Time integration: incompressible flow
by Yi Zhang
I try to consolidate last post using a scheme for incompressible flow under Lagrangian description. Those two condition make somehow make the plan much easier against general case. Since for the moment I am reading this paper, I will take some of its ideas, even though this whole modern time splitting scheme by projection methods owns its origin to Chorin and Temam in 1960’s.
Consider the governing equations:
(MEL-in mixed-Eulerian-Lagrangian form) or (Eulerian form)
For a specific material particle , both the displacement and velocity are functions of time:
Now, let’s suppose the variables at time are known as . For velocity we have the general time integration:
In equation above, partial derivative by means take the derivative within the spatial configuration of time . This spatial domain is obtained using velocity updating:
Parameter varies between 0 and 1. When , RHS is based on time , i.e. implicit scheme, while when we have explicit scheme. Generally, implicit schemes are prefered for its stability merit, which I adopt in this post. So we have
There are two choices to implement the discrete governing equations. One is two resolve all together, by coupling equation above with mass conservation equation of Eulerian form . The other one is try to update velocity separately, for this purpose we have to transform the mass equation, because it is this equation that couples velocities in different directions. On the other hand, we also want to decouple the pressure and velocity. Before introduce the scheme, let’s first take a look at the physical interpretation of the coordnate system.
When solving the dynamical problem numerically, we hope at one time step, we take care of time derivatives and space derivatives separately. Suppose I am examining a fixed time step , the material particle indexed by can actually be indexed by the spatial position (in Eulerian coordinates) . In other words, when we talk about some specific material particle, it can be labeled with location at time . So simply means the location of some particle which at occupies . By this notation, the momentum equation, assuming constant density and body force, is
and the mass equation is
In order to decouple velocity and pressure in momentum equation, we split it:
The two terms on both side form pairs accordingly. The second terms gives an implicit scheme for intermediat variable . Similar split in mass equation gives
which gives , then the first terms in split momentum equation is used to obtain :
The three equations above, are to be used to find sequentially. This means to solve two spatial domain PDEs for the first two equations, and take one derivative for the last.