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Improved quantitative visualization of hypervelocity flow through wavefront estimation based on shadow casting of
Applied Optics
|August 10, 2016
Summary
A novel optical probe non-interferometrically measures wavefront distortion in hypersonic flow. This technique enables quantitative recovery of density distribution around models using Fourier fringe analysis and tomographic inversion.
Area of Science:
- Fluid Dynamics
- Optical Metrology
- Aerospace Engineering
Background:
- Accurate measurement of hypersonic flow fields is crucial for aerodynamic design.
- Traditional optical methods often require complex interferometric setups.
- Characterizing density variations in high-speed flows presents significant challenges.
Purpose of the Study:
- To develop a simple, non-interferometric optical probe for estimating wavefront distortion.
- To enable quantitative recovery of density distribution in hypersonic flows.
- To validate the technique by comparing measurements with numerical simulations.
Main Methods:
- Utilizing a plane light wave to trans-illuminate hypersonic flow and project a sinusoidal grating shadow.
- Applying geometrical optics and eikonal approximation for wavefront distortion analysis.
- Employing Fourier fringe analysis (global FT and windowed WFT) and cross-correlation for processing grating and random-dot patterns.
- Implementing discrete cosine transform (DCT) and iterative methods for phase unwrapping.
- Using refraction tomographic inversion for quantitative density recovery.
Main Results:
- The probe successfully estimates wavefront distortion and enables space-continuous readout.
- Fourier transform (FT) inversion provided accurate and efficient results for small space-bandwidth products (SBP).
- Windowed Fourier transform (WFT) was necessary for accurate results with larger SBP wavefronts.
- Quantitative density distribution around a missile-shaped body at Mach 8 was recovered.
- Measurements showed good agreement with numerically estimated hypersonic flow field parameters.
Conclusions:
- The developed non-interferometric optical probe is effective for analyzing hypersonic flow fields.
- The technique offers a practical approach for quantitative density mapping in high-speed flows.
- The choice between FT and WFT depends on the wavefront's space-bandwidth product for optimal performance.
