Error Estimates for Finite Element Approximations of Drag and Lift in Nonstationary Navier-Stokes Flows

Error estimates are obtained for finite element approximations of the drag and the lift of a body immersed in nonstationary Navier-Stokes flows. By virtue of a consistent flux technique, the error estimates are reduced to those of the velocity as well as its first order derivatives and the pressure. Semi-implicit backward Euler method is used for the time integration and no stability condition is required. The error estimate in a square summation norm is optimal in the sense that it has the same order as the fundamental error estimate of the velocity. The error estimate in the supremum norm is not optimal in general but it is so for some finite elements.