On velocity errors due to irrotational forces in the Navier–Stokes momentum balance

This contribution studies the influence of the pressure on the velocity error in finite element discretisations of the Navier–Stokes equations. Four simple benchmark problems that are all close to real-world applications convey that the pressure can be comparably large and is not to be underestimated. In fact, the velocity error can be arbitrarily large in such situations. Only pressure-robust mixed finite element methods, whose velocity error is pressure-independent, can avoid this influence. Indeed, the presented examples show that the pressure-dependent component in velocity error estimates for classical mixed finite element methods is sharp. In consequence, classical mixed finite element methods are not able to simulate some classes of real-world flows, even in cases where dominant convection and turbulence do not play a role.