The incompressible NavierStokes equations are represented by the following sets of equations:
$$$$\frac{\partial \textbf{u}}{\partial t} + \textbf{u}.\nabla \textbf{u}  \alpha \nabla ^{2} (\textbf{u} + \textbf{u}^T) + \frac{\nabla p}{\rho} = \textbf{f}$$$$ $$$$\nabla . \textbf{u} = 0$$$$
where, $$$\textbf{u} = \begin{pmatrix} u \\ v \\ w \\ \end{pmatrix}$$$ = vector of flow velocity (m/s),
p = pressure (Pa),
$$$\alpha$$$ = kinematic viscosity ($$$m^{2}/s$$$),
$$$\rho$$$ = density ($$$kg/m^{3}$$$), and
$$$\textbf{f}$$$ = vector representing the body force per unit mass (N/kg).
In reality, the hydrodynamic pressure p, is the parameter that drives the flow. However, in practice, p is solved in a way that the incompressibility condition is maintained. When $$$\frac{\partial \textbf{u}}{\partial t} =0$$$, the above equation is termed as steady state incompressible navier stokes flow. And,additionally when the $$$\textbf{u}.\nabla \textbf{u} = 0$$$, the flow is termed as steady state stokes flow.
Incompressible NavierStokes(INS) in MOOSE Framework
Within MOOSE framework, the kernels are developed on the following structure: $$$$\rho \frac{\partial \textbf{u}}{\partial t} + \rho \textbf{u}.\nabla \textbf{u}  \rho \alpha \nabla ^{2} (\textbf{u} + \textbf{u}^T) + \nabla p = \textbf{F}$$$$ $$$$\nabla . \textbf{u} = 0$$$$
In the momentum equation (first equation), there are four terms in the left hand side. Similarly, body forces (for e.g. gravity) are represented by the term in the right hand side of the equation.The following table represents the corresponding kernels assigned for these terms:
S.N.  Term  Name of the Term  Representative Kernel 

1  $$$\rho \frac{\partial \textbf{u}}{\partial t}$$$  Time Derivative term  INSMomentumTimeDerivative 
2  $$$\rho \textbf{u}.\nabla \textbf{u}$$$  Convective term  INSMomentum 
3  $$$ \rho \alpha \nabla ^{2} (\textbf{u}+ \textbf{u}^T)$$$  Viscous term  INSMomentum 
4  $$$\nabla p$$$  Pressure term  INSMomentum 
5  $$$\textbf{F}_{g}$$$  Gravity  INSMomentum 
The continuity equation (second equation) is represented by the INSMass kernel.
S.N.  Term  Name  Representative Kernel 

1  $$$\nabla . \textbf{u}$$$  Continuity Equation's term  INSMass 
The following table can provide assistance for understanding the available source codes for BCs in NavierStokes solver:
S.N.  BC Type /Mathematical Representation  Variable(s)  Input Parameters  ClassName 

1.  Neumann BC  Pressure/Velocity  p/ $$$\textbf{u}$$$  ImplicitNeumannBC 
2.  $$$p + (\textbf{u}.\textbf{n}) \rho u_i$$$, i=x,y,z.  Pressure/Velocity  NSMomentumInviscidSpecifiedPressureBC 

3.  $$$\rho(\textbf{u}.\textbf{n})$$$  Velocity  $$$\rho u_n$$$  NSMassSpecifiedNormalFlowBC 
4.  $$$\rho u_n$$$  Velocity  inputs for NSIntegratedBC 
NSMassBC 
5.  $$$\rho(\textbf{u}.\textbf{n})$$$  Velocity  inputs for NSIntegratedBC 
NSMassUnspecifiedNormalFlowBC 
6.  $$$\rho_s \vert \textbf{u} \vert (\textbf{s}.\textbf{n})$$$  Velocity  inputs for NSIntegratedBC andNSWeakStagnationBC 
NSMassWeakStagnationBC 
7.  $$$\rho u_i  \rho v_d$$$  Velocity  desired velocity ($$$v_d$$$)  NSImposedVelocityBC 
many  
more 
Note: a.)The variable velocity is organized as momentum variable in NS kernel
b.) The usual Dirichlet and Neumann BCs can be applied by default for p and $$$\textbf{u}$$$ variables within MOOSE framework.