Initialization and Inflow for LES and DES

The specification of realistic initial and inflow boundary conditions is of paramount importance in Large Eddy and Detached Eddy Simulations of spatially developing flows.

As an inadequate amount of information introduces sources of error, a number of methods have been proposed for the specification of initialization and inflow, with varying degrees of complexity and computational cost.

The simplest approach consists of superimposing Gaussian noise to the mean velocity. However, this approach has the disadvantage of producing a decorrelated velocity field that is incapable of sustaining the growth of coherent turbulent structures.

In Simcenter STAR-CCM+, the following methods are implemented:

  • Synthetic Eddy Method, which provides turbulent eddies across inflow boundaries and initializes the flow domain with a perturbated flow field.
  • Anisotropic Linear Forcing, which generates a perturbated flow field in a volume.

Synthetic Eddy Method (SEM)

The Synthetic Eddy Method, proposed by Jarrin et al. [368], retains the conceptual basis of the vortex method [366], but is more flexible and virtually mesh-independent.

Within the SEM framework, the turbulent flow field is seen as a superposition of spinning eddies, whose spin (clockwise or counterclockwise), and position, are drawn from a normalized uniform distribution. The size of the eddies is the characteristic scale of turbulence. As eddies must span at least two cells in order to produce a correlated signal, the minimum mesh-spacing imposes a limit on this turbulence scale.

A user-assigned Reynolds stress tensor provides the correlation function that the method requires. If this stress tensor is not known, the correlation function uses only normal components that are directly derived from a given turbulence intensity (assumption of isotropy).

The synthetic eddies generated at the inflow are convected and recycled in the computational domain with the mean inflow velocity.

The turbulent structures that SEM produces are only an approximation of real turbulence, and must be allowed to develop the proper correlations. The correlations develop naturally as the eddies are convected downstream. To allow this behavior to occur, it is important to leave sufficient distance between the inflow boundary and the region of interest. The necessary distance varies from case to case, but in channel flow simulations, a distance of at least ten half-heights is required.

For more details, see Theory Guide—Synthetic Eddy Method.

Note The Synthetic Eddy Method is incompatible with motion. If you attempt to combine SEM with a motion, an error message is displayed.

Anisotropic Linear Forcing (ALF)

The Anisotropic Linear Forcing (ALF) is as an efficient way of generating turbulence fluctuations in a volume for scale-resolving simulations.

The ALF method proposed by De Laage de Meux et al. [369] is a good alternative to other synthetic turbulence generation methods, such as digital-filtering-based methods [372] or synthetic-eddy methods [370], [371]. The principal of the ALF is to force the mean (statistical) properties of a scale-resolving simulation towards target values for mean velocity and mean Reynolds-stresses. The target values can be obtained from RANS simulations of the same flow field.

For more details, see Theory Guide—Anisotropic Linear Forcing.