# Boundary Layer Parameters

Boundary layer parameters can only be applied in the Mesh Attribute panel for surface groups. To activate boundary layer specification, the Boundary layer flag must be turned On.

- First element height
- Total layer height
- Growth rate
- Number of layers

At least three of these four attributes need to be specified in Full control mode. The fourth attribute is then computed internally by AcuConsole.

## Resolve

The selection for Resolve should be set depending on which attribute you want AcuConsole to compute. If you provide the First element height, Total layer height and Number of layers, AcuConsole resolves the Growth rate.

## Boundary Layer Type

When Boundary layer type is set to Match outer layer, only the First layer height and the Growth rate are specified. Boundary layer elements will be grown until the mesh size of the top layer matches the mesh size of the volume into which the boundary layer elements are grown.

- Full control: The full control setting requires you to specify three of the boundary layer parameters described above. The fourth parameter is then computed automatically by AcuConsole.
- Growth rate and layers: When specifying growth rate and layers, you have no control over the first layer height of the elements. Using this technique, you enter the growth rate of the boundary layer mesh and the total number of layers desired. The mesher then computes the initial layer height that yields the growth rate specified, number of layers specified, and has the last element in the boundary layer mesh matching the volume of the surrounding volume mesh.

## Boundary Layer Elements Type

The Surface Mesh Attributes panel also has an option to select the Boundary layer elements type. The default value is set to Tetrahedron, which means that the mesh generator splits the boundary layer elements into tetrahedrons.

If you want the mesher to generate prism/hexahedral elements in the boundary layer, then Mixed should be selected. Note that the meshing algorithms used between the mixed and tetrahedron options are slightly different. So, slight variations in the behavior are expected. You are also cautioned that using mixed elements on walls that are adjacent to inlets will cause complications with the inflow types of mass_flux, flow_rate, and average_velocity. These are special inflow conditions that are enforced on each surface of the inflow. If two mesh types are present at the inflow, this will result in two surfaces and undesirable behavior when using these boundary conditions.

## Boundary Layer Blends Flag

Boundary layer blends is a method of meshing sharp edges where diverging normals are present in the boundary layer element stack. When this option is turned on, a radial array of elements is created around the sharp edge to provide better resolution of the flow features. The sweep angle is used to control how many degrees each radial division spans.

## Boundary Layer Propagate Flag

The boundary layer propagate flag is used to anisotropically grade the unstructured mesh at the last layer of boundary layer elements to provide a smoother transition in size with the surrounding volume mesh. Note that this feature can lead to large levels of refinement if there is a large discrepancy in size in the wall normal direction between the last layer of elements in the boundary layer stack and the surrounding volume mesh.

## Estimate Y+ Method

Finally, this panel has an option to compute the Estimated Y+ Method based on the flow type. The options are None, Flat Plate, Pipe or User.

- None: Perform no estimation of the y+ based on the first layer height that is specified.
- Flat Plate: Estimate the y+ value based on a flat plate correlation and the parameters specified for first layer height.
- Pipe: Estimate the y+ value based on a pipe flow correlation and the parameters specified for the first layer height.
- User: Estimate the y+ value based on a user defined correlation and the parameters specified for the first layer height.

When selecting Flat Plate the options shown below are available.

When selecting flat plate as the estimated y+ method, the following correlation is used to estimate the Y+:

Y = 5.81*Y+*(nu/U)*Re^0.1

- Velocity Scale: The freestream velocity over the flat plate (U in the equation above)
- Length Scale: Distance from the beginning of the boundary layer. Used to compute the Reynolds Number in the above expression.
- Estimated Y+: The estimated y+ that results from the parameters input above and the first layer height that is specified in the meshing controls. Note that the viscosity is extracted based on the material model that you specify.
- Estimated Reynolds Number: The estimated Reynolds Number that results from the parameters that are entered.

When selecting Pipe the options shown below are available and the following correlation is used for the estimation of Y+:

Y = 5.03*Y+*(nu/U)*Re^0.125

- Velocity Scale: The mean velocity in the pipe (U in the equation above)
- Pipe Diameter: The hydraulic diameter of the pipe.
- Estimated Y+: The estimated y+ that results from the parameters input above and the first layer height that is specified in the meshing controls. Note that the viscosity is extracted based on the material model that you specify.
- Estimated Reynolds Number: The estimated Reynolds Number that results from the parameters that are entered.

When selecting User the options shown below are available. This option allows you to customize the expression that is used to estimate the y+. The following general form is used for the expression:

Y = A*Y+*(nu/U)*Re^B

- Velocity Scale: The mean velocity to use in the computation (U in the equation above)
- Length Scale: The length scale to use in the expression above. Used when computing the Reynolds Number.
- Coefficient: A in the above equation
- Power: The exponent used in the above equation
- Estimated Y+: The estimated y+ that results from the parameters input above and the first layer height that is specified in the meshing controls. Note that the viscosity is extracted based on the material model that you specify.
- Estimated Reynolds Number: The estimated Reynolds Number that results from the parameters that are entered.

## Material Model

The material model is needed as part of the y+ estimation process to enable computation of the viscosity. The available options will depend on what material models have been defined in the database.