A numerical simulation study was carried out to investigate the suspension of cell colonies in bioreactors of stirring and orbiting shaking tanks. The cell colonies in a culture liquid were modeled as rigid spherical solid particles. The shear stresses acting on these particles were analyzed. The simulation results have shown that the maximum shear stress acting on the particles in the stirring tank is higher than that on the particles in the orbital shaking tank. However, the average values of the shear stress on the particles in both tanks are comparable. The simulation results also show that more particles accumulate at the bottom of the orbital shaking tank in which the particle aggregation obeys the Einstein's tea leaf paradox. The simulation results are evaluated based on the predicted shear stress profiles and tank configuration.