$.","problem_ref":null,"references":"","settled_by":"Systematic experiments in one apparatus varying $\\Gamma$, Pr and controlled boundary-layer perturbations, mapping Ra* against $\\mathrm{Re}_{s}$.","status_note":"Reported onsets range from $\\mathrm{Ra}\\sim 10^{11}$ to above $10^{14}$; Shishkina and Lohse (2024, arXiv:2407.16573) argue onset occurs in all large-Ra data sets at experiment-dependent Ra, as expected for a non-normal nonlinear instability.","title":"What sets the Rayleigh number of the transition to ultimate 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