Reducing Drag of Vehicles with Half-stepped Cylinder Surfaces Part I: Numerical Optimization

Drag reduction using passive means could provide up to 10% lower energy consumption in industrial applications. Riblets, cavities, and bio-inspired surfaces are among methods that have shown promising results in controlling hairpin vortices or relaxing no-slip boundary conditions for reducing surface shear stress and the overall drag of vehicles and machinery. Here we are proposing numerical optimization for developing an optimized half-stepped cylinder surface for reducing turbulent shear stress and the overall drag of a turbulent boundary layer. We hypothesize that with an optimized half-stepped cylinder surface we can control the hairpin vortices with ridge spacing between adjacent half-stepped cylinders for reducing turbulent shear stress and create micro-cavities between cylinder rows for relaxing the no-slip boundary condition for significant overall drag reduction. Our preliminary experimental results with a row of half-stepped cylinders placed inside a turbulent boundary layer at zero pressure gradient have shown a significant drop in skin friction coefficient immediately downstream. We estimated more than 10% fuel (energy) savings with our optimized geometry.