1 Abstract The Detached Eddy Simulation (DES) and steady- state Reynolds-Averaged Navier-Stokes (RANS) turbu- lence modeling approaches are examined for the incom- pressible flow over a square cross-section cylinder at a Reynolds number of 21,400. A compressible flow code is used which employes a second-order Roe upwind spatial discretization. Efforts are made to assess the nu- merical accuracy of the DES predictions with regards to statistical convergence, iterative convergence, and tem- poral and spatial discretization error. Three-dimensional DES simulations compared well with two-dimensional DES simulations, suggesting that the dominant vortex shedding mechanism is effectively two-dimensional. The two-dimensional simulations are validated via com- parison to experimental data for mean and RMS veloci- ties as well as Reynolds stress in the cylinder wake. The steady-state RANS models significantly overpredict the size of the recirculation zone, thus underpredicting the drag coefficient relative to the experimental value. The DES model is found to give good agreement with the experimental velocity data in the wake, drag coefficient, and recirculation zone length. Introduction The flow around bodies at high Reynolds number, es- pecially buff bodies, can be unsteady with turbulent ed- dies shed and detached from the boundary layer flow. The usual steady-state Reynolds-Averaged Navier- Stokes (RANS) equations become inappropriate for these flows. The Large Eddy Simulation (LES) ap- proach is becoming a popular technique to model these flows. In LES, the larger structures (eddies) in the turbu- lent spectrum are resolved, and the smaller structures are modeled. This approach is computationally expen- sive and there are still modeling issues that are being in- vestigated. A subgrid-scale model must be used. The classic subgrid-scale model was introduced by Smagor- insky, 1 while the dynamic model approach of Germano et al. 2 has become very popular. This paper is concerned with wall bounded flows where additional modeling is required near the surface. The unsteady turbulence mod- eling technique investigated herein is the hybrid RANS/ LES model Detached Eddy Simulation (DES). 3 There have been a number of reviews of the LES ap- proach. 4-7 In the reviews by Rodi 6 and Piomelli, 7 the modeling problem in the near wall region has been dis- cussed. There have been various approaches proposed to model the near wall flow with the LES approach. Some of the earliest work on this problem was done by Pi- omelli et al. 8 One approach uses boundary conditions at the first mesh point away from the wall that approxi- mate the flow behavior near the wall. This approach is similar to the wall function method used with the RANS equations and is computationally very efficient. The more accurate approach is to refine the mesh near the wall with a significant increase in computational re- sources. Recently there have been several papers that suggest a blending of RANS and LES. This approach has been discussed by Speziale, 9 Germano, 10 Arunajatesan et al., 11 and Spalart et al. 3 From a practical point of view, the blending of RANS and LES approaches is attractive because the majority of industrial computational fluid dynamics codes already employ RANS-type turbulence models. Furthermore, steady-state RANS models can provide good predictions for a wide variety of flows, while LES can provide additional accuracy, at increased cost, in regions where the steady-state RANS models fail (e.g., massive flow separation). Bluff-Body Flow Simulations using Hybrid RANS/LES Christopher J. Roy, † Lawrence J. DeChant, ‡ Jeffrey L. Payne, § and Frederick G. Blottner ¶ Sandia National Laboratories * P. O. Box 5800, MS 0825 Albuquerque, NM 87185-0825  † Senior Member of Technical Staff, E-mail: cjroy@sandia.gov, Se- nior Member AIAA ‡ Senior Member of Technical Staff, E-mail: ljdecha@sandia.gov § Principal Member of Technical Staff, E-mail: jlpayne@sandia.gov, Member AIAA ¶ Distinguished Member of Technical Staff, E-mail: fgblott@sand- ia.gov, AIAA Fellow * Sandia is a multiprogram laboratory operated by Sandia Corpora- tion, a Lockheed Martin Company, for the United States Depart- ment of Energy under Contract DE-AC04-94AL85000. This paper is declared a work of the U. S. Government and is not subject to copyright protection in the United States. AIAA Paper 2003-3889