Serdar Gorumlu

Holographic microscopy and microfluidics platform for measuring wall stress and 3D flow over surfaces textured by micro-pillars

Understanding how fluid flow interacts with micro-textured surfaces is crucial for a broad range of key biological processes and engineering applications including particle dispersion, pathogenic infections, and drag manipulation by surface topology. We use high-speed digital holographic microscopy (DHM) in combination with a correlation based de-noising algorithm to overcome the optical interference generated by surface roughness and to capture a large number of 3D particle trajectories in a microfluidic channel with one surface patterned with micropillars. It allows us to obtain a 3D ensembled velocity field with an uncertainty of 0.06% and 2D wall shear stress distribution at the resolution of ~65 μPa. Contrary to laminar flow in most microfluidics, we find that the flow is three-dimensional and complex for the textured microchannel. While the micropillars affect the velocity flow field locally, their presence is felt globally in terms of wall shear stresses at the channel walls. These findings imply that micro-scale mixing and wall stress sensing/manipulation can be achieved through hydro-dynamically smooth but topologically rough micropillars.

Engineered bio-inspired coating for passive flow control

Significance Flow separation on moving bodies has a negative effect on energy efficiency. Reducing recirculating regions is key in the design of energy-efficient systems. Efficient design decreases fuel consumption and pollutant emissions, including the systems’ carbon footprint. The engineered bio-inspired coating presented here aims to contribute in that direction. The relative ease of manufacturing and installation and its cost effectiveness, as well as its functionality under both wet and dry conditions, make it a versatile solution of potentially high impact in a broad range of applications, including transportation, wind power, and underwater vehicles. Flow separation and vortex shedding are some of the most common phenomena experienced by bluff bodies under relative motion with the surrounding medium. They often result in a recirculation bubble in regions with adverse pressure gradient, which typically reduces efficiency in vehicles and increases loading on structures. Here, the ability of an engineered coating to manipulate the large-scale recirculation region was tested in a separated flow at moderate momentum thickness Reynolds number, Reθ=1,200. We show that the coating, composed of uniformly distributed cylindrical pillars with diverging tips, successfully reduces the size of, and shifts downstream, the separation bubble. Despite the so-called roughness parameter, k+≈1, falling within the hydrodynamic smooth regime, the coating is able to modulate the large-scale recirculating motion. Remarkably, this modulation does not induce noticeable changes in the near-wall turbulence levels. Supported with experimental data and theoretical arguments based on the averaged equations of motion, we suggest that the inherent mechanism responsible for the bubble modulation is essentially unsteady suction and blowing controlled by the increasing cross-section of the tips. The coating can be easily fabricated and installed and works under dry and wet conditions, increasing its potential impact on a diverse range of applications.

Sticking to rough surfaces using functionally graded bio-inspired microfibres

Synthetic fibrillar adhesives inspired by nature, most commonly by the gecko lizard, have been shown to strongly and repeatedly attach to smooth surfaces. These adhesives, mostly of monolithic construction, perform on par with their natural analogues on smooth surfaces but exhibit far inferior adhesive performance on rough surfaces. In this paper, we report on the adhesive performance of functionally graded microfibrillar adhesives based on a microfibre with a divergent end and a thin soft distal layer on rough surfaces. Monolithic and functionally graded fibre arrays were fabricated from polyurethanes and their adhesive performance on surfaces of varying roughness were quantified from force–distance data obtained using a custom adhesion measurement system. Average pull-off stress declined significantly with increasing roughness for the monolithic fibre array, dropping from 77 kPa on the smoothest (54 nm RMS roughness) to 19 kPa on the roughest (408 nm RMS roughness) testing surface. In comparison, pull-off stresses of 81 kPa and 63 kPa were obtained on the same respective smooth and rough surfaces with a functionally graded fibre array, which represents a more than threefold increase in adhesion to the roughest adhering surface. These results show that functionally graded fibrillar adhesives perform similar on all the testing surfaces unlike monolithic arrays and show potential as repeatable and reusable rough surface adhesives.

