## 10.2.3 Single Rotating Reference Frame (SRF) Modeling

Many problems permit the entire computational domain to be referred to as a single rotating reference frame (hence the name SRF modeling). In such cases, the equations given in Section  10.2.2 are solved in all fluid cell zones. Steady-state solutions are possible in SRF models provided suitable boundary conditions are prescribed. In particular, wall boundaries must adhere to the following requirements:

• Any walls which are moving with the reference frame can assume any shape. An example would be the blade surfaces associated with a pump impeller. The no slip condition is defined in the relative frame such that the relative velocity is zero on the moving walls.

• Walls can be defined which are non-moving with respect to the stationary coordinate system, but these walls must be surfaces of revolution about the axis of rotation. Here the so slip condition is defined such that the absolute velocity is zero on the walls. An example of this type of boundary would be a cylindrical wind tunnel wall which surrounds a rotating propeller.

Rotationally periodic boundaries may also be used, but the surface must be periodic about the axis of rotation. As an example, it is very common to model flow through a blade row on a turbomachine by assuming the flow to be rotationally periodic and using a periodic domain about a single blade. This permits good resolution of the flow around the blade without the expense of model all blades in the blade row (see Figure  10.2.2).

Flow boundary conditions in FLUENT (inlets and outlets) can, in most cases, be prescribed in either the stationary or rotating frames. For example, for a velocity inlet, one can specify either the relative velocity or absolute velocity, depending on which is more convenient. In some cases (e.g. pressure inlets) there are restrictions based upon the velocity formulation which has been chosen. For additional information on these and other boundary conditions, see Section  10.7 and Chapter  7.

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