Manish K. Mathur, Anagh S. Bhanu, Murali R. Cholemari, Srinivas V. Veeravalli
This study investigates the influence of wind tunnel ground conditions (stationary/moving) on flow topology and passive scalar dispersion in the wake of the Ahmed body with rear slant angles, $\phi$ = 25 $^\circ$ and 40 $^\circ$ . We implement field measurements of both velocity and scalar concentrations in the wake, for both the ground conditions, within the same experimental set-up, allowing for structural correlation between wake topology and scalar dispersion. Particle image velocimetry measurements reveal the existence of a third spanwise vortex (vortex G) near the stationary wind tunnel ground, due to the floor boundary layer, for both of the Ahmed bodies ( $\phi$ = 25 $^\circ$ , 40 $^\circ$ ). Concentration field measurements performed using quantitative smoke visualisation show higher scalar dispersion in the wake of both Ahmed bodies for the stationary ground condition. Comparing the velocity and concentration fields further identifies vortex G as the primary physical driver for the enhanced vertical dispersion of the scalar, observed in stationary ground conditions. To quantify the dispersion and characterise these effects, we introduce dispersion parameters, such as non-dimensional dispersion ( $\mathscr{D}$ ) and dispersion length scales ( $\mathscr{L}_y, \mathscr{L}_z$ ). These parameters confirm that, while lateral dispersion remains relatively insensitive to wind tunnel ground conditions, the presence of vortex G in stationary ground conditions leads to an overestimation of vertical dispersion by up to $\approx$ 29 % ( $\phi$ = 25 $^\circ$ ) and $\approx$ 49 % ( $\phi$ = 40 $^\circ$ ). This study quantifies the overestimated dispersion, identifies the vortical structures responsible for scalar redistribution, provides physical insight into the wake dispersion phenomenon and highlights the importance of correct wind tunnel ground conditions in the vehicle wake dispersion studies.