Observations and Measurements of Air Flow over Water Waves

Weissman, Michael A.

doi:10.1007/978-1-4684-8980-4_24

Cited by 8 publications

(13 citation statements)

References 15 publications

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“…Banner and Melville (1976) were the first to establish that airflow separation occurs only if a stagnation point exists at the air/water interface and this kinematic condition corresponds to incipient breaking. Though separation may also occur when the waves are not breaking (Weissmann 1986), it seems likely that breaking waves induce separation far more strongly and consistently. Detailed information on the structure of the flow were first given by Kawai (1981Kawai ( , 1982 who clearly visualized airflow separation patterns over short wind-waves.…”

Section: Introductionmentioning

confidence: 95%

Air Flow Structure Over Short-gravity Breaking Water Waves

Reul

Branger

Giovanangeli

2007

Boundary-Layer Meteorol

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Despite its importance for momentum and mass transfer across the air-sea interface, the dynamics of airflow over breaking waves is largely unknown. To fill this gap, velocity and vorticity distributions above short-gravity breaking waves have been measured in a wind-wave tank. A Digital Particle Image velocimetry technique (DPIV) was developed to accomplish these measurements above single breaking waves, propagating in mechanically-generated wave groups and forced by the wind. By varying the wind speed and initial characteristics of the groups, the airflow structure was captured over waves at different stages of the breaking process, and breaking with various intensities. The instantaneous airflow that separates from a sharp breaking crest is very similar to that occurring over a backward facing step. The separation bubble is however strongly unsteady: the steeper the wave crest and the larger the Reynolds number based on the crest-height, the higher the separated layer and the farther downwind the reattachment point. Instantaneous flow topology displays specific features of three-dimensional separation patterns. The tangential stress above the wave profile does not exhibit spikes at reattachment but grows progressively downwind from zero at reattachment to a value at the next crest approximately that found at the upwind breaking crest. Static pressure measurements revealed that large pressure falls are generated by vortices in the separated layer, as found in separated flows over solids. This study may provide useful data for theoretical and numerical modelling of the flow and associated phenomena.

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Section: Introductionmentioning

confidence: 95%

Air Flow Structure Over Short-gravity Breaking Water Waves

Reul

Branger

Giovanangeli

2007

Boundary-Layer Meteorol

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“…However, early flow visualization techniques indicated that airflow patterns resembling the separated flow over solid obstacles may occur over non-breaking waves (Kawai, 1981(Kawai, , 1982Weissman, 1986). According to Weissman (1986), the separation criterion utilized by Gent & Taylor (1977) and Banner & Melville (1976) is not appropriate when the surface is moving, as airflow separation over moving boundaries may have stagnation points away from the wall. As described by Weissman (1986); "the "separation" we are concerned with is that of the shear layer closest to the surface...".…”

Section: Introductionmentioning

confidence: 99%

“…According to Weissman (1986), the separation criterion utilized by Gent & Taylor (1977) and Banner & Melville (1976) is not appropriate when the surface is moving, as airflow separation over moving boundaries may have stagnation points away from the wall. As described by Weissman (1986); "the "separation" we are concerned with is that of the shear layer closest to the surface...". Buckley & Veron (2016) considered the airflow as separating "if the near-surface, high vorticity layer characteristic of an attached boundary layer is ejected away from the water surface and the surface vorticity is near zero or negative".…”

Section: Introductionmentioning

confidence: 99%

Experimental investigation of intermittent airflow separation and microscale wave breaking in wavy two-phase pipe flow

2019

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We perform an experimental analysis of co-current, stratified wavy pipe flow, with the aim of investigating the effect of small scale wave breaking (microscale breaking) on the airflow. Particle image velocimetry is applied simultaneously in the gas and liquid phases. Active wave breaking is identified by high levels of vorticity on the leeward side of individual waves, and the statistics of the airflow above breaking and non-breaking waves are extracted from the gas-phase velocity fields. Keeping the liquid superficial velocity constant ($U_{sl}=0.1~\text{m}~\text{s}^{-1}$), we consider two experimental cases of different gas flow rates. The lowest flow rate ($U_{sg}=1.85~\text{m}~\text{s}^{-1}$) is slightly higher than the onset of microscale breaking, while the higher flow rate ($U_{sg}=2.20~\text{m}~\text{s}^{-1}$) is within the regime where wave breaking is observed to be frequent, and the root-mean-square interface elevation $\unicode[STIX]{x1D702}_{rms}$ is independent of gas flow rate. Results show that for the lowest gas flow rate considered, active wave breaking has a stabilizing effect on the airflow above the waves, reducing the sheltered region on the leeward side of the wave and the turbulence above the wave crest compared with non-breaking waves at similar steepness. At the higher gas flow rate the effect of active wave breaking is found to be small, and the main geometrical properties of the waves are found to dominate the evolution of the separated flow region.

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“…This is in part due to the difficulty of making measurements within one wave height of the mean water level. To date there are few studies of airflow patterns and separation over water waves [e.g., Chang et al , 1971; Banner and Melville , 1976; Kawai , 1981, 1982; Csanady , 1985; Weissmann , 1986; Kawamura and Toba , 1988; Banner , 1990; Reul et al , 1999; Kudryavtsev and Makin , 2001], and two‐dimensional velocity measurements showing the separation of the airflow above the surface waves are limited, to our knowledge, to the work of Kawai [1982] and Reul et al [1999]. Kawai [1982] performed low‐resolution velocity measurements but stress estimates from his data were not possible.…”

Section: Introductionmentioning

confidence: 99%

Measurements of the viscous tangential stress in the airflow above wind waves

Véron¹,

Saxena²,

Misra³

2007

Geophysical Research Letters

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The stress and drag at the surface of the ocean are crucial parameters for both short term forecasting and the modeling of long‐term global climate trends. However, the partition between viscous, turbulent, and wave stresses, and in particular the effects of airflow separation are not well understood. We present direct measurements of the velocity in the airflow above wind‐generated waves. We observe intermittent separation of the viscous sublayer past the crest of the wind waves leading to dramatic along‐wave variability in the surface viscous tangential stress. These results hold for wind speeds that would normally be considered low to moderate. These viscous stress measurements in the airflow above the wavy surface, and within the separated region are, to the best of the authors' knowledge, the first of this kind.

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Observations and Measurements of Air Flow over Water Waves

Cited by 8 publications

References 15 publications

Air Flow Structure Over Short-gravity Breaking Water Waves

Air Flow Structure Over Short-gravity Breaking Water Waves

Experimental investigation of intermittent airflow separation and microscale wave breaking in wavy two-phase pipe flow

Measurements of the viscous tangential stress in the airflow above wind waves

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