Structure of the airflow above surface waves

Date
2015
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
We present a laboratory investigation of the structure of the turbulent airflow above water surface gravity waves. Specifically, we investigate the intimate coupling of the wind with the waves, and we examine the role of the turbulent airflow kinematics for the momentum flux across the air-water interface. The airflow dynamics above the air-sea interface are believed to have a significant impact on the fluxes of momentum and scalars across the ocean surface. We present an experimental study of the turbulent structure of the airflow above waves, including instantaneous, mean, and wave-phase-averaged airflow characteristics. Measurements, taken at a fetch of 22.7 m in University of Delaware’s large wind-wave-current facility, are reported. We present results for a total of 17 different wind-wave conditions, including 5 wind wave experiments with 10-m extrapolated wind speeds spanning from 2.19 m s−1 to 16.63 m s−1. By combining winds with mechanically generated swells, we were able to achieve a wide range of wave ages Cp/u∗, from 1.4 to 66.7, where Cp is the peak wave phase speed, and u∗ the air friction velocity. In order to complete this study, we developed a complex imaging system, combining particle image velocimetry with laser induced fluorescence techniques. High resolution two-dimensional (18.7 x 9.7 cm2) velocity fields were measured as close as 100 μm above the air-water interface (on average). In addition, we acquired high spatial and temporal resolution wave field data simultaneously with the airflow measurements. The mean velocity profile follows the law of the wall in low winds (U10 = 0.86 m s−1, no waves detected). Over wind waves, the aerodynamic roughness of the airflow increases with increasing wind speed. Using our imaging system, we were able to measure airflow velocities within the viscous sublayer of the airflow boundary layer. Viscous sublayers remain intact and coherent upwind of wave crests at least up to a moderate wind speed of U10 = 9.41 m s−1. We were able to measure two-dimensional near-surface spanwise vorticity fields in the airflow. We observe direct evidence of airflow separation events past the crests of wind waves, starting at low to moderate wind speeds (U10 > 2.19 m s−1). With increasing wind speed, the contribution of viscous stress to total wind stress decreases exponentially (in favor of form drag), and the frequency of airflow separation events increases. At high wind speeds (U10 = 16.63 m s−1), over 85% of the waves experience airflow separation. Airflow separation causes dramatic along-wave variations in viscous stress. In all 17 experiments, the turbulent boundary layer in the air is characterized by numerous velocity sweeps and ejections, accompanied by intense downwind-tilted spanwise vorticity (shear) layers stemming from the surface. We were able to directly observe these turbulent events, and estimate their statistical significance using quadrant analysis. These events become phase-locked in the presence of waves, and over young wind waves (Cp/u∗ < 3.7), they are replaced by intermittent airflow separation events past wave crests. The mean airflow is subjected to a sheltering effect past wave crests, above the critical height c (defined by hu ( c)i = Cp). Mean along-wave turbulent momentum and energy in the airflow are also phase-locked. Intermittent airflow separation events past young wave crests cause free high shear layers to generate on average intense turbulence downwind of crests. We observe an opposite, upwind sheltering effect below the critical height. The airflow within the critical layer is strongly coupled with the wave orbital velocities at the water surface, starting at relatively low wave ages (Cp/u∗= 6.5). Preliminary instantaneous field measurements are consistent with our laboratory results.
Description
Keywords
Citation