Effects of Optical Turbulence and Density Gradients on Particle Image Velocimetry

Area of Science:

• Fluid dynamics
• Optical physics
• Experimental methods

Background:

• Particle image velocimetry (PIV) is a standard laboratory technique for measuring fluid velocity and turbulence.
• Optical turbulence arises from variations in refractive index due to temperature or salinity gradients.
• Previous studies on optical turbulence effects in PIV focused on high Mach number flows in air, not isotropic conditions in water.

Purpose of the Study:

• To investigate the influence of optical turbulence on PIV imaging in water.
• To quantify the effects of varying path lengths and turbulence strengths on PIV measurements.
• To explore mitigation strategies for optical turbulence in PIV data.

Main Methods:

• Experiments were conducted in a large Rayleigh-Bénard tank.
• Particle image velocimetry (PIV) was used to capture flow dynamics.
• Optical turbulence was induced by varying temperature or salinity gradients.

Main Results:

• Optical turbulence was found to significantly affect PIV measurements.
• The impact of optical turbulence depends on its strength and the optical path length.
• Post-processing can reduce noise and recover mean velocity but sacrifices high-frequency turbulence data.

Conclusions:

• Optical turbulence poses a challenge for accurate PIV measurements in fluids with significant refractive index variations.
• Mitigation strategies exist but involve a trade-off between signal fidelity and noise reduction.
• Further research may be needed to fully characterize and compensate for optical turbulence effects in PIV.