Mechanism of drag reduction on a three-dimensional model vehicle using a passive control device

It has been well known that the boat-tail device reduces drag on a three-dimensional vehicle. However, its detailed mechanism is not clearly known yet. To understand this mechanism, we conduct an experiment for flow over a three-dimensional model vehicle in ground proximity. We consider various lengths ($l/H = 0.1 \sim 0.5$) and slant angles ($\theta=0^{\circ} \sim 40^{\circ}$) of the boat tail, and conduct velocity measurements near the boat tail and oil visualizations on the boat-tail surface. We find that the slant angle is an important parameter for drag reduction. The maximum drag reduction occurs at $\theta=12.5^{\circ}, 15^{\circ}$ and $15^{\circ}$ for $l/H = 0.1, 0.3$ and $0.5$, respectively, and the amounts of maximum drag reduction are 20, 41 and 45\%. For the case of $l/H = 0.3$, separation starts to occur from $\theta= 6^{\circ}$ at the leading edge of the boat tail. This separated flow reattaches on the boat-tail surface and forms a small secondary separation bubble, which provides strong near-wall momentum and delays main separation down to the trailing edge of boat tail. The size of secondary separation bubble increases with increasing $\theta$. At $\theta>16^{\circ}$, main separation occurs at the leading edge of boat tail, and drag increases from the minimum value and reaches that of no control at large $\theta$'s.