Multicanonical analyses of bluff airfoil drags in subcritical flows

We apply the multicanonical procedure in conjunction with an improved multi-resolution lattice Boltzmann model to evaluate the probability distribution function of the drag resulting from small but rarely occurring statistical deformations of a bluff airfoil NACA (National Advisory Committee for Aeronautics) 0050 and on a more streamlined NACA0030 airfoil with a larger aspect ratio. This analysis enables the effect of surface roughness on drags in a subcritical fluid flow to be examined in detail and demonstrates that the magnitude of the pressure drag, which is the primary component of the bluff airfoil drag, correlates with the width of the Fourier spectrum of the perturbation on the airfoil, while surface smoothness correlates with the frictional drag reduction in the subcritical fluid flow. Moreover, the multicanonical analysis differentiates between bluff and streamlined bodies without separately evaluating frictional and pressure drags.