Fiber-Optic Probe Array for Measuring Spatial Distributions of Air Volume Fractions in Bubbly Flows

Area of Science:

• Fluid Dynamics
• Optical Sensing Technology

Background:

• Bubbly flows are prevalent in coastal engineering and oceanography.
• Accurate measurement of air volume (void) fractions is essential for understanding these phenomena.
• Existing methods may lack the spatial resolution required for complex flow fields.

Purpose of the Study:

• To develop and validate a fiber-optic sensing system for measuring spatial void fraction distributions.
• To investigate the impact of wave and breakwater parameters on void fraction distribution.
• To provide a reliable tool for studying air entrainment in wave breaking.

Main Methods:

• Calibration of a single-probe fiber-optic sensor to correlate signal time fraction with void fraction.
• Development of an eight-probe array system for spatial void fraction mapping.
• Systematic experiments in a wave flume simulating breaking waves near a submerged breakwater.

Main Results:

• A high correlation (coefficient) was achieved between the time fraction ratio and void fraction during calibration.
• The system demonstrated capability to measure local void fractions up to 18%.
• Spatial void fraction distributions were successfully mapped on the lee side of the breakwater, showing dependencies on wave height, period, and breakwater width.

Conclusions:

• The developed fiber-optic sensing system is effective for measuring local void fractions in bubbly flows.
• The system provides valuable data on the spatial distribution of air entrainment caused by breaking waves.
• This technology offers a robust method for studying complex air-water interactions in coastal environments.