Implementation of 3DPTV for turbulence analysis and subgrid-scale stress model testing of a backward-facing step flow

Abstract

Large-Eddy Simulation (LES) is a constantly-expanding field with many new applications and models being introduced on a regular basis. The active nature of this field establishes the need for high-resolution, 3-dimensional experimental data for assessment and development of Subgrid-Scale (SGS) Models. An experimental technique well-suited to this application is Three-Dimensional Particle Tracking Velocimetry (3DPTV) utilizing the epipolar line triangulation method is an accepted method of quantifying volumes of 3-Dimension 3-Component (3D3C) velocity vector fields. This study adapted the epipolar line search triangulation methodology, as it was applied in micro-scale systems, to a backward-facing step flow in a small-scale water tunnel testing facility. The camera system, consisting of three 4008 $\times$ 2672 CCDs, was aligned and calibrated using a custom grid and dot target plate mounted on a purpose-built rig, containing a precision single-axis translation stage. Dual-pulsed, Nd:YAG lasers at 532 nm, 120mJ/pulse, illuminated the $28\text{ mm} \times 18\text{ mm} \times 4.5\text{ mm}$ volume of interest, located downstream of a $2.858\text{ cm}$ step in a $15.24\text{ cm} \times 30.48\text{ cm}$ cross-section water tunnel. The turbulent flow, u$_\text{in}=$22cm/s, $Re_h=6274$, $R_\lambda\approx130$, and $ER=1.208$ was seeded with TiO$_2$ particles, $<$5$\mu$m diameter, to maintain one-way coupling. The resulting 3DPTV system was shown to have uncertainty comparable to that of previous experimentation. Utilizing the acquired data, \textit{a priori} testing of universally notable LES SGS Models, including the Smagorinsky, Similarity, Mixed, Coherent Structures, and Dynamic Models was accomplished and results are presented and discussed. This application of 3DPTV to a turbulent, backward-facing step flow and the results presented herein not only establish the technique as a promising source of experimental data in the development of LES, it lays a foundation for future study of the phenomena-rich backward-facing step flow and the testing and development of new LES SGS models.