In our previous work, the overset grid functionality of OpenFOAM (Open-source Field Operation And Manipulation) was applied to simulate the motion of a rigid body in a high-speed flow induced behind a moving shock wave. The result showed that the grid spacing significantly affected the body’s motion. In the present work, we performed numerical simulations of two-dimensional flows around a diamond-shaped aerofoil moving with a Mach number of 1.5 by using the overset grid method implemented in OpenFOAM v1812 in order to investigate the effect of the grid spacing on the aerodynamic force acting on the moving body. Wave drag forces on the diamond-shaped aerofoil were evaluated from the simulations using three grid systems with different grid spacings, and the grid convergence for the evaluated wave drag was systematically studied based on the generalized Richardson extrapolation. It was found that the relative errors of the obtained wave drag forces with respect to its analytical value were less than 1 %, and the wave drag force asymptotically converged to the analytical value with the decrease in the grid spacing.