Flow Characteristics over Double Delta Wings at Low Reynolds Numbers

Within the scope of this study, a quantitative investigation was carried out employing the particle image velocimetry (PIV) technique to determine vortical flow characteristics of 70°/40°, 70°/50°, and 70°/55° double delta wings (DDWs). Experiments were performed in four chordwise sections ($x/c=0.5$, 0.6, 0.8, and 1) at two Reynolds numbers ($\mathsf{R}=1\times {10}^4$ and $2.5\times {10}^4$) and different angles of attack ($\alpha =5°$, 10°, 15°, and 25°). It was observed that the secondary vortex is dominant on flow characteristics of double delta wings at low Reynolds numbers. The strengths of vortices increase with increases of both the wing sweep angle and Reynolds number. Furthermore, the location of wing vortex breakdown moves farther downstream as the wing sweep angle increased. Depending on the angle of attack, Reynolds number, and the wing sweep angle, either dual (strake and wing) or triple (strake, wing, and secondary) leading-edge vortices were observed. At a Reynolds number value $\mathsf{R}=1\times {10}^4$ and angles of attack in the range of $\alpha \le 15°$, the interactions of the vortices influence the flow structure significantly. Dominant frequencies of the vortices are detected near the shear layer formed between the strake vortex and secondary vortex, generally. Moreover, the spectral analyses indicated that bursting of the vortices causes complicated flow patterns over double delta wings, including small-scale vortices. It was found that the transverse Reynolds normal stress concentrates at chordwise sections where secondary vortices are dominant, whereas the locations of peak magnitude of vertical Reynolds normal stress coincide with the strake vortex cores.