The effect of wind-generated bubbles on sea-surface backscatter
conference paper
Predictions of sea-surface back-scattering strength are needed for sonar
performance modelling. Such predictions are hampered by two problems. First, measurements of surface back-scattering are not available at small grazing angles. These are of special interest to low-frequency active sonar since they mainly contribute to long range propagation. Second, existing theoretical models based on a bubble-free interface underestimate the surface back-scattering strength at larger grazing angles. We investigate whether wind-generated bubbles can explain this deficit. For this purpose, we develop a theoretical description that includes the effect of refraction and scattering of sound by wind-generated bubbles. The comparison of the theoretical predictions to
Critical Sea Test measurements show that a good fit is obtained between theoretical predictions and measurements for wind speeds up to 10 m/s. For larger wind speeds, the surface back-scattering strength critically depends on the population density of large (radius > 1 mm) bubbles. This provides an opportunity to estimate the number of large bubbles. We observe a change in the spectral slope in the bubble population model for
large bubbles that is in agreement with high-speed camera observations in breaking waves and with the Hinze scale.
performance modelling. Such predictions are hampered by two problems. First, measurements of surface back-scattering are not available at small grazing angles. These are of special interest to low-frequency active sonar since they mainly contribute to long range propagation. Second, existing theoretical models based on a bubble-free interface underestimate the surface back-scattering strength at larger grazing angles. We investigate whether wind-generated bubbles can explain this deficit. For this purpose, we develop a theoretical description that includes the effect of refraction and scattering of sound by wind-generated bubbles. The comparison of the theoretical predictions to
Critical Sea Test measurements show that a good fit is obtained between theoretical predictions and measurements for wind speeds up to 10 m/s. For larger wind speeds, the surface back-scattering strength critically depends on the population density of large (radius > 1 mm) bubbles. This provides an opportunity to estimate the number of large bubbles. We observe a change in the spectral slope in the bubble population model for
large bubbles that is in agreement with high-speed camera observations in breaking waves and with the Hinze scale.
TNO Identifier
462537
Source title
Proceedings 3rd International Conference and Exhibition on Underwater Acoustic Measurements - Technologies & Results - UAM 2009, 21-26 June 2009, Nafplion, Greece