Coherentstructuresareessentialforthemomentumexchangeandturbulenceproductioninwall-boundedturbulentflows.persifiedcoherentstructureshavebeenobservedinturbulentboundarylayers,andhairpin-basedvorticesdominatemostoftherelevantliterature.However,thereisnoconsensusyetontheoriginandformingmechanismofhairpinvortices.Herein,fivecornerstonespertainingtotheframeworkofhairpin-basedcoherentstructuresarereviewed,andthreedifferenthairpingenerationmechanismsareclarified.Next,thetime-resolvedtomographicparticleimagevelocimetry(Tomo-PIV)isusedinanearlyturbulentboundarylayer(Reθ=420)toinvestigatetheoriginofhairpinvortices.Thetimelinesrevealatriangularbulgeinthelow-speedstreak(LSS),andtheinitialroll-upoccursattwosidesofit.Meanwhile,thematerialsurfacesmanifestasathree-dimensional(3D)wavestructureintheLSS,whichmaysupportthemodelofasoliton-likecoherentstructure(SCS).Subsequently,themethodofLagrangian-averagedvorticitydeviationisusedtodetectearlyvortices.Wefindthatthe3Dwavestructureisflankedbytwovortices,thusconfirmingtheroll-upoftimelinesanddemonstratingtheadvantageoftheLagrangiancriteriaincapturingstructuresincomplexflows.Theseresultssuggestthatvariouscoherentstructuresmayevolvefromthemetamorphosisof3Dwavestructuresandtheirlaterinteraction.Finally,thelimitationsoftraditionalexperimentalandpost-processingtoolsarediscussed.
