Abstract
A new method is presented to measure the separation distance between probing volumes of closely spaced multi-foci Focused Laser Differential Interferometers (FLDI). The accuracy and precision of this distance measurement directly translate into the quality of convection velocity measurements performed by means of arrays of FLDI. The suggested method is based on the detection of a propagating weak blast wave, generated with a simple and inexpensive apparatus using an automotive spark plug. Demonstration is conducted using an FLDI with two foci (D-FLDI). The generated blast wave is probed at multiple distances from its source to verify its weakening into an acoustic pulse, which offers ideal conditions to the proposed methodology. D-FLDI separation distance measurement using the new approach is compared to measurements using beam profiler images and to the alternative currently established in the literature, based on the FLDI response to a moving weak lens. Tests are made on varying internal configurations of the D-FLDI, while the distance between the two systems is kept constant. Results show the present method to have improved accuracy and robustness in comparison with the moving lens approach, while requiring significantly less effort. Measured separation distances obtained from blast wave detections in a single location are within 0.5 % of the reference value measured through the beam profiler. This procedure is therefore a practical and reliable alternative to the measurement using beam profiler imaging, with similar quality. Its advantages concern associated costs, flexibility when measuring in constrained spaces such as in proximity to walls, and applicability to systems in which beam imaging is not an option, such as multi-point line FLDI.
Highlights
Focused Laser Differential Interferometry (FLDI) is a non-invasive measurement technique capable of detecting flowfield density fluctuations with remarkable spatial and temporal resolution, being especially suited to the field of experimental hypersonics (Parziale et al 2013)
In the precursor exploration by Jewell et al (2016), velocity estimates of second-mode instability wavepackets in a hypersonic boundary layer consistent with typically expected values were obtained using double-foci FLDI (D-FLDI)
The same modified lens displacement vector is used for both FLDIs in any given case, since the separation distance between them is constant and independent of this uncertainty
Summary
Focused Laser Differential Interferometry (FLDI) is a non-invasive measurement technique capable of detecting flowfield density fluctuations with remarkable spatial and temporal resolution, being especially suited to the field of experimental hypersonics (Parziale et al 2013). It is possible to use FLDI as a velocity measurement tool by producing two closely spaced probing volumes to obtain a double-foci FLDI (D-FLDI), as shown by Jewell et al (2016). In the precursor exploration by Jewell et al (2016), velocity estimates of second-mode instability wavepackets in a hypersonic boundary layer consistent with typically expected values were obtained using D-FLDI. Jewell et al (2019) presented velocity measurements of compressible turbulent jets. Results agreed to hot-wire measurements and a velocity decay model if a certain distance from the jet exit was observed, large uncertainties were reported. Bathel et al (2020) used a carefully adjusted D-FLDI with parallel optical axes to probe a laser-induced breakdown shock wave and a conical hypersonic boundary layer with second-mode instabilities. Measured shock wave convection velocity was in close agreement to the reference obtained from simultaneous high-speed schlieren, and with lower uncertainty. Reasonable agreement was verified for the velocity of the instability wavepackets, for which FLDI and high-speed
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
