Micro air vehicles are the size of insects, and are often named after them. It makes sense therefore to look at insects themselves for ways to improve vehicle design that can generate increased propulsion. Smaller flying insects such as thrips or fairy flies use the so-called fling and clap method to drive themselves in flight, a method which uses unsteady fluid flow structures to produce high propulsive capabilities. These insects produce unsteady flows because the bristles on their wings — flexible short hairs attached to the wing frame that radiate outward­­ — have gaps between them.

Key to understanding the flow structure and subsequent aerodynamic performance is understanding the role played by gap flow. Researchers report on a numerical model of gap flow caused by bristled wings in Physics of Fluids and show that these aerodynamic characteristics are heavily dependent on Reynolds number.

The authors examined how the value of the Reynolds number affects the vortices formed within the gaps as well as the aerodynamic force on each bristle. They used a two-dimensional bristled-wing model with multiple bristles.

The Reynolds number affects the termination and direction change of vortices within the gaps during unsteady wing motion; these changes occur more rapidly at low Reynolds number. Reynolds number values also affect bristle behavior. For high Reynolds number, the bristles act independently because the viscous shear layer in the gaps is thin. Conversely, according to the work, the bristles act as one continuous body at low Reynolds number due to the formation of virtual fluid barriers in the gaps.

Source: “Aerodynamic characteristics of unsteady gap flow in a bristled wing,” by Seung Hun Lee, Mohsen Lahooti, and Daegyoum Kim, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5030693.