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 [1]”.

Reference [1]: Hosseini, Seyed Mohammad Javad, Ataallah Soltani Goharrizi, and Bahador Abolpour. “Numerical study of aerosol particle deposition in simple and converging–diverging

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 velocity contours show different patterns of flow behavior in the microchannel under slip and non-slip situations. The velocity profile shows the traditional parabolic distribution in the non-slip regime, with zero velocity at the walls and a maximum value at the channel centerline. The contour plots’ darker red areas show that the maximum centerline velocity is noticeably larger in the non-slip scenario than in the slip flow condition. A key variation from this typical tendency is seen in the slip flow regime, where the velocity at the wall boundaries remains finite, hence lowering the velocity gradient close to the walls. The molecular-level interactions typical of microscale flows, where the mean free path becomes equivalent to the channel dimensions, are accurately represented by this phenomena, which was caught by the applied UDF.

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

The red (slip) and blue (non-slip) lines represent the sectional velocity profiles, which offer quantitative information about these different flow patterns. The slip condition exhibits a wider velocity distribution with lower peak amplitude at the centerline but higher speeds close to the walls, even though the profile shapes in both scenarios remain similar. Because of the non-zero wall velocities, the slip scenario shows greater average flow rates but a maximum velocity that is around 15-20% lower. This leads to a more uniform velocity distribution over the channel width. This microscale flow physics is effectively captured by the UDF’s implementation of the slip boundary condition.

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