The research has the objective to develop and apply an innovative new passive technique for producing significant drag reduction in the turbulent flows. By treating a solid surface to make it superhydrophobic it is possible to dramatically affect how this solid surface interacts with a flowing liquid. Preliminary experiments have shown that superhydrophobic surfaces can be utilized to reduce drag in laminar flows through microchannels by up to 40%. The new research is aimed to show through the proposed experiments that these surfaces can also be used to delay the transition to turbulence and produce substantial drag reduction in both internal and external turbulent flows. The development of the proposed technology could have a profound effect on a huge variety of existing technologies, resulting in benefits ranging from a reduction in the pressure drop in pipe flows to an increase in range and speed of ships.
Intellectual Merit:
Superhydrophobic surfaces are engineered by taking materials with micron or nanoscale surfaces roughness and chemically treating them to make them hydrophobic. Because of the hydrophobicity of these microscale and nanoscale protrusions, when water is brought in contact with a superhydrophobic surface, it does not fully wet the surface. Instead, it remains in contact with only the peaks of the surface topology resulting in a shear free air water interface. The PI has shown through direct velocity measurements and comparison to numerical simulations and analytical theory that drag reduction in laminar flows results from the reduction in the effective surface area of the solid in contact with the flowing fluid and the presence of this shearfree air water interface. By inducing an effective slip, reducing the effective surface area and reducing the wall shear stress in a flow, these super-hydrophobic surfaces should also produce considerable drag reduction in turbulent flows. The preliminary experiments show upwards of 50% drag reduction in turbulent channel flows, however the work was narrowly focused and limited. With a wealth of design space still to be explored this number could increase substantially. This technique represents a new passive approach to turbulent drag reduction that does not require any modification of the transported fluid or active control by the device only the development and use of hydrophobic micro and nanopatterned surfaces. Various patterns and superhydrophobic materials will be tested to maximize turbulent drag reduction using durable, long-lived and easily applied superhydrophobic surfaces. These surfaces will be investigated in a number of different flows including channel flow, flow past a flat plate and flow past a number of different blunt and streamlined objects.
In this proposal we will i) explore a number of different designs of superhydrophobic surfaces to attempt to better understand the origins and scaling or turbulent drag reduction, ii) extend the turbulent drag reduction measurements to high Reynolds numbers to investigate if the superhydrophobic drag reduction phenomena is a transition effect or robust at higher speeds, iii) investigate the importance of air-water interface shape and deflection and iv) investigate the impact of superhydrophobic surfaces on the flow past blunt or streamlined bodies where stagnation points are present and separation is expected.
Broader Impacts:
The proposed research program bridges fundamental experimental wetting phenomena and fluid dynamics with commercial and industrial applications where drag reduction could save significant amount of money, reduce the countrys dependence on fossil fuels and dramatically reduce the CO2 footprint of commercial and military shipping. The results from this research program should lead to a whole new way of thinking about turbulent drag reduction. The project has several educational components including involvement of undergraduates in the research, K-12 outreach and the recruitment of students from underrepresented groups through the NEAGEP centered at UMASS.
Under this research program we developed and applied an innovative new passive technique for producing significant drag reduction in the turbulent flows. We demonstrated that by making a surface superhydrophobic it is possible to dramatically affect how it interacts with a flowing liquid. In our previous work, we have shown that superhydrophobic surfaces can be utilized to reduce drag in laminar flows through microchannels by up to 40%. Under this grant, we showed that superhydrophobic surfaces could also be used to delay the transition to turbulence and produce substantial drag reduction in turbulent flows. We believe the work performed under this grant can have a profound effect on a huge variety of existing technologies, resulting in benefits ranging from a reduction in the pressure drop in pipe flows to an increase in range and speed of ships. Superhydrophobic surfaces are engineered by taking materials with micron or nanoscale surfaces roughness and chemically treating them to make them hydrophobic. Because of the hydrophobicity of these microscale and nanoscale protrusions, when water is brought in contact with a superhydrophobic surface, it does not fully wet the surface. Instead, it remains in contact with only the peaks of the surface topology resulting in a shear-free air-water interface. We have shown through direct velocity measurements and comparison to numerical simulations and analytical theory that drag reduction results from the reduction in the effective surface area of the solid in contact with the flowing fluid and the presence of this shear-free air-water interface. By inducing an effective slip, reducing the effective surface area and reducing the wall shear stress in a flow, we demonstrated that these superhydrophobic surfaces can produce more than 50% drag reduction in turbulent channel flows. In addition to drag reduction in turbulent channel flow, the research performed under this grant investigated flow past a number of bluff bodies treated to make them superhydrophobic. This included superhydrophobic cylinders and superhydrophobic hydrofoils. Our experiments showed that slip along a superhydrophobic cylinder does not reduce its drag significantly, but it does have a big impact on the size, shape and strength of the vortices shed from the cylinder in cross flow. One major impact was a reduction in flow induced vibrations which could have a big influence on the design and implementation of underwater cables and risers which are prone to fail due to vibration induced stresses. In our work studying superhydrophobic hydrofoils, we found up to a 20% reduction in drag. These results could, for example, translate into significant increases efficiency and power generated by underwater tidal stream turbines.
