WAVE-RESOLVING NUMERICAL SIMULATION OF LANGMUIR CIRCULATIONS

In open-water environment, the interaction of progressive surface waves with the shear turbulence generated by wind-induced surface shear stress causes the generation of Langmuir circulations – a flow phenomenon frequently occurring in oceans and large lakes. The study of the Langmuir circulations plays a special role in understanding the mass, momentum, and energy transfer between the air-water interface and the complex vertical mixing effects in the upper ocean layer. In this study, the detailed generation and growth processes of the Langmuir circulations by the wave-turbulence interaction is simulated using a high-fidelity wave-resolving direct numerical simulation (DNS) approach. The complex flow motions are decomposed into an irrotational wave potential field and a rotational turbulence field. The surface wave potential and elevation are prescribed using classical Airy wave theory. The wave field decay due to the turbulent dissipation is avoided in this simulation. The statistical characteristics of the wind-driven shear turbulent flow are investigated. Compared with the turbulent statistics of the turbulent channel flow with no-slip boundaries, the mean streamwise velocity profile of the stress-driven turbulent flow with the free-slip boundary still satisfy the low of the wall and the logarithmic law but having a lower offset constant in the logarithmic layer, and appreciable differences are found in the behaviors of Reynolds stress components and TKE budget. With the wave field being imposed at the domain surface, the DNS solves the evolution of the turbulence field under wave distortion. This DNS model is found to successfully capture the incipient generation and evolution of the Langmuir circulations caused by wave-turbulence interaction. In this dissertation, both the time evolution of the instantaneous flow field obtained from the current DNS and the statistical analysis results are presented. The Langmuir circulations based on popular Craik-Leibovich equations (CL2 model) are also performed as a reference. The obtained instantaneous turbulent flow fields are found to be very similar when using both the present wave-resolving method and the CL2 model. Nevertheless, quantitative analysis reveals that the turbulent statistics like the Reynolds stress components and the TKE production terms are underestimated for the CL2 model.