Visualize Boundary Layer and Y-Plus parameter

Ever wondered how to visualize the invisible forces shaping your designs? SOLIDWORKS Flow Simulation has options to visualize the boundary layers and y-plus parameters.

In the realm of computational fluid dynamics (CFD), understanding the behavior of the boundary layer and y-plus values is crucial for optimizing your designs. These parameters provide insights into the flow characteristics near surfaces, which can significantly impact performance and efficiency.

What is Y-plus?

Y-plus is a dimensionless parameter used in computational fluid dynamics to describe the distance from the wall to the first cell center in the mesh. It is important for accurately predicting the behavior of the boundary layer, which is the thin region of fluid near a solid surface where viscous forces are significant.

Unlike in other CFD programs, with SOLIDWORKS Flow Simulation we do not have to worry about the y-plus value or the cell size near the wall as they are handled internally by the software. This internal handling is one of the many reasons why SOLIDWORKS Flow Simulation is easier to set up than other CFD software programs.

Default Boundary Layer in SOLIDWORKS Flow Simulation

The default boundary layer type is determined by SOLIDWORKS Flow Simulation based on the Reynolds number and defined by the equivalent hydraulic diameter. The layer thickness is determined by the effective wall length and is governed by the Reynolds Number.

The boundary layer parameters at the model walls near inlets are always specified by default in SOLIDWORKS Flow Simulation. In the external flow analysis example shown in the image below, we have a local mesh defined around the frisbee. The finer mesh around the object will allow for higher fidelity and better visualization of the boundary layer. The y-plus parameter is an indication of the proximity of the first cell to the surface or wall. If the boundary layer is not resolved adequately, the values of y-plus won’t be accurate. This is because it depends on the velocity gradients on the wall, which itself depends on the mesh resolution. Per the SOLIDWORKS Flow Simulation technical reference: the software provides a “Two-Scale Wall Function” model that consists of two approaches to couple the boundary layer calculation with the main flow properties.

One method is the “thin boundary layer approach” which describes boundary layers on a coarse mesh with six or fewer cells across the boundary layer. In this approach, the Prandtl boundary layer equations are already integrated along the normal for the wall. This starts from zero and extends to the dynamic boundary layer thickness solving along a fluid streamline near the wall. The same approach is used for the thermal and concentration boundary layers where each boundary layer is considered with its thickness.

To describe boundary layers on a fine mesh with six or more cells across the boundary layer the “thick boundary layer” approach is used. In this approach, the calculation of parameters of laminar boundary layers is done via the Navier-Stokes equation, and for the turbulent boundary layer, a modification of the well-known “wall function” approach is employed.

We don’t have to manually choose one of the approaches since an appropriate boundary layer approach is automatically selected by SOLIDWORKS Flow Simulation. Once the analysis is complete, you can display the boundary layer by enabling the option in Cutplot.

Displaying boundary layer in Cut Plot

Viewing Y-plus

To view the y-plus, you can define a custom visualization parameter using shear stress, density, wall distance, and dynamic viscosity. Y-plus is a non-dimensional parameter and can be plotted using a surface plot. A low value of y-plus between one and thirty is suitable for most practical applications balancing result accuracy and computational cost.

Y-plus Surface Plot

Mesh Quality for Accurate Flow Simulations

To determine if the mesh is good enough for an accurate solution you can include an additional goal such as force or torque to monitor the change in its value when the mesh is refined in subsequent iterations. If the value doesn’t change much with further mesh refinement, then the mesh and boundary layer (including y-plus) are accurately resolved. Understanding y-plus values is essential for accurate and efficient flow simulations, particularly in applications involving complex geometries and turbulent flows.

If you are interested in learning more about SOLIDWORKS Flow Simulation or want to be connected with one of our technical experts, Contact Us here!