## 10.7.1 Choosing the Relative or Absolute Velocity Formulation

It is recommended that you use the velocity formulation that will result in most of the flow domain having the smallest velocities in that frame, thereby reducing the numerical diffusion in the solution and leading to a more accurate solution.

The absolute velocity formulation is preferred in applications where the flow in most of the domain is not rotating (e.g., a fan in a large room). The relative velocity formulation is appropriate when most of the fluid in the domain is rotating, as in the case of a large impeller in a mixing tank.

Example

A problem with stationary outer walls and a rotating impeller can be solved in a single reference frame. The example is illustrated in Figure  10.7.2.

In case A, it is expected that only the flow near the impeller would be rotating and that much of the flow away from the impeller would have a low velocity magnitude in the absolute frame. Therefore, solving in the absolute frame is recommended. In case B, most of the flow is expected to be rotating with a velocity close to that of the impeller. By solving the case in the relative frame, the flow speed would be very low relative to the impeller speed. Hence, the relative velocity formulation is appropriate.

In a situation between case A and case B, either of the formulations may be used.

 If the velocity formulation is switched during the solution process, FLUENT will not transform the current solution to the other frame, which can lead to large jumps in residuals. If changing the frame is necessary, it is recommended that you first reinitialize, and then solve.

 When one of the coupled solution algorithms is used, the absolute formulation is always used; the relative velocity formulation is not available in the density-based solvers.

For velocity inlets and walls, you may specify velocity in either the absolute or the relative frame, regardless of whether the absolute or relative velocity is used in the computation. For pressure boundary conditions, however, FLUENT imposes several restrictions on how total pressure and flow direction are specified in rotating reference frames.

The total pressure and flow direction at a pressure inlet must be specified in the absolute frame if the absolute velocity formulation is used. For calculations using relative velocities, the total pressure and flow direction must be specified with respect to the rotating frame.

For pressure outlets, the specified static pressure is independent of frame. When there is backflow at a pressure outlet, however, the specified static pressure is used as the total pressure. For calculations using absolute velocities, the specified static pressure is used as the total pressure in the absolute frame; for the relative velocity formulation, the specified static pressure is assumed to be the total pressure in the relative frame. As for flow direction in reverse flows, FLUENT assumes the absolute velocity to be normal to the pressure outlet for the absolute velocity formulation; for the relative velocity formulation, it is the relative velocity that is assumed to be normal to the pressure outlet.

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