RANS/LES/DES/DNS: The Future Prospects of Turbulence Modeling

While we were discussing the rapid increase in Large Eddy Simulation LES papers submitted to the Journal of Fluids Engineering JFE several years ago, Joe Katz, the editor in chief of JFE, suggested that we needed to organize more in depth symposia to sieve through LES papers as well as to educate the newly emerging large groups of LES users in almost all areas of engineering applications. We also talked about coming up with some sort of a quality assessment criteria to make a judgment of the overall quality of papers that were claimed to be LES. That is where the idea of publishing a special issue on mostly LES applications was started. This special issue is mostly the end result of that discussion. With the hard work of all of the individuals listed in the acknowledgments, we have successfully organized a symposium on “Advances and Applications of LES/DNS. ” More than 40 papers were submitted. After a thorough review process, 24 papers were accepted to be presented at the symposium held as part of the Joint Fluids Engineering and Heat Transfer Summer Conference, June 11–17, 2004, Charlotte, NC. These papers were further thoroughly reviewed for publication in this special issue according to the standards of JFE. Finally, 13 full papers and one technical brief were recommended for publication. It was difficult to categorize the papers in this issue into specific classes, though we have tried to order them roughly so that they start with more general review papers and more fundamental topics then proceed towards more applied research papers. Since the fashion for LES was so popular, we wanted to explore first what was going to happen to the good old Reynolds Averaged NavierStokes RANS models. The first paper by Hanjalic attempts to answer this very question. It should be viewed more as a perspective from a prominent individual who has devoted his carrier spanning close to half a century to development of classical turbulence models. Recent assessments have shown see, e.g., Rodi 1 , Froehlich and Rodi 2 , and Temmerman et al. 3 especially for wall bounded turbulent flows, that a good LES is almost a DNS. That is, a proper resolution of wall layers and prediction of transition to turbulence in boundary layers requires extremely fine grid resolution. For that, one has to resort to some sort of wall functions, and/or a RANS model to save time and perform calculations in an acceptable turn around time for practical engineering applications such as flow around ship hulls. That requires inevitably a hybrid LES/RANS or DES approach. That, in turn, requires a good RANS model. Moreover, even a very crude RANS turbulence model seems to do a good job as a subgrid scale SGS model in LES see Ref. 4 . SGS models constitute a major component of LES calculations. It is shown by Chen et al. 5 see also Meneveau et al. 6 that the balance of predictive capability may change when the stressstrain relationship becomes convoluted under transient rapid straining/destraining conditions. Their a priori analysis indicates that in some cases the dynamic Smagorinsky model does not perform as well as the classical Smagorinsky model, contrary to past experience and expectations. This, therefore, points towards the need to perform more experimental and numerical work to assess