Penalized approximation of the vibration frequencies of a fluid in a cavity

Here we point out some difficulties arising in the approximation of the vibration frequencies of a fluid in a cavity in the case of non convex polygonal domains. Since the eigensolutions must satisfy an irrotationality condition, a classical way to face the problem is to consider a penalized formulation. Unfortunately standard conforming finite elements fail to give good results. We intend to justify this failure and to suggest a finite element method based on a reduced integration strategy able to give reasonable results.