We continue the study of the nonconforming multiscale finite element method
(Ms-FEM) introduced in [17, 14] for second order elliptic equations with
highly oscillatory coefficients.
The main difficulty in MsFEM, as well as other numerical upscaling methods,
is the scale resonance effect. It has been show that the leading order resonance error can be
effectively removed by using an over-sampling technique. Nonetheless, there is still a secondary cell
resonance error of O(e2h2).
Here, we introduce a Petrov-Galerkin MsFEM formulation with nonconforming multiscale trial functions
and linear test functions. We show that the cell resonance error is eliminated in this formulation
and hence the convergence rate is greatly improved. Moreover, we show that a similar formulation
can be used to enhance the convergence of an immersed-interface finite element method for elliptic
interface problems.