Drag Reduction Using Microcavity on Flat Surface

July 8, 2018

The study of drag reduction is considered as the most effective means of enhancing energy efficiency in marine application, aircraft and road transports. The drag reduction methods are classified into active and passive methods. Active methods require external energy source unlike passive methods. In this study, the study of drag reduction using a microcavity, a passive method is carried out. The study is inspired from taro and lotus leaves, considered to be superhydrophobic surfaces. The application of superhydrophobic surface is only valid for fluid flow and it has a short period of effectiveness which limits the application of superhydrophobic surface. This study considers microcavity in the surface of a 2d flat plate inorder to overcome the shortcomings of the existing superhydrophobic surfaces.

 

A maximum of 39.34% reduction in shear stress for laminar flow and 35.4% for turbulent flow under the application of microcavity is found. A microcavity surface is usually defined as a surface textured with microscale holes. The fundamental phenomenon behind the reduction in shear stress for both laminar and turbulent flow may be comprehended to be due to the appearance of vortices inside the cavity which results in the instigation of a slip velocity at the interface of the vortices inside the cavity and the fluid flow. However, it has also been found that in comparison to smooth surface, the overall drag for the cavity surface is higher in the laminar flow regime and a maximum of 24.57% change in overall drag reduction is observed in the turbulent flow regime. The physical reason behind the appearance of a higher overall drag in the laminar flow regime could be because of the earlier appearance of flow separation. It is also observed in the turbulent flow regime, that the strength of turbulent fluctuations of the smooth surface is far stronger than that of the cavity wall thereby resulting in the reduction of shear stress, delay in the transition to turbulent flow and flow separation.

Drag reduction study of the microcavity has been carried out for an aspect ratio of 3.663 and solid fraction of 50%. The effect of the aspect ratio and solid fraction on the drag reduction is yet to be explored. However, this study provides important information about the phenomenon behind the drag reduction observed using microcavities. The drag reduction capability using the cavity on a smooth surface is promising to be a revolutionary research and it can be validated by suitable experimental results to fully understand its capability. This could have potential impacts for offshore developments such as supply vessels, AUV’s, FPSO’s and other plated structures.

 

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© 2018 by M A Hanann: All rights reserved.

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