  The dynamics of red blood cells (RBCs) in simple shear flow was studied using
a theoretical approach based on three variables: a shape parameter, the
inclination angle $\theta$, and phase angle $\phi$ of the membrane rotation. At
high shear rate and low viscosity contrast of internal fluid, RBCs exhibit
tank-treading motion, where $\phi$ rotates with swinging oscillation of shape
and $\theta$. At low shear rate, tumbling motion occurs and $\theta$ rotates.
In the middle region between these two phases, it is found that synchronized
rotation of $\phi$ and $\theta$ with integer ratios of the frequencies occurs
in addition to intermittent rotation. These dynamics are robust to the
modification of the potential of the RBC shape and membrane rotation.
  Our results agree well with recent experiments.
