Details

Autor(en) / Beteiligte

• Ceruzzi, Andrew P.
• Cadou, Christopher P.

Titel

Interpreting single-point and two-point focused laser differential interferometry in a turbulent jet

Ist Teil von

• Experiments in fluids, 2022-07, Vol.63 (7), Article 112

Ort / Verlag

Berlin/Heidelberg: Springer Berlin Heidelberg

Erscheinungsjahr

2022

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Focused laser differential interferometry (FLDI) and its relative two-point FLDI (2pFLDI) are used to make density fluctuation and velocity measurements, respectively, in a canonical, round turbulent air jet ( U jet = 300  m/s, d = 3.2  mm, R e d = 8.6 × 10 4 ). Both techniques are seedless, non-intrusive, inexpensive (< $5k), and insensitive to vibrations. The FLDI signal is proportional to the phase difference between two closely spaced laser beams passing through the flow. The phase difference is created by index of refraction gradients in the flow, integrated along the beam paths. Transfer functions for interpreting the FLDI signal are proposed as an accurate method for predicting the response in an arbitrary flow. A procedure for applying these transfer functions to a turbulent jet is developed. The procedure is able to model FLDI’s response to fluctuations in the jet with error on the order of 10–50% across a ∼ 100  kHz band. The transfer functions provide a simple method for estimating the FLDI & 2pFLDI spatial resolution along the optical axis, which is a strong function of disturbance scale, based on three instrument parameters: (1) the laser wavelength, λ 0 , (2) the beam separation, Δ x 1 , and (3) the beam radius at the focus, w 0 . For the 2pFLDI employed in this work ( λ 0 = 633  nm, Δ x 1 = 145 μ m, w 0 = 3 μ m), the resolution ranges from 1cm for a 0.9mm disturbance wavelength to 5cm for a 5.5mm disturbance wavelength. The spatiotemporal resolution depends on the convection velocity of the disturbances, as well as the spatiotemporal amplitude variation in the disturbances themselves. We model the velocity and spatial amplitude distribution with Gaussian functions based on historical jet studies and we measure the amplitude variation with frequency directly via FLDI. This leads to 1cm resolution for the smallest timescales measured (100 kHz) up to 5cm resolution for the largest time scales measured (1 k Hz). 2pFLDI’s relatively large spatial resolution complicates comparisons of velocity measurements to those made using hot-wires. The methods, modeling, and procedures outlined in this work provide a framework for interpreting future FLDI and multi-point FLDI measurements. Graphical abstract