Equation 3-244 through
Equation 3-246 must be solved using a Time Dependent study. This can be computationally expensive. The Rotating Machinery, Fluid Flow interfaces therefore support the so-called frozen rotor approach. The frozen rotor approach assumes that the flow in the rotating domain, expressed in the rotating coordinate system, is fully developed.
Equation 3-242 then reduces to
Frozen rotor is both a study type and an equation form. When solving a rotating machinery model using a Frozen Rotor study step, the Rotating Machinery, Fluid Flow interfaces effectively solve Equation 3-247 and
Equation 3-248 in a “rotating” domain, but “rotating” domains do not rotate at all. Boundary conditions remain transformed as if the domains were rotating, but the domains remain fixed, or frozen, in position. As in the time-dependent case, the Rotating Machinery, Fluid Flow interfaces solve for the velocity vector in the stationary coordinate system,
u, rather than for
v.
To make Equation 3-244 and
Equation 3-245 equivalent to
Equation 3-247 and
Equation 3-248, the Frozen Rotor study step defines a parameter
TIME, which by default is set to zero (
TIME appears in the Parameters node under Global Definitions).
Equation 3-246 is replaced by
Since TIME is a parameter and
x is a function of
TIME,
∂x/∂T evaluates to its correct value. Finally,
∂ρ/∂T
= 0 and the mesh time derivative of the velocity is replaced by
The frozen rotor approach can in special cases give the same solution as solving Equation 3-244 through
Equation 3-246 to steady state. This is the case if, for example, the whole geometry is rotating, or if the model is invariant with respect to the position of the rotating domain relative to the nonrotating domain. The latter is the case for a fan placed in the middle of a straight, cylindrical duct.