Numerical Study of the Instability and Flow Transition in a Vortex-Ring/Wall Interaction

Instability and fl w transitionof a vortex ring impinging on a wall were investigated by means of large-eddy simulation for two vortex core thicknesses correspondingto thin and thick vortex rings. Variousfundamentalmechanismsdictatingthefl w behaviours, suchas evolutionof vortical structures,instabilityandbreakdownof vortex rings,developmentof modalenergies, andtransition from laminar to turbulent state, have been studied systematically. Analysis of the enstrophy of wrappingvortices andturbulentkineticenergy (TKE) infl w fiel indicatesthattheformationand evolutionof wrappingvorticesareclosely associated withthefl w transitiontoturbulentstate. Itis foundthatthetemporaldevelopmentofwrappingvorticesandthegrowthrateofaxial fl wgenerated aroundthecircumferenceof thecoreregionforthethinringarefasterthanthoseforthethickring. Theazimuthalinstabilitiesof primaryandsecondaryvortex ringsareanalysed andthedevelopment of modalenergies reveals thefl w transitiontoturbulentstate. The law of energy decay follows a characteristick−5/3 law,indicatingthatthevorticalfl w hasbecometurbulent. Theresultsobtained inthisstudyprovide physical insightintotheunderstandingof theinstability mechanismsrelevant tothevorticalfl w evolution.