Spectral analysis and cross-correlation techniques for photon counting measurements on fluid flows

Area of Science:

• Fluid dynamics
• Optical measurement techniques

Background:

• Photon counting techniques are employed for measuring turbulent fluid flows.
• Conventional Fourier methods applied to Laser Doppler Velocimetry (LDV) count correlation records can lead to significant errors when transforming data to the frequency domain.

Purpose of the Study:

• To analyze errors in photon counting techniques for turbulent flow measurement.
• To present and validate alternative schemes for accurate frequency domain transformation of count correlation records.

Main Methods:

• Utilizing high-resolution spectral techniques to transform count autocorrelation records.
• Employing count cross-correlation between signals from two detectors.
• Conducting theoretical analysis of the count cross-correlation process.

Main Results:

• Demonstrated that conventional Fourier methods can cause considerable errors in LDV measurements.
• Two alternative methods (high-resolution spectral techniques and count cross-correlation) were presented to overcome these errors.
• Experimental results in air flow showed close agreement between the turbulence levels predicted by the two new methods.

Conclusions:

• The developed photon counting techniques provide accurate measurements of turbulent fluid flows.
• Alternative spectral and cross-correlation methods are superior to conventional Fourier methods for LDV data analysis in turbulent flows.