Experimental investigation of flow field behind a bluff body controlled by an oscillating upstream micro-rod
Résumé
In the current study, the flow field due to an aerodynamic interaction between two non-identical cylinders arranged in tandem has been investigated experimentally. The considered system consists of an upstream micro-cylindrical rod oscillating in the transvers direction while the downstream cylindrical bluff body was held stationary. The experiments are carried out at a Reynolds number of 12537 based on the free stream velocity and the main circular cylinder diameter, D = 75.8 mm. The micro-rod is placed in-line to the main cylinder such that the ratio is set to be 2.5 times the formation length of the vortices generated by the small rod. A physical wake analysis is performed to illustrate the mechanism of such active flow control in altering the aerodynamic performance of a circular cylinder. Both the influences of the upstream micro-rod diameter, as well as the oscillation parameters (frequency and amplitude) are deeply examined using a hot-wire anemometry technique. The time mean velocity fields, turbulence intensity and skewness factor are measured for two micro-rod diameters (d/D=2.5% and d/D=5%) and two oscillating amplitudes (A/D=6.6% and A/D=13.2%). Moreover, the vortex shedding frequency of the baseline stationary cylinder is established by hotwire anemometer technique and this frequency is being used to oscillate the upstream micro-cylinder with its sub-harmonic, harmonic and super-harmonic frequency ratios laying in the range of [0 -7]. All these parameters are analysed and their corresponding effects on drag force and turbulence statistics resulted from this flow interference problem. The main results are reported at proper gap spacing which have showed that the oscillating micro-rod significantly contribute to a drag reduction compared to the baseline single cylinder. Furthermore, the wake formation behind the stationary cylinder is influenced by the interaction induced from the oscillatory micro-rod. It is also noticeable; that the oscillating micro-rod is more dominant in drag reduction for the higher diameter as well as the bigger oscillating amplitude especially within the lock-on frequency range. To sum up, placing a micro-rod as an upstream active control in tandem arrangement with larger cylinder has given noteworthy aerodynamic force reduction in term of overall system drag that can reach 35% at optimal oscillating parameters.
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