Micro-Fiber Coatings for Bluff Body Flow Control

A hair-like structure is introduced as a biomimetic drag reduction coating. The hair-like structure consists of an array of micro-fibers that is introduced as a passive drag-reduction device. With the use of a hair-like structure, two methods for bluff-body flow control are hypothesized to reduce drag: (1) The hair-like structure feeds disturbance into a laminar boundary layer to induce boundary layer transition if it is applied before the separation point. (2) The hair-like structure suppresses reverse flow if it is applied after the separation point. Three research topics are studied to investigate the proposed hypotheses: development of the fabrication method of the hair-like structure herein named the micro-fiber coating, the effect of the micro-fiber coating on flow over a bluff body when it is applied before the separation point, and the effect of the micro-fiber coating when it is applied after the separation point.

The fabrication method of the micro-fiber coating consists of electrostatic flocking, which allows for control of the length and the area of the micro-fiber coating. Aerodynamic testing is performed using a circular cylinder as a bluff body within a subcritical flow regime. To address the proposed hypotheses, the micro-fiber coating is applied separately before and after the cylinder’s separation point.

It is found that the micro-fiber coating induces boundary layer transition when the micro-fiber coating is applied before the separation point. The separation point of the cylinder with added micro-fiber coating is delayed compared to that of the baseline. It is shown that both the angular position and length of the micro-fiber coating determines whether there is an increase or decrease in drag. It is also found that the flow features downstream of the cylinder are impacted by the coating's angular position. The dominant wake-frequency and the Strouhal number of the cylinder with added micro-fiber coating is higher than that of the baseline. The location of the dominant wake-frequency moves further downstream when the micro-fiber coating is applied.

It is found that the micro-fiber coating suppresses reverse flow when it is applied after the separation point. The velocity magnitude corresponding to the region of reverse flow is reduced and the recirculation length is elongated relative to the baseline. The vorticity distribution corresponding to the separated shear layer is also elongated. It is shown that both the angular position and the length of the micro-fiber coating play an important role in drag performance. It is also found that the flow features in the wake region are impacted by the coating's angular position. The dominant wake-frequency and the Strouhal number of the cylinder with added micro-fiber coating is higher than that of the baseline. The location of the dominant wake-frequency moves further downstream when the micro-fiber coating is applied.