Effect of the angle of attack on flow around an elliptic cylinder near a moving wall

Abstract

The present work investigates the combined effect of the angle of attack (AOA) (the angle between the semi-major axis of the cylinder and the vertical axis), and the gap ratio G/D (where G represents the distance between the cylinder center and the moving wall and D denotes the semi-major axis of the elliptic cylinder) on the flow around an elliptic cylinder near a moving wall. Here, AOA covers ±15°,±30°, and ±45° with G/D ranging from 0.6 to 2.5. The Reynolds number (based on the free-stream velocity and the semi-major axis) is fixed to 150. The resulting Kármán vortex street, the two-layered wake, and the secondary vortex street have been investigated and visualized. The resulting patterns have been classified and mapped out in the (G/D, AOA)-space. At small gap ratios, a clockwise rotation of the cylinder (negative AOA) leads to a stronger suppression effect between the moving wall and the backside of the cylinder. This results in more transitions between the different wake patterns than for the counterclockwise rotated cylinder (positive AOA). As the cylinder is rotated more, either clockwise or counterclockwise (i.e., as |AOA| increases), the crest value of the drag coefficient decreases while the crest value of the lift coefficient first increases and then decreases for G/D≥1.0. For a given G/D, the time-averaged drag coefficient decreases with the increased rotation of the cylinder, except for an increase observed as AOA increases from 0° to 15° for G/D≤0.9. Moreover, the lift force is directed upward and downward for positive and negative AOA, respectively, and its magnitude decreases with the increase in G/D when the cylinder is counterclockwise rotated, while it increases when the cylinder is clockwise rotated.