A Numerical Investigation on the Aerodynamic Properties of the Wavy Circular Cylinders

Driven by the need to mitigate the drag and vibration of the typical cables structures, the current thesis is dedicated to a numerical investigation of the aerodynamics associated with the flow around wavy circular cylinders. This is inspired by the successful flow control of the span-wise waviness found in several marine mammals and their biomimicry innovations. Based on the promising flow control efficacy of the wavy cylinder as has been reported by previous researches, the current work embark upon a detailed analysis of the wavy cylinder at two more complicated conditions. Firstly, the stationary wavy cylinder subjected to the inclined incoming flow is considered. Three inclination angles, 0◦, 30◦ and 45◦, together with 2× 2 combinations of shape parameters, namely λ/Dm = 2 and 6, a/Dm = 0.1 and 0.15, have been taken into consideration. The aerodynamic force coefficients in terms of both the span-wise averaged and sectional values are discussed at length. It is revealed that the growing inclination angle invites not only a surge in their span-wise averaged values, but also an enlargement in the sectional difference of the force coefficients. The span-wise correlation of the lift force is found to be enhanced for the wavy cylinders with the increase of inclination angle, while the opposite is true for the normal cylinders. By examining the mean wake properties, it is disclosed that the increase in the span-wise averaged drag force coefficients is closely related to the shrinkage in the vortex formation length and the regression of the base pressure, whereas the sectional distribution of the drag coefficients is largely affected by the stagnation pressure. The mean wall shear stress fields are exploited to shed light on the flow topology of the cylinder surfaces. It is discovered that the wavy cylinders, especially the short-wavelength one, exhibit significant span-wise variation in the separation structure in the inclined flow. Besides, the chaotic surface flow in the rear side of the noninclined cylinders could be regulated to maintain symmetry by the secondary axial flow in the near wake of the inclined cylinders. The instantaneous three-dimensional vortical structures are visualized by the Q isosurfaces at last to collaborate the previous discussions. A preliminary mechanism for the cessation of flow control efficacy for the wavy cylinders in inclined flow is also presented. The second contribution of this thesis is that for the first time, the vortex-induced vibration of the wavy cylinder is systematically studied. To this end, the wavy cylinder is mounted flexibly on a spring-damper system and its aeroelastic response to the normal incident stream is studied over a range of interested reduced velocities. It is discovered that the wavy cylinder, although almost eliminates the Kármán vortices in the fixed configuration, shows only limited efficacy on the mitigation of the flow induced vibration in the case of zero structural damping, and the typical initial-upper-lower type response curve is manifested. The aerodynamic forces of the wavy cylinder also magnify significantly in the flexibly mounted cases during synchronization. Moreover, the phase lag between the lift coefficient and the displacement displays a clear change from 0◦ at the initial branch to 180◦ at the lower branch. The association of the 2S and 2P vortex shedding modes to the different branches is also similar to that of the normal cylinder, in which the 2S mode corresponds to the initial branch and the 2P to the lower branch. In general, despite the absence of the primary shedding frequency in the fixed configuration, the flexibly mounted wavy cylinder exhibits many features that is also found in the normal cylinder. This implies that the vortex induced vibration may not be initiated by the Kármán vortex shedding, and thus may defy the conventional view on the mechanism of the vortex induced vibrations. Additional simulations are performed with non-zero structural damping. It is disclosed that with sufficiently high structural damping, the vibration of the wavy cylinder could be reduced more efficiently than the normal cylinder.