No-Slip - Boundary Condition

No-Slip boundary condition postulates that the velocity of the fluid layer directly in contact with the boundary is equal to the velocity of the boundary itself. As a result, there is no relative movement between the boundary and this fluid layer, leading to the absence of slip at the boundary.

No-Slip sets the velocity of the fluid at the wall to be equal to the velocity of the wall itself, typically zero, meaning the fluid sticks to the wall with no relative motion between the fluid and the surface. This condition is essential for accurately modeling the behavior of fluids near solid boundaries, ensuring that the shear stress and velocity gradients are correctly represented in the simulation.

The physical explanation behind this boundary condition is that particles in proximity to a surface experience limited motion relative to the flow when adhesion outweighs cohesion. This occurs at the fluid-solid interface, where the adhesive forces between fluid particles and solid particles dominate over cohesive forces among fluid particles.

As a consequence of this force disparity, the fluid velocity diminishes to zero. The No-Slip boundary condition is applicable only in scenarios of viscous flows, and when the continuum concept is applicable.

To define No-Slip boundary for a stationary wall, the velocity components must be set to 0. The alternative way is by using the Fixed Value boundary condition and manually setting all velocity components to 0.

No-Slip can also be applied for moving walls. In this case, the relative velocity must be considered, ensuring that the velocity of the wall and the fluid are the same.

In contrast to No-Slip, which sets the fluid velocity to zero at the boundary, Slip enforces a zero mass flux through the wall. This is achieved by mathematically ensuring that the normal velocity gradient (perpendicular to the wall) is set to zero, along with a zero gradient for the tangential component.

Graphically, these two conditions can be represented as in Figure 1.

The No-Slip is one of the most commonly used boundary conditions for wall boundaries. It fixes the fluid velocity to zero on the walls. It is important for any viscous flow to correctly determine drag and lift, energy dissipation, turbulence and many other physical properties. Below are a few examples demonstrating how this boundary condition can be used and how to correctly interpret its meaning.

SimFlow supports the User in definition of boundary condition. When a domain boundary is marked as Wall type in MESH panel, it is automatically assignes as No-Slip - Figure 2.

In this section, we propose boundary conditions that are alternative to No-Slip. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.