In this paper we discuss the third stage of our research on the nova outburst as described in the preceeding paper (Kutter and Sparks, page 290), i.e. we accrete hydrogen-rich material (normal solar composition) with Keplerian velocity onto a helium white dwarf at a rate of 10-8 M⊙/yr. When material with high angular momentum from a circumstellar disk is accreted onto a white dwarf with negligible angular momentum, a tremendous shear instability is created, and hydrogen-rich material is mixed with helium-rich material of the white dwarf on a scale that is short compared to the accretion time scale. Following Kippenhahn and Thomas (1978), we assume that in the mixing region marginal stability is established. Mathematically this is expressed by setting the Richardson number (the ratio of the work done against buoyancy to the kinetic energy of the turbulence) equal to ¼.