A Boundary Layer Thickness CFD simulation is the starting point for understanding how air slows down near a solid surface. When a fluid flows over a wall, the particles touching the wall stop moving completely. This creates a thin layer of slow fluid called the “boundary layer.” Understanding this layer is critical because it creates drag, which slows down cars and airplanes. A precise Boundary Layer Thickness Fluent analysis allows engineers to measure this invisible layer without using expensive wind tunnels.

This report details a fundamental Momentum thickness Fluent simulation. We simulate laminar flow over a flat plate to calculate three critical values: the standard boundary layer thickness (δ), the Displacement Thickness (δ*), and the Momentum thickness CFD parameter (θ). The displacement thickness tells us how much the air “pushes” the outer flow away, while the momentum thickness tells us how much energy is lost due to friction. By using ANSYS Fluent, we can compare these digital results against the famous theoretical “Blasius Solution” to prove the software is accurate. For more basics on how fluids move and interact with walls, please check our fluid mechanics tutorials: https://cfdland.com/product-category/engineering/fluid-mechanics-cfd-simulation/

Figure 1: The 2D computational domain used for the Boundary Layer Thickness CFD study

Simulation Process: Fluent Setup for Laminar Boundary Layer Simulation

The simulation process starts with a 2D model of the cylinder designed for structured grid generation to ensure high accuracy near the walls. We used ANSYS Meshing to create a high-quality mesh with 198,400 cells using a proper blocking strategy that aligns the grid with the flow direction. The main goal of this CFD simulation is to calculate the boundary layer thickness, momentum thickness, and displacement thickness at angles from 0° to 90° on the cylinder surface. This structured grid allows ANSYS Fluent to capture the velocity gradients very precisely in the near-wall region, which is essential for accurate boundary layer analysis.

Figure 2: The structured mesh generated for the Momentum thickness Fluent analysis.

Post-processing: Flow Separation and Drag Force Analysis

The first step in our analysis is to quantify the forces stopping the cylinder, based on the Force Report. The simulation calculated a Total Drag Force of 0.0084639514 N. It is crucial to look at the two parts of this force. The Pressure Force is 0.0082157322 N, while the Viscous Force is only 0.00024821928 N. This is a major engineering finding. The pressure drag is 33 times larger than the viscous (friction) drag. This happens because the flow separates from the back of the cylinder, creating a low-pressure vacuum that pulls the object backward. For a designer, this means that polishing the surface (reducing friction) will not help much. To reduce drag, they must change the shape to stop the flow from separating.

Next, we analyze the growth of the boundary layer using the plots in Figure 1 and Figure 3. We calculate the Displacement Thickness () using the governing equation

In Figure 1, looking at the 0° angle, the thickness is very small and stable because the flow hits the front of the cylinder directly. However, as we look at the 90° angle, the curve shoots up dramatically. This indicates that the Boundary Layer Thickness CFD value is becoming huge because the flow is detaching from the wall. We see the same trend in Figure 3 for Momentum Thickness (θ), calculated by

The momentum loss is small at 10° or 20°, but at 90°, the loss is maximum. This confirms that the energy of the air is destroyed as it tries to turn around the top of the cylinder.

Figure 3: Displacement Thickness profiles at various angles (0° to 90°) from the Boundary Layer Thickness CFD Study, showing rapid growth near 90 degrees.

Figure 4: Momentum Thickness variation vs. radial distance for different angles, calculated using ANSYS Fluent.

Figure 5: Velocity contours and streamlines visualizing the flow field and wake formation around the cylinder.

Figure 6: Skin Friction Coefficient plot along the cylinder surface, indicating the acceleration and subsequent flow separation.

Finally, these results help manufacturers predict the “Separation Point.” By comparing the Displacement and Momentum thickness, we derive the Shape Factor (). In the plots, the gap between the curves grows wider as the angle increases. At the top of the cylinder (near ), the boundary layer cannot stick to the surface anymore. This confirms why the Pressure Drag we found in the first paragraph is so high. The simulation proves that for a blunt body like a cylinder, the “Form Drag” (caused by separation) is the dominant force. An engineer using this Momentum thickness Fluent simulation would recommend adding a “fairing” or tail to the cylinder to keep the boundary layer attached and reduce the massive 0.0082 N pressure force.