Simultaneous Measurements of Scalar and Velocity in a Mach 5 Turbulent Boundary Layer using Naphthalene PLIF and PIV

Abstract

Scalar transport in a Mach 5 boundary layer is investigated using planar laser-induced fluorescence (PLIF) of a low-temperature sublimating ablator (naphthalene). Naphthalene vapor is introduced into the flow by ablation of a solid naphthalene plug located upstream of the imaging field of view and mounted flush with the wind tunnel floor. The naphthalene PLIF technique is used quantitatively, and is employed simultaneously with PIV to acquire two-dimensional fields of naphthalene mole fraction and velocity. The naphthalene mole fraction was measured with an uncertainty of ± 20%. The turbulent scalar structures between 0 < y/δ < 0.2 had a naphthalene mole fraction on the order of 1.5 × 10-4, which is approximately 6% of the saturation mole fraction in the boundary layer. The naphthalene vapor structures were primarily confined within y/δ < 0.4 and these large-scale naphthalene vapor structures appeared to coincide with regions of relatively low streamwise velocity. However, a correlation between naphthalene mole fraction and wall-normal velocity was not obvious from a visual inspection of the simultaneous image sets. Profiles of velocity and naphthalene mole fraction were both shown to exhibit logarithmic behavior from 100 < y+ < 300 when plotted in wall units, in agreement with several previous works. Root-mean-square profiles of the naphthalene mole fraction and velocity in the boundary layer were also acquired and the two profiles appeared similar, with the peak r.m.s. values occurring at the wall, then exhibiting a steady decay away from the wall. Lastly, by calculating the covariance of with respect to and , it was demonstrated that regions of high scalar coincided with negative fluctuations in streamwise velocity and positive fluctuations in wallnormal velocity away from the wall, indicating that an ejection mechanism is transporting low-momentum, high-scalar-concentration fluid away from the wall.