{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"a2aa56c002508762e13c6d3a6e729297a6763737cd2fef40eda3428bde126a76","created":"2026-10-03T07:18:04Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"7752c59dc3c1ac088a63617def1d196489000e2236d701a2d1f69c7b085521a4","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"fluid.wall-turbulence.roughness-drag","kind":"well-posed","literature_status":"open","n":"1","parents":[],"plain":"Rough surfaces such as fouled ship hulls or corroded pipes raise turbulent friction, and engineers summarize the effect by one number, an equivalent sand-grain roughness height. Nobody can yet compute that number from a measured map of a surface without testing the surface in a flow.","posed_since":"","precise":"In fully rough wall turbulence the mean-velocity shift is $\\Delta U+ = (1/\\kappa)\\ln k_s+ + \\mathrm{const}$, which defines the equivalent sand-grain height $k_s$. Determine whether $k_s/k$ ($k$ a roughness height) is a function of a finite set of topographic statistics (rms height, effective slope, skewness, correlation lengths) for generic random surfaces, give that function, and predict the transitionally rough curve $\\Delta U+(k_s+)$. Answer: a relation with error well below 10 percent on surfaces not used for calibration.","problem_ref":null,"references":"","settled_by":"Blind predictions of $k_s$ for new random surfaces confirmed by roughness-resolved DNS or experiments in the fully rough regime.","status_note":"Prediction uncertainty for full-scale drag is at least 10 percent (Chung, Hutchins, Schultz, Flack, Annual Review of Fluid Mechanics 2021).","title":"Predicting rough-wall drag from surface topography alone","topic_ref":"4f4b48fcb04737e7db5b7791039558ed9bddcd8c13b10abe847d72acfa6e0d0c"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"26fb2cad6889c6d13d8d299bb6f8baae90c5607ae567744f125bfe9cc38483ea","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"d9ccec27fab9a6f997e9511833e8658af229dfe0ff3b726f51890bf7f6d79a46","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"7NGRynHtMaOwOWSeEoVsCNOw9qfKk2bIxu32DXhRyIqMwO4_4gxi-8BplzH_ghEtwmlGnTW2W9c2Rf9-ZPuyCQ"},"schema":"pubphys.envelope/1"},"record_hash":"26fb2cad6889c6d13d8d299bb6f8baae90c5607ae567744f125bfe9cc38483ea","leaf_index":1454}