Boundary-Layer Control by Hydrophilic Surfaces

Effects due to hydrophilic surfaces are investigated on an airfoil by means of both experimental and numerical approaches. Boundary-layer velocity profiles are measured by embedded laser Doppler velocimetry along the NACA0012 airfoil upper-side surface on two identical wings (one of them being coated using hydrophilic treatment). Present experimental results clearly show that the hydrophilic treatment induces a positive viscosity gradient at the wall that significantly delays the boundary-layer separation occurrence and thus provides a favorable effect on lift-coefficient increase and drag-coefficient reduction. From solution of time-dependent Navier-Stokes equations, the numerical approach is conducted by accounting for the modification of the surface boundary condition as a slip-velocity condition. Different values of the slip velocity are investigated and compared with the reference case of a no-slip boundary condition. Calculated lift-and-drag coefficients well confirm the improvement due to a velocity slip along the airfoil boundary. The calculation results also show that the slope of velocity profiles at the airfoil surface is increased due to slip and that separation has partly or totally been avoided. Finally, the present experimental and numerical results indicate that aerodynamic benefits are clearly provided by hydrophilic surface treatments.