For a one-dimensional fluid with viscosity $\eta$ subject to a homogenous acceleration $a$ in periodic boundary conditions, in my understanding the momentum equation is
$$\rho\left(\frac{\partial u}{\partial t} + u\frac{\partial u}{\partial x}\right) = \rho a + \eta \frac{\partial^{2} u}{\partial x^{2}}$$
Qualitatively, I expect that starting from zero velocity $u(x, 0)=0$ the fluid should arrive at a terminal velocity given by the equilibrium between the homogenous acceleration and the viscous force. But I can not see this directly by looking at the above equation. In my understanding the effect of the viscosity (or "diffusive") term is to smoothen out and eventually suppress gradients in the velocity profile. The physical reason for this is thermal fluctuations and collisions at the molecular level of the fluid. However, for the exactly same reason I expect the fluid to reach a terminal velocity upon applying a homogenous acceleration. But clearly the viscous term above only affects the velocity in the presence of spatial gradients.
So what am I missing ?
In principle, one could argue that Navier-Stokes equation is not required for this problem and one could just consider a point mass subject to constant force and a drag force given by Stokes formula. This leads to the terminal velocity. However this would not be sufficient for me as I am considering a situation where spatial profiles or gradients of the velocity might eventually enter.
The full problem I am looking at is the following : A compressible fluid within a fixed cubic container is compressed at time $t=0$ by a spatially homogeneous force against one face of the cube. After a long time the mass distribution of the fluid should develop a gradient along the direction of the force. The equilibrium mass distribution should be an exponential, in a similar way to the barometric equation. But directly after turning on the force, and before the fluid has found the equilibrium distribution, intuitively there should be some density oscillations between the two faces of the cube, against which the fluid is compressed.
The density gradient should evolve on a time-scale given by the viscosity $\eta$ and the homogenous force $F$, roughly defined as the ratio of the box size $L$ by the Stokes terminal velocity. The evolution of the density profile can be modelled by the conservation of mass (i.e Fokker-Planck equation for the density) assuming a drift velocity equal to Stokes terminal velocity. This is done for example here J. Phys. 77, 240–243 (2009).
But the transient density oscillations are not considered there, and for that the momentum equation should considered. My thought was to add the homogenous force to the momentum equation, and simultaneously solve the momentum and mass conservation equation, with using the velocity field as a convection term in the mass conservation equation.
But adding the homogenous force to the momentum equation does not lead to the terminal velocity.
Best Answer
The solution of the Burger's equation with your initial condition is just: $$ u = gt $$ As you've mentioned yourself, viscosity only acts when your velocity field is inhomogeneous. Even if your initial condition were inhomogeneous, viscosity would act in the transient regime. There is no source of inhomogeneity (PBC, homogeneous force), you therefore don't have a balance of forces. Your heuristic argument is invalid.
For your compressible fluid, if I understand correctly, you are still using Burger's equation and adding the density continuity equation to close your system. I think that it's best to address it in a separate question.
Hope this helps.
Answer to comment
Your starting point is incorrect, there is no natural terminal velocity in your problem. Let's analyse the usual case of an object moving through a fluid. There is a natural constant imposed velocity inhomogeneity due to the no slip boundary conditions. At the surface of the object, the velocity of the fluid is the velocity of the object, while far from it, the fluid is at rest. It is therefore impossible to homogenise the velocity of the fluid, giving you a friction force. This force can be balanced by a force on the object to give a stationary solution.
Since your problem has a homogeneous solution, there is no friction force. The gravity is unbalanced resulting in an increasing velocity. If you want to create some kind of terminal velocity, you could add no slip boundary conditions on the side, and you'll get a Poisefeuille flow.
Your momentum equation is correct, assuming that there is no pressure (this is why it is Burger's equation not Navier-Stokes). If you want to add pressure, you'll need to add a state equation relating pressure $p$ and density $\rho$ to close your system.