Three-dimensional vorticity measurements in the wake of a yawed circular cylinder

Abstract

Using a phase-averaged technique, the dependence of the wake vortical structures on cylinder yaw angle α (=0°–45°) was investigated by measuring all three-velocity and vorticity components simultaneously using an eight-hot wire vorticity probe in the intermediate region (x/d=10) of a yawed stationary circular cylinder wake. For all yaw angles, the phase-averaged velocity and vorticity contours display apparent Kármán vortices. It is found that when α≤15°, the maximum coherent concentrations of the three vorticity components do not change with α. However, when α is increased to 45°, the maximum concentrations of the coherent transverse and spanwise vorticity components decrease by about 33% and 50%, respectively, while that of the streamwise vorticity increases by about 70%, suggesting that the strength of the Kármán vortex shed from the yawed cylinder decreases and the three dimensionality of the flow is enhanced. The maximum coherent concentrations of u and v contours decrease by more than 20% while that of w increases by 100%. Correspondingly, the coherent contributions to the velocity variances ⟨u2⟩ and ⟨v2⟩ decrease, while that of ⟨w2⟩ increases. These results may indicate the generation of the secondary axial vortices in yawed cylinder wakes when α is larger than 15°. The incoherent vorticity contours ⟨ωxr2⟩ are stretched along an axis inclining to the x-axis at an angle β in the range of 60°–25° for α=0°–45°. The magnitudes of ⟨ωxr2⟩∗ and ⟨ωyr2⟩∗ through the saddle points are comparable to the maximum concentration of the coherent spanwise vorticity ω̃z∗ at all cylinder yaw angles, supporting the previous speculation that the strength of the riblike structures in the cylinder wake is about the same as that of the spanwise structures, even in the yawed cylinder wakes.