Session: 05-02-01: Advances in Aerodynamics and Aerospace Propulsion Systems

Paper Number: 144187

144187 - Drag Reduction by Riblet-Structures With Limited Tip Sharpness – an Experimental and Numerical Survey – 

The increasing scarcity of fossil energy sources as well as the impact on the global climate associated with their use poses new challenges in the areas of energy, mobility and transportation. The politically set and economically necessary goals for a sustainable energy supply can only be achieved through the increased use of renewable energies such as solar, water, wind and geothermal energy, combined with a significant reduced energy consumption. The latter can partly be realized by increasing the energy efficiency of technical systems. However, in view of the fact that for example aircraft engines or stationary gas turbines already operate quite efficiently with component efficiencies of more than 90%  enormous challenges are posed for further improvement. Therefore, all known measures must be exploited synergistically.

A passive method for reducing the aerodynamic loses of flow-exposed components is the application of functional surface structures, inspired by the dermal denticles of the skin of fast swimming sharks, so-called riblet-structures. These longitudinal surface grooves in the microscale range are able to affect the dominating vortex structure in the turbulent boundary layer and thus reduce the wall shear stress in the range of up to 10%, offering high potential to increase the aerodynamic efficiency for a wide range of applications. Unfortunately, the achievable effectiveness strongly depends on the riblet tip width to spacing ratio, also referred to as the riblet tip sharpness. Smaller ratios are thereby associated with higher drag reducing potential. Therefore, the actual drag reduction attainable strongly depends on the manufacturing process applied, or more precise on its limitations to produce infinitely thin riblet tips.

Within the scope of the present study, wind tunnel measurements on a flat plate test specimen are conducted to evaluate the drag reducing potential of riblets with variable tip sharpness. In addition to self-adhesive riblet films manufactured by the company temicon, laser-applied riblets, produced by the Laser Institute of the University of Mittweida (LHM), were investigated. The overall range of the riblet tip width to spacing ratio reaches from t/s = 0.08 – 0.15. The experimental results reveal that compared to previous studies on riblets with varying riblet sharpness collected in the oil channel of the German Aerospace Centre (DLR), higher drag reduction can be achieved for riblet tip width to spacing ratios above t/s=0.1 provided that the riblet height corresponds to its ideal value. To confirm the results, numerical flow simulations over single riblet-structures are performed based on the viscous theory approach. Therefore, a velocity profile in form of a Coquette flow is embossed representing the time averaged flow field in the viscous sublayer. Using the computed protrusion heights of the riblet tips in the longitudinal and transversal flow, the wall shear stress reduction can be directly derived on the basis of existing correlations, confirming the experimentally measured influence.

Presenting Author: Konrad Hartung Institute of Sustainable Energy Supply, Jade University of Applied Sciences

Presenting Author Biography: Konrad M. Hartung graduated in 2016 with a master's degree in mechanical engineering at the Leibniz University of Hannover. Since 2017, he has been working as a research assistant at the Institute for Sustainable Energy Supply at the Jade University of Applied Sciences. His area of expertise is in the field of fluid dynamics and functional surface structures for viscous drag reduction.

Authors:

Konrad Hartung Institute of Sustainable Energy Supply, Jade University of Applied Sciences
Karsten Oehlert Institute of Sustainable Energy Supply, Jade University of Applied Sciences