This section describes the three algorithms available in FLUENT to model the fluctuating velocity at velocity inlet boundaries.
The stochastic components of the flow at the velocity-specified inlet boundaries are neglected if the No Perturbations option is used. In such cases, individual instantaneous velocity components are simply set equal to their mean velocity counterparts. This option is suitable only when the level of turbulence at the inflow boundaries is negligible or does not play a major role in the accuracy of the overall solution.
To generate a time-dependent inlet condition, a random 2D vortex method is considered. With this approach,
a perturbation is added on a specified mean velocity profile via a fluctuating vorticity field (i.e.
two-dimensional in the plane normal to the streamwise direction). The vortex method is based on the
Lagrangian form of the 2D evolution equation of the vorticity and the Biot-Savart law. A particle
discretization is used to solve this equation. These particles, or "vortex points'' are convected randomly
and carry information about the vorticity field. If
is the number of vortex points and
is the area of
the inlet section, the amount of vorticity carried by a given particle
is represented by the circulation
and an assumed spatial distribution
Since the vortex method theory is based on the modification of the velocity
field normal to the streamwise direction, it is imperative that the user creates an
inlet plane normal (or as close as possible) to the streamwise velocity
The spectral synthesizer provides an alternative method of generating fluctuating velocity components. It is based on the random flow generation technique originally proposed by Kraichnan [ 186] and modified by Smirnov et al. [ 338]. In this method, fluctuating velocity components are computed by synthesizing a divergence-free velocity-vector field from the summation of Fourier harmonics. In the implementation in FLUENT, the number of Fourier harmonics is fixed to 100.