The projection method is an effective means of numerically solving time-dependent incompressible fluid-flow problems. It was originally introduced by Alexandre Chorin in 1967 as an efficient means of solving the incompressible Navier-Stokes equations. The key advantage of the projection method is that the computations of the velocity and the pressure fields are decoupled.
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The algorithm
The algorithm of projection method is based on the Helmholtz decomposition (sometimes called Helmholtz-Hodge decomposition) of any vector field into a solenoidal part and an irrotational part. Typically, the algorithm consists of two stages. In the first stage, an intermediate velocity that does not satisfy the incompressibility constraint is computed at each time step. In the second, the pressure is used to project the intermediate velocity onto a space of divergence-free velocity field to get the next update of velocity and pressure.
Helmholtz–Hodge decomposition
The theoretical background of projection type method is the decomposition theorem of Ladyzhenskaya sometimes referred to as Helmholtz–Hodge Decomposition or simply as Hodge decomposition. It states that the vector field
Thus,
since
This is a Poisson equation for the scalar function
This is the essence of solenoidal projection method for solving incompressible Navier–Stokes equations.
Chorin's projection method
The incompressible Navier-Stokes equation (differential form of momentum equation) may be written as
In Chorin's original version of the projection method, one first computes an intermediate velocity,
where
One can rewrite this equation in the form of a time step as
to make clear that the algorithm is really just an operator splitting approach in which one considers the viscous forces (in the first half step) and the pressure forces (in the second half step) separately.
Computing the right-hand side of the second half step requires knowledge of the pressure,
It is instructive to note that the equation written as
is the standard Hodge decomposition if boundary condition for
For the explicit method, the boundary condition for
The explicit treatment of the boundary condition may be circumvented by using a staggered grid and requiring that
A distinguishing feature of Chorin's projection method is that the velocity field is forced to satisfy a discrete continuity constraint at the end of each time step.
General method
Typically the projection method operates as a two-stage fractional step scheme, a method which uses multiple calculation steps for each numerical time-step. In many projection algorithms, the steps are split as follows:
- First the system is progressed in time to a mid-time-step position, solving the above transport equations for mass and momentum using a suitable advection method. This is denoted the predictor step.
- At this point an initial projection may be implemented such that the mid-time-step velocity field is enforced as divergence free.
- The corrector part of the algorithm is then progressed. These use the time-centred estimates of the velocity, density, etc. to form final time-step state.
- A final projection is then applied to enforce the divergence restraint on the velocity field. The system has now been fully updated to the new time.