The transition from laminar to turbulent flow in the boundary layers on aerial and underwater vehicles is typically associated with a significant increase in drag and decreased efficiency. However, if properly tailored, this phenomenon can be beneficial to prevent laminar flow separation and stall. Turbulence tripping can be induced by isolated or discrete surface roughness elements. Specifically, the size, shape, and position of the roughness element in the flow play an important role on the mechanisms of boundary layer transition. In this work, we develop an active self-morphing device that is able to accelerate boundary layer transition on underwater surfaces. This device is actuated by bio-inspired Twisted Spiral Artificial Muscles (TSAMs). Inspired by cephalopods’ papillae, these TSAMs are manufactured from inexpensive and commercially available polymer fishing lines which are twisted into flat Archimedean spirals. When electrothermally actuated, the TSAMs can provide vertical displacements ranging from a few microns to several centimeters. Our self-morphing device consists of a TSAM placed on a flexible elastomer substrate and covered by a thin layer of silicone rubber. When the TSAM is actuated, the device acts like a roughness element that can be protruded into the flow with a specific size and shape. The amount of vertical displacement is tailored using a theoretical model we recently developed. Preliminary underwater tests using fluorescein dye are performed to demonstrate the ability of the device to selectively trip the laminar boundary layer. Thus, in a vehicle application, it could activate when needed – to prevent boundary-layer separation – and otherwise retract in order to eliminate parasitic drag.  These promising results pave the way for the development of smart self-morphing skins that can be wrapped around underwater vehicles or robots to tailor turbulence tripping in an adaptive fashion.


Boundary Layer Transition Induced by Bio-Inspired Twisted Spiral Artificial Muscles

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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