Editing Large eddy simulation (section)

== Boundary conditions of large eddy simulations ==
Inlet boundary conditions affect the accuracy of LES significantly, and the treatment of inlet conditions for LES is a complicated problem. Theoretically, a good boundary condition for LES should contain the following features:<ref name="Li_P">{{cite journal |last1=Li |first1=P. |last2=Eckels |first2=S. |last3=Mann |first3=G. |last4=Zhang |first4=N. |title=A Method of Measuring Turbulent Flow Structures With Particle Image Velocimetry and Incorporating Into Boundary Conditions of Large Eddy Simulations |journal=Journal of Fluids Engineering|publisher=ASME International |year=2018 |volume=140 |issue=7 |at=071401-071401-11 |doi=10.1115/1.4039256 }}</ref>

(1) providing accurate information of flow characteristics, i.e. velocity and turbulence;

(2) satisfying the Navier-Stokes equations and other physics;

(3) being easy to implement and adjust to different cases.

Currently, methods of generating inlet conditions for LES are broadly divided into two categories classified by Tabor et al.:<ref>{{cite journal |last1=Tabor |first1=G. R. |last2=Baba-Ahmadi |first2=M. H. |year=2010 |title=Inlet conditions for large eddy simulation: a review |journal=Computers & Fluids |volume=39 |issue=4 |pages=553&ndash;567 |doi=10.1016/j.compfluid.2009.10.007 }}</ref>

The first method for generating turbulent inlets is to synthesize them according to particular cases, such as Fourier techniques, principle orthogonal decomposition (POD) and vortex methods. The synthesis techniques attempt to construct turbulent field at inlets that have suitable turbulence-like properties and make it easy to specify parameters of the turbulence, such as turbulent kinetic energy and turbulent dissipation rate. In addition, inlet conditions generated by using random numbers are computationally inexpensive. However, one serious drawback exists in the method. The synthesized turbulence does not satisfy the physical structure of fluid flow governed by Navier-Stokes equations.<ref name="Li_P" />

The second method involves a separate and precursor calculation to generate a turbulent database which can be introduced into the main computation at the inlets. The database (sometimes named as ‘library’) can be generated in a number of ways, such as cyclic domains, pre-prepared library, and internal mapping. However, the method of generating turbulent inflow by precursor simulations requires large calculation capacity.

Researchers examining the application of various types of synthetic and precursor calculations have found that the more realistic the inlet turbulence, the more accurate LES predicts results.<ref name="Li_P" />