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AcuSolve contains a set of boundary conditions that automatically sets a boundary layer profile at an inlet boundary. When using the inflow boundary condition types of mass_flux, flow_rate, and average_velocity,AcuSolve computes an appropriate boundary layer profile for the velocity and turbulence fields based on the the distance from no-slip walls, and estimated Reynolds Number. The profile is re-computed at each time step such that deforming meshes are properly accounted for in the calculation. This boundary condition provides a robust method of automatically specifying physically realistic inlet conditions. It is much more realistic than specifying a constant velocity condition for internal flow applications.
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.