### 12.1.8. Unsteady Inlet/Interface Turbulence

Classical LES requires providing unsteady fluctuations at turbulent inlets/interfaces (RANS-LES interface) to the LES domain. This makes LES substantially more demanding than RANS, where profiles of the mean turbulence quantities ( and or and ) are typically specified. An example is a fully turbulent channel (pipe) flow. The flow enters the domain in a fully turbulent state at the inlet, so you are therefore required to provide suitable resolved turbulence at such an inlet location through unsteady inlet velocity profiles. The inlet profiles have to be composed in such a way that their time average corresponds to the correct mean flow inlet profiles, as well as to all relevant turbulence characteristics (turbulence time and length scales, turbulence stresses, and so on). For fully turbulent channel and pipe flows, this requirement can be circumvented by the application of periodic boundary conditions in the flow direction. The flow is thereby driven by a source term in the momentum equation acting in the streamwise direction. By that trick, the turbulence leaving the domain at the outlet enters the domain again at the inlet, thereby avoiding the explicit specification of unsteady turbulence profiles. This approach can be employed only for very simple configurations. It requires a sufficient length of the domain (at least ~8–10h, see Figure 12.8) in the streamwise direction to enable the formation inside the domain of turbulence structures independent of the periodic boundaries.

In most practical cases, the geometry does not enable fully periodic simulations. It can however feature fully developed profiles at the inlet (again typically pipe/channel flows). In such cases, you can perform a periodic precursor simulation on a separate periodic domain and then insert the unsteady profiles obtained at any cross-section of that simulation to the inlet of the complex CFD domain. This approach requires either a direct coupling of two separate CFD simulations or the storage of a sufficient number of unsteady profiles from the periodic simulation to be read in by the full simulation.

In a real situation, however, the inlet profiles might not be fully developed and no simple method exists for producing consistent inlet turbulence. In such cases, synthetic turbulence can be generated, based on given inlet profiles from RANS. These are typically obtained from a precursor RANS computation of the domain upstream of the LES inlet.

There are several methods for generating synthetic turbulence. In ANSYS Fluent, the most widely used method is the Vortex Method (VM) (see Mathey et al., 2003 [15]), where a number of discrete vortices are generated at the inlet. Their distribution, strength, and size are modeled to provide the desirable characteristics of real turbulence. The input parameters to the VM are the two scales (k and or k and ) from the upstream RANS computation. CFX uses the generation of synthetic turbulence through suitable harmonic functions as an alternative to the VM (see Menter et al., 2009 [17]).

The characteristic of high-quality synthetic turbulence in wall-bounded flows is that it recovers the time-averaged turbulent stress tensor quickly downstream of the inlet. This can be checked by plotting sensitive quantities like the time-averaged wall shear stress or heat transfer coefficient and observing their variation downstream of the inlet. It is also advisable to investigate the turbulence structures visually by using, for example, an isosurface of the -criterion, (where is the Strain rate, and is the vorticity rate). This can be done even after a few hundred time steps into the simulation.

Because synthetic turbulence never coincides in all aspects with true turbulence, you should avoid putting an inlet/interface at a location with strong non-equilibrium turbulence activity. In boundary layer flows, that means that the inlet or RANS-LES interface should be located several (at least 3-5) boundary layer thicknesses upstream of any strong non-equilibrium zone (such as a separation). The boundary layers downstream of the inlet/interface need to be resolved with a sufficiently high spatial resolution (see Section 12.2.3.3).

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