Check pdf user's guide for details.
The cs_user_les_inflow.c user-defined function allows to generate the unsteady boundary conditions for the LES by the Synthetic Eddy Method. The basic principle of this method is illustrated in the following figure.
In the figure above,
is the inlet boundary,
the virtual box and
the advection velocity of the eddies.
The turbulent fluctuations at the inlet are generated by a set of synthetic eddies advected across the inlet boundaries. The eddies evolve in a virtual "box" surrounding the inlet boudaries and each of them contributes to the normalized velocity fluctuations, depending on its relative position with the inlet faces and on a form function characterizing the shape of the eddies. By this way, the Synthetic Eddy Method provides a coherent flow with a target mean velocity and target Reynolds stresses at LES inlet.
In the current version of code_saturne, the Synthetic Eddy Method is not available through the GUI but only through the cs_user_les_inflow.c user file.
Use of these functions is illustrated in the generation of synthetic turbulence at LES inlets page.
The number of synthetic eddies in the "box" might be adjusted, depending on the case (in particular the size of the inlet plane and the level of turbulence). As a general rule, the greater is the better since an insufficient number can lead to an intermittent signal while some numerical tests have shown that this parameter does not have a great influence beyond a threshold value. Given the inlet of size h<up>2</up> of a shear flow at a given Reynolds number
, an appropriate number of eddies can be evaluated by
(Re and 50 approximates respectively the size, in wall unit, of the largest and the smallest synthetic eddy.
of the synthetic eddies in the i Cartesian direction. One has:
is a reference size of the grid, in order to assume that all synthetic eddies are discretized. In the implementation of code_saturne, it is computed at each inlet boundary face F as:
the subset of the vertices of the boundary face F and C the cell adjacent to F.For the sake of comparison, other LES inflow methods are available, in addition to the Synthetic Eddy Method: