Drag on sphere at Re = 0.2

At very low Reynolds number ($Re < 1$, creeping flow) inertia is negligible compared with viscous forces, and the drag on a sphere is given by Stokes' law:

$$F_D = 3\pi\mu V d$$

where $\mu$ is the dynamic viscosity, $V$ the velocity of the sphere relative to the fluid, and $d$ the diameter. This holds for $Re = \rho V d/\mu \lesssim 0.2$ to $1$.

Proof that $C_D = 24/Re$

By definition the drag coefficient is the drag divided by the dynamic pressure times the projected area:

$$C_D = \frac{F_D}{\tfrac12\rho V^2 A},\qquad A = \frac{\pi d^2}{4}$$

Substituting Stokes' drag:

$$\begin{aligned} C_D &= \frac{3\pi\mu V d}{\tfrac12\rho V^2\cdot\dfrac{\pi d^2}{4}} = \frac{3\pi\mu V d\times 8}{\rho V^2\pi d^2} \\ &= \frac{24\mu}{\rho V d} = \frac{24}{Re} \end{aligned}$$

Hence for $Re = 0.2$:

$$C_D = \frac{24}{0.2} = 120$$

Result: $F_D = 3\pi\mu V d$ and $C_D = 24/Re$ for the Stokes range.