Bioinspired Mushroom-Like Fiber Adhesives

Mushroom-like synthetic microfibers have been shown to generate strong adhesion to smooth surfaces, resulting in adhesive performance on par with the natural adhesive pads of the gecko. Theoretical and numerical investigations regarding the pull-off mechanism of this family of fibers suggest that adherence is primarily controlled by the geometry of the cap. Here, we designed an experimental study to determine the effect of tip size and edge angle on pull-off stress for comparison with existing theoretical/numerical results. An adhesion measurement system was designed to measure pull-off force of an individual fiber in an array. A fabrication technique was developed to fabricate microfibers with varying cap diameters and constant edge angle. Pull-off experiments performed with fibers of two different edge angles with varying tip sizes showed that pull-off stress, which reached as high as 2.08 MPa, increased with decreasing tip diameter and are in agreement with theoretical expectations. Repeatability experiments performed with a fiber employing a small cap (tip-to-stalk ratio = 1.3) showed that the fiber lost only about 10% of its initial adherence after six consecutive pull-off measurements. Conversely, a microfiber with a larger cap diameter (tip-to-stalk ratio = 2) lost 66% of its initial adherence after the same number of runs. Our findings suggest that fibers with smaller caps are better adhesives due to their enhanced durability and pull-off stress.

Flow Around an Autonomous Underwater Vehicle With Bio-Inspired Coating

Flow separation is a major factor in the form drag experienced by a moving object. In particular, suppressing or reducing flow separation is critical in the reduction of energy expenditure of autonomous underwater vehicles. Previous research suggests that bio-inspired micro-fibrillar structures are capable of reducing the boundary layer separation in a turbulent flow. Here, we present laboratory measurements using particle tracking velocimetry near the wall of two submersible vessel models: one coated with an array of micro-fibers and a second one with smooth walls. The flow around the vessels was enticed by the ordered micro-fibers to retain higher velocities near the wall of the vessel. The experiments suggest that separation of the flow may be reduced by the use of the bio-inspired micro-fibers. Keywords—Bio-inspired, Drag reduction, Flow Separation,

Flow modulation by a mushroom-like coating around the separation region of a wind-turbine airfoil section

The flow over a mushroom-shaped microscale coating was experimentally inspected over a diverging channel that followed the pressure side of a wind turbine blade (S835). High-resolution particle image velocimetry was used to obtain in-plane velocity measurements in a refractive-index-matching flume at Reynolds number Reθ ≈ 1200 based on the momentum thickness. The results show that the evolution of the boundary layer thickness, displacement thickness, and shape factor change with the coating, contrary to the expected behavior of an adverse pressure gradient boundary layer over a canonical rough surface. Comparison of the flow with that over a smooth wall revealed that the turbulence production exhibited similar levels in both cases, suggesting that the coating does not behave like a typical rough wall, which increases the Reynolds stresses. Proper orthogonal decomposition was used to decompose the velocity field to investigate the possible structural changes introduced by the wall region. It suggests that large-scale motions in the wall region lead to high-momentum flow over the coated case compared to the smooth counterpart. This unique behavior of this surface coating can be useful in wind-turbine applications, with great potential to increase the power production.The flow over a mushroom-shaped microscale coating was experimentally inspected over a diverging channel that followed the pressure side of a wind turbine blade (S835). High-resolution particle image velocimetry was used to obtain in-plane velocity measurements in a refractive-index-matching flume at Reynolds number Reθ ≈ 1200 based on the momentum thickness. The results show that the evolution of the boundary layer thickness, displacement thickness, and shape factor change with the coating, contrary to the expected behavior of an adverse pressure gradient boundary layer over a canonical rough surface. Comparison of the flow with that over a smooth wall revealed that the turbulence production exhibited similar levels in both cases, suggesting that the coating does not behave like a typical rough wall, which increases the Reynolds stresses. Proper orthogonal decomposition was used to decompose the velocity field to investigate the possible structural changes introduced by the wall region. It suggests that ...