Direct numerical simulation of wind turbulence over breaking waves

Abstract

We study wind turbulence over breaking waves based on direct numerical simulation (DNS) of two-fluid flows. In the DNS, the air and water are simulated as a coherent system, with the interface captured using the coupled level-set and volume-of-fluid method. Because the wave breaking is an unsteady process, we use ensemble averaging over 100 runs to define turbulence statistics. We focus on analysing the turbulence statistics of the airflow over breaking waves. The effects of wave age and wave steepness are investigated. It is found that before wave breaking, the turbulence statistics are largely influenced by the wave age. The vertical gradient of mean streamwise velocity is positive at small and intermediate wave ages, but it becomes negative near the wave surface at large wave age as the pressure force changes from drag to thrust. Furthermore, wave-coherent motions make increasingly important contributions to the momentum flux and kinetic energy of velocity fluctuations (KE-F) as the wave age increases. During the wave breaking process, spilling breakers do not influence the wind field significantly; in contrast, plunging breakers alter the structures of wind turbulence near the wave surface drastically. It is observed from the DNS results that during wave plunging, a high pressure region occurs ahead of the wave front, which further accelerates the wind in the downstream direction. Meanwhile, a large spanwise vortex is generated, which greatly disturbs the airflow around it, resulting in large magnitudes of Reynolds stress and turbulence kinetic energy (TKE) below the wave crest. Above the crest, the magnitude of KE-F is enhanced during wave plunging at small and large wave ages, but at intermediate wave age, the transient enhancement of KE-F is absent. The effect of wave breaking on the magnitude of KE-F is further investigated through the analysis of the KE-F production. It is discovered that at small wave age, the transient enhancement of KE-F is caused by the appearance of a local maximum in the profile of total momentum flux; but at large wave age, it results from the change in the sign of the KE-F production from negative to positive, due to the sign change in the wave-coherent momentum flux. At intermediate wave age, neither of these two processes is present, and the transient growth of KE-F does not take place.

A Unified Breaking Onset Criterion for Surface Gravity Water Waves in Arbitrary Depth

“…The laboratory work of Saket et al (2018) showed that B th ≈ 0.85 also segregates breaking from nonbreaking crests in the presence of wind forcing in deep water breaking. A number of high-fidelity two-phase flow simulations of breaking waves in the presence of wind forcing (e.g., Tang et al, 2017;Yang et al, 2018) have been recently performed. Detailed quantification of the effect of direct wind forcing on the proposed breaking onset criterion in shallow water is left for future study.…”

The Impact of Nonbreaking Waves on Wind‐Driven Ocean Surface Turbulence

“…Our measurements or analysis were not equipped and did not make any special provisions to deal with these instances any differently than other cases. In terms of the airflow separation, this can be of relatively low importance, based on the fact that short wave breaking (or even spilling breaking) is not expected to significantly impact the airflow pattern over the dominant wave (Yang et al, ). However, there is no doubt that these small breakers impose some additional vortex force near the surface, as well as dissipate their energy through breaking, and thus make an impact on the turbulence investigated in this study.…”

A Review on the Meandering of Wind Turbine Wakes

“…To achieve site-specific optimal wind farm design and operation [86,87], further research from both fundamental and applied points of view needs to be carried out. Such work should focus, among others, on: (1) the role and interaction of the two wake meandering mechanisms for different turbine operational and atmospheric conditions; (2) the physics and models for wake meandering in complex terrain, under offshore conditions with breaking waves [88] and in coastal areas [89]; and (3) extension of the meandering models to the wake of marine and hydrokinetic energy devices (may be of complex configuration [90]) interacting with sediment transport [91,92] and different types of turbulent eddies [93,94].…”