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Abstract
This experimental study investigates the effect of imposed rotary oscillation on the flowinduced vibration of a sphere that is elastically mounted in the crossflow direction, employing simultaneous displacement, force and vorticity measurements. The response is studied over a wide range of forcing parameters, including the frequency ratio f_{R} and velocity ratio α_{R} of the oscillatory forcing, which vary between 0 ≤ f_{R} ≤ 5 and 0 ≤ α_{R} ≤ 2. The effect of another important flow parameter, the reduced velocity, U∗, is also investigated by varying it in small increments between 0 ≤ U∗ ≤ 20, corresponding to the Reynolds number range of 5000 ≲ Re ≲ 30 000. It has been found that when the forcing frequency of the imposed rotary oscillations, f_{r}, is close to the natural frequency of the system, f_{nw}, (so that f_{R} = f_{r}/f_{nw} ∼ 1), the sphere vibrations lock on to f_{r} instead of f_{nw}. This inhibits the normal resonance or lockin leading to a highly reduced vibration response amplitude. This phenomenon has been termed 'rotary lockon', and occurs for only a narrow range of f_{R} in the vicinity of f_{R} = 1. When rotary lockon occurs, the phase difference between the total transverse force coefficient and the sphere displacement, φ_{total}, jumps from 0° (in phase) to 180° (out of phase). A corresponding dip in the total transverse force coefficient C_{y(rms)} is also observed. Outside the lockon boundaries, a highly modulated amplitude response is observed. Higher velocity ratios (α_{R} ≥ 0.5) are more effective in reducing the vibration response of a sphere to much lower values. The mode I sphere vortexinduced vibration (VIV) response is found to resist suppression, requiring very high velocity ratios (α_{R} > 1.5) to significantly suppress vibrations for the entire range of f_{R} tested. On the other hand, mode II and mode III are suppressed for α_{R} ≥ 1. The width of the lockon region increases with an increase in α_{R}. Interestingly, a reduction of VIV is also observed in nonlockon regions for high f_{R} and α_{R} values. For a fixed α_{R}, when U∗ is progressively increased, the response of the sphere is very rich, exhibiting characteristically different vibration responses for different f_{R} values. The phase difference between the imposed rotary oscillation and the sphere displacement φ_{rot} is found to be crucial in determining the response. For selected f_{R} values, the vibration amplitude increases monotonically with an increase in flow velocity, reaching magnitudes much higher than the peak VIV response for a nonrotating sphere. For these cases, the vibrations are always locked to the forcing frequency, and there is a linear decrease in φ_{rot}. Such vibrations have been termed 'rotaryinduced vibrations'. The wake measurements in the crossplane 1.5D downstream of the sphere position reveal that the sphere wake consists of vortex loops, similar to the wake of a sphere without any imposed rotation; however, there is a change in the timing of vortex formation. On the other hand, for high f_{R} values, there is a reduction in the streamwise vorticity, presumably leading to a decreased total transverse force acting on the sphere and resulting in a reduced response.
Original language  English 

Pages (fromto)  703735 
Number of pages  33 
Journal  Journal of Fluid Mechanics 
Volume  855 
DOIs  
Publication status  Published  25 Nov 2018 
Keywords
 flowstructure interactions
 vortex streets
 wakes
Projects
 1 Finished

Wake Transitions and FluidStructure Interactions of Rotating Bluff Bodies
Hourigan, K., Lo Jacono, D., Sheridan, J., Thompson, M. & Leweke, T.
Australian Research Council (ARC), Monash University, CNRS  Centre National de la Recherche Scientifique (French National Centre for Scientific Research), Université PaulSabatier (Paul Sabatier University)
1/01/15 → 6/11/18
Project: Research