Numerical Study of the Passive Motion of Airfoils in Porous Media

This study proposes adding a porous medium layer to the leading edge of a rigid airfoil and investigates the influence of various porous medium properties on the passive propulsion performance of the airfoil in a Kármán vortex street, using numerical simulation methods. By employing a control variable approach, the study compares the force and motion characteristics of airfoils with nine different porosities and permeabilities under both the incoming flow mode and Kármán vortex street conditions. Porosity (ε) is a parameter that describes the fraction of void volume relative to the total volume in a porous medium, ranging from ε=0.2 to ε=0.92. This parameter determines the ability of fluid to penetrate the medium, with higher porosity allowing more fluid flow, thus significantly affecting the flow field structure. Permeability (α), ranging from 10−12 m2 to 10−8 m2, indicates the ease with which fluid can pass through the porous medium and is a key factor in determining the flow resistance. Both porosity and permeability play crucial roles in the flow field and aerodynamic performance of the airfoil, and their interaction jointly regulates the formation of flow patterns and propulsion efficiency. The results show that under the incoming flow mode, the porous medium can significantly reduce the drag on the airfoil, with lower permeability leading to smaller aerodynamic forces. For high-permeability airfoils, drag reduction can be further achieved by lowering the porosity. In the Kármán vortex street, as both porosity and permeability decrease, the airfoil experiences reduced thrust, with an increase in horizontal displacement and a decrease in lateral deviation after release. This study deepens the understanding of passive propulsion phenomena in natural Kármán vortex streets and provides theoretical guidance and technical recommendations for related engineering applications.