LES

Large Eddy Simulation is a numerical technique for simulating turbulent flows first introduced by Smagorinsky cite{smagorinsky1963} in 1963. LES requires less computational power than DNS but more than RANS. Kolmogorov's (1941) theory of self similarity implies that large eddies of a flow are dependent on geometry, while smaller eddies are self similar and have a universal character. Therefore, it becomes a practice to solve only for large eddies explicitly and to model the effect of the smaller and more universal eddies on the larger ones. In LES, the large scale motions of the flow are calculated, while the effects of the smaller universal scales (the so called sub-grid scales) are modeled through the use of a sub-grid scale (SGS) model. The main advantage of LES over computationally cheaper RANS is the increased level of detail that LES can deliver. While RANS provides averaged results, LES can predict instantaneous flow characteristics and resolve turbulent flow structures. In engine research, this is particularly valuable in simulations involving chemical reactions. While the averaged concentration of chemical species may be too low to trigger a reaction, there can be localized areas of high concentration in which reactions will occur. LES is also significantly more accurate than RANS for flows involving flow separation or acoustic prediction.