Particle dispersion in the turbulent wake of an Ahmed body: An experimental and computational study

Moving vehicles pick up the particulates/dust on the road surface, and the wake structures behind the vehicle determine the dispersion of these particles. This study aims at understanding the characteristics of particle dispersion in the turbulent wake of a moving vehicle. The flow over a simplified vehicle model known as the Ahmed body is analysed. Experimental measurements and computational simulations of particle dispersion in the near wake of Ahmed body are presented. Unsteady simulations are performed for the prediction of the flow topology and particle dispersion around the Ahmed body, at Rel=0.7x105\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Re_l=0.7\times 10<^>5$$\end{document} (Reynolds no. based on vehicle model length 'l'), the rear slant angle of Ahmed body considered is phi=25o\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\phi =25<^>o$$\end{document}. Governing equations solved are unsteady Reynolds Averaged Navier-Stokes (URANS) equations, and the turbulence model employed was SST k-omega\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k-\omega $$\end{document}. Experiments involved the release of fog particles (Ethylene-Glycol, dp approximate to 1 mu m\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_p\approx 1\;\upmu m$$\end{document}) from a source behind the model. Dispersion of the particles was measured in the wind tunnel using a laser sheet for fog particle visualisation at three downstream locations, X=1, 2, and 3, at Rel=1.4x105\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Re_l=1.4\times 10<^>5$$\end{document}. It was observed that the flow structure in the near wake of the model consisted of two counter-rotating spanwise vortices and a pair of trailing vortices. URANS simulations capture both the turbulent flow features in the near wake. Experimental results were compared with the URANS simulation results, and it was found that the dominant turbulent structures (recirculation vortices and trailing vortices) in the near wake contribute to particle dispersion. The dispersion patterns obtained are dependent on the size and strength of turbulent structures. The URANS simulations perform reasonably well in capturing the vertical dispersion of particles. However, they do not accurately estimate the lateral dispersion of smoke particles. The lateral dispersion is underestimated by the URANS simulations.