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AcuSolve uses a semi-implicit predictor/multi-corrector time integration algorithm that places no stability limits on the time step size. Therefore, the time step size is selected based on the physics of the problem, and not the stability constraints of the numerical method. This allows users to perform transient simulations with ease without the need to iterate on solution settings. When performing unsteady RANS simulations, the timestep should be selected to resolve the transient phenomena of interest in the simulation. For example, in the case of vortex shedding behind a bluff body, the timestep would be set to resolve the shedding frequency. For most cases, resolving the shedding cycle with 30 time steps per period is sufficient to get a good estimate of the magnitude and frequency of the fluctuating forces on the body. When performing LES and DES simulations, the timestep size needs to be set according to the size of the turbulent eddies that you expect to resolve. For LES and DES, the size of the turbulent structure that the model can resolve is closely tied to the local element size. Therefore, the time step size should be related to the element size. Therefore, it is suggested that the following formula be used to determine the appropriate time step size: Î”t = CFL* Î”x/umean Where: Î”t=time step size, Î”x=characteristic element size, and umean=mean velocity, CFL = Courant-Friedrichs-Lewy number. The CFL should be set to ~5-10 or less for DES simulations. For LES simulations where very small turbulent structures are of interest, the CFL number should be set to approximately 1 for the highest accuracy. It should be noted that these are only guidelines to use for setting an initial time step size. A time step sensitivity should be done to determine when the solution becomes time step independent.
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.
AcuSolve supports three different techniques of modeling turbulent boundary layers. The first, and most accurate technique is to fully integrate the equations directly up to the no-slip wall. When the user selects "Low Reynolds Number" for the turbulence wall type, AcuSolve uses this procedure to model the boundary layer. When using this technique, it is important that the user constructs the mesh such that the first node off of the wall is within the laminar sublayer (i.e. y+ <= ~8). If the y+ exceeds this value, large errors in the accuracy of the shear stress can be introduced. Note that these guidelines are valid for the DES models and Spalart-Allmaras RANS model. When using LES, the first node off the wall should be at a y+<1.0. The wall normal mesh spacing should increase with a stretch ratio of ~1.3 until it smoothly blends into the surrounding volume mesh. The second type of treatment for turbulent boundary layers is the use of a wall function. When the Turbulence Wall Type is set to wall function, AcuSolve uses the well known "Law of the Wall" to model the boundary layer. When employing this technique, the first node off the wall should be placed between a y+ of 1-300. For extremely high Reynolds Number flows, the upper bound of the y+ limit may be extended beyond 300 without sacrificing accuracy. Note that AcuSolve has no lower limit on the near wall spacing when using the wall function. When in the viscous sublayer, the wall function recovers the Low Reynolds Number solution. This type of wall function can be used with either the DES or RANS models, but is not suggested for use with LES models. The third type of wall model that is offered by AcuSolve is the running average wall function. When this model is employed, the wall function is evaluated using the running average velocity field and not the instantaneous field. The meshing requirements for this model are the same as for the standard wall function. This approach is typically used with LES and DES models, but may also be used with RANS if appropriate. Note that this requires the Running Average field to be turned on in the simulation.