Mitsugu Hasegawa

A wake downstream of a circular cylinder is governed by the separated shear-layer development, vortex formation, and turbulent momentum transport, which together determine the unsteady characteristics of bluff-body flows. This study experimentally investigates the modification of near-wake turbulence by a microfiber-coated strip applied to the rear surface of a circular cylinder. Planar particle image velocimetry measurements were performed at a Reynolds number of 6.1 × 104, corresponding to the subcritical flow regime, to quantify the mean-flow field and turbulence statistics downstream of uncoated and microfiber-coated cylinders. The microfiber coating substantially altered the post-separation wake development. Compared with the uncoated cylinder, the coated cylinder exhibited an elongated recirculation region, slower wake recovery, and a downstream shift in the spatial development of velocity fluctuations. The maximum streamwise turbulence intensity was reduced by up to 29%, while the peak transverse turbulence intensity along the wake centerline was reduced by approximately 25% and shifted farther downstream. The two-dimensional estimate of turbulent kinetic energy and Reynolds shear stress were also reduced in the separated shear-layer region, indicating weaker in-plane turbulent fluctuation energy and reduced cross-wake momentum transport. These results suggest that the microfiber coating does not simply suppress local shear but modifies the spatial development of the separated shear layers and shifts the amplification of wake fluctuations downstream. This study extends previous drag-focused investigations by providing a turbulence-statistical characterization of wake modification by microfiber coating.

A hybrid anti-/de-icing system combining a superhydrophobic coating and an electrothermal heater is an area of active research for aircraft icing prevention. The heater increases the temperature of the interaction surface between impinging droplets and an aircraft surface. One scientific question that has not been studied in great detail is whether the temperatures of the droplet and the surface or the temperature difference between the two dominate the anti-/de-icing performance. Herein, this scientific question is experimentally studied based on the mobility of a water droplet over a superhydrophobic coating. The mobility is characterized by the sliding angle between the droplet and the coating surface. It was found that the temperature difference between the droplet and the coating surface has a higher impact on the sliding angle than their individual temperatures.

An artificial neural network (ANN) was constructed and trained for predicting pressure sensitivity using an experimental dataset consisting of luminophore content and paint thickness as chemical and physical inputs. A data augmentation technique was used to increase the number of data points based on the limited experimental observations. The prediction accuracy of the trained ANN was evaluated by using a metric, mean absolute percentage error. The ANN predicted pressure sensitivity to luminophore content and to paint thickness, within confidence intervals based on experimental errors. The present approach of applying ANN and the data augmentation has the potential to predict pressure-sensitive paint (PSP) characterizations that improve the performance of PSP for global surface pressure measurements.

The recent development of a superhydrophobic surface enhances the droplet shedding under a shear flow. The present study gives insights into the effects of shear flow on a pinned droplet over a superhydrophobic surface. To experimentally simulate the change in the size of a sessile droplet on an aerodynamic surface, the volume of the pinned droplet is expanded by water supplied through a pore. Under a continuous airflow that provides a shear flow over the superhydrophobic surface, the size of a pinned water droplet shed from the surface is experimentally characterized. The air velocity ranges from 8 to 61 m/s, and the size of pinned droplets shed at a given air velocity is measured using an instantaneous snapshot captured with a high-speed camera. It is found that the size of the shedding pinned droplet decreases as air velocity increases. At higher air velocities, shedding pinned droplets are fully immersed in the boundary layer. The present findings give a correlation between critical air velocity and the size of pinned droplets shed from the pore over the superhydrophobic surface.

An investigation has been made to see the effect of permeability of a microfiber coating on cylinder drag. The coating has a hair-like structure that can be taped over a cylinder surface. Microfiber coatings with different surface densities, designated as “thin”, “medium”, and “thick”, are prepared. The microfiber coating is applied to entire/leeward surface of the cylinder. Aerodynamic drag of the cylinder with the microfiber coatings is evaluated at the Reynolds number of 6.1× 104 based on the cylinder diameter. The entirely-covered cylinder with thick coating of length k = 1.0 mm achieved 15% of drag reduction. The entirely-covered cylinder with thin coating of k = 4.0 mm showed 16% of drag reduction. The permeability is defined by the combination of coating length and coating density, and is an important factor to achieve drag reduction on an entirely-covered cylinder, while drag is moderately reduced regardless of surface density for the leeward-covered cylinder.

A microfiber coating having a hair-like structure is investigated as a passive flow control device of a bluff body. The effect of microfiber length is experimentally studied to understand the impact of the coating on drag on a cylinder. A series of microfiber coatings with different lengths are fabricated using flocking technology and applied to various locations over the cylinder surface under the constant Reynolds number of 6.1 × 10⁴ based on the diameter of the cylinder. It is found that the length and the location both play important roles in the drag reduction. Two types of drag reduction can be seen: (1) when the relative length of the microfiber, k/D, is less than 1.8%, and the coating is applied before flow separates over the cylinder; and (2) k/D is over 3.3%, and the coating is applied after the flow separation location on the cylinder. The maximum drag reduction for the former type is 59% compared to that from the cylinder without the microfiber coating. For the latter type, the maximum drag reduction is 27%.

The biomimicry of using a hair-like structure is introduced as a drag reduction coating. The hair-like structure consists of an array of microfiber that is introduced as a passive drag reduction device. An effective flow control for a transition delay or a flow attachment is expected via an interaction or counteraction of flexible fibers, compared to the existing passive methods that use a solid or rigid surface device. The effect of the microfiber coating on drag reduction over a bluff-body was experimentally investigated using a circular cylinder in a wind tunnel at Reynolds number of 6.1 × 10⁴. A drag reduction of 32% was obtained when the microfiber coating with a length of 0.012D was located at 40° from the stagnation point. Smoke flow visualization showed that flow separation delay was induced by the microfiber coating when the drag reduction occurred.