Parallel flow solver schemes

The following schemes are available for the parallel flow solver.

Fractional Step

Solves mass and momentum equations separately once per timestep or steady state iteration.

Fully Coupled Pressure-Velocity

Solves mass and momentum equations simultaneously for each timestep or steady state iteration. For each timestep or steady state iteration, the solver iterates on the pressure and velocity solution until convergence is achieved.

Recommended use cases for Fractional Step

The factional step scheme is usually faster than the fully coupled scheme. It saves you time when you work on long transient problems under the following conditions:

• The density variation is insignificant: Δρ/ρ0 << 1 where ρ0 is a scale of the density field and Δρ is the variation of density for the problem.

• The timestep required to resolve the transient behavior to the desired accuracy is sufficiently small to imply a CFL number < 10. In this case, you require the same timestep size for both schemes. But, because the fractional step scheme is less computationally expensive on a per-timestep basis, it is faster than the fully coupled scheme. This is typically true of bluff body aerodynamics problems, for example, where you want to capture vortex shedding. In cases where the velocity field is not inherently strongly transient, but where the unsteady behavior is instead due to something like a time-varying heat load, it is often possible to obtain accurate results with the fully coupled scheme using a much larger timestep than with the fractional step scheme. Because the fully coupled scheme is generally more robust, and may even be computationally less expensive due to its ability to resolve the problems with a coarser timestep, for this class of problems, use the fully coupled scheme.

Note:

The following classes of problems are not supported by the fractional step scheme:

• High speed flows

• Models with rotational periodicity

• Flap and bursting membrane

Recommended use cases for Fully Coupled Pressure-Velocity

For the following classes of problems, the fully coupled scheme is a better choice than the fractional step scheme:

Steady state problems

The fractional step scheme generally requires a smaller timestep size to reach convergence and is less stable. In some instances, the fractional step scheme appears to converge to a steady pressure field that is demonstrably incorrect when you use a timestep that is too large. Reducing the timestep sufficiently eliminates CFL warning messages.

Models with rotational frames of reference

Because the momentum equations are solved in a segregated manner with the fractional step scheme, the flow solver treats the Coriolis acceleration fully explicitly. Unless the rotation rate is very small or the Reynolds number is very low, you require a timestep consistent with a CFL number less than unity to avoid divergence or blowup.

Models with porous blockages

The fractional step scheme gives reasonable flow field results in the near-interface region only when the blockage resistance coefficients are quite low.

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