Importance of Refractive Index Matching of Fluids for PIV and PLIF Measurements in Buoyant Jets

On the errors and their mitigation in planar laser induced fluorescence

“…Many of the studies [14,23,24,26,27] suggest the use of a wavelet transform followed by filtering the Fourier transform of the data using a Gaussian filter proposed by Münch et al [28]. However, it was found during the present investigation, that this procedure, while removing the horizontal striations, introduced additional artefacts in the form of altered structures in the image.…”

“…Some studies have used a reference cell with known concentration on the beam path ahead of the region of interest [11,23]. It is not always possible to have a reference cell depending on the complexity of the experimental setup [24][25][26]. Hence, studies have shown to take measurements under the 'optically thin' assumption [22], and invert to concentration based on the captured intensity of the fluorescence.…”

“…Crimaldi [17] has given an extended method of taking such measurements in his detailed review. An implementation of this method can be seen in [24]. Briefly, a 'calibration' procedure is defined to determine the limit of linearity in equation ( 2) and the extinction coefficient, ζ, for the experimental configuration in addition to defining a pixel-wise map of relation between concentration and fluorescence which is later used to map back from captured fluorescence to actual concentration during the actual experiment.…”

“…Generating a light sheet typically introduces optical aberrations in the form of striations which may be attributed to the imperfections in the lenses used (see figure 5(a)). These striations are commonly found in the studies involving quantitative concentration measurements [23,24,26,27]. Hence the process of attenuation correction is affected by the presence of striations unless the integration is done parallel to them.…”

“…This situation corresponds to the case where pixel wise map of calibration curves is taken for a supposed inherent attenuation correction [17,24]. Clearly, equation (20) shows that this is not true, although the error is less than neglecting attenuation completely, if an appropriate value of C cal is used.…”