1 Drag Reduction in Turbulent and Laminar Flows

This study is an extension of our previous work that was conducted on the aerodynamic characteristics of an ultra light weight airplane, which could be a human-powered airplane or an unmanned-aerial vehicle. One of the major aerodynamic tasks which should be done in this type of airplane is to reduce drag coefficient. The mainly effectual strategy for drag reduction is to contemplate on the components that make up the largest percentage of the overall drag and take into account that small improvements on large quantities can become significant aerodynamic improvements. Our experience showed that the use of light material in constructing human-powered airplanes and unmanned-air-vehicles has some side effects on the aerodynamic characteristics of the wings. One important side effect is unwanted deflection on the wing shell which is because of high flexibility and low solidity of the light material, which covers the wing skeleton. In this work, we numerically simulate the flow over a HPA wing with and without considering the generated deflection on its shell. Our study is shown that the curvature on the wing surface between two supporting airfoil frames causes total drag coefficient to be reduced. Indeed, this drag reduction is automatically achieved without benefiting from additional drag-reduction devices and/or drag-reduction considerations. The current survey has been conducted on a HPA wing with E593 airfoil section. This study was performed in Reynolds numbers of 105 to 106. We also studied the influences of drag reduction in both laminar and turbulent flows. For simulation of turbulent flow case, we have used Spalart-Allmaras and k-e turbulence models. The current results have shown that the drag coefficient drastically decreases in laminar flow. In turbulent flow both turbulence models show slight decrease in the drag coefficient which is about 3 % in all angles of attack. Nomenclature α = angle of attack (AOA) Cd = drag coefficient for 2D Cl = lift coefficient for 2D CD = drag coefficient for 3D CL = lift coefficient for 3