Slip and Non-slip Flow Inside a 2D Microchannel CFD Simulation Using UDF | ANSYS Fluent Training

In slip flow, the fluid molecules don’t stick to the solid boundary completely like in non-slip flow. Instead, they slip slightly at the solid boundary. This effect often happens in microfluidic systems or when gases run through microchannels because the molecules’ mean free path gets about the same size as the channel. In slip flow, the molecules of the fluid face less resistance at the boundary. This leads to faster flow rates and different velocity patterns than in non-slip flow. This difference in behavior has a significant impact on how fluids move and behave in microscale systems. It affects things like lab-on-a-chip devices, nanofluidics, and gas flow through porous surfaces.

Simulating slip flow requires further effort in ANSYS Fluent. In this project, it is aimed to study the difference the Slip or non-slip flow regime causes inside a microchannel. This is done regarding a valid reference paper entitled “Numerical study of aerosol particle deposition in simple and converging–diverging micro-channels with a slip boundary condition at the wall”.

Simulation Process

Regarding the ultimate goal of the present project, a simple 2D channel is designed using ANSYS Design Modeler software as the computational domain. Then, a structured mesh grid is generated considering denser mesh near the wall regions.

More importantly, a user-defined function (UDF) must be written to apply slip flow regime conditions to the microchannel walls because the ANSYS Fluent itself doesn`t have predefined governing equations.

Post-processing

The simulation study was conducted to investigate the flow characteristics of slip and non-slip regimes within a microchannel. According to the following figures, the velocity profiles adjacent to the channel walls in the slip flow regime displayed a notable departure from the no-slip boundary condition found in the non-slip flow. As both Figure 1 and 2 indicate, the maximum velocity in the middle of the channel is higher when the non-slip flow regime dominates, however, it can be different in average velocity through the channel.

Figure 1- Velocity distribution along the microchannel a) non-slip b) slip flow regime

Figure 2- Velocity profile on a section line – red and blue lines indicate slip and non-slip flow, respectively