The Green's functions of Stokes flow represent solutions of the continuity equation $\nabla\cdot {\bf u}=0$ and the singularly forced Stokes equation $$-\nabla P+\mu \nabla^2{\bf u}+{\bf g}\delta({\bf x-x_0})=0 \tag{*}$$
where ${\bf g}$ is an arbitrary constant, ${\bf x_0}$ is an arbitrary point, and $\delta$ is the three-dimensional delta function. Introducing the Green's function ${\bf G}$, we write the solution of (*) in the form $$u_i({\bf x})=\frac{1}{8\pi\mu}G_{ij}({\bf x,x_0})g_j$$
The following is how to find $G_{ij}$ in the free-space infinite unbounded flow from C. POZRIKIDIS's Boundary Integral and Singularity Methods for Linearized Viscous Flow
Replace the delta function with $$ \delta(\hat{x})=-\frac{1}{4\pi}\nabla^2\left(\frac{1}{r}\right) \tag{1} $$ where $r=|\hat{x}|$, $\hat{x}=x-x_0$. Recalling that the pressure is a harmonic function, and balancing the dimensions of the pressure term with those of the delta function in (1), we set $$ P=-\frac{1}{4\pi}g\cdot\nabla\left(\frac{1}{r}\right) \tag{2} $$ Substituting (1) and (2) into (*) we obtain $$ \mu\nabla^2 u=-\frac{1}{4\pi}g\cdot(\nabla\nabla-I\nabla^2)\left(\frac{1}{r}\right)\tag{3} $$ Next, we express the velocity in terms of a scalar function $H$ as $$ u=\frac{1}{\mu}g\cdot(\nabla\nabla-I\nabla^2)H \tag{4} $$ It will be noted that the continuity equation is satisfied for any choice of $H$. Substituting (4) into (3) and discarding the arbitrary constant vector g we obtain $$ (\nabla\nabla-I\nabla^2)\left(\nabla^2H+\frac{1}{4\pi r}\right)=0\tag{5} $$ Clearly, (5) is satisfied by $\nabla^2H=-1/(4\pi r)$. Using (1), we have $\nabla^4H=\delta(\hat{x})$. Thus $$ H=-\frac{r}{8\pi} \tag{6} $$ Substituting (6) into (4) we find $$ u_i(x)=\frac{1}{8\pi\mu}G_{ij}(\hat{x})g_j $$ where $$ G_{ij}=\frac{\delta_{ij}}{r}+\frac{\hat{x}_i\hat{x}_j}{r^3} $$
There are several places I don't understand:
- Why can we write the pressure function $P$ in (2)? What does "Recalling... we set" mean?
- Why can we express the velocity $u$ as (4)? How do we know it can be of this form?
- What's the difference between $\nabla\nabla$ and $I\nabla^2$?