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AcuSolve supports a variety of turbulence modeling options ranging from steady RANS to LES. The following list provides a description of each model. 1.) Spalart-Allmaras (spalart_allmaras or spalart) This is a general purpose single equation RANS model that solves for the transport of a modified eddy viscosity. This model has been shown to perform extremely well for a broad class of industrial flows. This model can be run in steady or transient mode. By default the model utilizes the rotation and curvature correction proposed by Spalart. Users may disable this feature by deactivating the -trc command line option of AcuSolve. 2.) Classical LES (large_eddy_simulation or les) This model corresponds to the fixed coefficient Smagorinsky subgrid scale LES model. This is an algebraic closure that requires no stagger to solve. Using this model, the large scale turbulent fluctuations in the simulation are resolved in time and space. This models requires that the Smagorinsky coefficient be changed for different types of flows. The Smagorinsky coefficient may be modified via the -smagfctcommand line on AcuSolve. This model may only be run in transient mode and requires sufficient mesh density to resolve the turbulent structures for accurate results. 3.) Dynamic LES (dynamic_model or dynamic) The dynamic subgrid LES model uses a filtering procedure to determine the appropriate Smagorinsky constant to use for specific flows. The filtering process is based on the Germano identity, and was further refined for unstructured meshes by Carati and Jansen. Using the dynamic model, the model coefficient varies in time and space to set the appropriate level of viscosity at each location in the flow. This model is also an algebraic closure that requires no stagger to solve. This model may only be run in transient mode and requires high levels of mesh density to resolve turbulent structures. 4.) Detached Eddy Simulation (detached_eddy_simulation or des) This model is a hybrid RANS/LES model based on the single equation Spalart-Allmaras RANS model. This model treats attached flow regions in RANS mode, and separated flow regions as LES. Starting withAcuSolve V1.7c, the default DES model uses the Delayed Detached Eddy Simulation (DDES, 2005) closure of Spalart. If users prefer the original DES formulation of Spalart (1997), the -ddes command line option of AcuSolve can be set to false. The DES models also utilize a constant coefficient subgrid model in LES regions. The value of this coefficient can be modified via the -desfct command line option ofAcuSolve. This model does require the solution of a turbulence stagger. This model may only be run in transient mode, and requires high levels of mesh density in separated flow regions to resolve turbulent structures.
General Applications The starting point for most applications should be the steady state Spalart-Allmaras model. For most industrial applications, this model provides sufficient accuracy. For applications involving massive separation, the DES model may be used if a higher level of accuracy is required. Unsteady Simulations For the simulation of unsteady flows, users have the option of unsteady RANS (URANS), DES, or LES. Depending on the goal of the simulation, different turbulence models may be used. If the unsteadiness in the flow is driven by some type of thermal transient, then the use of URANS (i.e. the Spalart-Allmaras model in unsteady mode) is typically sufficient. If the unsteadiness is due to large scale separation and bluff body vortex shedding, the DES model or LES model should be used. For cases where small scale turbulent structure is of interest, the Dynamic LES model should be used.