Flow-actuated Spatially-graded Micropillars for Flow Control

Flow separation is a prevalent problem in many aerodynamic applications, leading to increased

drag, loss of lift, and decrease in efficiency over control surfaces. One known biological example

of mitigating this issue is shark denticles, scale-like features that cover the sharks body. These

denticles offer a passive solution for flow control, controlled by flow separation: in body locations

that experience high adverse-pressure gradients, denticles passively ‘bristle’ (pivot upward) when

the reversing flow reaches sufficient magnitude. This generates micro-vortices that bring high

energy flow back to the surface, delaying flow separation and encouraging flow reattachment.

Inspired by shark denticles, this work aims to create a passive solution for flow control by

designing a simplified dermal denticle in the form of micropillars. Previous studies on denticles

primarily use fixed or free-to-rotate rigid micropillars. In this study, micropillars are fabricated

using two-stage reactive polymers (TSRPs). TSRP networks are an emerging dual-cure polymer

platform where cross-linking density, and consequently material stiffness, can be spatially

modulated with light. This allows us to design micropillars with a modulus gradient over the

micropillar height. In doing so, this allows the micropillar to retain flexibility at the base but

achieve rigidity near the top, more closely aligned to the construction of shark denticles.

An array of staggered flexible micropillars made of TSRPs are partially cured using UV light.

This array is mounted on a flat plate wing at angles of attack of 0 to 35 degrees, at a constant

chord-based Reynolds number of 20,000. The micropillars are expected to delay flow separation

and promote quicker flow reattachment. By simplifying the structure and integrating tunable

material properties, these micropillars can pivot and adapt to various flow conditions, allowing

for a much greater range of flow separation control.