Near-wall effect on flow around an elliptic cylinder translating above a plane wall

Abstract

In this work, the flow over an elliptic cylinder near a moving wall is investigated for Reynolds numbers less than 150. Here, the ratio between the gap (i.e., the distance between the cylinder and the wall) and the length of the semi-major axis of the elliptic cylinder varies from 0.1 to 5. This ratio is hereafter denoted as the gap ratio. The resulting Kármán vortex street, the two-layered wake, and the secondary vortex street have been investigated and visualized. Numerical simulations show that for the steady flow, the wake is composed of two asymmetric recirculation vortices, while a decrease in the gap ratio suppresses the vortex shed from the lower part of the cylinder. For the unsteady flow, the wake can be classified into four different patterns based on the wake structures (the Kármán vortex street, the two-layered wake, and the secondary vortex street). The regions of these wake patterns are given in the gap ratio and Reynolds number space, showing that the critical Reynolds number for the transition between different patterns increases as the gap ratio decreases. An overall increase in the mean drag coefficient with increasing gap ratios is observed, except for a sudden drop that occurs within a small gap ratio range. Moreover, as the gap ratio increases, the onset location of the two-layered wake first decreases due to a decrease in flow velocity in the gap and then increases due to the weakening of the wall suppression effect.

Three-dimensional wake transition behind an elliptic cylinder near a moving wall

“…In the present work, the near-wall effect leads to the two-layered wake moving upstream, thus suppressing the near-wake instability. This upstream movement of the two-layered wake caused by the near-wall effect was previously demonstrated by Zhu et al 20 for two-dimensional flow past an elliptic cylinder near a moving wall.…”

“…for wake pattern D identified by Zhu et al 20 for two-dimensional flow. Thus, this flow, depicted in figure 18, is denoted as the three-dimensional wake pattern D. For G/D = 0.2, the following wake transitions are found: For Re ≤ 120, the flow is twodimensional and steady without vortex shedding.…”

“…Four streamwise vorticity pairs (figure 17b) are present along the cylinder in the spanwise direction, corresponding to a spanwise wavelength of λ z = 9D, which is larger than that λ z = 6D observed in the three-dimensional steady flow regime for a circular cylinder near a moving wall 13,18 for G/D ≤ 0.22. It should be noted that this flow regime does not occur for flow around an elliptic cylinder at G/D = 0.2 because the critical G/D for the onset of the unsteady flow is larger for an elliptic cylinder than for a circular cylinder 20 . x for Re = 200.…”

“…In a previous work of Zhu et al 20 , the two-dimensional wake pattern behind an elliptic cylinder near a moving wall has been investigated for G/D ∈ [0.1, 5] and Re ≤ 150. At small gap ratios, a significant near-wall effect was found on the wake structures (including the Kármán vortex street and the two-layered wake).…”

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

“…Moreover, this flow configuration can be used to investigate the effect of gap ratio on the transition from the Kármán vortex street to the two-layered wake and the transition from the two-layered wake to the secondary vortex street which do not occur for a circular cylinder translating above a plane wall at the same Re range 18 . Four wake patterns have been classified by Zhu et al 23 for flow around an elliptic cylinder with AR = 0.4 and AOA = 0 o near a moving wall for Re ≤ 150 with G/D ∈ [0.1, 5]; i) at relatively large G/D, the flow, which is denoted as wake pattern A, contains the Kármán vortex street, the two-layered wake and the secondary vortex street; ii) a decrease of G/D suppresses the secondary vortex street, forming the wake pattern B; iii) a further decrease in G/D leads to the break-down of the Kármán vortex, resulting in a pair-wise vortex shedding (denoted wake pattern C) or iv) forming a quasi-steady near-wake region (with time-independent lift and drag coefficients) and a pair-wise vortex shedding farther downstream (denoted wake pattern D). They also found that the critical Re for the transition between the different wake patterns increases as G/D increases.…”