1. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
2. Laboratoire d’Analyse Nume´rique, Universite´ Pierre et Marie Curie, Boı^te courrier 187, 75252 Paris, Cedex 05, France
3. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
4. ASCI-CNRS Orsay, and INRA Rocquencourt M3N, B.P. 105, 78153 LeChesnay Cedex France