Abstract
The effect of a boattail angle on the structure of the wake of an axisymmetric model was investigated at low-speed condition. Four conical boattail models with angles of 0° (blunt-based body), 10°, 16°, and 22° were selected for this study. The Reynolds number based on the diameter of the model was around 1.97×104. Particle image velocimetry (PIV) was used to measure the velocity of the wake flow. The time-averaged flow characteristics including the length of recirculation of the afterbody, turbulent intensity, and Reynolds shear stress were analyzed and compared among those boattail models. The experimental results showed that the length of recirculation decreases with increasing boattail angle to 16°. At a boattail angle above 16°, the flow was fully separated near the shoulder and near-wake structure was highly changed. The turbulent intensity at a boattail angle of 22° showed a similar level to that in the case of the blunt-based body. Flow behavior on boattail surface should be accounted as an important parameter affecting the wake width and drag of the model. Power spectral density and proper orthogonal decomposition (POD) analyses showed that a Strouhal number of StD=0.2 dominated for the boattail model up to 16°. The fully separated flow was dominated by a Strouhal number of StD=0.03−0.06, which was firstly presented in this study.
Highlights
An axisymmetric blunt-based body is featured by a large separation around the base
Since this study focuses on the effect of the boattail angle in the nearwake structure, we did not conduct measurement with freelevitated system
The development of the boundary layer near the base edge, which leads to weaken of the free-shear layer when the boattail angle increases from 0° to 16°, is the other reason for the decrease of streamwise turbulent intensity
Summary
An axisymmetric blunt-based body is featured by a large separation around the base. The high turbulent region behind the base causes a significant decrease in the base pressure and an increase in the base drag, noise, and structure fatigue [1]. Merz et al [7], who investigated the nearwake of an axisymmetric semielliptical afterbody at lowspeed conditions, observed a significant decrease on the rear recirculation region comparing with that of the blunt-based body. They showed that the wall curvature produces a large effect on the pressure drag and the flow just outside the shear layer is highly convex near the separation position. Mariotti et al [6], who investigated contoured boattail models of 26.7° and 40.4°, reported two flow types on the boattail surface, which are fully attached and separated conditions They showed that the turbulent intensity of the wake reduces with increasing boattail angle, whenever separation occurs on the boattail surface or not.
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