Airflow measurements at a wavy air–water interface using PIV and LIF

Abstract

geophysical sphere, which largely motivates this study, fluxes of momentum and scalars across the wavy air-sea interface provide boundary conditions for both the atmosphere and the oceans and are therefore, pivotal in controlling the evolution of weather and climate. These fluxes are affected by fine-scale, coupled dynamics above and below the wavy ocean surface. In fact, the surface waves significantly modify the boundary layers on both sides of the interface and it is now well established that it is through waverelated dynamical processes that the air and water boundary layers are coupled (see Sullivan and McWilliams (2010) for a review on the topic).On the water side, for example, it has been shown that the interaction between the wave-induced Lagrangian mass transport, i.e. the Stokes drift, and the surface shear current, leads to the generation of Langmuir circulations, causing significant mixing of the water column (Skyllingstad and Denbo 1995;McWilliams et al. 1997;Noh et al. 2005;Li et al. 2005;Harcourt and D'Asaro 2008; Grant and E 2009;Veron et al. 2009;Kukulka et al. 2010;Belcher et al. 2012;D'Asaro 2014). When surface waves break, the turbulence injected into the water column also significantly enhances surface mixing Melville et al. 1998;Veron and Melville 1999;Melville et al. 2002;Thorpe et al. 2003;Gemmrich and Farmer 2004) and leads to substantial deviations from the classical theories (Agrawal et al. 1992;Thorpe 1993;Melville 1994;Anis and Moum 1995;Melville 1996;Terray et al. 1996; Veron and Melville 2001) and causes significant energy dissipation (Banner et al. 2014;Thomson et al. 2016;Schwendeman et al. 2014;Zappa et al. 2016; Melville 2013, 2015).On the air side, it is clear that surface wave processes also play an important role in the kinematics and dynamics of the boundary layer (e.g. Janssen 1989; Komen et al. 1994;Belcher and Hunt 1998;Hristov et al. 1998; Abstract Physical phenomena at an air-water interface are of interest in a variety of flows with both industrial and natural/environmental applications. In this paper, we present novel experimental techniques incorporating a multi-camera multi-laser instrumentation in a combined particle image velocimetry and laser-induced fluorescence system. The system yields accurate surface detection thus enabling velocity measurements to be performed very close to the interface. In the application presented here, we show results from a laboratory study of the turbulent airflow over wind driven surface waves. Accurate detection of the wavy air-water interface further yields a curvilinear coordinate system that grants practical and easy implementation of ensemble and phase averaging routines. In turn, these averaging techniques allow for the separation of mean, surface wave coherent, and turbulent velocity fields. In this paper, we describe the instrumentation and techniques and show several data products obtained on the air-side of a wavy air-water interface.

Surface viscous stress over wind-driven waves with intermittent airflow separation

“…The dynamics in the airflow above wind-generated waves is crucial for wind-wave coupling and for the air-sea momentum flux as a whole (Janssen 1989;Komen et al 1994;Belcher & Hunt 1998;Edson & Fairal 1998;Janssen 1999;Sullivan & McWilliams 2002;Sullivan et al 2008;Mueller & Veron 2009;Sullivan & McWilliams 2010;Grare, Lenain & Melville 2013a;Suzuki, Hara & Sullivan 2013;Hara & Sullivan 2015;Grare, Lenain & Melville 2018). Detailed experimental investigations of the airflow structure and wind stress above wind waves remain rare, however, largely because of the technical challenges involved with acquiring high resolution measurements very close to a rapidly moving interface (Buckley & Veron 2017). Yet, it is now accepted that surface waves in general, and airflow separation and wave breaking processes in particular, play an important role in the momentum flux between the ocean and the atmosphere (Melville 1996;Sullivan & McWilliams 2010).…”

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

“…where τ is the total wind stress, ρ a is air density, and u * is wind friction velocity, which is alternatively expressed in terms of a more convenient and readily available wind speed at z = 10 meters height, U 10 , and a corresponding empirical drag coefficient parameter C D . Original and most commonly used parameterizations of C D depend only on wind speed (e.g., Large & Pond, 1981;Yelland & Taylor, 1996, among others) without specifying and hence effectively averaging over the wave field variability. It has been recognized, however, that the drag coefficient, which is related to the surface roughness length, z 0 , by the law of the wall.…”

Scanning stereo-PLIF method for free surface measurements in large 3D domains

“…In this case, the typical error of the reference system is estimated as the error induced by a single pixel shift, which corresponds to 0.1 mm or approximately 1-3% of the amplitude. The errors of the stereo-camera system are comparable to typical PLIF systems (e.g., Buckley and Veron 2017;Duncan et al 1999).…”

“…No surface seeding is required when there are naturally present contaminants (Gomit et al 2015) or when the reflection of colored light is used (Dabiri and Gharib 2001). Otherwise, a dye is required to make the liquid fully opaque (Cobelli et al 2009;Tsubaki and Fujita 2005) or to make the liquid fluorescent (André and Bardet 2014;Buckley and Veron 2017;Duncan et al 1999). For fully opaque fluids, fringe projection techniques can be applied to obtain accurate, two-dimensional free surface height measurements in large three-dimensional domains (Cobelli et al 2009).…”

“…The technique offers accurate free surface height measurements, without altering the fluid properties or the need for fixed patterns. Furthermore, per-pixel wave height measurements can be obtained along a line (Buckley and Veron 2017). In contrast, stereo-correlation-based approaches use particles or features and are limited by the distribution of these particles or features over the surface.…”