Absorption focused laser differential interferometry for near-simultaneous steady and fluctuating flow property...

Area of Science:

• Optical diagnostics
• Fluid dynamics
• Spectroscopy

Background:

• Accurate measurement of flow density fluctuations and steady flow properties is crucial in wind tunnel research.
• Traditional methods often require flow seeding, which can interfere with measurements.
• Molecular oxygen (O2) is a common gas in wind tunnel facilities, offering a potential non-intrusive measurement target.

Purpose of the Study:

• To develop and test a focused laser differential interferometer (FLDI) system for near-simultaneous, colinear measurements.
• To measure high-frequency flow density fluctuations and steady flow properties using molecular oxygen absorption.
• To evaluate laser modulation techniques for optimizing interferometer and absorption measurements.

Main Methods:

• Assembly and laboratory testing of a focused laser differential interferometer (FLDI) system utilizing a single tunable diode laser near 760 nm.
• Probing the A-band absorption spectrum of molecular oxygen for non-intrusive measurements.
• Testing both non-focused LDI and FLDI systems in a pressure-controlled test cell with 100% O2 and a compressed air jet.

Main Results:

• Both LDI and FLDI systems accurately measured pressure in O2 from 0.01 atm to 0.3 atm, with FLDI showing higher uncertainty.
• LDI demonstrated constant sensitivity to a compressed air jet, while FLDI showed signal filtering away from the focal plane.
• Direct absorption and wavelength modulation techniques were demonstrated, with a comparative analysis of their benefits and drawbacks.

Conclusions:

• The developed FLDI system shows promise for non-intrusive, simultaneous measurements of flow density fluctuations and properties.
• The system's ability to probe molecular oxygen absorption avoids the need for flow seeding.
• Further optimization of laser modulation techniques is recommended for enhanced measurement accuracy and performance.