For time-dependent flows, the discretized form of the generic transport equations is of the following form:

 (25.4-25)

 where = conservative form of transient derivative of transported variable = density = velocity vector (= in 2D) = surface area vector = diffusion coefficient for = gradient of (= in 2D) = source of per unit volume

The temporal discretization of the transient derivative in the Equation  25.4-25 is described in Section  25.3.2, including first-order and second-order schemes in time. The pressure-based solver in FLUENT uses an implicit discretization of the transport equation (Equation  25.4-25). As a standard default approach, all convective, diffusive, and source terms are evaluated from the fields for time level n+1.

 (25.4-26)

In the pressure-based solver, the overall time-discretization error is determined by both the choice of temporal discretization (e.g., first-order, second-order) and the manner in which the solutions are advanced to the next time step (time-advancement scheme). Temporal discretization introduces the corresponding truncation error; , , for first-order and second-order, respectively. The segregated solution process by which the equations are solved one by one introduces splitting error. There are two approaches to the time-advancement scheme depending on how you want to control the splitting error.

In the iterative scheme, all the equations are solved iteratively, for a given time-step, until the convergence criteria are met. Thus, advancing the solutions by one time-step normally requires a number of outer iterations as shown in Figure  25.1.1 and Figure  25.4.1. With this iterative scheme, non-linearity of the individual equations and inter-equation couplings are fully accounted for, eliminating the splitting error. The iterative scheme is the default in FLUENT.

The Frozen Flux Formulation

The standard fully-implicit discretization of the convective part of Equation  25.4-26 produces non-linear terms in the resulting equations. In addition, solving these equations generally requires numerous iterations per time step. As an alternative, FLUENT provides an optional way to discretize the convective part of Equation  25.4-25 using the mass flux at the cell faces from the previous time level n.

 (25.4-27)

The solution still has the same order of accuracy but the non-linear character of the discretized transport equation is essentially reduced and the convergence within each time step is improved.

To use this feature, turn on the Frozen Flux Formulation option in the Solver panel.

 This option is only available for single-phase transient problems that use the segregated iterative solver and do not use a moving/deforming mesh model.